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          Introduction         

The Fal estuary is located in the southwest of England. The estuary is a ria (drowned river valley), and was formed at the end  of the last ice age, during which glacial movement cut a channel through the area. Since then, the sea level has risen by 120m (Bindoff et al, 2007) due to eustatic change, resulting in the formation and subsequent flooding of a river valley. The melting of the glaciers after the ice age has resulted in isostatic rebound. The removal of ice cover over Northern Europe has allowed the land to rise, causing the south of the UK to sink.

The Fal estuary is classed as a Special Area of Conservation (SAC). This is mainly due to the high numbers of seagrass (Langston et al, 2006) and maerl beds at St. Mawes bank, providing important habitats for a number of species.

The Fal estuary is macrotidal, with a mean tidal range of 5m and maximum tidal currents below 2 knots.

In recent years, dredging of the seabed has been proposed in order to promote 'sustained long-term growth of Falmouth’s cruise business'. As the increased traffic may have an effect on the SAC, it is important to understand the many different processes occurring within the estuary in order to understand the full impact that this dredging may have on the area.

Aim: To gain an understanding of the physical, chemical and biological processes in the Fal Estuary and the surrounding coastal waters.

          Equipment & Methods         

          Estuary         

INTRODUCTION

Estuaries form an important transition zone between saline marine environments and fresh riverine environments, and their inherent physical, chemical and biological properties help us understand the processes which occur in the estuary. These semi-enclosed bodies of water typically have high nutrient values due to the freshwater input and therefore are zones of high productivity, making estuaries a very dynamic and interesting area of study.

As  two groups were sampling, one in the morning and one in the afternoon, this allowed them to sample the river more intensely with one group (AM) focusing on the upper part of the estuary and one group (PM) focusing on the lower part of the estuary. This allowed for a higher resolution when assessing the estuaries characteristics.

 

RESULTS

BIOLOGICAL - Chlorophyll

Station 1 has two similar peaks at depths 3.5m and 1m. Due to the sample area being shallow, deeper readings could not be done. A strong decrease in chlorophyll concentration can be observed at depths 2m and 5m. The 5m depth value at station 1, being 0.456 µg/l, is one of the lowest observed along the estuary. Only station 3 reaches a lower concentration of 0.0846 µg/l at 6m depth. Station 3 chlorophyll concentrations peak to an overall highest value of 2.154 µg/l at 12m. (Compared to studies by Langston et. al 2003, these values seem significantly lower. These studies showed measurements of around 20µg/l from the Truro river). From then on chlorophyll concentrations drop again, indicating a decrease in phytoplankton population density. Station 4 does not have the same fluctuations as observed in all other stations. Instead it has a slight increase in concentrations, followed by a smooth decrease with increasing depth. Station 5 had no depth recorded values and hence only the surface value was recorded and plotted. This may not allow for a full understanding of the changes with depth, but one can observe the surface changes of chlorophyll concentrations down the estuary. Between stations 4, 5 and 6 there is a clear gradual increase in surface chlorophyll concentrations, showing that phytoplankton population increases at surface with increasing distance down the estuary. At Station 6 high chlorophyll can be observed at surface and at 5m. One can observe a decrease of concentrations with depth.

Chlorophyll concentrations are dependent on various factors such as oxygen concentrations, turbidity and light intensity. Station 4 has a clear increase in concentration, which correlates to oxygen concentrations found at that site. Oxygen saturation decreases at around 5-10m (Figure 7), which is the similar depth at which chlorophyll rises in concentrations. This may be due to chlorophyll respiration, causing oxygen depletion (Johsnon et al., 2002). All stations experience decrease of chlorophyll with depth, while oxygen saturations show increases at 15m and beyond. This may then be related back to the turbidity of the water masses as well as light intensity. Looking at the Richardson graphs (Figure 26) for station 4, one can see that the water column is mostly turbid and well mixed. High turbidity would cause phytoplankton to be mixed too deep into the water column and hence reduce the amount of light exposure (Cho, 2007). This close correlation between chlorophyll and oxygen saturation can also be observed at station 1 and 3. At station 3 the saturation rises with depth due to a decrease in chlorophyll and station 1 has a clear fall-peak-fall chlorophyll pattern in the upper 5m of the water column, which matches the rise and fall of the oxygen saturation at similar depths.

 

BIOLOGICAL - Plankton

The Fal Estuary was sampled by two survey teams before and after high water (HW) (Table 1). Overall phytoplankton diversity was high with a greater abundance of 640 cells/ml approaching HW compared to 99 cells/ml post HW. An overall diatom dominance of 87% was typically representative of seasonal summer temperate waters (Figure 1-3)(Kraberg et al., 2010; Widdicombe et al. 2009). Both teams sampled phytoplankton at different depths and dissolved oxygen % saturation measured as a proxy of phytoplankton activity was lowest in the surface 10m water column at most stations. This activity was supported by a surface chlorophyll peak of 1-2 ug/L above 10 m however these levels were below the UK indicator value of 20 ug/L but above the 0.5 ug/L suggestive of a bloom decline (Widdicombe et al., 2010). It is suggested that the 210 µm mesh plankton nets sampled smaller diatoms representative of an intermediate bloom succession (Graph1)(Figure 4)(Langston et al., 2003).

 

Heterotrophic holoplankton copepod and meroplankton copepod nauplii dominated both the estuary and offshore zooplankton. Surface water zooplankton abundance of 1.0 x 10⁷ cells per m³ was more than double that sampled in the afternoon suggesting lateral travel on the ebbing tide to feed on phytoplankton prey (Figure 5)(Larick and Westheide, 2006). Phytoplankton sampling was limited to deeper waters in the afternoon and no night sampling was undertaken when zooplankton use the protection of darkness to vertically migrate to feed (Somerfield, Gee and Warwick, 1994)(Figure 4 & 5).

 

PHYSICAL - CTD

Figures 3a and 3b both give us insights into the vertical mixing processes in the estuary. They both show an established thermocline and halocline at station 1 - the station highest up the estuary - which leads us to believe that mixing in the upper section of the estuary is limited. However, as you travel further down the estuary, the gradient of the profiles decrease, indicating that the mixing is increasing and causing the water column to become more homogenous for both temperature and salinity. Station 6 shows a well mixed, homogenous layer from surface to bottom. This is reflected in the T-S plot (Figure 3c) which shows station 6 with a consistent salinity and only a small change in temperature and station 6 with a large change in both salinity and temperature. The fluorometer plot (Figure 3b) shows us that surface levels of chlorophyll a decrease from the riverine end of the estuary to the marine end, and shows some possible formations of deep chlorophyll maximums at the bottom/below the thermoclines.

 

PHYSICAL - Light Attenuation

The data shows that as you move down the estuary towards the sea, the average secchi disk depth increases from 2.85m (at station 1 – top of the estuary) to 6.9m (at station 7 – bottom of the estuary). The increase in (Z_SD) as you go towards the sea implies that transparency of the water increases (or turbidity decreases) as you move towards the sea. From average Z_SD for the stations, the euphotic zone depth was calculated and the calculations show that the depth of the euphotic zone increases moving towards the sea. The attenuation coefficient decreases as you move towards the sea which supports an earlier fact that the water becomes clearer as you move towards the sea.

During the practical, it was observed that the k decreases as you move toward the sea, meaning that the secchi disk depth increases (because k = 1.44/ Z_SD) suggesting an increase in the transparency of water (or a decrease in turbidity). This increase in transparency seems to cause an increase in the concentration of chlorophyll in the water column. The increase in chlorophyll concentration in the water column is an indicator for an increase in primary production. Increase in primary production is due to the increase in depth of the euphotic zone, meaning light penetrates further into the water hence, more phytoplankton activity. Rivers and estuaries are usually higher in turbidity than the open ocean; this is because river flow rates are sufficient to cause material on the seabed to be suspended in the water column. However, as the river reaches the estuary, its flow rate decreases. The decrease in flow speed causes deposition of material, therefore the transparency of the water in the estuary increases as you move down the estuary towards the sea, hence less light is attenuated by the water column. And the depth of the euphotic zone increases as less light is attenuated. 

PHYSICAL - ADCP

Station one (Figure 6a) was the furthest up the estuary and therefore had the shallowest water. At this station, the depth of the water column does not exceed 10m. The backscatter of the ADCP seems to increase from right to left starting off with about 0.121 m/s, increasing with depth to around 0.317 m/s – 0.416 m/s continuing to increase with decreasing depth to flows of up to 0.514 m/s. It is a very gradual increase in flow velocities that may be influenced, but not limited to, the small increase in depth in the centre.

At station 2 (Figure 6b), the depth of the estuary increases compared to station 1, however the pattern of increasing flow from right to left still seems persistent. Overall however, the flow is slower than previous. The right hand side has flow velocities between 0.017 m/s to 0.112 m/s and increases to values of about 0.399 m/s on the left. To the far left of the transect backscatter decreases again. The highest flow rates can be observed closer to the surface, whereas the bottom waters have a generally calmer flow between 0.208 m/s and 0.303 m/s.

Station 3 (Figure 6c) has no obvious patterns as previously observed. It appears to be slightly more uniform, where flow velocities lie in the range of 0.064 – 0.122 m/s. The flow seems a bit increased at the right of the transect, where depth decreases. At about 10m in the centre of the transect, there is a very obvious increase in backscatter showing an increase in velocity of the water masses present there. The increased velocity ranges from about 5-12m and has flow rates up to 0.237 m/s. Although high, it is not as high as previously observed values at station 1 and 2. The flow seems to decrease slightly towards the left of the transect.

At station 4 (Figure 6d) the water mass seems to have roughly the same velocity throughout the transect, with values ranging between 0.004 – 0.181 m/s. At the left of the transect a small patch of decreased backscatter can be observed, showing a decrease in flow velocities. The ADCP shows a deep caving, which is the beginning of the deep channel that runs through the Fal estuary. In there the flow rates seem to have decreased slightly. Surface waters in general are uniform, with quite similar velocities throughout the transect

Station 5 (Figure 6e) shows a blank spot which represents the deep channel that runs through the Fal estuary. Due to its depth being beyond 20m, the ADCP was not able to do any recordings. There was overall low backscatter recorded. The ADCP shows an up cropping bed form feature on the right and left of the deep channel. At that point the flow velocities recorded increase from around 0.010 m/s to 2.752 m/s. These velocities are still not as high as those observed in station 1. In general the surface waters seem to have a slightly higher flow rate than the bottom waters

Station 6 (Figure 6f) similar to station 5, the flow around the opening of the deep channel is low, at values of around 0.005 m/s. It also shows a small up crop that causes an increase in the water flow to 1.972 m/s. Towards shallower waters the backscatter increases showing a slight increase in flow rates. In general the surface waters seem to have a slightly higher flow rate than the bottom waters.  To the left of the transect the water has a slower flow than to the right. The water at station 6 is deeper than at station 5

As the 2 groups conducted their survey at different parts of the day (AM and PM) and high water was at 11:24, we can see the tidal velocity profiles of the estuary for the incoming flow tide in the morning (station 1-3) and the outgoing ebb tide (station 4-6) in the afternoon. As the tide at the top of the estuary is mesotidal, we can see that the tidal currents have less velocity than those at the bottom of the estuary, where the tide is macrotidal. These tidal currents and their magnitudes have a large effect on the physical structure of the estuary.

The ADCP on the boat can sometimes give us spurious results due to a number of different factors. Firstly the chop from the engine can cause air bubbles to form at the surface, changing the way that the ADCP sonar waves travel through the water and creating noise on the velocity and backscatter plots. Secondly things can block the ADCP sonar. For example at stations 5+6 you can see a lack of data in the channels in the estuary. This could possibly be due to the ADCP being taken during the CTD drop and obstructing the ADCP sonar.

 

PHYSICAL - Richardson Numbers

The Richardson number is a good indication of the stability of the water column in the estuary. As the CTD salinity measurements on the Conway vessel were not correctly calibrated, the salinities measured in-vitro were used  to calculate the densities of the samples taken to analyse the chemical composition of the estuary.  The densities were used to calculated Richardson numbers. This is why our Richardson numbers are only calculated for certain depths, and not at all for stations 2+4 (no water samples were taken on these drops).

Station 1 has a transition from strong mixing waters with Richardson values below 0.25 into stable water flow of Richardson values above 1. Between 2m and 3.5m the flow resides in the area between 0.25 and 1, indicating shear instability. The water flow in the top 2m of the water column has low Ri numbers which show that there is strong mixing. Between 3-5m one can see the decrease in flow and hence a decrease in mixing. The point at 2m could be an indication of the position of the thermocline, as the flow rate decreases causing stratification.

Station three had more samples taken than station 1 and so it was possible to create a clearer depth profile. Between 2-6m the values lie perfectly in the area between 0.25 and 1, indicating low mixing and the flow will be dependent on temperature and density. At 8m depth there is a high peak in the values, where the flow becomes stable and less mixed. The stability of the flow gradually decreases again as depth continues to decrease. The values then maintain themselves in the area between 0.25 and 1, again indicating well mixed waters. The middle can be assumed to be stratified and surrounded by two well mixed layers of water.

Station 5 shows decreasing Richardson number with depth in the top few metres, and slight increase beyond around 7m. The vast majority of the water column had a Ri of below 0.25- indicating a turbulent water column. At around 4m depth the flow lies in the area between 0.25 and 1 indicating that the flow is dependent of density and temperature. Stratification occurs in the surface layers due to thermal heating, forming a thermocline.

Station 6 appears to show turbulent surface waters, with Ri of less than 0.25 and gradual stratification with depth beyond around 13m. At around 20m the Ri is over 100, indicating a strongly stratified water column. One can assume the water column experiences a transition period, shown by the area between 0.25 and 1, which may be due to a thermocline.

 

PHYSICAL - Residence Times

 

The residence time of the Fal estuary was calculated by using the formula:

 

                                                                   

The mean estuarine salinity was calculated from all recorded values from station 1 through 6 (see table 2).The sea salinity was the marine end member recorded using a CTD at station 6. The total volume of the Fal is roughly 1.35×10⁷ m³. In order to find the discharge rate into the Fal, data was obtained from the Centre for Ecology and Hydrology. The database had daily readings of discharges into the Fal estuary since 1978. For the purpose of this investigation data from 2000 -2010 was used, allowing for a 10 year average (see table 3). 

	Station 1	Station 2	Station 3	Station 4	Station 5	Station 6
Salinity	25.8	31.2	32.3	35	33.9	34.5

Table 2 -  Salinity values recorded at each station along the estuary

Average Salinity is hence:  32.12

Year	Salinity
2000	2.98
2001	2.09
2002	2.33
2003	1.64
2004	1.76
2005	1.49
2006	1.48
2007	1.97
2008	1.94
2009	2.21
2010	1.59
Average	1.95

Table 3 – Average yearly discharge into the Fal from 2000 to 2010 and the final average value for the 10 year range

One should note that the values provided from the Centre were from the Fal at Tregony, which is beyond the point of sampling. Hence the calculated value may actually be an understatement of the actual residence time, due to lack of sampling further up the estuary.

The calculated residence time for the Fal estuary is hence:

The Fal estuary has a fairly short residence time, allowing for incoming pollutants and other harmful substances to be flushed out and allowing for uncontaminated water to come in.

 

CHEMICAL - Oxygen

Most stations show an increase in saturation at a certain depths, stations 1 and 4 show this increase in the first few metres, whereas station 3 shows an increase around 10m. Station 6, the most marine station shows a slight decrease with depth, however a surface value is absent due to error and a mid-depth value is significantly higher, suggesting an anomaly.

Oxygen concentrations are strongly influenced by phytoplankton abundance and blooms. Due to respiration phytoplankton will cause a decrease in oxygen saturation. Oxygen saturation decreases at around 5-10m, which is the similar depth at which chlorophyll rises in concentrations (see chlorophyll analysis – estuary). The saturation of oxygen can either rise or fall with depth. Rising would indicate a well-mixed water column, where surface oxygen is mixed to deeper layers. A fall in saturation could occur due to lack of replenishment to the depth. Concentrations of oxygen can also be impacted by high amounts of bacteria caused due to pollution. It is known that the Fal Estuary has major mining discharges into it, containing heavy metals that impact the biology as well as chemistry (Bryan, G. and Gibbs, P. 1983). One can see that oxygen saturation is fairly low at the top of the estuary at station 1. This could be due to eutrophication leading to high amount of bacteria, which would consume the oxygen in the upper water column and change the chemistry (Jorgensen and Richardson, 1996). It would cause a change in biological composition of the water. The oxygen saturation depth profiles can be more of a representation of the seasonal cycles of nutrients and chlorophyll, such as seen in station 4 (see chlorophyll analysis  - estuary).

CHEMICAL - Silicon

The composition of estuarine waters varies considerably, depending on the main source of dissolved salts (Olaussan and Cato 1980 p-72). Dissolved constituents that enter the estuary behave in accordance to the physical and geochemical characteristics of the estuary. For some constituents the estuary is a simple chemically conservative mixing interface between the rivers and the ocean (Loder and Reichard 1981). For others the estuary is an environment that acts as a reaction vessel, resulting in substantial depletion and addition.

Figure 8 displays the behaviour of dissolved silicon Si(OH)₄ in the lower part of the Fal estuary. Silicon is a key dissolved constituent in estuarine environments and is often correlated with phytoplankton production. Diatom populations are particularly vulnerable to silicon fluctuations because they require silicic acid to produce external skeletal material. Thus considerable variation in silicic acid concentrations can determine the dynamics in phytoplankton communities and regulate the amount of primary production in a given estuary. When diatom blooms occur in the spring and autumn when irradiance levels are sufficient for substantial for primary production, silicon concentrations plot below the theoretical dilution line (TDL), which indicates the dissolved constituent, is behaving non-conservatively. Substantial addition to the environment from anthropogenic or from naturally occurring physical processes can also result in non-conservative behaviour. In this case silicon concentrations will congregate above the TDL. Conversely when silicon values display linearity it is said to behaving conservatively i.e. dissolved silicon is behaving in relation to the physical mixing between the riverine and marine end member.

In this case samples collected from the lower end of the Fal estuary are behaving conservatively (R²= 0.9992), typical of a well-mixed estuary. Values are extremely low which is to be expected in the lower part of the estuary where any external addition of silicon is negligible and quickly diluted. Despite strong correlation there seems to be a possible outlier (i.e. an anomaly in the data set) at salinity value 34.7. Despite thorough preparation and data handling control human error is inevitable possibly skewing the data set. The riverine end member was collected prior to the investigation from the Truro tributary and was multiplied by 5 because of the dilution factor, yielding the true concentration. The marine end member was taken from the concentration at the highest salinity in this case 0.71mmol.

Conversely Silicon values found in the lower estuary lie above the theoretical dilution line, which suggests that silicon is behaving, non-conservatively (R²=0.8679). Values found between 30.1- 32.4 lie above the TDL, which suggests that silicon is been replenished from an external source, possibly from the various tributaries that flood into the estuary. Possible anthropogenic impacts from sewage run off could also cause fluctuations in dissolved constituents in the lower part of the estuary. Beyond >32.4 Psu silicon becomes depleted, which suggest biological utilisation in the intermediate salinities. However all these assumptions are based on a steady state environment, ignoring Flushing time and residence time values (in this case 7 days for the Tregony river). Also the relatively low freshwater inputs from the six main tributaries maintain high salinity values, which explains why all the samples were collected between 25- 35 psu. Thus the interpretation of theoretical dilution diagrams must be undertaken with an understanding of the variability in behaviour of dissolved constituents and their relationship to the mixing properties and flushing times (Loder and Reichard 1981).

 

CHEMICAL - Nitrate

Conservative behaviour shown by Nitrate along the estuary, general structure shows a decrease in concentration of Nitrate with an increase in Salinity. Data points follow TDL, which doesn’t indicate any addition or removal of nitrate. When comparing riverine end member concentrations with Langston et als (2003) nitrate concentrations in the Kennal and Truro river, our calculations show significantly lower nitrate concentrations than theirs. In order to convert their values in mgl/l to µmol/l we had to use the molecular mass of presumably NO₃ (unsure of whether Langston et al used nitrite or other). From their report, the Kennal river displayed values of 83mg/l compared to our end member concentrations of 32.83mg/l which is significant difference. This lower value could be due to long period of rainfall over the past few weeks, which has “diluted” the nitrate within the estuary.

 

CHEMICAL - Phosphate

Figure 11b shows how phosphate concentration varies across the Fal Estuary. The concentrations of the marine end member and the river end member are 0.0073umol/L and 0.5770umol/L respectively. The figure only shows a partial picture of the Fal Estuary, this is because concentrations for lower salinities were not obtained, and the lowest salinities recorded were 25.8. As a result of this, the river end member was provided at the lab. From Figure 10a can observe that the data point do not plot on or close to the theoretical dilution line (TDL), the all data points save one, lie above the TDL, this means that phosphate is added into the system as you move toward the sea (higher salinity end). This addition of phosphate means that it behaves in a non-conservative manner (conservative behaviour means that it will plot along the TDL).

To allow us to compare our values with other reports on the estuary, we had to convert our phosphate concentrations firstly to orthophosphate and then from µmol/L to mg/L using the molecular mass of the compound.

Comparing the riverine end member to Langston et al (2003) various orthophosphate concentrations in the Truro River, our end member appears to be lower than the orthophosphate concentrations in the Truro River, which are 0.1mg/l, whereas from our calculations we calculate orthophosphate concentrations of the end member to be 0.04mg/l. Looking at Langston et als other river concentrations our results show a low concentration of orthophosphate in the estuary, however it is still within the limits calculated by Langston et al.

Discussion:

The addition of phosphate into the system could be attributed to the fact that there is more than one river input into the system for example; River Fal, Truro River, Percuil River and Mylor. The inputs from these rivers are likely to add phosphate into the system due to the number of rivers and tributaries into the estuary.

Phosphate concentration can be affected what is happening at the sediment boundary. Changes in weather and sedimentation rates in the short term could affect the concentration of phosphate in the water temporarily. If there was a storm in the area, wind speeds and flow rates would be higher in the area. This means that there is more input into the estuary and higher wind speeds could stir up the sediments at the estuary bed and release nutrients into the water hence increasing the concentration of phosphates in the Fal Estuary.

The pH of the water could also be the cause of the addition of phosphate in the lower estuary. Adsorption is highest when pH is 3 – 7. Therefore, adsorption rates should decrease as you go towards the sea as the pH for seawater is about 8.

Concentrations of phosphate varies annually, it is observed that concentrations are highest in the summer. Phosphates are remineralized as it reacts with iron to form ferric phosphate which is insoluble. As summer progresses, the estuary becomes more anoxic due oxygen consumption being greater than oxygen production rates (from phytoplankton), this improves the rate of reduction of iron, which releases phosphate from ferric phosphate into the water. This would cause an addition of phosphate in the estuary.

 

CONCLUSION - Estuary

The data collected within the Fal estuary contains biological, chemical and physical parameters. The transition between different parts of the estuary was observed using the data collected.

Surface levels of chlorophyll were found to decrease from marine to the estuary end, the nitrate decreases towards the marine end and has been seen to been added to the water in the riverine end due to anthropogenic inputs. Turbidity increases with increasing salinity due to reduced flow speeds resulting in natural deposition of suspended material. Phosphate addition occurs most in the summer periods. Addition occurs due to inputs from more than one river and also oxygen production rates, reducing iron so added ferric phosphate to the water. All these can be affected by the spring plankton blooms. The zooplankton was dominated by copepod and copepod nauplii. Silicon behaves conservatively in the estuary and shows addition this can also be due to anthropogenic inputs.

The ADCP was essential in assessing the level of stratification in the estuary, indicated by the calculation of Ri numbers using the velocity magnitude. It also helped us understand the tidal processes in the estuary, helping us evaluate the impact of the tides on the vertical and horizontal structure of the estuary.

The depths of the estuary samples were quite shallow so making detailed depth profiles difficult.

 

 

(a) (b)

Figure 1a-b : (a) The Fal Estuary and its tributaries (b) study transects across the estuary.

Figure 2 - Average chlorophyll values calculated for each site depth profiles (Station 1 being the furthest up the estuary and Station 6 furthest down.)

(a)

(b)

Figure 3a-b: (a) Fal Estuary survey times, locations, duration and depth with a local tidal table indicating high water (HW) and low water (LW); (b) Estuary am (site 1 &2) and pm (site 3 & 4) zooplankton taxa

(c) (d)

(e) (f)

Figure 3c-f: (c) Phytoplankton taxa; (d) Dominant Fal estuary phytoplankton; (e) Fal estuary phytoplankton species; (f) Copepod abundance am and pm in Fal estuary

(a) (b)

(c) (d)

Figure 4a-d: (a) Estuarine Salinity-Depth Plot; (b) Estuarine Chlorophyll-Depth Plot; (c) Estuarine Temperature-Salinity Plot; (d) Estuarine Temperature-Depth Plot

Figure 5 - Table showing averaged Secchi disk depths, attenuation coefficient, and depth of euphotic zone at each station.

(a) (b)

(c) (d)

(e) (f)

Figure 6a-f: (a) ADCP Transect 1; (b) ADCP Transect 2; (c) ADCP Transect 3; (d) ADCP Transect 4; (e) ADCP Transect 5; (f) ADCP Transect 6

Figure 26. Ri for Station 1,3,4 and 6.

Figure 7 - Oxygen saturation depth profile for the estuary (Stations 1, 3, 4 and 6). Anomalous points for stations 3 and 6.

Figure 8a. Lower Estuary Silicon Theoretical Dilution Line (TDL)

Figure 8b. Estuary mixing diagram

Figure 10. Nitrate mixing diagram for Fal Estuary plus close-up.

(a) (b)

Figure 11a-b: (a) Phosphate against absorbance; (b) Phosphate depth profile

          Offshore         

INTRODUCTION

Vertical mixing in the water column has a large influence on primary production in the ocean.  During the seasons, variation in mixing processes will regulate the nutrient concentration in the surface layer of the ocean. For example during the summer months, the increase in solar radiation and the strengthening of the thermocline will lead to an increased level of phytoplankton production, in turn leading to depletion of nutrients in the area (e.g. Silicon and Nitrate). The degree to which the nutrients can be replaced will depend primarily on the amount of vertical mixing that will occur, drawing up nutrient rich water from deeper depths.

Our investigation was carried out on six stations in the western English Channel. Weather constraints (high winds leading to rough seas) prevented us from sampling areas further out into the channel, and therefore we sampled closer to the coast than initially expected (see Figure 12).

Each station consisted of a CTD drop and an ADCP profile, from which we analysed the water column structure and decided whether to deploy Niskin bottles to collect water samples and at what depths. If deemed necessary, the Niskin bottles were fired as the CTD ascended. The samples were then analysed at a later date in the lab where we tested for plankton abundance and species composition, nutrient concentration of N,Si and P, oxygen concentration and chlorophyll a.

RESULTS

Biological - Chlorophyll

Station 1 only had two depths sampled, hence not giving a detailed overview of the depth profile. However, it indicates a slight increase between depths of 11m to 30m. Station 2 shows high surface values which overall decrease with depth. A small peak can be observed at 22m, however the chlorophyll concentration at depth does not reach that of the surface values. Station 3 has a similar surface value to station 2, however at about 12m it peaks to the overall highest recorded concentration of 4.4 µg/l. The peak is followed by a sharp decline in concentrations as depth increases. Station 5 is a rather low chlorophyll area, having the overall lowest values recorded at surface. As with station 1 it does not allow for a detailed depth profile, but does indicate a slight increase with depth. Station 6 does not have fluctuations as strong as station 2 and 3. Chlorophyll concentrations increased slightly until around 16m, where after they fall to similar values of that found in other stations. Where it was possible to take samples of 50m depth or more the values of chlorophyll are fairly similar, showing an overall similarity of phytoplankton population distribution past certain depths.

Chlorophyll concentrations are dependent on various factors such as oxygen concentrations, turbidity and light intensity. Station 2 shows a correlation with oxygen saturation seen in Figure 20. At the very surface of the water column, concentrations drop and only increase again at depths of 30m. The oxygen saturation increases at the surface does the exact opposite to chlorophyll; it decreases at around 30m. This is due to respiration of the cells causing depletion of oxygen. The small peak in depth at station 2 may also be enhanced by the very light peak in irradiance that can be observed in Figure 25 at depth of about 15m (Violette, 1995). The low chlorophyll values observed in station 2 at surface may also be related to the stable flow of the water, indicated by the Richardson number (Figure 17).  Station 1 indicates low chlorophyll values, which can most likely be related to high stratification at those sights also due to high Richardson numbers. High stratification would cause a gradual nutrient depletion of the top water column and hence a lower concentration in chlorophyll.

Station 2- possible frontal system with two observed maxima, one at the surface and a second at 16m. This correlates with high backscatter recorded from the ADCP data at station 2 (Figure 16b) which is indicative of a tidal front system. Furthermore high levels of zooplankton were observed between 25-15m suggesting strong stratification at station 3, which correlates with the deepest chlorophyll maxima observed from the 6 stations. The stability of the water column increases the critical depth and depending on the irradiance intensity and turbidity of the water column, there will be a net gain in photosynthesis, thus producing chlorophyll maxima at depth.

 

 

BIOLOGICAL - Plankton

The most dominant taxa of diatom found in the offshore practical was Guinardia sritiata. The overall total of the samples contained 48% of this species. The least dominant taxa found were Eucampia sp, Leptocylindrus minimus and Stephanopyxia turris each having only 2% of the overall total of phytoplankton. Guinardia sritiata was also the most dominant species at station 1, 2 and 6, however at station 3 Guinardia flacida was most dominant and at station 5 Rhizosolenia delicatula was most dominant.

The chlorophyll maximum at station 2 corresponds with the peak in phytoplankton: 58% at a depth of 23m (seen in Figure 14c at station 2). This also corresponds with the lowest reading a phosphate for the same station at the same depth. At station 2 there is a sharp decrease in phytoplankton from 58% at 23m to 5% at 52m, showing a possible nutricline.

There were very low numbers of dinoflagellates counted at the offshore stations, (6 counted in total for all the sites)  possibly due to being consumed by the large amounts of copepods that were present.

The most dominant species of zooplankton were the copepod and the copepod nauplii with an abundance of 45% and 27% respectively. The least abundant zooplankton found was fish larvae. The highest numbers of copepods were found at depths of 12-18m at station 6 and 15-25m at station 2; both these stations were furthest from the land. Copepod nauplii were most predominantly found at station 1 between 0-15m and station 1 was located close to land. The chlorophyll maximum was found between 11 and 22m at all the stations, this is also where the majority of the copepods were found.

Diatoms together with planktonic algae provide good nutrition for the copepods. High diatom abundance can have a negative effect on copepod reproduction (Nejstgaard, et al (2001)). Copepod nauplii were found in higher abundance in areas away from the higher numbers of diatoms. Higher nauplii production occurs when there are blooms in a flagellate bloom, the flagellate numbers were extremely low.

Tidal fronts dominate shallow seas around the UK. They are dominated by diatom blooms and are marked by a chlorophyll maximum layer at the subsurface. (Franks, 1992). The chlorophyll maximum for the highest concentration of chl α was found at site 3 at a depth of 15m. The second highest was at site 2 and corresponds with a high count of copepods and a high count of phytoplankton. This is also the depths with the lowest phosphate and increasing amount of nitrate so this could indicate the presence of a front between sites 2 and 3.

 

PHYSICAL - CTD

Station 1 (figure 15e)- Temperature changes from nearly 14⁰C to 13.2⁰C between 5 and 10m depth, relatively shallow thermocline. Salinity increases from 34.65 to 35.10 as temperature falls at the thermocline, halocline corresponds with thermocline, then salinity remains  between 35.1 and 35.2 at greater depths. Turbidity increases at the lower depth of the thermocline. The fluorometer shows a Chlorophyll a max at 0.11V between 5 and 8m depth with a large sharp peak at 11m of the Voltage.

Station 2 (figure 15f) - Thermocline is deeper, temperature  falling from 13.5⁰C to 11.7⁰C between 20m and 28m then there is a small decrease from 11.7⁰C and 11.5⁰C between 38 and 40m describing a stepped thermocline. Fluorometer shows chlorophyll steadily increases from surface to 18m, chl max  shown as 0.15V between 18 and 23m. Fall in chl down to 0.08V from 23 to 28m and decrease in chl at 40m from 0.08V to 0.06V. Decrease of temp at thermocline corresponds to increase of Salinity. Turbidity jumps around between 0V and 4Vin the surface 1m as the CTD is lowered and swashed by swell. Turbidity peak at chl max of 3.94V.

Station 3 (figure 15g) - Strong thermocline between 10m and 20m and with a well mixed section between 20m and 37m with a second strong thermocline shown with temperatures falling from 12.3⁰C to 11.6⁰C between 38 and 40m. Chl peak of 0.18V at 15m, remains relatively steady between 20 and 38m and falls from .12V to 0.06V from 38 to 40m. Turbidity peaks at lower thermocline depths although only small changes in turbidity observed. Salinity increases slowly from 35.2 at the surface to 35.26 at 38m then increases more substantially from 35.26 to 35.4 between 38 and 50m.

Station 4 (figure 15h) - Well mixed upper water column with a reasonably consistent decrease in temperature from 12.8⁰C to 11.8⁰C between the surface and 50m. Shallow chl max of 0.18V at 10 to 12m and gradual decrease of chl to 0.10V at 25m, chl remains the same between 25 and 32m then decreases to 0.06V at 50m. Salinity increases uniformly from 35.24 to 35.28 between 0m and 25m, remains at 35.28 between 25 and 33m then increases with depth with a brief steady period at 35.32 between 38 and 45m. Turbidity is highest (3.9V) at 22m, decreases to 3.73V between 22m and 35m, turbidity higher in surface 30m.

Station 5 (figure 15i)- Shallow thermocline between 2.5m and 4m where temp changes from 12.7⁰C to 12.3⁰C. Salinity shows an increase between 2.5m and 4m from 35.22 to 35.28. Chl increases slowly from a 0.06V reading at the surface to 0.08V at 8m then increases more rapidly with depth from 0.08V at 8m to a peak of 0.12V at 12m. Turbidity shows an even value of 3.8V through the analysed water column. Surface turbidity change here describes the CTD entering the water.

Discussion

Thermal Stratification is a key aspect of offshore waters in early July. Increased solar radiation during the summer months increases solar radiation absorbed by the oceans resulting in increased warming of surface waters. Warmer sea water is less dense than cold sea water of the same salinity. Surface waters therefore increase in buoyancy. Thermal stratification occurs when the increase in buoyancy is strong enough to overpower vertical mixing of the water column. A stable warmer surface layer is formed, distinguished from the rest of the water column by a region of rapid temperature change, the thermocline (Smythe et al. 2009).

The most striking difference in the data displayed above is the levels of stratification experienced at the different Stations. Stations 2 and 3 are furthest offshore and show far deeper thermoclines (Figure 15a) indicating stronger thermal stratification. The thermocline at Station 5 for example is found between 2.5 and 4m due to disruption of formation of thermocline by tidal mixing. The thermocline for Station 2 is found at 20 to 25m with a second area of rapid temperature change found at 38 to 40m.

Stations 2 and 3 portray an interesting thermocline set up with several depths of rapid temperature change. This stepped thermocline results from a strong, deep longer term seasonal thermocline sitting at 40m, this is the result of long term seasonal changes in solar irradiance, surface water temperatures and mixing through wave and wind action. The shallower thermocline is a shorter term intraseasonal thermocline formed by rapid warming of surface waters during sunny days creating a layer of increased buoyancy above the previous thermocline (Liu et al. 2001). This thermocline is likely to be dissipated if there are several days of low solar radiation with large winds and waves. Or intensified and deepened if sunny weather and low wind stress.

Chlorophyll peaks are often found at or near to the depth of the thermocline. Surface waters are nutrient starved due to high light levels and thus high photosynthesis rates, stratification of the water column prevents replenishment of recycled nutrients. Below the thermocline, the water column is well mixed with higher nutrient levels. At the thermocline, therefore, nutrient levels and light levels are high so chlorophyll flourish. Below the thermocline light levels decrease and without the stabilising effect of thermal stratification, phytoplankton may be mixed to greater depths. Station 5 is near the coast in shallow water, the chl peak mimics the depth of its closest offshore stations, 3 and 4 peaking at 12m rather than following the shallow and weak thermocline. It is clear that above this depth, light levels are too high causing nutrient levels to stay too low to support a large phytoplankton population despite mixing by tide and wind. Further offshore Stations 2,3 and 4 show higher chlorophyll values (Figure 15d).

Turbidity levels are expected to coincide with chl max depths normally at the lower depths of the thermocline due to the extra particulate loading of the phytoplankton. Station 2 shows this clearly while other Stations show a less correlated relationship. Shallower Stations 1 and 5 are tide effected and Station 4 is well mixed so unlikely to show this. Station 3 shows little changes in turbidity through the water column.

 

PHYSICAL - ADCP

Station 1 shows a layer of increased backscatter at around 6-8m (Figure 16a), getting deeper near the end of the transect when it descends to the seabed at around 10-15m. The typical backscatter layer displays backscatter values of around 80-90 dBs. The velocity magnitude measurements show a consistent speed from the surface down to the sea floor of 0-0.2 m/s and the velocity direction is similar all the way through the water column.

Interestingly, station 2+3 both show evidence of double thermocline formation (See Figure 15a). This is reflected in increased backscatter occurring at multiple deeper depths in the water column (see Figure 16b+16c), 80dB for both stations at the depths of the possible second thermocline. Both station 3+4 showed little variation in flow magnitude, however station 3 showed a shear forming between the surface layer and the deeper layer caused by different directions of flow (Figure 16d).

Station 4 showed a weak thermocline, indicating a well mixed water column (See Figure 15a). The ADCP was not functioning correctly in this area, possibly due to the strong swell we were experiencing, and therefore it either did not record any data or the data was spurious (e.g. showing extremely high values for current velocity).

Station 5 was recorded in a shallow cove (see Figure 12). The headland of the cove has an influence on the flow inside, causing turbulent eddies to form in the water column (see Figure 16e, circled).

Station 6 shows different directional flow at 15m, which corresponds to a high backscatter reading (81 dB) also at 15m. The magnitude of the flow is consistent from surface to deep water.

Wind driven currents at surface, could cause mixing and enhanced primary production.

The backscatter from the ADCP shows the possible formation of a deep chlorophyll maximum at around 5-15 m, which would correspond with the depth of the thermoclines of the stations (Figure 15a). This occurs when zooplankton graze on the abundant phytoplankton which reside near the bottom of the thermocline where they have access to nutrients in the deeper layer (Cullen J. J, 1982).

A few stations (notably 2+3) both show the possible formation of a double thermocline. This is mirrored in the chlorophyll steps which are seen on the fluorometer reading (Figure 15d) at the bottom of both the thermoclines.

The shear created by the differing directions of water flow will lead to mixing in the area, combining nutrient rich water into the mixed surface layer (W.R. Young et al, 1982) and allowing the deep chlorophyll maximum to form. An example of this could be at station 6 where the surface water and the deeper water are flowing in different directions, creating a shear and increasing the backscatter at the same depth. This shear could be formed by wind driven currents at the surface, and this in turn would increase the mixing of nutrient mixing water into the surface layer.

PHYSICAL - Light

The depth profile of irradiance shows that light decreases exponentially with depth at all stations, with rapid decrease in the surface layers followed by slower decrease in deeper. Station 1 shows the lowest values with the fastest decrease, and station 3 the highest surface values. Station 5 was a shallow inshore location, and shows different results to other stations, as light can penetrate to the seabed.

Station 1 (black rock) is situated within the estuary, so the amount of particulate organic matter POM will be higher here than offshore stations. This POM will limit the amount of light penetrating beyond the surface layers.

In order to calculate the attenuation coefficient, the natural log of the depth sensor readings ln(Ez) is calculated and then plotted against depth, and from this, the slope of the lines for each station can be found. From the slopes, k (m^-1) can be calculated by 1/slope. The table shows the calculated attenuation coefficient values, k.

 

Station	Slope	K (m-1)
1	4.39	0.228
2	4.556	0.219
3	4.453	0.225
4	3.8641	0.259
5	6.79	0.147

 

 

 

A small attenuation coefficient suggests the water is relatively transparent.  Station 5 has the lowest k value suggesting low levels of POM, whereas station 4 shows the highest coefficient suggesting there are higher amounts of POM in this area. Looking at fluorometer depth profile, there is a possible chlorophyll maximum at around 12m, which could be an explanation for the high attenuation coefficient.

 

CHEMISTRY - Silicon

It is visible from the silicon vs. depth figure, that the concentration of silicon increases with depth. Values of <1 µmol/litre in surface waters at stations 2, 3 and 6, and values of around 4 µmol/litre beyond 50m. station 5 shows higher surface values , this station was inshore and shallow, which may explain the difference in  values.

Silicon is taken up by diatoms which use it to construct their frustules. Diatoms require light for photosynthesis, and therefore are generally found in the euphotic zone.

 

CHEMISTRY - Nitrate

Aim: To gain an insight into how Nitrate changes with depth away from the coastal environment.

Method: water samples taken using Niskin bottles around a CTD Rosette. 50ml water sample filtered into a glass bottle for analysis using flow injection method with a spectrometer measuring absorbance at 540nm. Standards of 0μM, 2.5μM, 5μM and 10μM Nitrate concentrations were run and plotted against their mean peak heights to gain a calibration equation to convert peak heights of samples to concentrations.

Nitrate conc. = Peak height (cm) / 0.84

Station 2 shows an increase in Nitrate concentration with depth in the surface 22m, steady concentrations for the following 20m and a large increase in Nitrate between 40 and 50m.

Station 3 describes Nitrate values undetectable using our equipment in the surface 12m then an increase in Nitrate from 0.296μM between 12 and 30m. Deeper than 30m, Nitrate values are shown to drop off to undetectable values by 55m.

Data for Stations 5 and 6 gave Nitrate concentrations too low to detect while the lower detection limit is bordered at 30m at Station 3 at .296μM with general lower detection limits described as <.3μM, concentrations at other depths at this Station, however are also too low to detect.

An increase in detectable samples would greatly improve this description of Nitrate behaviour at these Stations. Station 2 provides the most clear and useful results but does not give enough information to find, for example the deep chlorophyll maximum. It does however suggest highest phytoplankton photosynthesis rates at the surface, shown by the lowest Nitrate concentration at the Station. Photosynthesis rates are shown to slowly slow from the surface to 22m where rates remain steady in the next 20m of the water column. Below 40m, rates are seen to slow more dramatically with a jump from 0.65μM to 1.1μM between 40m and 52m.

Station 3 data displayed here is unlikely to present an accurate interpretation of Nitrate behaviour in the water column at this Station, the only detectable value is just on the detection limit, as such Nitrate concentrations at other depths may also be exceptionally close to the detection limit but just off the chart or may be considerably lower than the detection limit.

Equipment with a higher sensitivity that could detect lower Nitrate values would hugely improve this offshore analysis.

 

CHEMICAL - Phosphate

Concentration standards were mixed with water to produce known concentrations, after measuring the absorbance values of known concentrations, one can produce a calibration line on a graph which can give the concentration of a sample of a certain absorbance value (see figure 1). This is done by rearranging the formula of the calibration line (regression line) to solve for concentration:

Where, x = concentration of phosphate in sample (umol/L), y = absorbance value of sample

Phosphate samples were taken by the bottles attached on to the CTD at different depths which would give us it vertical distribution. The results seem to suggest that concentration of phosphate increases with depth generally. At station 4, the CTD was deployed; however no samples were taken at the station. For station 1 and 5, samples were only taken at 2 depths. Also, some samples had replicas however; some of those values differed slightly meaning that for those whose replica absorbance value differ an average of the two was taken. At station 1, the concentration increased from 0.089µmol/L at 11 metres to 0.209µmol/L at 30metres. At station 2, there is an initial drop in concentration from the surface (0.304µmol/L) to 22metres (0.078µmol/L). Phosphate concentration increases after that depth to 0.471µmol/L at 39 metres before dropping at 55 metres to 0.364µmol/L. Concentrations also increase with depth from 0.245µmol/L at 1.6 metres to 0.31µmol/L at 11.7 metre. Station 6 increases in concentration from 0.191µmol/L at 1.5m to 0.71µmol/L at 51.7m.

General trend of the figure shows that chlorophyll increases with depth at all stations however, stations 1, 3, 5 and 6 gives an incomplete picture of the phosphate distribution across the water column at the stations because only two or three depths were sampled. Across over 60 metres of water in depth, this amount of samples is not enough to give a high resolution picture.

At station 2, phosphate concentration decreases from the surface to 22 metres. The CTD data shows that roughly the same depth there is a chlorophyll maximum (see figure for station 2 CTD), this indicates that primary production is highest at that depth. This means that at that depth phosphate concentration is low because it is used up by the phytoplankton for growth. At this station, a tidal front seems to have been observed; stratification of water was also observed, the thermocline was also located at 17 to 28 metres which separates the nutrient depleted surface waters from the nutrient rich waters at depth.

At station 3 although the resolution is not very high, one can still observe the relationship between chlorophyll concentrations and phosphate concentrations. At station 3, phosphate minimum is at around 13 metres whilst the chlorophyll maximum occurs at 13.5 metres. A thermocline is also observed at those depths as well meaning that there is also a front at this station, however it is not as clear as at station 2 due to the fact that not very many depths were sampled here.

 

CHEMICAL - Oxygen

Generally oxygen saturation decreases with depth, over 100% saturation in the surface waters at stations 2, 3 and 6, and values below 90% beyond 50m.  Station 2 shows a slight increase with depth, followed by a dramatic decrease, showing a possible chlorophyll maximum. All stations show similar values, except station 5 (an inshore station) which shows slightly lower saturation values.

 

CONCLUSION - Offshore

Six stations were sampled; however weather constraints limited the distance offshore where samples could be taken, so samples were taken closer to the coast. Biological, chemical and physical parameters were measured at each station.

A strong thermal stratification was observed at the stations furthest from the shore. A chlorophyll peak was found close or near to the thermocline. Plankton was dominated by copepods and copepod nauplii and Guinardia sritiata and a possible tidal front was located between station 2 and 3. Low nitrate concentrations were found at the surface, this correspond with higher levels of plankton, showing nitrate is being consumed. Phosphate was found to generally increase with depth as did silicon, but oxygen decreased with depth.

The ADCP gave a valuable insight into the physical and biological structure of the offshore area which we studied. Backscatter graphs consistently showed the formation of deep chlorophyll maxima, largely around the areas where strong thermoclines were forming. The velocity magnitude and direction data also helped evaluate any possible shear which was forming, possibly leading to mixing processes.

Sampling of more depths with more precise equipment would have provided higher resolution data, so conclusions could have been more solid.

 

Figure 12 - Transect map for the Offshore route

Figure 13 - Average chlorophyll values plotted against depth with increasing offshore distance

(a) (b)

(c)

(d) (e)

(f) (g)

Figure 14a-g: (a) and (b) Total percentage of diatoms found for all five offshore stations; (c) Percentage of total phytoplankton for each station against depth; (d) percentage of zooplankton found for all 5 stations; (e) Zooplankton for each station at given depths (bottle 21 is station 1, bottle 22 is station 2, bottle 23 is station 3, bottle 24 is station 5 and bottle 25 is station 6); (f) Guinardia striata (Coale, 2007); (g) Copepod (Kils, 2002)

(a) (b)

(c) (d)

(e) (f)

(g) (h)

(i)

Figure 15a-i: (a) Temperature at all Offshore stations; (b) Salinity at all Offshore stations; (c) Temperature-salinity plots for all offshore stations; (d)  Fluorescence at all Offshore stations; (e) All parameters at Station 1; (f) All parameters at Station 2; (g) All parameters at Station 3; (h) All parameters at Station 4; (i) All parameters at Station 5

(a) (b)

(c) (d)

(e) (f)

Figure 16a-f: (a) Backscatter contour for Station 1; (b) Backscatter contour for Station 2; (c) Backscatter contour for Station 3; (d) Flow direction data for Station 3; (e) Velocity direction data for Station 4; (f) Velocity direction data for Station 5

(a) (b)

(c)

(d) (e)

Figure 17a-e: (a) Richardson Number for Station 1; (b) Station 2; (c) Station 3; (d) Station 4; (e) Station 5

Figure 25. Depth profile showing how the irradiance varies with depth at all offshore stations, except station 6 due to corrupted CTD files.

Figure 17. Silicon concentration as depth increases for all stations offshore, except station 4 where no bottles were fired.

 (a)

(b)

Figure 18a-b: (a) Nitrate standard; (b) Offshore nitrate concentrations with depth

(a) (b)

Figure 19a-b: (a) Phosphate standards; (b) Phosphate concentrations with depth at each station

Figure 20. Oxygen saturation depth profile at all stations offshore, except 4, where no bottles were fired.

          Pontoon         

Introduction

A YSI 6600-D V2 multiprobe, a Li-Cor Li-1400 datalogger and Valeport current meter were used to take quarter hourly measurements over a 3.75 hour period of an ebbing tide from a chain ferry pontoon on a shallow 5-6m reach of the upper Fal river which had experienced rainfall overnight.

Aim: Collect a time series of various seawater parameters over the tidal cycle.

Results & Discussion

The water level shoaled from 5 m to 3 m with the ebbing tide as a function of time and black blocks in Figures 21a-21i indicate seabed when incomplete data sets were taken as a result of this.  Salinity decreased from 34 to 30 at the surface as the tide ebbed with lower salinity, less dense freshwater overlying tidally driven seawater (Figure 21a). Initially surface pH became more alkaline as the tide ebbed, this extended throughout the water column by the end of the survey when the water depth had decreased from 5 to 3m (Figure 21b). The Fal river current speed at the pontoon was measured as 0.2-0.4 m/s (Figure 21c). Temperature at 14 ^o C was lowest at depth and increased from the surface throughout the water column with shoaling of the retreating tide and increased seasonally warmed riverine input (Figure 21e).The chlorophyll α concentration was highest at 7 µg/L at mid depths of 1.5-3 m and 6 ug/L at the surface when the water depth was 5 m decreasing to 5 µg/L for two quarter-hour periods possibly related to an intermittent increase in sunlight or dispersal related to a onside docking ferry.  As the water level decreased to 3m the peak moved towards the surface (Figure 21e). Initial dissolved oxygen levels were 96% at the water surface with lower levels of 92% at 5m depth; the water column homogenised at its 3 m minimum and reached a saturation peak of 98% within the surface 0.5 – 2 m. The latter two parameters are proxies of phytoplankton activity and suggest possible UV shading in the surface above 0.5 m. Depth profile of ln (irradiance µmol/m²/s) shows an exponential decrease with depth, the compensation depth above which phytoplankton photosynthesis is 10 µmol/m²/s (Miller, 2001). This is due to the attenuation of light as it moves through the water column, it is scattered and absorbed by various particles. A few points display higher than expected levels, these can be accepted as anomalies.

Figure 6, the attenuation coefficient plot, shows generally stable light penetration throughout the time series. The area of higher k values in the surface waters at 09.45UTC could represent an area of plankton dominance. Under this area is an area of lower k values, where the plankton has blocked the light from penetrating to deeper water. The area of high k values at around 3m at the beginning of the time series could represent sediment disturbance.

Pontoon conclusion

The pontoon station was used to collect chemical and physical data from one station on an ebbing tide.

Surface salinity decreased as the tide was ebbing and its influence was reduced and pH became more alkaline as surface river flux increased. Initially irradiance decreased with depth due to light attenuation but as the depth shoaled as a function of time the water column homogenised and the compensation depth was raised increasing the euphotic zone and phytoplankton proxies increased in response to this. 

Figure 26. Google earth image of Pontoon site.

(a) (b)

(c) (d)

(e) (f)

(g) (h)

(i)

Figure 21a-h: (a) YSI salinity profile; (b) YSI pH profile; (c) Irradiance with Depth; (d) YSI oxygen profile; (e) Directional flow profile; (f) Current velocity profile; (g) YSI chlorophyll a profile; (h) YSI temperature profile

Geophysics - View the e-poster here

A Geophysical survey was conducted along the coastline off Castle Head at the Western edge of the Fal estuary mouth involving Side Scan sonar (4 transects), grabs (3 along three of our 4 transects due to worries about proximity to shoreline danger to the vessel) and underwater video footage of the bed (across 2 of our transects) in order to establish biota and bed types across the survey area allowing the creation of habitat maps combining physical and biological properties. This habitat mapping exercise is aimed at likely implications on maerl and other life forms that may result from the planned dredging of the channel in the estuary at the proposed sites.

Maerl is an extremely slow growing (1mm/year) coralline algae, creating its own calcium carbonate structure upon which it can grow. It is only in recent years that dredging of maerl for use as fertiliser in the agricultural world was banned due to reports stating that maerl was too slow growing to be considered a sustainable resource while the benefits of living and dead maerl in providing shelter for juvenile species was realised resulting in maerl beds in the Fal estuary being given a protected status. Now there are exceptionally strict limitations on any disturbance of maerl beds across the estuary. Disturbance of sediment beds and habitat dynamics due to the proposed dredging the Fal channel and the resulting settlement of fine sediment are a worry.

TRANSECTS

Subsurface Duel Frequency Analogue Side Scan Sonar  at a frequency of 410 kHz (high resolution images) was used with a swath range of 200m. Transects were 100m apart and alongside one another so overlapped allowing full analysis of surveyed area. The tow fish was towed with a layback of 4m both vertically and horizontally, corrections were not made due to the small scale of these errors.

GRABS

3 grab samples along each transect line deemed deep enough to safely conduct the grab. Using a Van Veen grab allowed ground truthing comparing and confirming side scan data. Van Veen grab lowered to sea floor on a marine-grade stainless steel hydrographic line. Grabs then sieved through 1cm and 1mm sieves to analyse sediment sizes. Photographic evidence taken for later species identification.

VIDEO ANALYSIS

Video analysis of the bed over the area surveyed allows further ground proofing, helping to confirm the sidescan analysis as well as providing real images of the structure of the bed type and biota in their natural state. This is useful in exploring habitat dynamics. Camera was lowered from starboard side of the boat to 7m, just above the bottom. Boat drifted during video.

Video footage was available for only one expected boundary crossing upon analysis of the sidescan sonar, this boundary was evident with more coarse grained, maerl rich gravels covered by bivalve mollusc shells giving way to finer grained sediment between boundaries 6 and 5. Numerous starfish were seen along the video transect, small fish were seen to swim away from the camera preventing identification.

High resolution side scan sonar with relatively few artifacts allowed a clear representation of the sea bed in the survey area. Ground proofing through grabs and video footage has confirmed our analysis of a small area of the survey area, on reflection grabs could have been taken diagonally across the whole area in order to avoid ground proofing in the same boundary zone and increased video footage more fully describing the survey area would have provided tools for greater accuracy in our analysis of the side scan data.

 

Figure 21: Habitat map of transects and interpretation of side scan sonar showing boundary layers of bed forms, grab sites and path of video.

(a) (b)

(c) (d)

Figure22a-d: Tables of relevant coordinates and times for each individual grab made for the geophysical survey

Figure 23: Table of findings from the grabs

Figure 24: Table of coordinates and times for the video transect

          Conclusions         

Aim: To gain an understanding of the physical, chemical and biological processes in the Fal Estuary and the surrounding coastal waters.

A number of processes were used to gather information about the physical, biological and chemical processes within the Fal estuary and the surrounding coastal waters, information was collected in the field and studied in the laboratory.

The estuary samples gave a more horizontal profile as the depth was too shallow in parts to get a detailed vertical profile. The nutrients generally decreased with increasing salinity down the estuary whilst turbidity would increase due to deposition. The offshore study gave a more vertical profile as sampling occurred at deeper depths. A strong thermal stratification was found offshore. Low nitrate was found at the surface, phosphate increased with depth. We were also able to use an ADCP to analyse both physical and biological properties of the water column in the estuary and offshore. With this data we were able to determine a chlorophyll maxima and mixing profiles. The pontoon practical gained an insight into physical and chemical properties of an estuary as the tide was ebbing, giving a vertical profile at 1 station over time.

Zooplankton in both the estuary and offshore were dominated by copepod and copepod nauplii and the phytoplankton was dominated by Guinardia striata.

There were some constraints when conducting practical work such as weather, equipment and depth of water columns.

         Appendix         

 

Equipment and methods sections were included to avoid repetition occurring in our write ups. As the methods and equipment used in certain investigations were similar (e.g. chemical analysis for offshore and estuarine studies) we felt it would be appropriate to give a general overview of our processes to avoid repeating ourselves.

We would like to thank the boat crews, academic staff and demonstrators for all the help and advice given over the course of our survey. Their expertise was vital in the completion of our survey.

The view and opinions expressed in this report are that of our own, and do not necessarily reflect the views  of the University of Southampton.

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