The Team

Our team was made up of 10 second year students from the University of southampton, from Marine Biology and Oceanography undergraduate programs.

The Area

Falmouth is situated in the South coast of Cornwall, South-west England, and is the third largest natural harbour in the world. In the Middle Ages it was a small unimportant fishing village. Throughout the years it was considered a well situated, safe and sheltered town for vessels on trade routes to stop for repair and shelter. The major turning points in Falmouth’s gaining prosperity were the introduction of the Post Office Packet Station, which gave rise to the "Falmouth Golden Age", and the construction of the docks in 1859. Falmouth has been known through time for its mild all year round climate, with frost and snow being a rarity.

Phosphate (Fig.1)

Phosphate levels decrease along the estuary, from the riverine end member to the marine end member. At Site 2 and Site 3 phosphate is highest at the surface, with the highest values occurring in the surface waters at the top of the estuary (Site 2) of 0.75μmol/L. Site 1 was at the seaward end of the estuary so the riverine input of phosphate at the surface is not as high as at the other two sites. Instead the phosphate maximum occurs at 20m depth.

Fig.1. Phosphate samples from the length of the estuary. Click image to enlarge.

Fig.2. Dissolved oxygen samples from the length of the estuary. Click to enlarge

Dissolved oxygen (Fig.2.)

Site 2 and Site 3 both have high dissolved oxygen at 1-1.5m depth. Site 1 has fairly consistent dissolved oxygen levels of 101.8-102.2% until about 12m depth where it drops until it reaches 98.8%, which could indicate a well-mixed euphotic zone compared to the other two sites. All three sites show a decline in dissolved oxygen concentration with depth. Due to organic oxygen utilisation and no input of oxygen from the atmosphere like there is at the surface.

Silicon (Fig.3.)

Silicon concentrations are highest at site 2, with the greatest value of 8.9μmol/l found furthest upstream. Values then decrease with distance downstream and the lowest concentration of 1.1μmol/litre found at site 1. Within upper section of the water column (top 4 metres for sites 2 and 3 and top 6 metres for site 1) silicon concentration decreases, this decrease continues at site 3 until the greatest depth at 13m to a value of (2.9μmol/l). This vertical decrease is greatest at site 2 (6.6μmol/l) and smallest (0.4μmol/l) at site 1 with an intermediate value (4.2μmol/l) at site 3. For sites 1 and 2, silicon increases after the initial decrease between depths of 4m to 6m for site 2 and 6m to 12m for site 1. The greatest range of values with depth is found at site 2 and smallest range at site 1. This indicates a more uniform silicon concentration with depth at the mouth compared with the point furthest upstream which displays clear differences between surface and bottom waters.

Fig.3. Silicon samples from the length of the Estuary. Click to enlarge.

Fig.4. Chlorophyll samples from the length of the estuary. Click to enlarge.

Chlorophyll (Fig.4.)

The lowest and highest values are found at site 1, the lowest of 0.80μg/litre at a depth of 1.1m and highest of 6.85μg/litre at a subsurface depth of 6m illustrating the site with the greatest variation. At site 3 the highest concentration (4.90μg/l) can be seen at the surface unlike the other two sites which occur at a subsurface depth of 2m for site 2 (4.03μg/l) and 6m site 1 (6.85μg/l).  After this subsurface peak for sites 2 and 1 chlorophyll concentrations decrease with depth with site 3 showing a subsurface peak of 3.32μg/litre at the same depth as site 1 at 6m.

Vertical Profile Analysis

At Turnaware Point (CTD location 3), a much higher resolution sample was taken (Fig.5.), with multiple casts taken to aquire more water bottle samples.

Phosphate levels decrease with depth through the water column. At the surface the concentration is at its highest (0.5µmol/l). This decreases rapidly over the next 5m depth to 0.27µmol/l. There is then a small resurgence at 8m to 0.38µmol/l.  However, after this the concentration reaches its lowest point at the bottom of the water column. Dissolved oxygen follows a very similar pattern to phosphate. It is highest in the surface waters (103.78% at 2.7m) and declines significantly over the next few metres depth (100.4% at 6m). It then increases to 102.5% at 10m in a similar pattern to phosphate. It then declines to its lowest percentage of 98.5% at 13m, the bottom of the water column. Silicate follows a simple pattern through the water column. It is highest at the surface (8.9µmol/l at 1m depth) and declines sharply over the next 5m to 4.2µmol/l at 6m, less than half the original concentration at the surface. From 6m to 13m depth the decline in concentration is steady but slow, reaching a low of 2.8µmol/l at 13m. Chlorophyll was found at its highest concentration at the surface of the water column (4.89µg/l at 1m). There is a sharp decline to 2.69µg/l at 2.7m. The concentration then increases to reach a secondary peak of 3.31µg/l at 6m. This is followed by a decline to reach its lowest concentration at 10m depth (2.25µg/l).

Sample site 1 (Fig.6.)

Firstly, starting with temperature there is a very clear trend to notice, with increasing depth the temperature exponentially decreases. After approximately 5 metres depth the rate of decreasing temperature increases, it reaches a minimum of 12.5 at a maximum depth of 20m. The salinity data makes it clear to determine that the halocline is at around 7.5 metres, this is visible by the large increase in salinity from approximately 32.3 to 34.865 salinity units. Salinity values however are higher than would be expected even over the change with depth, this could indicate a calibration issue and problems of fouling with a sensitive piece of equipment.  The transmissometer data, showing turbidity throughout the water column shows an initial decrease in turbidity (from approximately 4 to 3.97), after this event the data begins to flatten out reaching approximately 3.99 at 20 metres. Finally the fluorescence data, albeit it having several outlying results (of 0.656, 0.841 and 0.496 respectively), shows a uniform value (with an average value of 0.268). These findings show the well mixed water column.
One dramatic comparison between two variables is that between the salinity (and therefore the position of the halocline) and the fluorescence values (which would therefore denote the abundance of chlorophyll and consequently phytoplankton as a proxy of this). The increase in fluorescence just above the halocline at 7.5m could be due to phytoplankton residing just above it where nutrient concentration will be highest. When compared to the temperature of the water column and therefore the position of the thermocline it becomes visible that the phytoplankton are residing just below the thermocline, again, where nutrient concentrations are highest. Another aspect of this data set that would support this hypothesis is the low level of turbidity therefore allowing for a higher irradiance (and thus higher potential for photosynthesis) at this point.

Fig.7. CTD data from the second CTD sample at the top of the estuary. Click to enlarge.

Sample site 2 (Fig.7)

Temperature in this graph follows the same trend as that from the first site, however, the average temperature (14.04) is greater; due to the fact that these results were taken from a shallower depth. At this site the salinity data shows a much smoother profile ranging from around 28.975 PSU to a maximum of approximately 33.25 at a maximum depth of 6.24m. The turbidity data also shows a much smoother distribution on the graph and between the depth range of 2 and 3m the turbidity data follows nearly the exact same pattern as the salinity.

The low salinities in this area are obviously attributed to this data set being the river-end member. There is a small spike in the fluorescence data below the thermocline; this is again to be attributed to the higher levels of nutrients at this point. There is also a turbidity maximum at the base of the thermocline, this will aid phytoplankton growth by bringing up bottom layer nutrients into the sub surface chlorophyll maximum (Vidussi et. Al. 2001), however, due to this growth and therefore the incurring increase in turbidity light will become the limiting factor for phytoplankton photosynthesis.

Sample site 3 (Fig.8.)

Once again the temperature readings follow that of sites 1 and 2 with a peak of 14.425 at the surface and a minima of 12.14 at a maximum depth of 13.149. Like site 2 the salinity and the turbidity follow approximately the same pattern; exponential decay with depth. The consistency of the fluorescence measurements is most apparent at this site as most values lie around 0.2; however, a large outlier of 2.043 brings the overall average up to 0.269.

The fresher water on the surface is causing the top water body to have a lower density, thus causing it to stay above the dense salt water below, creating a halocline, which is apparent at around 3 metres. This, in conjunction with the presence of a thermocline, caused by less dense warm water at the surface and more dense colder water at depth, further prevents mixing In the water column, therefore overall creating stratification.

Fig.8. CTD data from the third CTD sample at the narrow section at the mouth of the Fal river. Click to enlarge.

Fig.9. Theoretical dilution line for nitrate. Click to enlarge

Estuarine Mixing Diagrams

Estuarine mixing diagrams assume steady state for constituents, that end member concentrations are constant on a time scale greater that the residence time in the estuary and that there is only one riverine and marine end member, they are therefore affected by more rivers or waste discharge or pore waters changing composition of estuarine waters.

Both Nitrate(Fig.9.) and Silicate(Fig.10) show conservative behaviour in the estuarine system changing with the salinity. Phosphate(Fig.11) shows a non-conservative behaviour. Two riverine end members were taken from Truro, the different values can be related to the different freshwater volume inputs higher at the Kenwyn and lower at the Allen. Two are taken due to the difficulty accessing the river at another suitable place. An average was taken on the mixing diagrams to meet the assumption. The seaward end member for phosphate is 0.26 µmol/l and the riverine end member 0.68 µmol/l. Phosphate levels reach 0.75 µmol/l at a salinity of 30, a higher concentration than the riverine end member.

Values of 0.64 µmol/l at salinity 29.5 and 0.46 µmol/l at 32.4 are also further examples of phosphate levels that are clearly above the theoretical dilution line. The silicate river end member was 85.2 µmol/l and the seaward member was 1.3 µmol/l. The nitrate river end member was 1 µmol/litre and the seaward member was 33.5 µmol/l. It is important to remember the lab method in interpretation and the possibility of errors and difficulties in procedures. This was demonstrated in the preparation of standards the first set made was lower than expected then the second still showed variation, not always a linear increase in absorbance from concentration.

There are a number of factors that may lead to a more conservative behaviour of silicon and nitrate in the estuary such as lower biological activity. Conservative behaviour relates to the mixing of seawater and freshwater which is mainly regulated by river flow rate and magnitude of tidal currents (Hansen and Rattray, 1966). A short transit time of the fresh water component of the estuary from mouth to source, leads to lower removal from an estuary, because uptake and sinking are time dependent. The transit time in the Fal estuary is high however due to the low fresh water input. Monbet (1992) found that macrotidal estuaries (tidal range greater than 2 m) such as the Fal, show a greater tolerance to nutrient pollution

Fig.10. Silicate Theoretical dilution line. Click to enlarge.

Fig.11. Phosphate theoretical dilution line. Click to enlarge

despite high input from freshwater sources, so avoiding eutrophication. Cloern (1987) linked higher tidal mixing, current velocities, sediment resuspension, increased turbidity and decreased light penetration to lower primary productivity/biological activity and hence uptake of nutrients. Seuront and Schmitt (2005) noted that patchiness of plankton populations increases with a decrease in turbulence intensity and is higher during the ebb than during flood tide for a given turbulence intensity. This may be important in the upper Fal, where turbulence across the freshwater/seawater interface may lead to patchy distributions of phytoplankton and again lower levels of biological activity.

The Fal is found in an area of extensive farming both arable and pastoral. Changes of water chemistry in the system are therefore inevitable, as observed by Jarvie et al. (2008). This could be a potential reason for the non-conservative behaviour of phosphate. Sewage treatment works at Truro and Malpas can also lead to further addition of phosphates (Langston, 2003). Mine drainage sites and sediments are also a key phosphate sources around the Fal (Langston, 2003).

Biological

Two plankton samples were collected from Black Rock and pontoon. A plankton net was used for zooplankton and surface water from the flow through pump for phytoplankton. The net was deployed off of the back of the boat and was trawled for 5 minutes to collect the sample. Before deployment, the bottle attached to the end of the net was submerged to remove all air bubbles and when in the water, the rim of the net was fully submerged. At the end of the trawl, the net was washed with seawater from a hose in order to collect all zooplankton from the net.

50ml of surface water and a predetermined quantity of Lugol's iodine was added to a dark glass bottle for the phytoplankton sample. A pre-measured quantity of formalin was added to the plastic bottle collected from the plankton net for the zooplankton sample.

Phytoplankton samples were concentrated from 100ml down to 10ml using a vacuum pump without removing any phytoplankton. One ml of concentrated solution was then pipetted onto a Sedgewick-Rafter chamber and five columns of twenty squares each were observed, species identified and counted under a microscope with a 10x magnification. This number was then multiplied by ten to get an estimate of the entire volume.

Formalin was added to a 500ml container of the zooplankton sample to preserve organisms. From this container, each person extracted 10ml and analysed two 5ml volumes using a Bogorov tray under a microscope. The main organism groups and orders were identified and counted; the abundance per volume could then be calculated.

Fig.12. Phytoplankton species abundances from the estuarine stations. Click to enlarge.

Phytoplankton (Fig.12.)

Phytoplankton samples collected at the mouth of the estuary show a much higher abundance and range of phytoplankton present with a total of seven species, compared to only two found at the furthest point sampled upstream at site 2. The abundance of these two species (Thalassiosira rotula and Dinophysis) are both higher at site 1, with relative abundances of 430 per ml at site 1 compared to 380 per ml at site 2 and 190 per ml at site 1 compared to 160 per ml at site 2, with Thalassiosira rotula being the most dominant species at site 2. The most dominant species found at site 1 was Rhizosolenia stotterfothi with an abundance of 580 per ml and Rhizosolenia setigera was the least dominant species with an abundance of 90 per ml.

Zooplankton (Fig.13.)

The range of zooplankton groups and relative abundance of each are highest at site 2, furthest upstream, except for Chaetognatha which had an abundance five times higher than at site 1. A total of ten zooplankton groups were present at site 1 and a total of twelve for site 2, with echinoderm larvae and fish larvae not present at site 1. Copepoda  and Copepoda Nauplii are most dominant at both sites with relative abundances of 61600 and 5500 per m3 for site 1 and 126000 and 12000 per m3 for site 1 and 2. The least dominant species for each site differ with Cirripedia Larvae and Appendicularian for site 1 and Chaetognatha for site 2 with a relative abundance of 550 per m³ for each zooplankton group.

Discussion

Discrepancies seen for phytoplankton and zooplankton abundance between sites 1 and 2 may be explained by a variety of factors. When considering phytoplankton distribution, it is important to account for tidal state and range as phytoplankton have no or limited motility so are unable to control their position within the estuary. Sampling was undertaken in the Fal Estuary, a macrotidal estuary, on an ebbing tide, two hours after high water with a tidal range of 3.3m.

Fig.13. Zooplankton abundances in the estuary. Click to enlarge.

Across the estuary mouth (Fig.14.1), around Black Rock, there is a uniform velocity of around 0.2 to 0.4 ms^-1, this is somewhat dependant on the tidal state of the estuary, but would be expected to be lower than other transects due to the wider and deeper cross section of the mouth. There are small elements of increased backscatter at the surface of the water column, possibly indicative of higher plankton abundances or suspended particles. The direction of flow shows distinct differences either side of the central channel with the western edge and eastern side of the channel displaying a southern flow, and the western side a north eastern flow

In contrast, transect two (Fig.14.2), taken at the very narrow top part of the estuary shows an increase of velocity in the deeper parts of the channel , while the water in the shallows shows a lower velocity and a non determinate flow direction. This section also shows a greater range of velocities, from 0 to 1 meters per second. There is a large area of high back scatter in the western part of the transect, likely due to a high quantity of suspended particles in the high velocity shallower part of the channel.

Transects 3 (Fig.14.3) and 4 (Fig.14.4), the mid estuary transects show similar trends, the velocity at both transects is around 0.4 ms^-1, with little variance between shallower and deeper parts of the estuary, transect 3 shows an increase in backscatter above the deeper channels and transect 4 shows a minimal increase towards the west of the transect. The velocity direction in transect three is predominantly South West at the surface, however the waters nearer the bottom in the deep channel runs slightly South East reflecting the direction of the central channel.

In transect 5 (Fig.14.5), the velocity direction is dominated by south-westerly flow. However towards the eastern side of the estuary flow is multidirectional, possibly caused by turbulence. Backscatter is at a minimum again along this transect. Much of the data from this transect is lost due to the failure of the ADCP over 20 meters.

In transect 6 (Fig.14.6), the velocity magnitude is around 0.03-0.6ms^-1 and mainly in a southwesterly direction, but with a strong flow in other directions. There is some significant backscatter in the surface layer between 1-5m which goes up to 107dB. As the transect does not cross the main body of the estuary, this transect is relatively uniform, one of the reasons why there is no deep central channel or areas of increased depth.

1. Transect - 1
2. Transect - 2
3. Transect - 3
4. Transect - 4
5. Transect - 5
6. Transect - 6

Fig.14. ADCP graphs for the 6 different transects, see this map for the locations of the transects throughout the estuary

Fig.15. Richardson numbers throughout the water column provide an indication of flow state, numbers below 0.25 indicate turbulent flow and above 1 indicate laminar flow. Click to enlarge.

Richardson Numbers (Fig.15.)

At the mouth of the estuary the water column is relatively stable; Richardson numbers do not decrease beneath the 0.25 turbulent threshold. Although the top two measurements are in the intermediate zone where laminar and turbulent conditions can exist, the lowest measurement has a Ri number high enough that laminar flow is definite.

At Station 2 (Turnaware point), Richardson number patterns are complex throughout the water column.  In the surface 4 m shear flow instability occurs as figures are below 0.25. Between 4 and 8 m there is a zone where Reynolds numbers increase through to stable laminar flow zone and peak at 3.4. Below 8 metres figures fall back below 0.25, to similar figures to that of the surface 4 metres.

Richardson number patterns can be related to the data shown in ADCP plots for the same profile locations and theories deduced as to why Richardson numbers fluctuate throughout the water column.

Richardson data suggest that the entire water column entering the estuary is of stable flow with Richardson number increasing with depth, and hence the stability of flow increases with depth.

When taking light meter readings in a water column, two light meter units are used. One out of the water to measure incident light and one in the water which is lowered into the water column. Before any measurements are taken they both need to be calibrated against each other in air. Once calibration calculations have been completed on the data, you can divide the in air and in water values together to get a value between 0 and 1. Percentage attenuation can be calculated from this figure.

Fig.30. Contour plot generated from the time series data collected at the pontoon. Click to enlarge.

Fig.31. Contour plot generated from the chlorophyll time series data collected at the pontoon. Click to enlarge.

Dissolved oxygen (Fig.30.)

Dissolved oxygen remains low at depth throughout the time series, ranging between 95.0% and 96.0% saturation. However, the dissolved oxygen levels between the surface and 3.0m increase over time, increasing from 95.0% at 09:00 UTC, through 96.5% at 10:00 UTC, to a maximum of 97.5% at 11:00 UTC.

Chlorophyll a (Fig.31.)

Chlorophyll is variable over time and depth, but shows an overall increase across the time series. Surface values increase from 5.0mg/L at 09:00 UTC to 6.0mg/L at 11:00 UTC. There is also a high concentration of chlorophyll a at 4m depth between 09:40 UTC and 10:20 UTC.

pH (Fig.32.)

pH is relatively constant over both time and depth. The water column becomes slightly more alkaline over time, increasing from 8.66-8.67 at 09:00 UTC to 8.69 at 11:00 UTC, but there is no significant pH change.

Fig.32. Contour plot generated from the time series ph data collected at the pontoon. Click to enlarge.

Fig.33. Contour plot generated from the time series salinity data collected at the pontoon. Click to enlarge.

Salinity (Fig.34.)

Salinity decreases across the time series, with surface salinity values starting at 32.5 and ending at 30.5. On average, salinity increases with depth; the minimum salinity at the surface is 30.5, whereas the bottom water does not reach salinities below 31.5.

Temperature (Fig.34.)

The average temperature of the water increases across the time series, with the surface temperature increasing from 14.6°C at 09:00 UTC to 15.2°C at 11:00 UTC. Temperature consistently decreases with depth. There is an area of cool water (13.8°C) that occurs at 3.5-4.0m depth between 10:05 UTC to 10:25 UTC.

Discussion

The time series measurements taken at the pontoon were taken at the start of the ebb tide, when the riverine input was low but its influence increased towards the end of the time series. This means that there was a significant input of freshwater at the surface. This caused the decrease in salinity over time in the surface waters and, being less dense it flowed over the more dense salt water as the tide ebbed. This also explains the temperature increase as the temperature of the ocean is lower than that of the freshwater inputs. The increase in chlorophyll a, with time, is a proxy for an increase in phytoplankton. The input of nutrients from terrestrial and riverine sources could account for the increase in chlorophyll a with time. The phytoplankton will photosynthesise, causing the increase in dissolved oxygen at the surface.

Fig.34. Contour plot generated from the time series temperature data collected at the pontoon. Click to enlarge.

Fig.39. light attenuation time series from the pontoon. click to enlarge

Light attenuation

At 08:45GMT attenuation is at its lowest (0.24). This increases steadily to reach a peak of 0.36 by 11:15GMT.

Percentage Light levels show an exponential decrease with depth as it rapidly attenuated by water and suspended particles within it. Over time the general trend shown is that attenuation increases. One reason for this trend could be due to the tide. At 08:45 the tide is turning and is therefore still in slack water. The amount of suspended particulate matter will therefore decrease as the water has less energy to keep it suspended, hence the high levels of irradiance at this time. By 11:15, at the end of the time series, the ebbing tide is in full flow, it will therefore be carrying more particulate matter. This increase in particulate matter suspended in the water may have caused the decrease in irradiance through the water column.

Station 1 (fig.16.)

At station 1 there was a small range of phytoplankton species found. Quinardia delicatula was the dominant species and accounted for 30cells out of the 70cells per ml in total with only three species were found at the surface. At 24m only four species were identified in the water sample. These was dominated by Rhizosolenia imbricate and Guinardia flaccida which accounted for 90 and 120 cells per ml respectively out of a total of 240cells per ml.

Fig.16. Phytoplankton abundances from the offshore sample station 1. Click to enlarge.

Fig.36. Phytoplankton abundances at station 2. Click to enlarge.

Station 2 (Fig.36.)

In the surface waters at station 2 there was a large range of phytoplankton species (9 in total) which combined accounted for an average of 430cells per ml. The dominant species was Rhizosolenia alata which accounted for 220cells per ml.  At 10m the type of species changed with Guinardia flaccida the dominant species with 170cells per ml. The total cell count per ml dropped to 340cells per ml with only four species present. The biota changed once more at 27m with Rhizosolenia imbricate becoming the dominant species found with 340cells per ml out of a total of 540cells per ml. The range of species declined further to three species.

Station 3

Station 3 showed a large variety of species and abundance at various depths.  At the surface a total of 7 species were identified and accounted for a total of 440cells per ml. the dominant species found was Guinardia flaccida which accounted for 130cells per ml. At 19m depth a total of 8 species of phytoplankton were identified. The dominant species was Rhizosolenia stolterforthii which had a total of 160cells per ml. Two species to note that were identified were Nitzschia longissima and Ceratium furca but were found in low abundance (10cells per ml). The total abundance of phytoplankton cells at 19m was 430cells per ml. The range of species found at 32m decreased to seven. The dominant species was Guinardia flaccida which had an abundance of 110cells per ml. The total abundance of phytoplankton found at 32m was 270cells per ml.

Fig.37. Phytoplankton abundances at station 3. Click to enlarge.

Fig.38. Phytoplankton abundances at station 5. Click to enlarge.

Station 5

Five species of phytoplankton were found at the surface at station 5. The dominant species was Rhizosolenia stolterforthii which had an abundance of 180cells per ml. All five species accounted for 360cells per ml in the surface water. At 11m, six different phytoplankton species were found. The total abundance at this depth was 200cells per ml, of which the dominant species (Eucampia species) accounted for 90cells per ml. The range of species found at 23m remained at six but different species were present. They had a total abundance of 190cells per ml. The dominant species found at this depth was Rhizosolenia imbricate which had an abundance of 60cells per ml.

Discussion

Diatom species dominated at all sites most notably Rhizosolenia imbricata at sites 1, 2 and 4 and Guinardia flaccida at sites 1,2 and 3. A noticeable bloom of diatoms during the summer months has been known to occur with the dominant presence of small Diatom species Rhizosolenia and Pseduo-Nitzschia recorded in the Western English Channel (Widdicombe et al., 2010). Despite this, previous findings oppose this trend including surveys conducted by Holligan and Harbour (1977) who found dinoflagellates to dominate from June to August within the thermocline. They did however observe the presence of particular diatom species to extend into June; one of which (Rhizosolenia delicata) was recorded at site 5. The presence of Diatoms towards the end of June can be explained by the occurrence of favourable conditions that are able to support the growth of larger phytoplankton species (Holligan & Harbour, 1977).

Zooplankton (Fig.17.)

Copepods are dominant in site 1,3,5 and 2 (from 12m to surface), while site 2 (from 22m to 12m) is dominated by hydromedusae.  Copepods, Copepod larvae, Polychaeta larvae, Chaetognatha, Hydromedusae and Appendicularia are present at all stations. Echinoderm larvae, Fish larvae, Ctenophora and Cirripedia larvae are not found at all sites and where they are present, their abundance is very low. There is a significant difference in the abundance of Copepods between station 1 (sampled from 12m to the surface) and station 2 (sampled between 22m and 12m). At station 1, Copepods are 124 times higher than at station 2, with 1.28×10⁷ m3 at station 1 and 1.03×10⁵ m³ at station 2.  The abundance of Hydromedusae, is however almost 2 times greater at station 2 between 22 and 12m than it is at the same station between  12m to surface, in fact, between 22 and 12m the abundance is 5.96×10⁵ and between 12m and the surface the abundance is 3.04×10⁵.

Fig.17. Zooplankton abundances per cubic meter from the offshore excursion. Click to enlarge.

Station 2 (Fig.22.)

Temperature is highest at the surface at 13.0˚C, and lowest at the bottom at 11.9˚C, with a strong thermocline at about 13.8m. The fluorescence follows the thermocline and is matched by the chlorophyll, which peaks at 2.4mg/L. Nitrate is low in the surface waters, at 0.1mmol/L, and stays low moving down through the water column, increasing to 0.2mmol/L below the thermocline. Silicon also follows this pattern, starting low at 0.43mmol/L and reaching a peak of 3.66mmol/L at 27m depth. Dissolved oxygen decreases with depth, starting at 83.3% saturation at the surface, decreasing rapidly to 70.3% at the thermocline and remaining fairly constant after that, increasing only slightly with depth to 71.0%. Nitrate does not change much with depth, but does increase slightly below the thermocline, from 0.1mmol/L to 0.2mmol/L Phosphate is depleted at the surface and bottom, and peaks at 0.42mmol/L, just above the thermocline. Irradiance decreases exponentially with depth.

Discussion

The temperature decrease from the surface to the bottom is only 1.1˚C, but this occurs over only a few metres, producing a very sharp thermocline. This shows that the water column is stratified, likely due to the solar heating that occurs during the spring. The fluorescence and chlorophyll correlate very closely, indicating that there is an increase in phytoplankton mass at about 5m, which declines again approaching the thermocline. This pattern can be explained by the nutrient values; nitrate and silicon are both depleted in the surface waters due to biological utilisation, and high in the bottom waters, below the euphotic zone.

Fig.23. Water sample depth profile from station 3 with the relevant CTD cast data. Click to enlarge.

Station 3 (Fig.23.)

The temperature decreases by about 0.9˚C between the surface and 32m, with a weak thermocline at about 20m. Fluorescence closely matches temperature. Chlorophyll differs from fluorescence at the surface, but is similar at depth, and peaks on the thermocline at 2.12μg/L. Silicon increases constantly with depth, from 1.22μmol/L at the surface to 4.35μmol/L at depth. Phosphate is low throughout the water column, increasing by only 0.46μmol/L from the surface to the bottom of the profile. Nitrate is very low at the surface and at depth, at 0.3μmol/L, but shows a very high peak of 1.2μmol/L at the thermocline. Dissolved oxygen decreases with depth; it is high in the surface waters at 86.1%, declining rapidly between the surface waters and the thermocline to 62.4% and reaching a low of 54.3% at 32m depth.

Discussion

The temperature decrease through the water column is small but still significant, with a slight thermocline. This suggests some mixing, possibly caused by the strong winds present at the time, but not enough mixing to completely remove the stratification. The decrease in dissolved oxygen with depth is due to the shift in the ratio of production and consumption of dissolved oxygen. At the surface, the rate of photosynthesis is higher and so more oxygen is produced. At depth (below the euphotic zone) phytoplankton are unable to photosynthesise and the dissolved oxygen present is utilised by aerobically respiring organisms.

Station 5 (Fig.24.)

Station 5 does not show any pronounced thermocline. Over the 25m covered by the CTD the temperature only changes by 0.6°C between the surface and at the bottom of the water column. Fluorescence stays at a low level throughout the water column ranging from 0.1 at surface to 0.07 at depth. Nitrate concentrations are low at the surface of the column at 0.3µmol/l and stay low throughout the water column. There is however a relatively small increase in the concentration at 23m (0.3µmol/l to 0.4µmol/l). Phosphate behaves in a very similar way. Its concentration is lowest at the surface (0.25µmol/l) and stays low through the water column with a slight increase at 23m to 0.37µmol/l. Silicon, like the other nutrients is found at its lowest concentration at the surface (2.74µmol/l at 1m). The concentration then increases with depth to reach a peak of 3.99µmol/l at 23m depth. Chlorophyll concentrations vary significantly with depth; At the surface the concentration is high (1.22µg/l) but this decreases dramatically to 0.35µg/l at 11m. There is then a sharp rise between 11 and 23m from 0.35µg/l to 1.57 µg/l. Finally, dissolved oxygen at this station is very low, at 11m it is just 61%. This increases with depth to 70% at 23m. The third data set for dissolved oxygen was unusable.

Discussion

Station 5 produced a very varied data set. The graph shows that there is no distinguishable thermocline as the temperature only dropped by 0.6°C from the surface to the bottom.

Due to the increased swell in the area which caused the boat to move significantly from the original sampling point a lot of unnecessary noise was observed from the CTD at site 1, thereby making a graph incredibly difficult to generate.

Starting with station 2 (Fig.18.) and the temperature data, it is the same pattern that one would expect; there is a peak temperature at surface of approximately 13.0°C which stays relatively constant through to a depth of 13.0m where it then suffers a large decrease down to 12.3 at a depth of 13.8m, this is the thermocline. After the thermocline the temperature gradually decreases to 11.9°C at a depth of 27.8m. Due to this being offshore data after the first meter the salinity data is relatively constant with only a range of approximately 0.1 salinity units. However, a clear halocline is visible at 13.6m, where there is an increase from 35.1 to 35.3 salinity units.

In terms of fluorescence the general trend to notice is the dramatic increase from 0.105 to 0.133 at approximately 13m (at the same place where the temperature decreases dramatically). The irradiance data shows an exponential decay with depth and an asymptotic relationship towards 0. The turbidity data shows a similar relationship as that salinity data, after the first meter the data becomes relatively constant with and average value of 3.8 and a range of approximately 0.02.

At station 3 (Fig.19.) the thermocline is much less pronounced, however one could argue that it occurs at roughly 20m where the gradient of the temperature data is at a minimum. At this station the salinity data is much more variable and a clear halocline is less clear to determine. There is a lot of “noise” in the salinity data around 7 and 26 metres with ranges of 0.13 and 0.1 salinity units respectively.

Fluorescence shows a steady trend of decreasing with depth from a maximum of 0.143 to at around 3 metres to a minimum of 0.064 at a depth of 31.2 metres. Again the irradiance data shows an exponential decay with an asymptotic relationship towards 0. The turbidity data remains reasonably constant with a range of 0.2, it begins at  3.87 and then peaks at 3.88 at a depth of 17.1 metres and then follows a dramatic decrease to a minimum of 3.77 at a depth of 32.73m.

Station 4 (Fig.20.) shows a clear thermocline at 11.17m where the temperature decreases from 12.32°C to 12.11°C after this it remains reasonably constant with a range 0.1°C. this site illustrates one of the clearest haloclines with a salinity of  35.21 to 35.32 at a depth of 10.939m after this depth salinity is greater than surface values by over 0.8 salinity units. Under this halocline, the fluorescence data hits a minimum of 0.07 (a decrease from the value above the halocline by 0.58).

Irradiance shows the same trend here as the other two sites, however because the results were taken higher in the water column the overall irradiance is higher, as it has not yet been able to reach such low values that are illustrated in sites 1 and 2. Again, aside from the surface interference the turbidity values are held within a small range of 0.05 turbidity units.

Fig.21. CTD data depth profile from station 5, click to enlarge

Station 5 (Fig.21.) exhibits the same patterns in temperature, irradiance and fluorescence as stations 2,3 and 4 in that all changes that occur are very small thus demonstrating a highly mixed water column. In this data there does not appear to be any discernible thermocline, however there is a small halocline present at 7.34m from 35.26 to 35.28. below this small halocline fluorescence begins to decline to a minimum of 0.072.

Discussion

At station 2 it is clear that a greater temperature would extend down to greater depths during the summer months. They usually become thermally vertically stratified during these warmer months. The extent to which these waters become stratified in this way is due to the depth of the water and the tides, there type and there magnitude. The stratification parameter can be used to quantify the extent to which the waters are mixed (Pingree & Griffiths, 1978). Numerically this can represented:

As the stratification parameter = Log (h/u³)
Where d is depth and u is current velocity

Transect 1 (Fig.25.1) - Station 1 - 2

The maximum depth of the water column is between 20 -40m the faster moving waters are found at the edges of the transect and in the deeper waters in this instance. The higher velocities have an approximate value of 0.624m/s. The direction of flow is predominantly Northwards at the edge of the transect. Looking more towards the centre of the transect the flow direction is more toward the East. Backscatter increases as you move up the water column. It is approximately 68dB at 36m depth and towards the shallower depths the first 5m, the backscatter is between 90-101dB.

Transect 2 (Fig.25.2) - Station 2 - 3

The maximum depth is around 30m, with the higher velocities being in the first 20m, ranging from about 0.3-0.6m/s. It is clear looking at direction that there is shear in the water column, the flow is eastward in the bottom waters. It is predominately northward flowing, as you move up the water column it. There is a small proportion that is westward flowing in the mid-waters. This could cause shear in the water column. There is a relatively high amount of backscatter across this transect. It is virtually all 80dB with a single region falling below this. This region extends up near surface waters. On the contrary there are some areas of exceptionally high backscatter, these again are found in the surface 5m and have values up to 119dB.

Transect 3 (Fig.25.3) - Station 3 - 4

Along this transect the depth ranges from 30m to 20m. The velocity magnitude is highest in the mid and surface waters. With values ranging from 0.3-0.6m/s. There are some slower velocities found at depth with values in the region of 0.029m/s. The direction of flow is mainly Eastwards through the entire transect and water column. The Backscatter is similar to other transects observed in that there is high backscatter at the surface waters with a value of 119dB. The majority of the water column has a backscatter value of 72 dB.

1.Transect 1 (station 1 - 2)
2.Transect 2 (station 2 - 3)
3.Transect 3 (station 3 - 4)
4.Transect 4 (station 4 - 5)

Fig.25. ADCP transects for the offshore transect, between stations.

Station 1 (Fig.26.)

The Richardson number shows the water column is laminar at the surface and therefore is more stable. It also shows that turbulent flow is increasing as depth increases to 7.5m. This is likely to be attributed to wind stress at the surface. There is laminar flow between 7.5m and 17.5m, where a large thermocline occurs which contributes to more stable flow. Because of the fluctuations in temperature after the thermocline the water column becomes more unstable demonstrated by a re number less than 0.25. Due to no other influencing factors the Richardson number begins to increase after the thermocline, to a more stable flow. This shows that tidal affects are weak.  

Fig.27. depth profile of Richardson numbers at statin 3. Click to enlarge.

Station 3 (Fig.27.)

Between 0 and 30 meters there is a gradual thermocline thus contributing to the progressive increasing stability with depth. An Ri number of 0.05 which is less than 0.25 gives a turbulent flow at the surface, this is induced by wind stress. The water column becomes laminar (Ri > 0.25) at a depth of 6.3m. This shows that the water column is very well stratified in two layers; a surface turbulent layer and a deeper laminar layer.

Station 4 (Fig.28.)

For this station taken just off the headland the surface Ri number is in the uncertainty zone, between 0.25 and 1, so could be considered to be turbulent or laminar. The surface this value is more likely to be turbulent due to strong winds over the surface at the time of measurement. The rest of the water column is laminar, due to a clear thermocline over 4-10m, showing that the column may be stratified. The headland will have high acceleration of flow around it which could be an influencing factor on the data.

Fig.28. depth profile of Richardson numbers for station 4. Click to enlarge.

Fig.29. Depth profile of Richardson numbers for station 5. Click to enlarge.

Station 5 (Fig.29.)

The surface Ri number is 2.5m which is representative of turbulent flow. This again is due to wind stress on the surface and high velocities of flow. The flow becomes laminar and stays laminar between 5m and 20m, which are due to the gradual thermocline over this depth. The flow is turbulent below 20m, induced by high tidal velocities heading into the coast. This structure in the water column is indicative of a stratified flow, with two layers of turbulent mixing separated by a stable laminar layer. This station was just around from the headland so the stratified layers may be a result of the sheltered waters.

CTD (Conductivity, Temperature, Depth) rosette sampler

CTDs are used on most research vessels to establish vertical profiles of salinity and temperature, and can include packages containing sensors such as chlorophyll fluorometer and  transmissometers. These can be found on R.Vs Callista and Bill Conway and other sensors to measure dissolved 02, pH, light attenuation, sound velocity and more.

CTDs can be joined to a protective metal frame called rosette sampler to which Niskin bottles can be added to collect water samples for laboratory analysis of other parameters, such as nutrient concentrations. The opening or closure of the Niskin bottles can be controlled by the ship which sends an electric signal; the accumulated voltage triggers the release of a pin causing its closure. The temperature is measured using a platinum resistance thermometer, the pressure using pressure sensors, and salinity using water conductivity.

ADCP

ADCP are used on the back of research vessels, lowered into the water column, along with CTDs or attached to moorings.
ADCPs work on the principle of velocity, the current’s speed and direction is detected using sound transmitters, receivers and amplifiers . The current is measured using an effect called doppler shift. The transducer sends pulses of acoustic beams, pulses are scattered by flowing particles and this backscatter is then transferred to the receivers which record and analyse the acoustic pulses. The downward and outward directed acoustic beams allow a better detection of the frequency and gives continuous data on the direction and speed of flow (Herschy, 2009).

The ADCP mounted on a boat is used at depths greater than 1.5m. Flow velocity is recorded continuously along transects and the accuracy of the data transmitted to a computer (Herschy, 2009).

Secchi Disk

Secchi Disk is a simple method to measure light attenuation. It is a flat white disk which is lowered into the water while a human observer detects the depth at which it disappears (Preseindorfer, 1986). The deeper the Secchi disk, the less turbid the water and thus the better the clarity and vice versa. However, this technique is influenced by the outside light intensity and cloud cover.

Plankton Net

Plankton nets are cone shaped and made of a 200 micron mesh. The mouth of the cone has a diameter of 0.5m and the mesh will then filter the water flowing in the mouth. The part of the net with the smallest diameter is connected to a plastic bottle in which phytoplankton smaller than 200 microns are collected. The bottle has a cod end which allows plankton too large to go through the mesh to be restrained (Sigler and Sigler, 1990).

Some plankton nets, such as the one deployed on R.V. Callista, are closing plankton nets, meaning that they can target a specific depth interval in the water column, close up, and be recalled to the surface without collecting any more plankton.

Sidescan Sonar

Sidescan sonar are used in conjunction with a tow fish and launched from the back of a research vessel and uses acoustic waves from 1kHz up to 100 MHz to determine the topography of the seabed (Blondel, 2009).

An acoustic pulse is emitted from the transducer and reaches the seabed where it is backscattered by seafloor features to the transducer, amplified and transferred via a cable onto a recording unit onboard (Kenny et al., 2003). The scale of the seabed features depends on the different acoustic pressures that are backscattered to the hydrophone on the sonar.

The position of each seabed feature is recorded by the recording unit on the vessel pixel by pixel in order to be able to analyse features ranging from tens of centimeters to a hundred meters. The sidescan output is then printed either on thermal paper or electro sensitive paper.
However, the acoustic pulse can be affected by the distance it travels, the longer it travels from the source of emission, the more it dissipates. The fact that seawater is viscous also results in the attenuation of the acoustic wave. Once the sidescan paper is printed, seabed bed feature can be identified and interpreted  by human eye.

Remotely Operated Vehicles (ROVs)

Remotely Operated Vehicles can be manned or unmanned and have various purposes ranging from cleaning of  boats’ hull to construction and maintenance of drilling platforms.
ROVs that are camera mounted like the one from the London Natural History Museum used on the Xplorer, has a console to which a controller, a monitor and the submersible itself is attach. The controller is used onboard by the operator who controls the speed and direction of the submersible. The operator uses the monitor as a spatial indication as well as for identifying marine habitats. The submersible is equipped with thruster which allow quick manoeuver, the vertical thruster is used to keep the ROV at a stable depth and the altimeter present on the ROV enables an estimation of the distance from the seabed. The submersible is tethered to the vessel by a copper cable to allow transmission of the video signal onboard (Christ., 2007).

A drop camera was also utilised, hung on a cable below the ship, the camera was drifted just above the sediment surface, providing footage of the seabed.

YSI probe

The 6600 V2 YSI probe is a multi-probe measures conductivity (salinity), temperature, chlorophyll a, dissolved oxygen and pH. It can be deployed into the water by hand while values are displayed by a computer on deck.

Light Meter

The light meter consists of two sensors; the air sensor and the water sensor. When the two sensors are calibarated; the difference between them indicates light attenuation through the water column, percentage light levels certain depths can then be calculated.

R.V. Callista

RV Callista is a purpose built twin hulled research vessel commissioned By the University of Southampton and built in Finland. She was built to provide practical experience for students that are studying various elements of Oceanography at the University of Southampton. She is equipped with some of the latest technology for monitoring coastal and estuarine environments in situ, such as an on-board ADCP unit.  She has purpose built wet and dry labs, a galley area and is able to sleep up to 4 crew members.

R.V Conway

RV Conway is a University of Southampton owned multi-use research vessel. It can be outfitted with a variety of equipment to carry out specific scientific operations. It has an open rear deck area with a covered wheelhouse in which scientific computer based programmes can be run from onboard computers. Its primary use is for coastal and inshore work and is equipped with a 750kg A-frame with a trawl cable setup which extends up to 70 metres.

RV Xplorer

Xplorer is a vessel contracted to us to assist us in our studies of the Falmouth estuary and the surrounding area. She Is a purpose built MCA category 2 survey vessel built in 2002 by Gemini Workboats in Plymouth. Her primary role is as a survey and diving support ship, which makes her ideal for carrying out our side scan sonar work in the Falmouth Estuary. It has a large rear deck area with a 1 tonne capacity crane with facilities for dry labs in the wheelhouse.