Introduction

The aim of this survey was to explore the variability in chemical, physical and biological parameters up the estuary from the mouth up to the fresher river water in Truro.

Method

A CTD rosette loaded with Niskin bottles was used to collect data on temperature and salinity throughout the water column. Two samples were taken at both the top and bottom of the water column, and a further two samples were taken at a point of change within the column. This was generally the bottom of the thermocline; where chlorophyll concentrations should reach a theoretical maximum.

Subsamples from these bottles were used to calculate the concentrations of silicate, nitrate and phosphate throughout the column, as well as chlorophyll from all depths. Phytoplankton was sampled from the bottles taken at the surface, and oxygen concentration from the deepest sample.

4 ADCP transects were taken at different points along the estuary. Additionally, two zooplankton net trawls were performed; one at the mouth of the estuary, and another approximately halfway up the estuary.

Zooplankton

Zooplankton types and abundance was carried out by identifying and counting the plankton in 10ml of a sample under a microscope. The highest abundance was seen at station 5, and copepods accounted for over half of this number. This is not as was expected because station 5 was shown to have the lowest abundance of phytoplankton (Figure 2B), and since zooplankton consume phytoplankton. Moving from most saline (Station 1) up the estuary (station 5) zooplankton abundance appears to increase, however station 9 shows a drastic drop in number of zooplankton. As for the breakdown of types of zooplankton, copepods were the most abundant type of zooplankton, and represented approximately half of the zooplankton identified in each station’s sample. Siphonophorae only occurred in Station 1, which may be because this was the most saline station, and they are a type of cnidarian so it is likely that they are only suited to saline habitats. Three zooplankton types occurred in only one site each; Siphonophores and Echinoderm larvae were only seen in station 1’s sample, and Appendicularians were only seen in station 9.

Figure 3. Figure to display the species composition of zooplankton throughout the estuary from stations 1-9.

Chlorophyll

Moving up the estuary, surface chlorophyll increases, peaking at station 4, before falling again and subsequently rising again from station 6. Although data on chlorophyll at depth was only collected from stations 1 to 4, they appear to follow the same trend, at least at the mouth of the estuary. If more data had been collected, particularly from depth and in between stations to give a more detailed spatial scale, this pattern could be explained more definitively.

Figure 8. Graph displaying the surface chlorophyll (umol/L) for each station (1-9) on the Bill Conway and Winnie the Pooh, and the depth chlorophyll from each station (1-4) on Bill Conway.

Chemistry

Dissolved Oxygen

This shows that oxygen saturation of water in the estuary increases with station number (used as a proxy for salinity – higher station numbers are located further up the estuary). The generated line of best fit appears to confirm this. However, our data for this is limited- additional data from further up the estuary would have served to solidify this conclusion.

Figure 7. Graph displaying the oxygen saturation (%) for each station (1-4) on the Bill Conway, with a line of best fit.

Physics

Richardson Number

For Richardson number values above 1.00 turbulent mixing is suppressed and stratification occurs. For Richardson number values under 0.25 turbulent mixing is expected, resulting in well mixed conditions. Richardson number values between 0.25 - 1.00 are said to be in a transition between stratified and mixed conditions.

CTD

Salinity

For all stations, the salinity is lowest at the surface of the water column. This is due to the less dense freshwater from the riverine source, which has a lower salinity, and lies above the more dense seawater. However, the salinity profile for each station is very different and displays the different levels of stratification. At station 4, the water column is the most stratified displaying a salt wedge where there is a sharp boundary separating the upper less saline layer from the intruding more saline bottom layer. At the surface (1m depth) the salinity is as low as 32.3, this rapidly increases within the first few metres to reach 33.7 salinity at 4.0m depth, and 33.9 salinity by 10.7m depth. This is due to the strong riverine flow and weaker tidal currents as the station is located nearest the head of the estuary. This is also seen at station 3 where there is a sharp boundary seen from the surface (1m depth) at 33.15 salinity, to a higher salinity of 34.2 at 10m depth. However, stations 1 and 2 suggest that at the mouth of the estuary the water column is less stratified and therefore displays mixing. This is visible as there is less range in the salinity throughout the depth of the water column. For example, for station 1 at 1m depth, the salinity is 33.8 and only increases by 0.4 by 25m. This results in the less stratified and well mixed water column as there is greater tidal influence on the riverine input.

Nitrate

The most striking feature of Figure 5A is that station 4 appears to have much higher nitrate concentrations than any other station, with its minimum concentration approximately 2μmol/L greater than the next highest value. Stations 1 and 2 follow very similar patterns, staying fairly constant with depth; Station 1 shows a slight increase, whereas station 2 shows a slight decrease. This indicates they are neither being added or removed at significant quantities. Station 3 follows a more classic pattern of depletion in surface waters, with slight remineralisation at depth.

Overall, nitrates seem to exhibit fairly conservative behaviour,as shown in Figure 5B, with a few small additions at salinities of around 8psu and 18psu. The input at 8psu could be due to the proximity of two roads; car exhaust dissolving into the river could produce increased levels of nitrates in solution. The increase at around 18psu could be explained by the closing of some river gates just prior to sampling. This would have disturbed sediment which may have contained trapped nitrates and this would have resulted in suspension and subsequent dissolution of the nitrates.

Figure 5A. Graph showing the nitrate profile with depth through the water column at all stations in the Fal estuary.

Figure 5B. Graph showing the mixing profile of nitrate in the Fal estuary.

Map 1A. Map displaying the locations of the station sampled on 04/07/2017 in the Fal estuary on the Bill Conway and Winnie the Pooh. Map 1B. Map displaying the zoomed in view of the location of the stations 5 to 9 of which were sampled in the Fal estuary on the Winnie the Pooh.

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Phosphate

Figure 6A shows phosphate levels were highest at station 4 which is closest to the input of phosphate seen in the TDL graph, so this is as expected. Most stations do not appear to follow any of the expected patterns, including those of depletion at the surface and remineralisation at depth common in coastal waters; Station 2 does exhibit these patterns, however it is unknown why the other stations do not, especially when considering the lack of reflection of the patterns shown in the phytoplankton data (Figure 2C). This may be due to human error or an unknown input of contaminants.

The mixing diagram, figure 6B,  shows that phosphate shows non-conservative behaviour in the Fal estuary. Large amounts of phosphate addition are shown by the fact that most data points are falling above the Theoretical Dilution Line. This is probably due to a sewage outflow pipe located in the upper estuary, beginning at salinities of around 10psu.

Figure 6A. Graph showing the phosphate profile with depth through the water column at all stations in the Fal estuary.

Figure 6B. Graph showing the mixing profile of phosphate in the Fal estuary.

Phytoplankton

TThe CTD’s Niskin bottles were used to take surface water samples at different sites along the estuary in attempt to see how the biology changed with differing chemical and physical conditions. Lugol’s Iodine was added to each to preserve the phytoplankton for lab analysis; a microscope was used to record phytoplankton species richness and abundance for 0.1ml of samples, which was then multiplied up to give numerical data per 1ml. No phytoplankton data was collected from Station 11 because the Niskin bottle misfired.

Figure 2B shows that the abundance of phytoplankton did not follow a clear pattern along the estuary, and therefore seemed unaffected by increasing salinity and decreasing nitrate, silicate and phosphate. The station with the highest abundance was station 6 which had a moving gate and scrap metal heap; the numbers of phytoplankton found here were over double the abundance than that of the next highest site. Potentially, this could be a result of an increase in iron in the area, however, this is still a hypothesis since samples were not tested for metals. The site with the lowest abundance was station 5, which was at the mouth of what was first thought to be an inlet. After looking at a chart of the area we discovered that this was not an inlet, but was where a meander had been cut off by the main flow; movement in and out of this area may be a lot slower than the main channel of the estuary, which may result in a long residence time. The lack of phytoplankton found here could be explained by the potential low nutrient levels, and any nutrients that do flow in to this area are likely to be depleted quickly. However, looking at the nutrient mixing diagrams, there is no clear depletion in nutrients at this station to support this hypothesis.

The highest species richness was at station 1 which has 10 different phytoplankton types identified – this was at the mouth of the estuary in the most saline area. Station 7 had the lowest species richness as Cylindrotheca spp. was the only genus identified, yet was in huge quantities. We were unable to confidently identify the phytoplankton found to lower than a genus level, so an element of human error was involved as there may have been more than one species within the Cylindrotheca genus.

Station 6 had the highest abundance yet the third lowest species richness. Looking at species and abundance together on figure 2B highlights that they are not positively correlated. Station 8 was next to a sewage output (a large input of nutrients) and had a low abundance but high species richness, additionally there were some types of phytoplankton that were only seen in our station 8 sample, including the dinoflagellates Ceratium and Dinophysis, and the diatom Lauderia.

Figure 2C, shows the 5 most frequently occurring phytoplankton genera across all the station, and these clearly reflect the overall abundance for each station. All of these 5 types of plankton are diatoms, and this could account for the slight depletion in silicon in the estuary Figure 4B. Cylindrotheca spp. accounted for the enormous abundances seen at stations 6 and 7 as they were seen in the thousands, however this species was not very influential at other stations in comparison.

Diatoms made up over three quarters of all the phytoplankton recorded from each station. Ciliates and dinoflagellates were also seen, but not in the same abundance as diatoms. This may be because diatoms have a silica frustrule which acts as protection from zooplankton grazing and physical water movement damage.

Figure 2A. Pie chart displays the abundances of the 3 types of phytoplankton identified from all the samples.

Figure 2C. Bar chart displaying the five most frequently occurring species across all stations, and line displaying total abundance.

Figure 2B. Bar chart displaying the abundance at each station, and line displaying number of species seen at each station (species richness).

Site selection

Stations 1-4 were chosen because they were roughly equidistant along the bottom half of the estuary. Station 1 was chosen in particular to give an ocean endmember with which to compare other samples. We had enough equipment to definitely gather three lots of samples, with the fourth being taken on a time-dependent basis.

Station 5 was chosen because it was at the mouth of an inlet, approximately halfway between stations 4 and 6.

Station 6 was chosen because it was located close to moving gates and near a scrap metal heap; we wanted to investigate any effects these may have.

Station 7 was chosen because it was approximately halfway between stations 6 and 8, to give an intermediate salinity reading. It was also located near some mudflats, which may have caused some changes in nutrient levels via sediment suspension.

Station 8 was chosen as it was approximately 50m downriver of a sewage outflow pipe. We were interested in the effects of this input on nutrient levels in the water.

Station 9 was chosen as it was the lowest salinity we could access and was therefore the closest we could get to a true riverine end member. It was also at the joining of two tributaries, which may have increased the overall nutrient load of the water.

During the CTD deployment, Niskin bottles were fired at the bottom, surface and somewhere inbetween. We decided to fire the bottles at the base of the thermocline as this is where the deep chlorophyll maximum canbn sometimes be found if present in the water column.

Click on the picture to see a video of the deck work that occurred on the Bill Conway at the mouth of the Fal estuary

Biology

Silicate

Silicate tends to be higher further up the estuary, especially in surface waters. This is due to rock weathering in the upper part of the river valley releasing silicate into solution. However, throughout the estuary silicate usually decreases with depth, Figure 4A, perhaps due to the uptake of silicate by diatoms to make their frustules. The exception to this is at Station 1, which increases between 5m and 25m. This could be due to the breakdown of decaying diatom frustules, re-releasing the minerals into solution in the water column

The mixing diagram, Figure 4B, shows that silicate exhibits non-conservative behaviour in the Fal estuary. This is shown by the majority of data points falling below the Theoretical Dilution Line, suggesting slight removal of silicate throughout the estuary. This could be due to uptake by plankton such as diatoms, which use silicon to build their frustules.

Figure 4A. Graph showing the silicate profile with depth through the water column at all stations in the Fal estuary.

Figure 4B. Graph showing the mixing profile of silicate in the Fal estuary.

ADCP

ADCP data was collected from 4 stations starting at black rock at the mouth of the estuary and ending at the pontoon further upstream into the estuary. The ADCP identified stronger, more turbulent flow upstream with a decrease in flow rate and turbulence closer to the mouth of the estuary. This observation will be as a result of widening of the estuary channel, and a resulting increase in cross sectional area reducing flow rate. The calculated Richardson’s number for each site back this observation. The greatest flow rate are recorded at depth, the reason for this is due to the presence of the incoming tide.

Station 1

The naturally deep channel shows the highest velocity, Figure 9A, this is due to the water being channelled in a smaller area and thus has a higher hydrostatic pressure. A high concentration is also observed on the surface, due to its localised area it is most likely from a moving boat. The average Richardson number for station 1 is 0.091, as seen in Figure 13. As this value is lower than 0.25 it represents well mixed water.

All stick ship tracks show northern flows, Figure 9B, indicating that the tide is coming in, this correlates with the tide times from the Falmouth marina. The incoming tide may have been influential on the phytoplankton and zooplankton count with coastal water abundance spreading further up the estuary. Figure 9C shows a limitation when recording velocity using the ADCP. A depth of only 17.5m was recorded despite the channel reaching a maximum depth of 30.6 m.

Figure 9. Figures from results collected using ADCP on the Bill Conway at Station 1. Figure 9A displays the backscatter. Figure 9B displays the flow directions. Figure 9C displays the flow speed.

Station 2

Figure 10 shows the backscatter from station 2. Stratification is noted slightly in the deep channel which is backed up by the Richardson number which is greater than 0.25 at  a value of 0.27. High concentration hotspots are also recorded throughout the water column which may suggest algal blooms or marine vertebrate. activity.

Station 3

Figure 11 shows large spikes in average backscatter may be as a result of either large objects such as boats or buoys passing by or from high densities of marine life such as shoals of fish or plankton blooms. Due to its localisation and sharp cut off the high backscatter reading is most likely due to a boat or buoy.

Station 4

The backscatter seen at station 4, see figure 12A, similarly shows a spike in backscatter at the start of the transect. Whilst a boat or buoy being suggested as the cause to this as this pattern is repeated in the same place at a different location, this occurrence may be due to the equipment deployment and wave actions causing this reading to be observed.

ADCPs measure the absolute speed of the water, not just how fast one water mass is moving in relation to another, and is used as it can measure a water column up to 1000m long. This is ideal for station 4 as the figure shows a gradual increase in velocity viewed along the transect, due to proximity of the boat to a meander in the estuary, increasing the flow speed on the shallower river bank.

Figure 12C shows the stick ship track at site 4. The change in ship track from South to North, as demonstrated in figure 12C, may be the cause for the change and shift in velocity viewed.

Figure 12. Figures from results collected using ADCP on the Bill Conway at Station 4. Figure 12A displays the backscatter. Figure 12B displays the flow directions. Figure 12C displays the flow speed.

Figure 15A. Salinity profile with depth at 4 stations in the Fal Estuary. Data was collected using the CTD on the Conway.

Temperature

In general, all stations show a decrease in temperature with depth from the surface. Below 10m the temperature becomes more constant. For all stations, the surface temperatures remained within the range 15.5- 16.2oC; solar heating causes the water temperature to rise at the surface, causing the surface water to have the highest temperature in the water column. Stations 3 and 4 have the highest surface temperatures (above 16.0oC), in contrast to stations 1 and 2 with surface temperatures between 15.5oC and 16.0oC. This pattern continues throughout the water column, for example at 15.0m the temperature for station 1 is 13.8oC, whereas station 3 has a temperature of 14.4oC.

Stations 3 and 4 do not require as much heat energy to increase the temperature of the water column because they are further up the Fal Estuary, and therefore have a smaller volume of water to heat in contrast to stations in deeper water near the mouth of the estuary. The stations nearer the mouth also have a greater freshwater-seawater exchange, resulting in greater heat loss. The decrease in temperature of depth seen at all stations is due to stratification of the water column; with less dense freshwater overlying more dense seawater. The stratified bottom layer has lower temperatures because it does not experience the effects of solar heating.

Figure 15B. Temperature profile with depth at 4 stations in the Fal estuary, data collected using the CTD on the Conway.

Transmission

The transmission represents the variation in the turbidity levels along the Fal, i.e. as the transmission increases the turbidity level decreases (negative correlation). Transmission (%) is the percentage of the transmission for each depth relative to the 100% transmission measurement of 4.5. We removed very low anomaly points from surface (0m depth) from station 2 and 3. As the transmission is greater than 100%, this suggests the 100% transmission may not have been accurate, i.e. taken at a time of greater cloud cover.

In general, transmission increases between the surface and 10m, where the transmission then remains constant. As the lowest transmission occurs from 0-7m, this is where the greatest turbidity occurs; this is likely to be due to the boundary between less dense freshwater and denser seawater resulting in the mixing between the two layers which increases turbidity through increased dissolved matter within the water column. In the surface layers higher turbidity can also be attributed to increased phytoplankton, which is supported to Figure 2B. Stations 3 and 4, which are closer to the head of the estuary, have lower transmission at the surface than stations nearer the mouth of the estuary; this is due to the stronger tidal currents causing increased mixing near the mouth of the estuary, therefore resulting in increased turbidity. This is again supported by Figure 15B displaying greater mixing at stations 3 and 4. After the depth of 10m, the transmission and therefore turbidity becomes more stable. However at station 2 and 3, at the base of each depth profile there is an sudden decrease in transmission, this is likely due to suspended particulate matter from the seabed.

Figure 15C. Transmission profile with depth at 4 stations in the Fal estuary, data collected using the CTD on the Conway

Fluorescence

Fluorescence is mainly influenced by the levels of chlorophyll pigment, which represents the abundance of phytoplankton. The data for fluorescence displays a large amount of variation for all stations; this is likely due to fluorescence being influenced by a variety of pigments where the most influential is chlorophyll. For stations 2, 3 and 4 the highest fluorescence is seen at the surface between 0-5m depths, particularly for station 4 where the fluorescence peaks at 0.62v at 1.1m. The peak at station 4 corresponds to the peak in chlorophyll as seen in the Figure 8, indicating an increase in the abundance of phytoplankton. This is likely to occur because the riverine input is less dense and is therefore visible in the surface layers of the water column, this riverine input has a high nutrient concentration and therefore a high phytoplankton abundance; increased stratification also keeps the phytoplankton near the surface in the euphotic zone. However for station 1, the peak in fluorescence at 0.44v is seen at 9.3m depth, this is likely related to the chlorophyll maximum causing a phytoplankton bloom where there are optimum conditions for nutrients, and light, through maintained position in the euphotic zone. The highest fluorescence occurs at the stations closest to the mouth of the estuary, this is likely due to strong tidal currents causing increased mixing which increases the availability of nutrients to the surface allowing increased phytoplankton growth. The trend therefore suggests that the peak in fluorescence and therefore in chlorophyll, increases closer to the mouth of estuary, which is seen in the graph. Similar to Figure 15C, the seabed causes a sudden change to the level of fluorescence, this is likely due to the dead phytoplankton which has fallen to the seabed however still displays high chlorophyll level, and therefore high fluorescence. This is seen at station 1, 2 and 3, for example station 1 has high fluorescence of 0.58v at the bottom depth of 25.7m.

Figure 1. Graph displaying the state of the tide for each station relative to the time of sampling at each station.

Table 1. Table displaying the environmental conditions and times (UTC) of data collection within the estuary.

Figure 10. Backscatter from station 2.

Figure 11. Backscatter from station 3.

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Figure 13. Graph displaying the Log Richardson number profile with depth for each station (1-4) using data collected on the Bill Conway, as well as the vertical line for the Ri 1 and Ri 0.25.

Figure 15D. Fluorescence profile with depth at 4 stations in the Fal estuary, data collected using the CTD on the Conway

Table 2. Table showing the coordinates and time of sampling (UTC) at each station (1-9) in the estuary.

Disclaimer: The views and opinions expressed are solely those of the contributors, they do not reflect the views and opinions of the University of Southampton.

Residence Time

The residence time is used to calculate the time taken to completely replace the freshwater fraction in a water system. Calculating the flushing time can be used to help explain nutrient usage by phytoplankton and can be used to investigate the residence time of pollutants in the water column.

Flushing equation = Volume of water flushing (Surface area * Average depth)

                                                 Flux in (ADCP total Q value)

Surface area = 23,887,567 m^2

Average depth = 18.9

Total Q for site 1 = 1064.298

Flushing time = 5 days

The surface area was calculated using the ruler tool on Google Earth. The average depth was calculated by averaging the depths collected using the CTD. The flux in was taken from site 1 which is located at the mouth of the Estuary. The ADCP total Q from site 1 gave the flux in for the entire Fal estuary.

The calculated flushing time came to 5 days, this is considered higher than average. This may be as a result of the number of approximations used such as averaging the depth which was highly influenced by the deep natural channel which reached 35m at some points. Furthermore, the Q value was lower than usual as it was taken at slack water on a high tide.

Fig 14 - Fal Estuary surface area including Site 1

ESTUARY

Falmouth 2017 - Group 7