On the 05/07/17, surface water samples from various locations along the Truro River were collected on board the vessel RV Winnie the Pooh.

Map 1. Locations of 4 stations for the upper estuary (NOPQ) collected on 05/07/17 and 3 stations for the lower estuary collected on 11/07/17.

Table 1. Locations and time of the 4 stations on the 05/07/17 for the upper estuary

Methods Upper Estuary

A T/S probe was deployed at four stations to the same depth to record the temperature, salinity and PH of the locations.

Zooplankton tow nets were deployed at two stations against the incoming tide. The nets were left to collect the sample for a duration of 5 minutes as the vessel travelled between stations.

A vertical Niskin bottle was deployed at 4 stations along the estuary to collect water samples at the same depth. Once the samples were on board the vessel: ‘Winnie the Pooh’, we filtered the water samples to provide chlorophyll, nutrient and phytoplankton (with the addition of Lugols) samples for later lab analysis.

No ADCP values were collected on board the RV Winnie the Pooh, because there was no ADCP available. No Secchi disks were deployed on board the RV Winnie the Pooh.

Methods Lower Estuary

Nisken bottles, CTD and ADCP all mounted on a rosette sampler, was deployed at three stations along chosen sites of the Falmouth estuary. The CTD data was recorded at the water surface which was followed by the deployment into depth. This was visualised and recorded by computer on the Conway vessel. Once the rosette was retrieved back on board, the Niskin bottles which were fired at specific depths via remote trigger, were sampled to obtain the nutrient, chlorophyll and phytoplankton data from later lab analysis.

A secchi disk was deployed at each station to obtain the light penetration depth.

A single zooplankton tow net was deployed at the first station against the tide for a duration of 5 minutes.

Upper Estuary Results: Biology

Disclaimer: All the opinions expressed in this site are that of group 14 and not necessarily the University of Southampton or the National Oceanography Centre, Southampton.

Upper Estuary Results: Chemistry

Upper Estuary

Lower Estuary

On 11/07/17, 3 different stations were surveyed on board the RV Bill Conway.

Table 3. Locations and times of CTD deployment to surface on the 11/07/17 for the lower estuary.

Aim

To investigate the variation in components such as the; temperature, salinity, Transmission and Flourometery via CTD as well as the biological parameters, discharge rates and flow speed variation across the three stations.

Lower Estuary Results: Biology

Lower Estuary Results: Chemistry

Lower Estuary Results: Physical

Figure 1. Stacked bar graph showing percentage (%) composition of phytoplankton assemblage by number of individual cells per group. Samples taken from four stations (N, O, P, Q) up river Truro on RV Winnie the Pooh. See separate map for exact coordinates of locations. Phytoplankton groups contributing to < 1% of the assemblage at any one station have been removed for clarity.

Figure 2. Stacked bar graph showing percentage (%) composition of zooplankton assemblage by number of individual cells per group. Samples taken from two transects up river Truro. Samples taken with tow net. See separate map for exact coordinates of locations.

Table 2. Metadata collected on RV.Winnie the Pooh on 05/07/17

Table 4. Metadata collected on RV. Bill Conway on 11/07/17

Figure 4. Calculated chlorophyll concentrations with relation to the station numbers up the River Truro. The first station N1 shows highest amount of chlorophyll (> 1 µg/L) which decrease by around half at the next station O1. This trend of decreasing is repeated for P1 that is again half of the previous with Q1 being the lowest of all four stations.

Chlorophyll

Figure 5: Silicon (black), nitrate (blue) and phosphate (red) plotted against salinity. The data was collected from the Winnie the Pooh by groups 13 and 14 on the 5th July. The theoretical dilution lines (TDL) correspond to their dataset’s colour and are drawn between the river water endmember and the seawater endmember (obtained from group 5’s callista data which was collected on the 4th July as our group lacked the Bill Conway data).

The lower chlorophyll concentration at the sites further up the river could indicate a relationship between salinity and chlorophyll concentration. However It could also be due to the proximity of sites P and Q to the town Truro and the sewage treatment plant.

The difference in chlorophyll concentration between sites P and Q is much smaller than the difference between sites N and O. This may be due to the smaller distance between these sites, however that would still likely not account for the difference. Both sites N and O are located downstream of a river confluence.

Phytoplankton

Zooplankton

Estuarine Mixing

  (The seawater endmember is not a proper endmember due to lack of Bill Conway data).

  Silicon along the area of the Truro river that was sampled was mostly conservative, with some removal occurring around sites O,P&Q. This is almost certainly due to uptake of silica by diatoms such as Cylindrotheca, which accounted for 35.59% of all phytoplankton found at station Q.

  Nitrate appears to be non-conservative, with addition occurring along most of the course of the estuary. There seems to be a large input between the river endmember and our first datapoint. However, as the river endmember does not fit the otherwise consistent trend seen in the other data, and there is little that could account for such large inputs of nitrate, it could be an error in the endmember concentration.

  Phosphate has extreme non-conservative behaviour, with very large inputs around stations O,P&Q. This is almost certainly due to inputs from the sewage treatment plant located just 100m from station P.

Phytoplankton Discussion

Cryptomonas is typically most successful in fresh- and brackish waters, and is therefore an excellent competitor in estuaries. Cryptomonas can protect themselves using cell aggregations called palmellae, which protects them from predators [1] (Univ. Koeln Botany). Cryptomonas also competes well in changing light intensities, something not uncommon in turbid estuaries. Cryptomonas is selectively grazed upon by zooplankton like Ceriodaphnia, which may limit their success [2] (Gladyshev et al., 1999).

Cylindrotheca is likely highly abundant due to its competitive capabilities. Like other diatoms it competes best in high nutrient conditions, cool temperatures and changing light conditions [3][4](Spaulding and Edlund, 2008, Buchan et al., 2014). Cylindrotheca relies on relatively high silicate concentrations to grow, something commonly found in estuaries.  

Zooplankton Discussion

Copepods thrive in a range of environmental conditions, like salinity, temperature and nutrient loading [5]](Lawrence et al., 2004). Different species of the subclass thrive in different conditions, and seeing as species were not individually counted, this study shows the success of the entire subclass. Copepods seem to rely more on food availability and quality for their success [5] (Lawrence, et al. 2004), something that is relatively common in the middle of summer.
Copepod reproduction is hindered by diatom blooms [6](Miralto et al., 1999). As blooms typically occur in spring, reproduction (and presence of nauplii) is not hindered in the middle of summer.

Cladocera are typically most successful in fresh- and brackish waters, which explains their presence in estuaries [7](Irvine, 1986). Their abundance relies mostly on food abundance and quality, and interspecific competition [8](Sommer and Stibor, 2002). During this time of year, food will be abundant. Species were not discriminated against, which aids in their abundance as an infraorder.

Increased presence of hydromedusae zooplankton is likely due to aggregation within estuaries and other physical boundaries [9](Graham et al., 2001).

References

[1] http://www.uni-koeln.de/~aeb25/ecology.html  University of Koeln Botany department (Online).

[2] https://link.springer.com/article/10.1023/A:1009916209394   // Gladyshev, M.I., Temerova, T.A., Dubovskaya, O.P. et al. Aquatic Ecology (1999) 33: 347. doi:10.1023/A:1009916209394

[3]Spaulding, S., and Edlund, M. (2008). Cylindrotheca. In Diatoms of the United States. Retrieved July 12, 2017, from http://westerndiatoms.colorado.edu/taxa/genus/Cylindrotheca

[4]Buchan et al 2014 Nature Reviews Microbiology 12, 686–698 (2014) doi:10.1038/nrmicro3326

[5] Stewart.R. (2008) Introduction to Physical Oceanography

Chlorophyll

Figure 8. Bar graph showing chlorophyll concentration at surface at three separate stations in the Fal estuary. G45 showed low surface concentration, F46 showed a medium surface concentration and E47 showed a high surface concentration.

Chlorophyll represents primary activity by phytoplankton and other primary producers

Chlorophyll increased with depth at G45. G45 was far down the estuary, thus resembling truly marine environments. In marine environments, chlorophyll increases with depth due to more stratified top layers and nutricline, thus optimal growth being at lower light but higher nutrients. F46 was mid estuary, with higher nutrients. The higher concentration at surface is produced by optimal primary producers. Slight increase towards depth can be due to stratification or movement of phytoplankton by tide. E47 was far up the estuary. With high nutrient input from rivers, optimal position for primary production is at surface. This shows in the graph where the highest chlorophyll is at surface, and decreases with depth.

Figure 9. Line graph showing chlorophyll concentration against depth at three separate stations in the Fal estuary. Station G45 showed a positive correlation between concentration and depth, starting with a low surface concentration. Station F46 shows a zig-zag line which ends with a slightly lower concentration than at surface, which was medium-high. E47 showed a negative correlation between concentration and depth. It started very high and ended lower, at a relatively high rate.

Figure 10. Line graph showing silicon concentration against depth at three separate stations in the Fal estuary. All three stations showed a positive correlation between phosphate concentration and depth. Station G45 had a low surface concentration and increased at a very low rate with depth. Station E47 had a high surface concentration and increased rapidly with depth. Station F46 had a high surface concentration and ended with a much higher concentration. F46 had an outlying point around 7m where concentration spiked only to decrease afterwards.

Phosphate is also an essential nutrient for primary producer growth. It will decrease with high primary producer presence and increase with high input from waste or agricultural input. At all stations phosphate increased with depth. This suggests there was a high input from settlement or agriculture or that primary producers did not use up a lot of phosphate.

Phosphate

Nitrate

Nitrate is an essential nutrient for primary producers. Therefore, wherever primary producers are present, nitrate should be lower unless affected by an unaccounted for source. Sources like this could be sewage or waste from nearby settlements or industry or agricultural runoff. At G45 nitrate increased with depth. This could either be due to low primary producer presence, which does not use the nitrate up or it is likely that there is an input of nitrate from nearby settlements or industries. At F46, nitrate concentration increases slightly. There is a sharper increase after 11m. This shows no correlation with chlorophyll, which suggests that there is significant nitrate input or very low primary producer presence. At E47 there was a slight decrease with depth. This correlates with chlorophyll, as chlorophyll was high at E47. The primary producers use up the nitrate for growth, which lowers concentration.

Figure 11. Line graph showing nitrate concentration against depth at three separate stations in the Fal estuary. Two stations (G45, F46) show a positive correlation between concentration and depth, and one (E47) show a very slight negative correlation between concentration and depth. G45 increased rapidly with depth whilst F46 increased slowly with depth.

Silicon

Silicon is a nutrient used by certain primary producers, diatoms, and input mainly from rivers via rock erosion. Silicon is generally a conservative nutrient due to low input and high usage. At all stations silicon concentration decreased. This suggests high presence of diatoms which uses up the silicon. Silicon would increase at depth as diatom frustules sink and dissolve. As silicon decreases at depth probably means that the water moves too fast (via tides) for frustules to settle at depth and dissolve and are moved to sea.

Figure 12. Line graph showing silicon concentration against depth at three separate stations in the Fal estuary. At all three stations there was a negative correlation between silicon concentration and depth. Station G45 had a low surface concentration which decreased linearly to depth. Station E47 had a medium surface concentration which decreased linearly to depth, but was higher than G45 at the same depths. Station F46 had a high surface concentration which decreased linearly to depth, and increased much more rapidly than the other two stations.

Figure 13. Line graph showing phosphate, nitrate and silicon concentrations against depth at a single station (F46) in the Fal estuary. The graph has several x-axes to allow for accurate portrayal of nutrient concentrations. Silicon decreased with depth almost linearly, nitrate increased with depth almost linearly and nitrate decreased with depth until 7m where it started increasing again. At 18m the “increase” was still much lower than the initial surface concentration.  

ADCP data was collected at 3 transects of the survey beginning at Black rock at the mouth of the estuary and finishing further up the estuary at King Harry Pontoon. From analysing the data on WinRiver we can see that the flow speed is much higher and more turbulent further upstream, with flow speed being relatively low at the mouth of the Estuary. This is due to the estuary widening at the mouth of the river; increasing the volume of the water column and therefore decreasing the flow rate. We began our tidal flow on a slack tide and so as the survey progressed, the tidal flow increased.We see an increased flow rate from Station F45 to E47, this is due to the combination of the narrowing channel upstream of the estuary as well as the increasing tidal flow.

Figure 14. Station G45 Earth Velocity Magnitude from ADCP on Board RV. Bill Conway, transect start time 07:57 UTC. The high water was at 07:34 UTC, with our data being collected from 07:57 UTC; therefore, the tide was on the ebb. The magnitude of the water flow confirms that the tide was ebbing and moving out of the estuary. The water was travelling at a higher speed on the left hand side than the right hand side.

At Station G45 the average flow rate was generally between 0.18 and 0.3 m/s which although is low in comparison to the other stations, it is a moderately high flow speed.

Station G45

Station F46

Figure  15. Station F46 Earth Velocity Magnitude from ADCP on Board RV. Bill Conway, transect start time 09:13 UTC. The flow speed is fairly constant throughout this transect with flow speeds varying between 0.011 and 0.438 m/s.

Although this station is a lot further up the estuary than the last, it is still in a relatively wide section of the estuary. We do see an increased flow speed compared to the mouth of the estuary, however it may not be as high as you may expect for it’s location due to the width of the river.

E47

Figure  16. Station E47 Earth Velocity Magnitude from ADCP on Board RV. Bill Conway, transect start time 09:53 UTC. This transect was taken at the upper estuary near the pontoon; a location where the channel narrows and shallows. The flow speed is more variable through this transect, with flow speed varying from 0.015 – 0.3m/s at the sides to a much higher flow speed of around 0.375-0.7 m/s in the middle part of the channel.

At this time of the day, the tidal flow has had the time to mature and increase since High Tide at 07:34 UTC. This increased tidal flow increases the flow speed. The location of this transect has the biggest impact on the very high tidal speeds seen. This part of the estuary is a lot narrower and shallower; reducing the volume of the channel and therefore increasing the flow rate.

Richardson Number

Discussion

shows our profile of Richardsons number against depth at Station 45, 46 and 47 on a logarithmic scale. A logarithmic scale is used to clearly show the distribution above 1 and below 0.25. All points are below 0.25 apart from 1; except this 1 point from Station 46 has a Reynolds number between 0.25 and 1, and in this section there is some uncertainty. Overall from our calculations, it can be concluded that on this day, in this part of the estuary, the Fal estuary is experiencing turbulent mixing. A number of factors contributed to this turbulent mixing including stormy weather, large waves, high precipitation and the location being a very wide part of the estuary; very open to the elements.

As the majority of the data points reside past the 0.25 line we can conclude that the three stations were turbulent. The data points for all three stations are well below the 0.25 line at a range of depths. This would indicate that the water column at each station was turbulent throughout the water column. One Richardson number is above the 0.25 line indicating that at 6 metres depth at station 46 the water column was transitioning from laminar toward a turbulent water column. The transition was determined in this direction as the majority of the data points for station 46 was below the 0.25 boundary suggesting that this water column was at that point turbulent. The turbulent nature of the water columns at all three stations would mean that the three water columns were well mixed.

The aim of calculating Richardsons number along each transect in the Estuary is to make an approximation as to whether the Fal Estuary was undergoing turbulent mixing or laminar flow. To calculate Richardson’s number, the following equation was used: . An Ri </= 0.25 implies turbulent flow whilst an Ri >/=1.0 implies laminar flow [5]. To calculate , the change in shear was calculated from east and north velocities from the velocity from the ADCP data from WinRiver.

Figure 17. Profile of Richardsons number against Depth on a logarithmic scale for the 3 stations surveyed in the Estuary on the RV. Bill Conway. The graph shows that all richardsons values calculated for each station are less than 0.25. All values below 0.25 display turbulent flow, therefore all Stations show turbulent flow.

Figure 3. Images of the top 4 species of Phytoplankton found in the collected samples on board the RV. Winnie the Pooh  on 05/07/17

Table 6. Table showing number of zooplankton as recorded in lab and converted into m3 based on in-situ bongo net flowmeter measurements. “Zooplankton group” refers to taxonomic divisions of zooplankton to specific taxonomic level. Samples taken from the Fal estuary. Table shows highest abundance of copepods, cladocera and copepod nauplii. Samples showed an unusually high amount of cirriped and echinoderm larvae compared to other samples in the estuary.

Table 5. Table showing number of phytoplankton cells as recorded in lab and then converted into cells per ml. “Phytoplankton group” refers to taxonomic divisions of zooplankton to specific taxonomic level.  Sample was taken from the Fal estuary. Sample shows Pseudo-nitzschia dominance compared to any other phytoplankton group.

Figure 6. Images of the top 4 species of Phytoplankton found in the collected samples on board the RV. Bill Conway  on 11/07/17

The Dissolved Oxygen Saturation(%) is very similar for Station G45 and F46 at 100% and 100.2% respectively, with Dissolved Oxygen Saturation being slightly higher for Station E47 at a percentage of 104. Overall, there is little variation in dissolved oxygen saturation across the three transects at varying depths.

Dissolved Oxygen