Date: 03/07/17

Time: 9:30 - 13:00 BST

Vessels: Bill Conway and Winnie the Pooh

Low Tide: 06:45 UTC (1.55m)

High Tide: 12:57 UTC (4.07m)

Low Tide: 19:10 UTC (1.66m)

An Estuary is defined as a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage (Pritchard, 1952).

The aim of our study of the Fal estuary was to investigate the biological, chemical, and physical structure of the estuary and the degree of mixing from the interactions between the fresh water river input and the tidal seawater input.

To collect our data the vessels Bill Conway and Winnie the Pooh were used to take CTD measurements, ADCP transects, zooplankton nets, and water samples at 8 locations at different depths along the Fal estuary. We began our sampling at the estuary mouth at black rock and worked our way up the estuary.

To collect our water samples, we used a series of Niskin bottles attached to a CTD rosette which we lowered through the water column to a few meters above the sea-floor. As the CTD was lowered we determined from the profiles if there was an area of interest in the water column where water samples would be taken. We typically took water samples at 3 different depths, Deep, Mid and Surface, firing 2 Niskin bottles at each depth in case one mis-fired.

Samples taken were then analyzed in the lab to determine the phosphate, nitrate, dissolved silicate, chlorophyll, dissolved oxygen concentrations, and plankton counts.

Dissolved Oxygen:  we took samples to measure oxygen by ensuring the bottles had no air bubbles at all, and then added manganese chloride and alkaline iodide which binds and neutralizes the oxygen in the precipitate to be later released and analyzed in the labs.

Nutrients: For Nitrate, Phosphate and dissolved silicate we filtered each depth and stored the Nitrate and phosphate samples in plastic bottles and the dissolved silicate samples in glass bottles to prevent contamination.

Chlorophyll: For chlorophyll, we took 3 samples for each depth by filtering 50ml of the sample and then folded the filter paper in half and stored in a test tube of 90% acetone in the freezer.

Phytoplankton: the water sample was fixed with 100% toxic formalin and then stored with iodine dye so that the phytoplankton can be seen under microscopes in the labs.

Zooplankton: We deployed a 50cm diameter zooplankton net with a mesh size of 200 microns to take zooplankton samples at 3 different locations along the estuary.

Data points for the silicate samples fall below the theoretical dilution line suggest that there is a removal of silicate at the upper River Allen. The removal of silicate is correlated with high chlorophyll concentrations at the respective stations with high numbers of diatoms (up to 1548000 cells/L in station P). This indicate that silicate is used by diatoms for building their silicate test.

The riverwater end member was collected from the River Allen and the seawater end member was collected from the Falmouth estuary.

Data points collected for the nitrate samples fall above the theoretical dilution line in Stations P, O and Q and phosphate samples fall above the theoretical dilution line in Stations N, O, P and Q suggest that there is an addition of nitrate and phosphate to the River Allen. This could be due to sewage discharge along the river in the densely populated area contributing to the increase in nitrate and phosphate concentrations (Langston et al. 2003).

In order to assess the influence of tidal mixing, ADCP transects were taken during estuarine sampling of the estuary. Locations of the ADCP transects can be found on the site map.

A transect at Blackrock was planned, however the ADCP configuration wasn’t set for water depths in Falmouth this meant data was not able to be collected.

Comparison of ADCP transects show how varying tidal states influence flow in the estuary.

TRANSECT 1 STATION 2F

Date: 3/7/2017

Time: 11.27UTC
Start: 50 10.88274N      5 1.48919W
End: 50 10.41310N       5 2.59515W

ADCP data for this station shows a flood tide as expected with high water at 13:57BST.

We hypothesise that the differences in flow velocity (Figure 1) result from a mass of higher salinity seawater moving up the estuary at depth, with reduced flow velocity nearer the surface from freshwater river flowing out of the estuary on the eastern side of the river. This is supported by the salinity profile taken at station 2F where a halocline can be seen at around 5m (Figure 2) where a sudden change in velocity is also seen from the ADCP.

Investigating whether Coriolis has an effect on the flow of water concluded that a water column with a depth of 30m would need to be at least 153km wide to be influenced by Coriolis, so this would not account for the slightly higher velocities seen on the eastern side of the deeper channel.

ADCP data for this station shows a flood tide as expected with high water at 13:57BST.

We hypothesise that the differences in flow velocity (Figure 1) result from a mass of higher salinity seawater moving up the estuary at depth, with reduced flow velocity nearer the surface from freshwater river flowing out of the estuary on the eastern side of the river. This is supported by the salinity profile taken at station 2F where a halocline can be seen at around 5m (Figure 2) where a sudden change in velocity is also seen from the ADCP.

Investigating whether Coriolis has an effect on the flow of water concluded that a water column with a depth of 30m would need to be at least 153km wide to be influenced by Coriolis, so this would not account for the slightly higher velocities seen on the eastern side of the deeper channel.

TRANSECT 2 STATION 4E

Date: 3/7/17

Time: 12.41UTC
Start: 50 12.2494N      5 2.49751W
End: 50 12.26367N     5 2.11314W

High tide was 13.57 BST
There was no clear single direction of flow on ships track reflects this suggesting transect was taken at slack water.

As time approaches high water, the velocity of the flow is reduced as shown in the transect and flow is not in an exclusive direction indicating that soon slack water will be reached. Examining boat tracks at different depths of this transect however, shows that water at depth is still largely moving up the estuary. This most likely gives rise to the estuary-ward flow in the middle of the channel shown in the average boat track, as the channel is deepest in the middle.

At the upper estuary (Station 1G) it appears that the water column is most well mixed. Surface chlorophyll was low and there was a slight increase of chlorophyll at depth > 13m. Increased stratification was observed in station 2F which is followed by an increased in chlorophyll at the base of halocline at 3m. In station 4E, there was a slight increase in mixing and a chlorophyll maximum at depth of 5m.  In station 5D, the water column appeared to be the most stratified with a salinity difference of 2psu in surface 10m. A peak in chlorophyll at 3m was observed with low chlorophyll at the surface.

Station 1 was located at the end of the estuary (click here for station locations) and is most strongly influenced by tides and wave action which explains the well mixed structure. The riverine freshwater input is less dense than the tidal seawater input which results in the surface freshwater to lie above the saline water. This can be seen further up the estuary at stations 2F, 4E, and 5D. Station 5D receives freshwater inputs from the Truro river, Tresillian River and River Fal; station 4E receive freshwater inputs from the Channel's creek, Tolcarne creek and Pill creek. Station 2F receives

TRANSECT 3 STATION 5D

Date: 3/7/17

Time: 13.06
Start: 50 12.9131N      5 1.505631W
End: 50 12.93837N     5 1.67314W

Again here the ships track shows no one clear direction of flow in a similar situation to the station 4E transects.

freshwater inputs from the Restronguet creek and station 1G receives freshwater input from the Mylor creek and St Just creek. At the stratified waters in the upper estuary, a shallow halocline was observed and this result in a high average irradiance for phytoplankton growth as the mixing depth is shallow. This could be seen in Station 5D. As the water column mixes with depth, the mixed layer depth deepens and the average irradiance decreases as seen in stations 1G.

Zooplankton samples taken from River Allen. 106 individuals of Calonoid copepods per m-3 and 22 m-3 individuals of mollusc larvae were found. Limited zooplankton diversity observed could be due to the low salinities (21.1) where only a few species can be found.

A much greater number of phytoplankton was observed in the samples from the inner estuary as opposed to the outer estuary. The highest amount of phytoplankton was found at the uppermost station sampled from the River Allen (Station O). The abundance of phytoplankton then generally decreased as the stations moved out into the estuary, towards the sea, although there was a slight increase in abundance between stations 2F and 1G, the latter being the outermost station sampled (Figure . There is less phytoplankton observed in the deep water sample (5.26 m) than the surface water sample (0.98 m), which is expected, as there is less light energy available for phytoplankton to survive in deeper water. All other phytoplankton samples were taken from surface depths.

Overall, there was a clear distinction in the species composition of the samples taken from the outer estuary (Bill Conway samples) and the inner estuary, up the river (Winnie the Pooh samples). While Rhizosolenia sp. and Chaetoceros sp. were more commonly found in the outer estuary, the inner estuary was dominated by Cylindrotheca closterium, Nitzschia sp., and Prorocentrum sp. The samples from the inner estuary were also much more diverse, with more kinds of phytoplankton species being recorded, and in greater numbers. Cilliates, however, was a dominant species throughout the whole estuary, although they were much more abundant in the inner estuary.

2008). Copepods were found in much lower abundance (24 individuals m-3) compared to the upper estuary. Mysids were found dominating (54 individuals m-3) followed by Ceratium spp. (36 individuals m-3). Higher species diversity was found in this area which could be due increased predation on dominating species. This prevents species from outcompeting each other and encourages diversity.

Gao, Q., Xu, Z., Zhuang, P. and Chin. J. 2008, The relationship between zooplankton and salinity in the Changjiang Estuary', Chinese Journal of Oceanology and Limnology, 26, 178.

Pritchard, D.W. 1967. What is an Estuary: Physical Viewpoint. Pp. 3-5 in: Estuaries, G.H. Lauff (editor). American Association for the Advancement of Science (AAAS) Publication No. 83. Washington, DC: AAAS. 757 pp.

W.J. Langston, B.s. Chesman, G.R. Burt, S. J. Hawkins, J. Readman and P. Worsfold. 2003, Site characterisation of the South West European Marine Sites. Marine Biological Association.

Zooplankton samples taken from the Falmouth estuary (between station 1G and 2F). A higher abundance and species diversity were found in the outer estuary (208 individuals m) compared to the upper estuary (128 individuals m) (Gao et al.

To summarise, the estuary exhibited stratified behaviour further up the estuary, and well mixed nearer the sea.

Our data indicated that there was a removal of silicate to the upper River Allen which we suggest was used up by the high number of diatoms present to build their silicate test. The data also indicated that there was an addition of nitrate and phosphate to the river Allen which we have suggested was due to the sewage discharge from the surrounding population.

The overall dominant phytoplankton species in the estuary was Pseudo-nitzschia delicatissima, and the dominant zooplankton species was Calonoid copepods.

Estuary Phytoplankton:

Major species: Rhizosolenia sp., Chaetoceros sp., Cilliates.
Notable species: Nitzschia sp.

River Allen Phytoplankton:

Major species: Cylindrotheca closterium, Nitzschia sp., Cilliates, Prorocentrum sp.
Notable species: Unknown dinoflagellates, Bacillaria sp., Rhizosolenia sp., Karenia mikimotoi sp., Coscinodiscus sp.