On Monday 29th June a survey of the Fal river on the RV Bill Conway between 07:00 UTC and 11:00 UTC was conducted. The aim of this was to create a profile of the estuary from lower salinities upstream to higher ones further down the Fal. This was done by collating our data (upper estuary) together with another group who surveyed the lower estuary in the afternoon. This enabled us to see how physical, chemical and biological properties change with salinity. The weather was calm with no precipitation, a sea state of 0, 6/8 cloud cover, swell of 0, no wind and reduced visibility. Low tide was at 09:13 UTC, high tide was at 15:10.
A rosette mounted with niskin bottles and a CTD probe was deployed off the back of
the boat to collect water samples for nitrate, phosphate, silicate and chlorophyll
concentration at different depths. In addition, a temperature, salinity, turbidity
and fluorometry depth profile was taken at each site. The CTD was similar in function
to the CTD rosette on the Callista as used two days previous, however it needed to
be set manually (the set up and deployment of which is shown in Figure 3). 
The ADCP was also used to give a constant stream of physical properties whilst the boat was moving as well as transects between stations. A T/S probe was used to calibrate the ship surface measurement. A plankton net was dragged 7m behind the boat to catch a sample of zooplankton. This was treated with 10% formulin in situ.
We surveyed a total of four sample sites which we then combined with the other groups’ five to get a full picture. We did not necessarily sample chemical and biological properties at each site and depth due to
bottle limitations
Nitrate Theoretical Dilution Line (TDL)
Nitrate plots close to, but slightly below the theoretical dilution line. This may be due to removal by a particularly large phytoplankton bloom stripping nutrients from the water column. We collected no data for salinities less than 27 but it should plot close to the TDL.
The highest concentrations seen on this graph are at the lowest salinity values, and therefore are the values for the riverine end members.
Nitrate depth profile
The depth profile shows that nitrate concentrations are reasonably uniform and generally have a subtle decrease with depth amongst most stations sampled. Station 46 shows noticeably higher nitrate concentrations within the top 5 metres of approximately 24umol/L, but decreases to approximately 2.5umol/L at 4 metres. At Station 54 nitrate concentrations increased from 5umol/L at 3 metres before slowly decreasing with depth.
The high nitrate levels at low depths at station 46 may be due to its position furthest up the estuary where tidal mixing is at a minimum, thus riverine inputs dominate the surface.
The views and opinions expressed on this page are those of Group 3 and do not represent the views of the University of Southampton, the National Oceanography Centre or Falmouth Marine School.
Figure 1.3: Image showing samples being taken from niskin bottles and bottles being reset for deployment on board RV Bill Conway.
RV Bill Conway
Figure 1.1& 1.2: Map and table showing locations of sampling stations carried out on the Fal river in the RV Bill Conway for both groups who sampled on Monday 29th June.
Phosphate -
As with the other two nutrients, phosphate plots slightly below the theoretical dilution line. This can increase the confidence that nutrients are in fact being stripped from the water column by heavy phytoplankton usage at these high salinities. However, as afore mentioned the pattern may not follow through to lower salinities.
Phosphate depth profile
The profiles of phosphate concentration with depth show that phosphate concentration is lower near the surface and generally increases with depth in the water column. Station 46 shows a considerably higher phosphate concentration near the surface of approximately 0.27umol/L, and then decreases to approximately 0.07umol/L within the top 5 metre. At 5 metre depth, the phosphate concentration for Station 48 fluctuates sharply before then increasing steadily with depth.
Station 46 has the strongest river input, and the phosphate peak could be due to anthropogenic input from agricultural runoff and sewage treatment as this station was furthest up the river and thus anthropogenic nutrient inputs are likely to be at their highest.
Silicon -
All our silicon measurements lie fairly close to the theoretical dilution line. However, there is a slight deviation below the line, indicating that silicon may be being removed from the water column. We only have points at higher salinities, 27 was the lowest therefore we cannot know the behaviour of silicon at mid salinities but we can assume that they would plot also close to our TDL.
Removal of the silicon from the system could be due to flocculation, where the particles charges are cancelled out by the charge of the seawater ions, allowing them to clump together and fall out of solution. It could also be due to biological removal by phytoplankton, or by a variation in the riverine end members causing the appearance of removal without anything being lost from the system.
Silicon depth profile
The silicon depth profiles at stations 46 to 54, show relatively uniform profiles under approximately 2umol/L. However, stations 47 and 49 show considerably higher near surface silicon concentrations within the surface 5 metres of approximately 5umol/L and 5.5umol/L respectively, compared to the other stations sampled.
The uniformity at depth of the silicon profiles could be due to tidal mixing as the Fal is a tidally dominated estuary. Lack of rainfall recently may also lead to uniformity as inputs from river system are at a minimum due to lower river discharge.
Stations 46, 47 and 49 show a higher percentage of oxygen saturation near the surface, before decreasing steadily with depth at each station. Station 49 shows a small increase in oxygen saturation between 10 metres and 15 metres depth. The oxygen saturation percentage at Station 54 decreases slowly with depth. Only one oxygen sample was collected per station at stations 51, 52 and 53 due to a limited number of oxygen sample bottles available to us.
It can be expected that oxygen saturation will be highest at the surface where the interactions between the sea and air are at their strongest. As organisms respire, oxygen is used more quickly and is removed from the water column, leading to the lower saturation at depth. Water at depth also has very little interaction with the overlying air mass thus no oxygen can be dissolved directly from the air to the deeper water.
Acoustic Doppler Current Profiler (ADCP)
The ADCP transects for each estuary station show flow velocity throughout the water column rather than backscatter which was used in the offshore section. Due to problems only 5 ADCP transects were collected from station 48, 49, 51, 53 and 54. Higher velocities are indicated in light blue with lower velocities in purple.
At station 48 (figure 2.5) it can be seen that the highest flow velocities were at the bottom of the water column with flow generally in a northward direction and a peak velocity of 0.375 m/s. This makes sense given the tidal state (beginning of flooding tide) with low tide occurring at 09:13 UTC, approximately an hour before this transect. The influence of the flooding tide is observed nearest the bed due to saline waters having greater density and hence lying below less dense river water. The region of higher flow is relatively small as tidal influence is less, further up the river.
At station 49 (figure 2.6) the highest velocities are again seen towards the base of the water column, although this time extending more into the centre. The highest flow velocity is again in a general northward direction with a peak velocity of approximately 0.337m/s. The greater area of higher velocities would likely represent increasing tidal influence and strengthening flooding tide.
At station 51 the transect is now much wider and highest velocities are seen towards
the surface with a peak velocity of 0.218m/s and a net flux of 760 m3 s-
At station 53 we can see the potential influence that the Coriolis force has within an estuary. In the western side of the transect flow velocities are lower (less than 0.1m/s in general) than on the eastward side and in a general southward direction suggesting this is where river water is flowing out of the estuary. On the eastern side flow velocities are much higher as indicated by the blue patch reaching a peak velocity of around 0.31 m/s in this region and in a general northward direction. This would suggest that the continuing flooding tide is being deflected to the right in the estuary. The total net flux for the whole estuary is approximately 1450 m3/s to the north.
The transect at station 54 was taken at approximately high tide and so the total net flux for the estuary was extremely low at approximately 190 m3/s. The highest velocities were around 0.3m/s to the east of the transect with flow in the deepest part of the channel in a general northward direction and to either side in a southward direction.
Figure 2.1: Graph showing temperature versus depth down the Fal River and Estuary
Figure 2.2: Graph showing Salinity versus depth down the Fal River and Estuary
Figure 2.3: Graph showing Fluorometry versus depth down the Fal River and Estuary
Figure 2.4: Graph showing turbidity versus depth down the Fal River and Estuary
Across all stations, temperature was highest at the surface where there is more interaction between air and water. Stations 50 to 54 were further down the estuary in more saline waters, therefore the surface temperature is cooler. Stations 50, 51 and 52 show evidence of a thermocline which is not clear at other stations. As is to be expected, temperature is lower at depth at every station with the lowest depth temperatures at the more saline stations 53 and 54.
Station 46 shows a well mixed surface layer at 17.6oC, between 1-
Salinity profiles are much more uniform lower down the estuary, especially at stations 52, 53 and 54 where it is almost completely constant as depth increases. The more freshwater influenced stations are typically less saline at the surface due to less dense freshwater inputs. As the depth increases however, salinity does so steadily such that by around 15m depth salinity at the upper estuary stations are almost the same as the lower estuary salinity values.
Station 46 has a fresher surface flow in the top 1m at 30PSU, before it increases
rapidly at 2m to ~32 PSU. Due to recent weather conditions, freshwater flow is likely
to be lower, as there has been less rainfall. The upper estuary stations (46-
The well mixed profile at stations 53 and 54-
Once more, there is much higher variability in the fresher water stations than in the lower estuary stations. Fluorometry at stations 50 to 54 is almost constant whereas stations 46 to 49 show spikes in the surface 5m before levelling off at depth to similar levels as the other stations. This may be due to phytoplankton blooms near the surface, or more likely a large input of chlorophyll from the banks as there was very thick vegetation in the area either side of the estuary.
Stations 53 and 54 show uniform distributions, while station 46 peaks at 39 V and 2m depth. Stations in the upper estuary generally peak near 2m depth.
Turbidity was fairly consistent through all stations, with highest values at the surface, sometimes with a very strong peak as with station 54. High turbidity is caused by the presence of suspended particles, thus in areas where there is a strong peak there must be more suspension. The highest indicates a red tide, caused by an algal bloom so intense the waters are discoloured.
Cavitation from the propellers from the RV Bill Conway could have affected the results to some degree, giving the appearance of large volumes of suspended material. At station 49, we were closest to the pontoon where human particulate inputs may be higher than at our surrounding stations.
Copepoda and cirripedia larvae are the dominant zooplankton groups present at stations 46 and 49.
The species Chaetoceros, is the most dominant phytoplankton species at stations 46 and 49.
Silicon, phosphate and nitrate plot below the theoretical dilution line and show
non-
Phosphate concentration increases with depth at most stations, except for station 46 where the concentration decreases rapidly within the surface 5 metres.
Oxygen saturation generally decreases with depth at each station
The Richardson number can be used as a guide as to whether mixing is likely in a water column or whether stratification is present to prevent turbulence. If a value is below 0.25 turbulent mixing is likely, while values above 1 indicate mixing is unlikely. These two thresholds are indicated on each of the Richardson number graphs by a dashed and dashed/dotted line respectively. Between 0.25 and 1 there is a transition zone between turbulent and laminar flow.
At station 48 mixing would likely be prevented in the upper 3 metres as values lie above 1 (as high as 82 near the surface). This is unusual and could be as a result of extremely low density values recorded near the surface (as low as 1021.67 kg/m3) affecting the calculations. Below this, other than an extremely high value of 708 at 9m, the values lie in the transition zone or below 0.25 indicating turbulent mixing is likely in much of the water column.
At stations 49 and 51 the Richardson number values unusually fluctuate throughout the water column between values below 0.25 and values as high as 27 at 10.5 metres in station 51 and 41. It would seem likely only very small scale mixing is occurring between layers
At station 53 most of the values lie below 0.25 in the turbulent mixing region. This would suggest the water column is reasonably well mixed here with little thermal stratification preventing mixing. Between 5 and 7 metres the values lie within the transition zone which could indicate the presence of slight stratification developing resulting in a transition from turbulent to laminar flow. At 12m there is a peak in Richardson number of about 20 before values go back below 0.25 most likely due to the influence of the seabed.
At station 54 the Richardson number fluctuates between turbulent values below 0.25 and values as high as 5.4 in the upper 5m, indicating that while some very small scale mixing may occur in thin layers, overall mixing is prevented in this region. Below 5m most of the values lie in the turbulent mixing region indicating that overturning can occur at lower depths. The low Richardson number values may result due to a turbulent boundary layer at the seabed rising through the water column.
Figure 2.10: Station 48
Figure 2.12: Station 51
Figure 2.11: Station 49
Figure 2.13: Station 53
Figure 2.14: Station 54
Figure 3.1: (Above) Nitrate mixing diagram
Figure 3.2: (Left) Nitrate depth profile
Figure 3.3: (Above) silicon mixing diagram
Figure 3.4: (Right) Silicon depth profile
Figure 3.5: (Above) Phosphate mixing diagram
Figure 3.6: (Below) Phosphate depth profile
Figure 3.7: Oxygen depth profile
The phytoplankton data from station 46 indicates that Chaetoceros sp. is by far the most common group, with counts an order of magnitude larger than other phytoplankton species in the same sample. For having such a dominant species, there is also a surprising amount of diversity, though the other groups were only found in low numbers.
Station 49 shows a much lower diversity system than station 46, with much lower counts. The lack of individual organisms indicates that the area sampled was not ideal for phytoplankton growth, which would also explain the lack of diversity.
Zooplankton
The bottle collected from the Station 46 zooplankton net shows a low diversity system with only 5 different groups identified. The system is primarily dominated by two groups, the cirripedia larvae and the copepoda. Other groups are only present in low numbers but most groups are not present at all.
The zooplankton system at station 49 is significantly more diverse than at station 46. Though the system is still dominated by cirripedia larvae and copepoda there are also significant numbers of appendicularia and cladocera along with small numbers of other groups.
The domination of these systems by such few groups could be a result of the method of collection. Because of the lack of net flow, the zooplankton net was dragged behind the boat for a set distance. The resulting sample is therefore only from a single depth. Whilst the copepoda and cirripedia larvae could be dominant at the depth that the sample was taken, the community structure could change dramatically with depth.
Figure 4.1: Phytoplankton count for station 46
Figure 4.2: Phytoplankton count for station 49
Figure 4.3: Zooplankton count for station 46
Figure 4.4: Zooplankton count for station 49
Figure 2.6: Flow of water at the ADCP transect at Station 49.
Figure 2.8: Flow of water at the ADCP transect at Station 53.
Figure 2.5: Flow of water at the ADCP transect at Station 48.
Figure 2.7: Flow of water at the ADCP transect at Station 51.
Figure 2.9: Flow of water at the ADCP transect at Station 54.