Four transects, approximately 2 km in length running parallel to shore, were surveyed using side-scan sonar operated at 500 kHz to producing 150 metre swaths. Transects were overlapping to facilitate positioning of features relative to one another. Data collected was transferred to the computer onboard the ship. Time, position, weather conditions and other observations were recorded in order to assess effects on results, e.g. of wash from passing vessels.

The trace produced by the data was used to look at the sediment type and any bedforms present. Four areas of particular interest were noted during transects and were returned to for benthic sampling using a Van Veen grab. The sample was analysed for substrate type and species of flora and fauna contained within them. Fauna present were identified, photographed and returned to the water. A sample of the grab was placed into a 2mm sieve above a 1mm sieve.  The sample was then rinsed and shaken through to ascertain the average grain size of the sample and the approximate percentages of the different grain types present. A video camera was also deployed to look at the overall composition of the seabed in the area.

Side-scan traces were analysed the following day in the laboratory. Traces were laid out and the boundaries of areas with differing bedforms delineated, and size and nature of bedforms were determined. On the boat, the distance between the GPS and the tow-fish had also been measured to calculate layback, which was accounted for in bedform calculations. Surfer was used to create a track plot, and a series of calculations performed on the trace plot features to transpose the information to the track plot, producing a map of the bathymetric features seen in Falmouth Bay.

Survey line positions

Track plot

Side-scan Sonar

The surveyed area was found to be predominantly sand waves of 0.2m height and 1.6m wavelength. The orientation of the sand waves running parallel to current direction, and bifurcations observed indicate these features are formed by tidal currents.

Areas of anchor marks were observed at position 180760, 29370 (120m by 80m), at position 180990, 30080 (190m by 140m) and in position 181600, 30800 (70m by 300m). The Fal estuary is one of the deepest natural occurring harbours in the world with strong shipping links due to oil refinery in the area. Many ships were observed anchored in the bay just outside the harbour while our study was being conducted. The effects on the seabed from these large ships can be seen by the large scour marks picked up by the side scan sonar.

Flat areas were observed at position 180700, 29500 (240m by 250m), at position 181450, 30190 (60m by 150m), and position 181790, 30600 (60m by 150m), and alternating flat/sandy areas were observed at position 181200, 29800 (150m by 330m) caused by variation in tidal velocity due to geometry of the seabed.

Van Veen Grabs

Grab 1 (050º 08.29 N 005º 03.19 W, in area of sand waves) was composed of 85% maerl and 15% bivalve shells. 98% of the maerl was dead. 3 amphioxus were observed, and 2 common brittle stars, Ophiotrix fragilis.

Grab 2 (050º 07.95 N 005º 03.59 W, in area of sand waves) was composed of 85% maerl and 15% bivalve shells. 99% of the maerl was dead. 4 amphioxus were observed.

Grab 3 (050º 07.82 N 005º 03.75 W, in flat area) was composed of 95% dead maerl and 5% muds. More than 99% of the maerl was dead. 1 amphioxus was observed.

Grab 4 (050º 07.57 N 005º 04.05 W, in palaeovalley area) was composed of 93% dead maerl and 7% pebbles. 1 polychaete tube and 1 dead urchin were observed.

Video footage indicated that live maerl was present in the troughs of the sand waves, and the surrounding sediment was mostly dead mearl with sand.

The decrease in amphioxus in samples may be due to less favourable conditions created by the mud.

Sorting through the grab sample. Ophiotrix fragilis (brittle star). A bivalve found in the grab.

Maerl

Maerl is loose-lying, normally non-geniculate coralline red algae that forms a calcareous substratum. A large proportion of the surveyed area consisted of dead maerl beds composed of unattached corallines forming accumulations, with or without terrigenous material. Grab samples suggest that the sediment was composed of a maximum of 2% living maerl, and this contrasted with the video footage which suggested greater quantities of living maerl in the troughs of waves. This area has been classified by Natural England as dead maerl however, as in this investigation, grab samples may have been unrepresentative of the area sampled. Examination of this area could be improved by carrying out replicate sampling at each station in conjunction with video camera deployment.

Analysis of the side-scan sonar trace suggests that the bed substratum of the studied area was mainly composed of coarse sediment, which was identified as dead maerl by ground truthing with Van Veen grab.  The trace also provided evidence of anchor marks, sand waves, flat area and a palaeovalley.  Sand waves were the dominant feature observed, with heights of 0.2m and wavelengths of 1.6 metres.

All of these samples contained high proportions of maerl, with only 1-2% living.  Due to the high quantity of maerl present throughout the sampling, the grabs suggest that the majority of Falmouth Bay is comprised of maerl, with smaller quantities of sand and mud.  Despite the sampling from the grabs showing the actual benthic environment, the samples had lost their vertical structure and so results are still not completely reliable.

The video camera deployed provided an in situ visualisation of the benthic fauna and sediment composition.  Most of the fauna was observed to be located in the troughs of the sandwaves, suggesting bias of the grab samples.  Troughs provide a more sheltered environment with less stresses due to lower current velocities.  A higher benthic diversity was observed in the video camera than in the grab samples.

A map of Transects done by R/V Bill Conway

050º08.568

005º02.502

08:53:31

005º02.694

09:57:13

005º02.444

11:04:28

005º02.854

11:45:34

005º02.221

12:09:50

005º01.804

12:23:05

005º01.647

12:58:55

050º13.342

005º01.477

13:06:03

005º00.973

13:32:21

050º13.618

005º00.885

13:34:35

 The graph from CTD cast 1 shows moderate stratification with a permanent thermocline and halocline at around 5.5m. There is a steep decrease in temperature in the surface waters that shows an influence of river flows, so in general warm riverine water is overlying cooler seawater. As can be seen on the graph there is a constant temperature above the thermocline accompanied by a shallow increase in salinity; this can be explained by the presence of mixing in the upper 5m of the water column.

The temperature salinity graph for CTD 2 clearly shows by the red line that there is a constant temperature in the surface waters, but a general warming closer to the surface. It can be seen that there is a shallow increase in salinity in the surface waters and with increasing depth. These properties can be explained by observing that the water was well mixed and there was little riverine influence.

The graph for CTD cast 3 shows a steep increase in salinity at the surface with a slight decrease in temperature with increasing depth. This suggests well mixed waters.

The graph for CTD cast 4 is showing well mixed waters above and below the thermocline, this is due to a strong influence from riverine inputs. It can be seen that temperature is uniform throughout the column and there is a shallow increase in salinity with increasing depth. The thermocline is observed to be at approximately 7m.

The graph for CTD cast 5 shows that the water column here was well mixed above and below the thermocline, situated at around 4m depth. In general there was an overall increase in salinity with depth. When looking closely at the surface metre of water, salinity decreases with depth, therefore being higher at the very surface.

It can be seen on the graph from CTD cast 6 there is a very strong thermocline at around 1m depth. There is a large increase in salinity in the surface 1m of water down to the thermocline, after which there is an increase in salinity with depth but not such a great increase as observed above the thermocline. The fairly uniform salinity below the thermocline suggests well mixed waters. 

CTD1

Well mixed waters shown by the tiny change in temp & salinity (range of 0.7^oC & 0.3 respectively).

No stratification or thermocline present, as a result of strong turbulence, created by wind and tidal mixing.

Mixing in surface waters caused by riverine input, which creates a shear between the 2 different layers of water.

CTD 2

Mixed layers above & below the halocline, as shown by a minimal change in temp & salinity. Temperature is uniform in the mixed surface layers, but varies more below the halocline.

Mixing in the surface waters is caused by shear stress between the riverine input over the more dense seawater.

Large decrease in temperature and increase in salinity in the surface waters above the thermocline are caused by a large input of riverine water from a major tributary and also runoff from surrounding land. Warmer river water (14.6^oC) is overlying the cooler, more salty seawater. Riverine water diffuses down into the seawater causing a decrease in temperature with depth, and an increase in salinity with depth.

CTD 3

Water is well mixed below the thermocline, as shown by the uniformity in temp and salinity. This is caused by the shear between the overlying river water and underlying seawater.

CTD 4 

There is a steady decrease in temperature with depth and a increase in salinity to a homogeneous layer below. There is a smaller range in salinity than in CTD 5 and 6.

CTD 5 

CTD cast 5 has a slight decrease in temperature from the surface to 8 metres depth of approximately 0.5 degrees centigrade. There is a salinity of 24 at the surface increasing to 31 at 8 metres depth. The profile shows more stratification and around 2m a defined layer showing a thermocline structure.  The fresher water at the surface is due to tributary input and runoff from surrounding fields diluting the water already present in the lower reaches of the river channel.

CTD 6

CTD cast 6 has a high amount of freshwater (salinity of approximately 20) at the surface and increases in salinity with depth to 31. There is slight increase in temperature at a depth of around 2m of approximately 0.3 of a degree centigrade;  but then decreases in temperature after 2m depth by 0.5 of a degree centigrade.

Track 1: Falmouth_08041 

From the ADCP data displayed above, it can be noted that the flow is dominated by outflow, which is both largest with respect to volume and velocity magnitude.  This can be confirmed by the fact that sampling took place on an ebb tide.  A small inflow of water can be noted by the deepest section of the channel, which is expected to be the last remnants of the seawater flood tide.  Outflow of water is seen to be at a maximum on the western side of the estuary, due to larger output of water from around Falmouth docks.  The opposing effects of these two flows are expected to create a strong shear (vertical velocity of gradient) creating a small Richardson number for the whole water column (an average value of 0.196 was calculated) indicating strong turbulence. 

Observations of the average backscatter show that there are two maximums, one at the sea surface and one at the seabed. The surface backscatter is believed to be due to the presence of zooplankton in the water column, while the seabed effect is expected to be caused by suspended sediments. Because of the penetration of light in the water (only to a maximum of about 12 metres deep), there is little phytoplankton, and thus zooplankton are not expected to be present. The high velocities associated with the flood and ebb tides are generally expected to cause sediment resuspension and result in higher backscatter values.  However, previous research into the Fal estuary system has shown that these increased velocities can trap the sediment on the seabed, storing the kinetic energy.  As the velocities decrease towards slack water, the sediment is no longer trapped at the seabed and the stored energy allows resuspension to occur.  This is believed to be the cause of the seabed backscatter shown.

Track 2: Falmouth_08042

The ADCP data for transect 2 shows that most of the estuarine flow is seawards, with very small variations in direction at the western seabed. This variation could be due to small scale turbulence picked up by the equipment. Flow has its highest velocity at the western surface waters of the transect. This could be due to the effects of the headland of the tributary.  The water depth is also noted to become shallower at this point, thus large volumes of water may become pressured into a smaller area, resulting in higher velocities.  Richardson numbers are calculated to generally between 0.1 and -0.02, however at a depth of 2.65m, the Richardson number has been evaluated as 4.1.  This suggests that flow at this depth is more stable with less overturning of the water column.  The Richardson number value of -0.02 at 5.65 metres deep indicates that localised overturning is occurring in the water column, due to more dense water overlying less dense water.  

Track 3: Falmouth_08043 

Transect 3 also shows that the majority of flow is seawards, which some direction variability at the eastern section, due to the input of water by a tributary.  Velocities are again noted to be higher in the surface waters.  Much lower velocities of less than 0.125m/s are noted in the deep channel.  This could be due to the effects of wind forcing combining with tidal velocity magnitude.  Observations of the Richardson number suggest that flow at the seabed (0.979) and surface (0.44) is more stable than that in the centre of the water column (an average value of 0.2).  However, more overturning of the water is noted at 5.65m deep, shown by the negative value of the Richardson number (-0.0593).  Strong shear values of around 0.025m/s are shown near the deeper channels of the transect against the surface water outflows. 

From the backscatter information, zooplankton blooms are again reduced with increased isolation of high backscatter surface points.  This reduction in zooplankton can be explained by the drop in nutrients measured and thus lower prey phytoplankton populations.  Large volumes of resuspended sediment can also be noted in the bottom of the main channel, due to the reduction of velocity reducing the flows trapping effects.

Track 4: Falmouth_08044 

The transect of line 4 was taken along the deepest channel following the river upstream.  Because of this there is little variability of the data, but velocities are noted to be slower in the south western end where water depth is at its highest.   

Zooplankton blooms have little penetration this far up the estuary with only a small signature located around 5m deep in the south western section.  It should also be noted that backscatter at the estuary bed has become reduced, with little backscatter in the deepest sections of the transect.

Track 5: Abandoned 

Track 6: Falmouth_08047 

Transect 6 was taken along the estuary’s width crossing over the site of transect 4. Velocities have increased, with flow still flowing seawards in a south westerly direction.   

Backscatter values are also noted to be higher outside of the main channel, with readings reaching up to 70dB.  This increase in the shallower regions is expected to be due to the increased turbulence in the water column. This suggested by the fact the backscatter values for this section are constant through the water column, which indicates a well mixed site.  Backscatter is reduced to 64dB in the deeper channel because it is too deep for turbulence to produce a well mixed water column.

Track 7: Falmouth_08048 

At transect 7, variability in the velocity of flow increases with flows of 0.50m/s and under 0.125m/s recorded at the east and west sections of the line. This is due to the presence of a meander in the estuary’s shape, which increases flow velocity on the outside bend, while reducing flows on the inside.  The main direction of flow has also changed to around 270^o, which compensates the change in direction of the sea with the meander in the estuary.  The water column is also noted to become more stable, with Richardson numbers over 0.25, indicating laminar flow and vertical stability of the water column. 

Backscatter results show values of 70dB for most of the transect, with a small gap in the centre of 66dB separating the effects of turbulence in the upper layer.  This gap suggests that there could be a surface plankton bloom present, and that the lower layers of backscatter are caused by the resuspension of sediment.

Track 8: Falmouth_08049 

Transect 8 covers the entrance of Lamouth Creek into the Fal estuary.  Velocities in the surface 2 metres reach 0.25m/s, with a reduction to under 0.125m/s at the deeper depths of the main channel.  The direction of flow varies from south east in the northern section, to southerly in the southern section of the transect.  There is also strong evidence of shear along the whole length of the transect at the interface of flow velocity at 5 metres deep. 

Track 9: Abandoned 

Track 10: Falmouth_08051 

Velocities at transect 10 have become reduced to a maximum of 0.311 m/s in the surface waters, with directions of 220^o.  Richardson numbers for this transect are generally high, suggesting flow is laminar.  Strong shear is indicated at around 5 metres deep by the differences in velocity magnitude.  Average backscatter readings of 70dB have been recorded throughout the transect, once again suggesting a well mixed water column with a high sediment content.

Track 11: Falmouth_08052 

Transect 11 of the estuarine study is parallel the Truro River section of the estuary.  Velocities have dropped, with a maximum of 0.25m/s recorded in the surface waters.  The direction of flow is generally southwards down the estuary towards the sea, with some seabed variability. Backscatter once again suggests a well mixed water column with readings of 70dB recorded.  Water depth has also become reduced to less than 10m deep

Track 12: Falmouth_08053 

Nitrite and nitrate estuarine mixing diagram

The mixing diagram for nitrate and nitrite shows that the concentrations reduce with increasing salinity.  The fact that the samples plot below the theoretical mixing line suggest that these nutrients do not behave conservatively in the estuarine system.  Removal is most likely caused via nutrient uptake by phytoplankton for photosynthesis.  However this cannot be concluded solely from the conducted study. Variability in the end member concentration, and the presence of other tributary streams in the Fal estuary will act to reduce the conservative behaviour of both nitrate and nitrite.

Vertical profile of nitrite and nitrate

Phosphate estuarine mixing diagram

The CTD2 profile shows little change in phosphate concentration with depth as in this area phytoplankton have utilised all nitrate and have become limited by nutrients. Further up the estuary at the CTD3 cast site the profile shows higher concentrations in surface waters and then decreases from 4m down. This could be due to lots of riverine input flowing over the surface carrying nutrients from land run off containing fertilisers. A surface maxima is observed in the profile of CTD5, with a value of 1µmol/L which decreases down to 0.8µmol/L at 8m. The last station follows the same trend as the previous station however its surface maximum is much higher with a reading of 1.3µmol/L but a similar concentration at depth.

Vertical profile of phosphate in the four CTD casts

Silicon estuarine mixing diagram

The estuarine mixing diagram shows that all the points lie close to the theoretical dilution line, possibly showing conservative behaviour in the Fal estuary. Silicon concentration decreases towards the mouth, as salinity increases. Possible anomalous results lie between salinities 15 and 25, as they show more deviation from the TDL. More data will need to be collected to conclude if any addition or removal is taking place.

Addition of silicon may occur in the upper reaches due to the release of sewage effluents. Phytoplankton present due to the previous spring bloom may be responsible for some removal of silicate for diatom growth occurring near the mouth of the estuary, as it is utilised. Silicon concentrations decrease as salinity (and closeness to estuary mouth) increases. This is because silicon is added to the water from the riverine end.

In general, silicon concentrations decrease as depth increases. The only cast that does not show this is cast one, which increases from the surface to 4m deep, and then decreases again. This may be an anomaly. Only casts 2,4,5 and 6 are shown as these were the only casts that Niskin bottles were deployed on. The other two casts only used a YSI probe.

Vertical profile of silicon

Vertical profile of dissolved oxygen at the four CTD casts

Oxygen levels in surface waters suggest that higher levels of chlorophyll relate to lower levels of Dissolved oxygen possibly due to bacteria and zooplankton present feeding on the living and dead Phytoplankton respiring and using up oxygen. There also seems to be a correlation between salinity and oxygen levels with the sea water higher oxygen levels than the river water,

A scattergraph of oxygen concentration against chlorophyll concentration

The chlorophyll profiles analysed seem to show a general decrease in the concentration of chlorophyll with depth except at CTD 2 where there is an increase up to 4 meters and then a slight decline. Chlorophyll levels also increased as we proceeded up the estuary. Suggesting higher levels of Phytoplankton in the less saline and surface waters possibly due to the higher levels of light and river inputted nutrients in the surface waters.

The main taxonomic group found within the estuary was Copepoda, at a maximum of 70% of the taxa found at one station alone.  This is due to the fact that they have high tolerances to fluctuating levels of salinity, temperature and nutrients. As long as they have a food source, being phytoplankton, they will survive virtually anywhere.  This is providing that the only food source is not solely toxic forms of phytoplankton dinoflagellates, being Alexandrium, but as seen from our phytoplankton results of Falmouth estuary, diatoms dominate all habitats ranging from seaward end member to riverine end member.

The results from bottles 54, 101, 22 and 006 which were taken from transects  within the lower part of the estuary show high amounts of diatoms, in particular Rhizosolenia setigera and Chaetoceros.  This correlates well with CTD cast one where results found show depleted levels of silicate, particularly in the surface waters, as diatoms use silicate in the production of their tests.  When looking at mid estuarine locations, typically sample bottles 102, 012 and 109, these sites are dominated by Alexandrium in the lower part of the mid estuarine system and in the upper part of the mid estuarine system, Chaetoceros prevails over other species.  There are high levels of Alexandrium at rib casts 102 and 109 due to an influx of nitrogen into these waters from eutrophication from surrounding agricultural fields.  This can be correlated to our graph of the vertical profile of nitrite and nitrate, showing depleted levels of these nutrients in the same location that these samples were taken.  Higher up the estuary from bottle 12 where there are lots of Chaetoceros present.  At this site silicon is abundant, potentially due to high fresh water input from the surrounding tributaries.  Chaetoceros requires little silicate to survive, hence the high levels of silicate at rib cast 3.  The top of the estuary is dominated by diatoms, this is due to the high silicate content of the freshwater input here from riverine sources passing through high energy, rocky environments.

Chart showing the locations of the CTD casts.

CTD1

Well mixed waters shown by the tiny change in temp & salinity (range of 0.7^oC & 0.3 respectively).

No stratification or thermocline present, as a result of strong turbulence, created by wind and tidal mixing.

Mixing in surface waters caused by riverine input, which creates a shear between the 2 different layers of water

CTD2

Mixed layers above & below the halocline, as shown by a minimal change in temp & salinity. Temperature is uniform in the mixed surface layers, but varies more below the halocline.

Mixing in the surface waters is caused by shear stress between the riverine input over the more dense seawater.

Large decrease in temperature and increase in salinity in the surface waters above the thermocline are caused by a large input of riverine water from a major tributary and also runoff from surrounding land. Warmer river water (14.6^oC) is overlying the cooler, more salty seawater. Riverine water diffuses down into the seawater causing a decrease in temperature with depth, and an increase in salinity with depth.

 Water is well mixed below the thermocline, as shown by the uniformity in temp and salinity. This is caused by the shear between the overlying river water and underlying seawater. 

CTD3

This is the first of the time series casts, showing the thermocline and halocline occurring between 25 and 30m, with a chlorophyll maxima at 15m and a steep drop at the thermocline. The phytoplankton are remaining in the surface waters, in the euphotic zone, due to the thermocline. The irradiance shows a uniform attenuation through the water column with the euphotic zone ending around 20m, but phytoplankton are restricted to higher waters by the thermocline.

The thermocline seemed fairly stable all through the series, with a slight variation of around 5m due to an internal wave. The surface temperature showed the most fluctuation and this is mostly due to the decreasing cloud cover and gradual warming over the time series. Towards the end of the series the fluctuations became more stable.

The initial fluorescence is relatively high at the surface, compared to the rest of the series, possibly due to lower light and temperature levels suggesting more mixing in the surface waters over night when there was no light input.  The rest of the time the fluorescence was more stable showing the zonation of the chlorophyll maxima, which was fairly constant throughout the time series above the thermo and haloclines.

Site 4: 

1040-1112 GMT

The ADCP was activated during the CTD deployment to take a time series of the waters characteristics while sampling took place. The velocity is noted to be highest at the surface, with measurements of 0.6m/s recorded. This trend of higher surface velocities is due to forcing effects of the south westerly winds combined with the tidal flows into the estuary. These increased velocities are only noticed down to about 10 metres deep, with the deeper water fairly homogeneous in velocity and direction.

The backscatter results show high return values of 0.73dB at the surface, due to the high acoustic impedance of bubbles.  Backscatter levels are reduced around 18 metres deep, due to the presence of phytoplankton spring blooms. Above and below this there are high backscatter returns of 69dB. These are believed to be caused by zooplankton blooms feeding on the phytoplankton from above and below. There are also some higher levels of backscatter return in the deeper water below the zooplankton bloom, which may have been caused by turbulence in the water column. 

Calculation of the Richardson number over the time-series resulted in values all over 1, implying that flow is laminar with little mixing. At the start of the time series, 1040 GMT, the water is most stable at the seabed with a Richardson number of 1614.9.  s time progresses, the Richardson number at mid-depth increases to a measured maximum of 2966.9 at 22.6 metres occurring at 10:56. At the end of the time-series, the Richardson number is reduced at all depths, indicating instabilities are beginning to occur in the water column.

 The vertical profile of nitrate and nitrite for each of the five CTD casts.

In the vertical profile of nitrate, there is a general increasing trend in concentration with depth. The highest nitrate concentration is 4µmol/L at 50m and 70m deep in casts 3,5 and 8. Concentrations are lowest in surface waters as this is where there is a high population of phytoplankton due to high light levels. From the chlorophyll maximum of 18m downwards, there is a lower consumption rate of nitrate. This is because there are less phytoplankton populating this area, due to lower light levels preventing them from carrying out photosynthesis. However, the chlorophyll maximum has been slowly deepening over the past two weeks, as phytoplankton migrate down to deeper waters, possibly due to the depletion of nutrient levels in surface waters and increased turbulent mixing (as shown here). The nutrients have been depleted by the recent spring bloom, causing phytoplankton present at this time to migrate deeper to more nitrate rich deep waters.

The trend of CTD cast 7 showed an increase in nitrate levels to the chlorophyll maximum, followed by a sudden decrease in deeper waters. This is a contradictory result to the other casts and may be due to strong tidal mixing, pushing the nutrients lower. Without further investigation it is difficult to conclude the exact reason for this, and is more likely to be an anomalous result.

Vertical profile of phosphate for each of the five CTD casts

The phosphate concentrations for this area are generally high in the deeper waters, with low concentrations in surface waters. This is due to utilisation in these upper waters by phytoplankton during photosynthesis. In CTD casts 3, 7 and 8 there was a general trend of increase in phosphate concentration to intermediate waters then a slight decrease in deeper waters. The highest phosphate concentration is in CTD cast 3 with 3.7µmol/L at 18m depth. At this time in the tidal cycle the current would have been more turbulent, creating a more stressful environment and therefore a temporary decrease in primary productivity. Less phosphate would have been consumed and more entered the water column from the tidal flow causing levels to increase. However in CTD casts 4 and 5 the intermediate waters showed the lowest concentration. This is the location of the chlorophyll maximum, indicating high phytoplankton productivity and thus causing low nutrient levels as they are consumed. The lowest concentration of phosphate was shown in CTD cast 5 at 0.5µmol/L at a depth of 18m.

A vertical profile of silicon for each of the five CTD casts

Silicate generally decreases in surface waters down to the chlorophyll maximum at 18m, then increases with depth. The lowest silicate concentration is found in cast 5 with 0.7µmol/L at 18m deep. At this point there is a high population of phytoplankton, most likely dominated by diatoms. This would explain the decrease in silicate at this depth as it is consumed by them for use in test formation. This confirms the pattern previously suggested of phytoplankton migrating deeper, searching for increasing nutrient concentrations. There is little variation in silicate concentration between the casts during the time series.

Due to an error in experimental procedure the results observed for dissolved oxygen were not as expected. This occurred as the bottles were not inverted immediately after the reagents were added therefore chemicals could not react correctly. The results were not full enough to form a conclusion from, however a graph of the results obtained was generated.

A vertical profile of chlorophyll for each of the five CTD casts

The graph of chlorophyll concentration shows a peak in the mid waters at about 20m, with chlorophyll concentrations at least 1µmol/l higher than the surface waters. This depth was sampled as a result of the indications provided by the fluorometer and for all stations shows a significantly higher value than the other points sampled. We can see that the water column is significantly stratified, however with an average Secchi disk depth of less than 5m we would expect the euphotic zone to end at around 20m. This ties in well with our sampled chlorophyll maximum, and also our deep water samples as we would expect no large values from here. The deep water sample collected from CTD 7 contains higher levels of chlorophyll than expected. As previously discussed, phytoplankton are currently experiencing a period of turbulent mixing due to recent stormy weather along with a depletion of nutrients that may have forced some phytoplankton into deeper waters. The surface values indicate the changing light intensity as the cloud cover cleared in the afternoon, this is represented in the last sample as the surface value changes from its slow decline to a steep increase.

When looking at our offshore zooplankton data, Copepoda dominate each sample site with an average of 44% of the total sample, most significantly Callista 1 at Black Rock, with the second most abundant in each sample being Hydromedusae with an average of 17% of the total species recorded. Sample Callista 2 was taken at the observed chlorophyll maximum in the water column while Callista 3 was taken in the upper surface layer where a backscatter maximum was observed. After processing the numbers of individuals present, the sample which was taken at the chlorophyll maximum contained 22% more species than that of the sample taken in the surface waters.  This higher amount of individuals is due to the greater abundance of phytoplankton as a food source on the thermocline, compared to the surface waters where phytoplankton numbers are lower.

All bottle samples except from bottle 009 were taken at the chlorophyll maxima which ranged in depth from site to site, bottle 009 was sampled from the surface waters at the mouth of the estuary.  Phytoplankton in deep, thermally stratified water will be most abundant at the thermocline.  This is because above the thermocline nutrients are depleted and below the thermocline light intensity is too low to support photosynthesis, promoting phytoplankton growth.  Station 3 where the sampling of bottle 105 occurred was nearest to the dumping ground.  This could explain the high abundance of phytoplankton (147 individuals perm ml) due to the anthropogenic input of nutrients into the water column, providing a basis for which phytoplankton growth can thrive.  Rhizosolenia alata and Ceratium furca show obvious fluctuations in numbers over the time series.  Bottle 070 shows a high diversity of phytoplankton species.  However this may be due to the accidental mixing of the water column by the CTD undulation from casts 5 to 9.