Content |
Map depicting sampling locations (offshore stations shown by crosses) |
Callista. |
Group 4: |
Preparing the CTD for deployment on board Callista. |
Rhizosolenia setigera. |
George and Lizzie enjoying sea winds from the bow. |
AbstractThe Fal estuary is a semi enclosed body of water open to the western English Channel. Using National Oceanography Centre vessels Callista, Coastal Research, Ocean Adventurer, Bill Conway and the Falmouth Diver RV.Grey Bear, a range of data was collected in order to investigate the physical, chemical and biological aspects of the upper, lower and offshore reaches of the estuary. Between the 3rd and 16th July 2006 discrete oxygen, nutrient (nitrate phosphate silicate) and chlorophyll samples were collected at appropriate positions and depths and analysed in the laboratory. Phytoplankton and zooplankton samples were collected at predetermined positions and depths and numbers and species analysed in the laboratory. ADCP profiles were generated and analysed and a geophysical profile of an area adjacent to Restronguet Creek was created using SideScan sonar. A bathymetric map was then created including physical and biological features of interest discovered. This work presents the findings with emphasis on the physical structure and nutrient chlorophyll profiles throughout the estuary. Zooplankton and phytoplankton numbers and distribution are discussed with relation to the euphotic zone. A strong thermocline was evident offshore while a homogenously mixed water column is described at the mouth of the estuary with a partially mixed regime identified further upstream in the Truro River. Conservative nutrient chemistry is identified in the mid estuary area. Phytoplankton species which dominate in the estuary and offshore included Nitzschia and Rhizosolenia and reasons for this are discussed along with turbidity and the key elements which control turbidity in the region.
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IntroductionThe Fal estuary in south western England drains six major catchment areas. The main body of the Fal Estuary is known as Carrick Roads and is the deepest part of the ria with depth decreasing inland. There is no delta at the estuary mouth, in rias the sediment is deposited in the submerged river system. At low tide a substantial amount of the estuary is exposed leaving dendriform channels and large areas of inter-tidal sediment. The inter-tidal areas are predominately mud-flats with salt marshes in the upper creeks.
The Fal estuary (Figure 0.1) and immediate coastal areas were sampled
across four boat practicals; the RIBs, Bill Conway, Grey Bear and Callista.
The aim of the study was to investigate the biological, chemical,
physical and geophysical features of the Fal estuary system over two weeks
in the beginning of July to identify the vertical mixing processes that
affect the structural and functional properties of the estuary. Stratification is the presence of layers of differing densities,
pycnoclines, within the water column. It develops when there is little
turbulence in the atmosphere-ocean boundary layer resulting in a reduction
in mixing of the upper 100m of the water column. In temperate latitudes
stratification is highly seasonal, being chiefly controlled by temperature.
Between September and April increased storminess and thus greater turbulence
result in an isothermal water column and retaining of heat by the water.
Throughout the rest of the year there is less turbulence and increased
surface heating resulting in a stratified water column and the development
of a thermocline. Tidal fronts are often associated with phytoplankton blooms (Pingree,
1975). Blooms occur because the well mixed water column provides abundant
nutrients to the restricted, and thus concentrated, phytoplankton
populations in the stratified water column. Boat ProcedureSalinity and temperature were recorded using the YSI probe on the RIBs and the CTD on the Bill Conway and Callista, with subsequent seawater samples taken at each sampling point. A filtered 50 ml sample was collected in a glass bottle at each location for later nitrate and phosphate analysis. As well as this, a 35 ml sample was stored in a plastic bottle to later analyse for silicate concentration (plastic rather than glass to avoid silica contamination). Each filter was stored in a test tube containing acetone to use for chlorophyll analysis. Irradiance penetration of the water column was also recorded at each station using the Secchi disk. This is a straightforward but useful measurement of light penetration, which may be used to calculate the euphotic depth. The depth of the Secchi disk must be multiplied by three to obtain a depth for the euphotic zone. To determine the biology of the estuary and surrounding coastal area, both phytoplankton and zooplankton samples were taken. Phytoplankton samples were fixed using Lugols iodine, with the species and number of individuals determined in the lab. As well as sampling stations, zooplankton tows were undertaken using a zooplankton net with 200 µm mesh and a diameter of 50cm. Oxygen samples were taken using Niskin bottles, either with the CTD rosette on the Bill Conway and Callista or by hand on the RIBs. A glass bottle was filled with the water from the Niskin and allowed to overflow. Manganese chloride and alkaline iodide were added in 1ml measures to preserve the oxygen concentration in the bottles. The bottles was then sealed and stored submerged in water to avoid contamination. On the Bill Conway an ADCP was used to determine the magnitude and direction of the currents on transects across the estuary; this was done to determine the physical oceanography of the lower estuary. The ADCP on Callista was used to determine the physical structure of the water column both whilst stationary, to pick up internal waves, and moving along the transects between our stations.
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OffshorePrimary production is strongly influenced by vertical mixing as this supplies essential nutrients to the surface layer from the bottom water. When surface waters are nutrient depleted and deeper waters nutrient rich, the rate at which nitrate and silicate can be transported up above the thermocline is vital to phytoplankton cell growth. | |||||||||||||||||||||||||||||||
Lighthouse at Black Head. |
RV. Callista in Carrick Roads. | ||||||||||||||||||||||||||||||
RV. Callista
IntroductionThe area of investigation was chosen for sampling due to its potential for interesting physical data; the Lizard Peninsula is known to interfere with local currents so we expected to find features such as eddies there. We chose to study 4 locations, the first being Black Rock which has been sampled by all groups, so that a continuous data set can be constructed. Station 2 was the inshore location on the south east of the peninsula; stations 3 and 4 are offshore stations to the South, where the sampling depth was much greater. AimsThe aim of this study is to identify how the vertical structure of the water column influences the plankton communities; therefore the water column was sampled with respect to its physical structure and using discrete chemical and biological sampling. Objectives
The same sampling procedure used for the previous boat practicals was implemented, with the addition of purple harnesses to stop James and George falling off the boat. Station 1
Fluorescence: Fig 3.1 shows that at the surface, there is a value 1.95v, peaking to 2.5v at 7.7m depth, then dropping to 1.4v at 13.2m depth. Inferred chlorophyll maximum is at 7.7m depth. The fluorometer is a qualitative measure of chlorophyll concentration, obtaining a fluorescence value in volts that reflects the amount of organic pigment in the water column. The 1.9m value of 1.95v reflects the high cell count of Rhizosolenia delicatula, 120 000cellsL-1, seen in the phytoplankton sample taken at 1.3m. The fluorescence peak at 7.7m depth corresponds to high counts of diatoms and dinoflagellates, 43 000cellsL-1 and 41 000cellsL-1 respectively. This peak is subsequently followed by a decrease in fluorescence to 1.4v at 13.2m, potentially caused by predation of phytoplankton by herbivorous zooplankton, 133cellsm-3 of Copepoda were counted in a tow between 13m and 5m depth at this station. Temperature:There is a variation of 3.2°C between 1.9m and 13.2m depth. This indicates that the water column is fairly well mixed with no obvious thermocline and a decrease of only 3.2°C between the surface and 13m depth. The near-linear decline in temperature with depth is indicative of surface warming of a mixed water column. Salinity:The salinity varies from a value of 35.17 at the surface, peaking at 35.5 at 7.5m depth then decreasing to 35.27 at 13.2m depth. Low surface salinity values reflect the influence of freshwater input to the estuary. Lower salinity water is less dense and thus remains at the surface until completely incorporated into the water column. The increase in salinity at mid-depth (7.9m) appears to be a characteristic salinity spike sourced from the rapid drop in temperature detected by the CTD. The maximum range of salinity between 1.9m and 13.2m depth is only 0.33 so any inference made is regarding very small variations in salinity. Silicate:Fig 3.1 shows that the silicate concentration at 1.3m is 2.92umolL-1, rising to 4.13umolL-1 at 7.9m depth and then falling to 2.21umolL-1 at 14.2m depth. At 1.3m depth, silicate concentration is relatively low, 2.92umolL-1. Analysis of the phytoplankton present at this depth shows that diatoms – particularly Rhizosolenia delicatula – dominate and that silicate removal to create diatom frustules results in low concentrations. At 7.9m depth, silicate concentration is relatively high at 4.13umolL-1. At 7.7m depth, chlorophyll concentration (from the fluorometer) peaks at 2.5. Discrete analysis of phytoplankton indicates co-dominance by diatoms (particularly Rhizosolenia delicatula) and dinoflagellates (Karenia mikimotoi) with 43 000 and 41 000 cellsL-1 respectively at 7.9m depth. Silicate removal will not be as rapid as at 1.3m because the number of diatom cellsL-1 at 7.9m depth is less than a quarter of the number at 1.3m depth (43 000 compared to 219 000cellsL-1) resulting in higher silicate concentration. At 14.2m depth silicate concentration and the number of diatom cellsL-1 are both low. There may have been a recent departure from bloom conditions, a decrease in the number of diatom cells, leaving a low silicate concentration or this depth may be influenced by a silicate deficient water body outside of our sample area. Nitrate and Phosphate:Phosphate concentration at 1.3m depth is 0.802umolL-1, falling to 0.100umolL-1 at 7.9m depth and then rising to 0.100umolL-1 at 14.2m. Nitrate concentration follows a similar pattern, starting at 0.658mmolL-1 at 1.3m depth, falling to -0.292mmolL-1 at 7.9m depth (indicating removal) and then rising to 0.197mmolL-1 at 14.2m depth. The primary source of nitrate and phosphate is fluvial input - lower salinity water is less dense and thus remains at the surface giving us the high concentrations measured at 1.3m depth. The greatest number of phytoplankton cells, 221 000 cells L-1, are at this depth. If cell numbers have increased recently then nitrate and phosphate concentrations can be expected to fall from our sampled concentration. If cell numbers have been established for a period of time then we can assume consistently high nutrient replenishment from fluvial sources. The chlorophyll maximum (from the fluorometer) is at 7.7m depth so the low phosphate concentration, 0.100umolL-1, and nitrate concentration, -0.292mmolL-1, in our discrete sample at 7.9m depth are expected because of removal by phytoplankton. Nutrient replenishment results in a slight increase in nitrate and phosphate concentration at 14.2m depth. Phytoplankton numbers are lower at this depth, as this station is below the euphotic zone, which only extends to 9m. Nitrate and phosphate concentration can be seen to be increasing, because of nutrient replenishment by bacterial activity and decomposition of organic matter below the euphotic zone. Nutrient analysis is limited by a lack of discrete samples at each depth and the number of depths sampled – these limit both the detail of the profile produced and prevent the inclusion of relative error bars (which would highlight any discrepancies in our analysis). PlanktonStation 1- at 1.3m, dominated mainly by large amounts of diatom cells, especially those of the Rhizosolenia sp. This correlates well with the lower silicate concentrations seen in the water sample taken in the near surface waters. -at 7.9m, both diatoms and dinoflagellates are present in similar numbers, although the total cell count is less than half of that in the surface sample. The chlorophyll maximum is present at around this depth, we would therefore expect the phytoplankton concentration the be highest, this is not so. This may be explained by the fact that the dinoflagellates found, Karenia mikimotoi, are of a smaller number but of a larger size and also contain increased amounts of chlorophyll per cell. -at 14.2m, 52 000 phytoplankton cells per litre were identified, half of that of the previous station. This supports the lower chlorophyll concentration identified by the fluorometer on the CTD at this depth. Station 2
Temperature, salinity and fluorometry:Station 2 ( 49°59.751N, 005°08.163W) is located between Black Head and Lizard Point. The vertical profile (fig. 3.2) down to a maximum depth of 30.7m shows a prominent thermocline from 16.49°C at the surface to 13.64°C with a range of 2.48°C and a small salinity spike at 15m, salinity from 35.19 at surface to 35.24 at 26.8 m with a range of 0.5. Fluorescence increases with depth from 1.17 V at surface to 2.12 V at 26.8m At this station we were in particular looking for signs of eddies and fronts caused by the movement of water around the shape of the land as was previously observed by group 9. Nitrate:Nitrate is seen to decrease through the water column with depth, from -0.49 µmol/L (removal) at 8.3m depth to -0.18 µmol/L at 26.8m depth. This is a result of nutrient depletion in the euphotic zone by phytoplankton and lack of mixing due to stratification . Therefore nutrient transfer from bottom waters is reduced. Phosphate:The concentration of phosphate increases with depth
correlating with fluorescence as a result of nutrient replenishment by
terrestrial runoff and cycling. However the difference between values of
the two samples is very small (range 0.13µmol/L). Silicate:Silicate concentration decreases with depth from 1.68 µmol/L at 8.3m to 1.51µmol/L at 26.8m. This is because there is lack of mixing of bottom waters from stratification and silicate has been depleted by phytoplankton, diatoms in particular use lots of silica to build the frustules. PlanktonLarger numbers of phytoplankton were seen deeper in the water column at this station, this is supported by the increase in fluorescence seen at depth. Phytoplankton are able to populate greater depths within the water column at this station, due to the euphotic zone extending to 27m. Karenia mikimotoi dominate the phytoplankton species seen at 26.8m, with a total count of 28 000 cells per litre. Domination of the phytoplankton species sampled by these dinoflaggellates is to be expected during the summer months as they prefer a stratified water column, which is exemplified well in the above temperature profile. Diatoms are the only species found in the sample at 8.3m, which would explain the lower silicate concentration found at this depth as they utilize this nutrient in their frustules. Zooplankton tow- was conducted between 25m and 15m, which of the 2 tows conducted, possessed the largest number of individuals, dominated by hydromedusae and copepods, of numbers 2971 and 1521 per m3 respectively. This ties in well with the larger numbers of phytoplankton also found at this station, which are an abundant food source able to support the large zooplankton population. -another conducted between 10m and the surface which sourced significantly smaller numbers of zooplankton per m3 in comparison with that of the deeper tow. This can possibly be explained by the lack of food source, namely phytoplankton, to support a large zooplankton population. Station 3
Temperature and salinity:There is a well developed seasonal thermocline with a temperature range of 5.79 °C from 17.16 °C in the surface waters, to 11.37°C at depth, as shown in fig 3.3. Salinity does not show such a pronounced vertical variation with values ranging from 35.17 to 35.54; such small variation within the whole water column suggests that it is well mixed with respect to salinity. Chlorophyll:The chlorophyll profile shows a peak of 3.17µg/L at 15.86m – just below the upper depth of the thermocline. The thermocline often separates water bodies with different nutrient concentrations; phytoplankton can thus thrive on one side of it but not the other, leading to peaks above or below the thermocline. Plankton:In the surface waters, 6.3m depth, the diatoms Rhizosolenia stelgera and Eucampia Sp, dominate with 35 000cellsL-1 total. There are also a large number of zooplankton present at this depth: 1700cellsm-3 approx of herbivorous zooplankton and 3950cellsm-3 of carnivorous zooplankton. At 31m depth, diatoms of the genus Nitzschia, primarily Nitzschia longissimus and the dinoflagellate Karenia mikimotoi are co-dominant. There are less zooplankton towards the base of the euphotic zone, possibly because the phytoplankton present, Nitzschia longissimus and Karenia mikimotoi are respectively very small and highly toxic. At 69m depth there are large numbers of Nitzschia longissimus which, coupled with an increase in silicate and nitrate concentrations, could be attributed to the effect of mixing – raising nutrients from depth and mixing phytoplankton to depth. There was rough weather in the three days prior to our sampling and our station is in an area of fast tidal currents and turbulent flow, potentially increasing mixing. NitrateNitrate concentration decreases from 0.6290µmolL-1 at 6.3m depth to -0.4643µmolL-1, or a negligible concentration, at 31m depth; before rising to 1.3195µmolL-1 at 69m depth. Within the photic zone - surface to 31m depth - significant removal of nitrate occurs, high numbers of phytoplankton, 160 000cellsL-1 primarily diatoms of the genus Nitzschia, . Nitrate concentration decreases from the surface waters to the base of the photic zone and then increases to peak at 1.32µmolsL-1 at 69m depth. At this depth, nutrient replenishment by bacterial decomposition of particulate organic matter may be responsible for the nitrate increase. PhosphatePhosphate concentration shows an increase in concentration with depth, phosphate increases from 0.0731µmolL-1 at 6.3m to 0.3080µmolL-1 at 31m depth and 0.393µmolL-1 at 69m depth. In surface waters, phosphate is low in concentration because of phytoplankton removal and subsequent lack of replenishment due to stratification of the water column (limiting mixing). The increase in concentration beneath the thermocline results from nutrient replenishment and reduced phytoplankton removal. SilicateSilicate concentration increases with depth, gradually from the surface down to 31m with an increase from 1.249µmolL-1 at 6.3m depth to 1.336µmolL-1 at 31m, and more rapidly from 31m down to 3.8197µmolL-1 at 69m depth. Low silicate concentrations above the thermocline indicate removal by diatoms and lack of nutrient replenishment because of increased stratification and reduced mixing. Concentration increases beneath the thermocline because of dissolution of diatom frustules and replenishment from depth. Station 4
Temperature and Salinity:Clear stratification exists with a strong seasonal thermocline the base of which is at 20 m. The temperature drops from 18.18 to 11.02 between the surface and 20m.There is, as expected, a salinity spike at the thermocline then below 20m temperature and salinity are fairly constant down to 80m. Chlorophyll:There is a double peak of chlorophyll just below the thermocline and zooplankton found in higher numbers in the shallower water are carnivores (chaetognaths and hydromedusae) so not directly associated with the phytoplankton. More zooplankton in the gap between peaks included high numbers of copepods (grazers) which could have caused the split peak by consuming the central area of phytoplankton. Nitrate:The nitrate profile is unusual in that it peaks and then reduces again with depth. Nitrate concentration is at a maximum slightly above the chlorophyll maximum and is depleted within the chlorophyll max as it is taken up by the plant cells. Nitrate would be expected to be at a maximum below the thermocline. SilicateThe silicate profile at station 4 is only drawn from 3 data points so any conclusions should be drawn cautiously, had more time and resource been available more accurate profiling may have been achieved with repeated sampling at the same depths and creating a scatter graph of data points. The lowest silicate reading is the closest sample point to the surface and above the thermocline. The middle value is just above the thermocline with the highest reading below the thermocline and in between the 2 chlorophyll peaks. PhytoplanktonThe low level near the surface could be due to the end of a diatom bloom with dinoflagellates in water where the deeper sample was taken. Dinoflagellates are commonly found near to fronts they prefer less turbulent waters and waters with higher nutrients. Diatoms are generally less particular about mixing and can be found both earlier in the season and higher in the water column, their stronger structure allows for a more dynamic environment. Samples were taken at 6, 13 and 20 m. Bottle 96 was taken at 20 m and its main constituents include the flagellate Karenia mikimotoi, and the dinoflagelatte Prorocentrum. Bottle 64 was collected at 15.9 m and included very high numbers (102,000 cells per litre) of the flagellate Karenia mikimotoi and the dinoflagelatte Prorocentrum. ZooplanktonGroups well represented in the samples include Hydromedusae, Copoda, Polychaeta larvae and Chaetognatha. ADCP analysisSeveral interesting features were observed, both at stations and whilst undertaking various transects. Data recorded from a stationary period at 49º54.892 N : 5º11.954 W showed the presence of an internal wave in the water column highlighted by plankton presence on along the thermocline. The backscatter image, fig 3.5 below, clearly shows the waves progression along the thermocline as time elapses:
Isolating the vertical component of the water highlights cells of upward flowing water, as seen in Fig 3.6 below. These cells are an indicator of the wave’s propagation.
A further example was found during a transect between 49º55.172 N, 5º10.636W and 49 º52.468 N , 5º11.410 W. Smaller oscillations can be seen at depths of approx 87 to 174m. Above 50m there appears to be a continuous oscillation at the chlorophyll maxima. This is shown in fig 3.7 below.
Another phenomena that occurred during the cruise was seen whilst completing a transect between 49°55.172 N , 005°10.636 W and 49°52.468 N , 005°11.410 W. The sudden drop in sea floor depth results in a fast current down the shelf break, as seen in fig 3.8 below.
A further point to note is the cell of slower moving water behind the faster flowing current. This area appears to be noticeably slower than the faster current. Also, by isolating the East component of the velocity in fig 3.9 below, it can be seen that the water is also moving in the opposite direction to the general flow.
SummaryAll the stations sampled were affected by a lack of discrete samples
for nutrients and chlorophyll. Salinity spikes are a common feature on
the vertical profiles constructed from the CTD data; these are caused by
rapid changes in temperature. |
Geophysics
Sorting through grab samples. IntroductionA geophysical survey of the middle estuary was undertaken using SideScan Sonar. This indicated any interesting areas of sedimentary structure for further analysis using a Van Veen Grab.AimsTo survey the seabed in the Middle Estuary to establish the benthic structure and the biota that live there.Objectives- Use a SideScan Sonar to identify seabed type and structure- Use a Van Veen Grab to establish the seabed sedimentary structure and benthic biota MethodWe deployed the fish about a metre below the water surface and undertook 3 transects across the middle estuary, just outside of Restronguet Creek. The SideScan Sonar emitted an acoustic pulse, which was then reflected back to the fish and recorded both a computer and paper copy. This indicated any sea bed features.Once the transects had been completed, 3 sites were picked for benthic analysis using a Van Veen Grab. Sites that looked interesting were chosen. The Van Veen Grab collects a seabed surface sample approximately 10cm thick, giving an indication of the surface sediment stratification and biota. This was then analysed by identifying the species and describing the types of sediment present. ResultsVan Veen Grabs:
Grab 1: 50°11.5383N 005°03.0280W, 11:06GMT Benthic Biology:Dead Maerl (cream/white) ConclusionsThe 3 tracks crossed over a channel and we calculated the depth and detail
of some of the features. At its deepest point, the channel depth was
calculated to be 24m. It narrowed the further north we surveyed. The main
feature we found was what looked to be a dredging track running parallel to
the east of the channel; this we calculated to be approximately 90cm deep. |
Examples of samples from Van-veen grabs. Left to right: Nereis sp. Turret shell. Maerl. Warty Venus. Cowrie shell. Grey top shell. |
ConclusionThe Fal estuarine system is complex and composed of a number of dynamic
environments, as are most estuarine systems. Having observed the biological,
chemical and physical parameters which make up this river? over the past two
weeks, a number of conclusions can now be drawn. |
Esther! |
[caption] |
ReferencesAdmiralty (2006). Admiralty Leisure Charts - SC5602: The West Country; Falmouth to Teignmouth. 7th Edition. Miller, C.B. (2003) Biological Oceanography. Blackwell publishing Ltd. Pingree, R.D. (1975). The advance and retreat of a thermocline on a continental shelf. Journal of the Marine Biological Association of the United Kingdom. Vol.55: pp965-974. Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon. Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH. Johnson K. and Petty R.L. (1983) “Determination of nitrate and nitrite in seawater by flow injection analysis”. Limnology and Oceanography 28 1260-1266. |
Disclaimer: The data presented on this page was collected and analysed
by students of the University of Southampton. Views expressed do
not necessarily reflect those of the University. All data is provided "as is" with no
guarantee. Use at your own risk. |