Content

Abstract

Introduction

Estuarine (RIBs and Conway)

Offshore

Geophysics

Conclusion

References

Map depicting sampling locations (offshore stations shown by crosses)

Callista.

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Group 4:
James Clarke, Hayley Jane Essex, Esther Hughes, Alex Hutter,
George Neville-Jones, Lizzie Nicol, Ellie Parsons, Shaun Villa.

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Preparing the CTD for deployment on board Callista.

Rhizosolenia setigera.

George and Lizzie enjoying sea winds from the bow.

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Abstract

The 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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Introduction

The 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 main sub-tidal areas of the estuary are wide and deep with a central channel and only a narrow inter-tidal area around the edge. The width of the inter-tidal area increases as the central channel narrows in the estuaries upper reaches. The central channels in the upper reaches are maintained partially by dredging, there are two major ports, dredged shipping channels and commercial shellfish beds farming the native oyster Ostrea edulis. The sub-tidal zone is submerged throughout the tidal cycle and is thus the more stable of the tidal environments. In the Fal Estuary the bottom sediments consist of sand and fine silt deposits intermingled with oyster shells and other detritus. An extensive maerl colony is found on the eastern side of the Carrick Roads. Past mining of the Falmouth area produced great amounts of mining waste and heavy metal contamination increasing siltation. China clay extraction and purification also release large amounts of fine sediment into the fluvial system.
 

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.
The seasonal development of distinct layering within the water column has significant effects on the phytoplankton biomass. In the winter months, phytoplankton productivity is limited by a lack of insolation despite high nutrient availability. During summer, the presence of predominately temperature induced density layers within the water column restricts the movement of near-neutrally buoyant particles like diatoms. This can have a variety of effects on photosynthetic plankton: they can be isolated in oligotrophic layers at the top of the euphotic zone, in eutrophic layers beneath the photic zone or in eutrophic conditions at the top of the photic zone. It is this last case that, along with isolated mixing events like summer storms, can trigger summer blooms
 

Boat Procedure

Salinity 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.

a:

b:

c:

Figure 0.1: (a) Chart of the Truro and Fal rivers (b) chart of Upper Fal estuary and (c) Fal estuary showing sample points and transect lines.  Offshore sample points are shown in the figure that forms the menu at the top of this page. Purple items denote areas sampled using RIBs on 4th July 2006. Green items denote areas sampled using Bill Conway on 11th July 2006 (Admiralty, 2006).

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Estuaries

The overall aim of the estuarine project is to investigate the transition zone between freshwater and saline sea water in the Fal Ria system (Fal, Truro, Tresillian and Carnon rivers). Measurements from onboard instruments on both the RIBs and Bill Conway were analysed to determine the chemical, biological and physical processes taking place within the Fal estuarine system

Additional to the procedure stated above, on the RIBs and Bill Conway the plankton net was towed along the surface for 5 minutes at 1 knot on the RIBs and for 2 minutes at 1 knot on the Bill Conway before being recovered, and the sample water collected and stored in a plastic container. The zooplankton species were identified and counted for analysis.

Mussel aquaculture on the Truro river, just south of King Harry Ferry.

Ellie and Sean on Coastal Research.

RIBs

Date: 4/7/06
Location: Truro (50°15.712N, 05°02.862W) to Turnaware Point (50°12.348N, 05°02.257W) along the Truro River, Fal Estuary System.
Time: started 1100GMT, finished 1400GMT
Tides: 0500GMT 1.90m
1100GMT 4.24m
1720GMT 2.10m

Introduction

The RIBs were used to survey the Upper Estuary, from Truro to Turnaware Point along the River Truro, Fal estuary. The main aim was to collect data necessary to determine the biological, chemical and physical processes occurring in the Upper Estuary.

Sample points were selected with the intention of sampling up the salinity gradient at intervals of 2. The two RIBS then sampled every other station in a leap frog pattern to maximize time utilization. This meant two YSI probes were used, both of which were inter-calibrated with the CTD on the Bill Conway.

Results

Silicate Concentration Figure 1.1 below shows the change in silicate concentration with salinity. It shows addition in the Upper Estuary between 1.2 to 25 salinity, with the maximum silicate concentration of 67.38umol/l seen at 4.8 salinity. In the Lower Estuary conservative behaviour is observed between 30 and 34 salinity, with the minimum concentration seen in the seawater end-member at 6.4umol/l.

Phosphate Concentration Figure 1.2 shows that maximum phosphate concentration is seen in the Riverine end-member at 0.28umol/l. Removal is seen between 1.8 and 13.85 salinity, with it decreasing to 0.158umol/l at 4.6 salinity before increasing again to 0.192umol/l at 13.85 salinity. Massive addition is seen in the middle estuary between 13.85 and 33 salinity, reaching a high of 0.222mol/l at salinity 18.9. The lowest concentration is seen in the seawater end-member at 0.0189umol/l.

Nitrate Concentration Fig 1.3 shows addition in the Upper Estuary between 4.6 and 13.85 salinity, reaching a maximum of 336.83umol/l at 9.75 salinity. Massive removal is seen in the middle estuary between 13.85 and 32 salinity, with it decreasing to 24.73umol/l at 18.9 salinity. Between 32 and 33.89 salinity nitrate behaves conservatively, decreasing to a minimum of 4.24umol/l in the seawater end-member.

Fig 1.1 Estuarine Mixing Diagram for Silicate along the Truro River, Falmouth

Fig 1.2 Estuarine Mixing Diagram for Nitrate along the Truro River, Falmouth

Fig 1.3 Estuarine Mixing Diagram for Phosphate along the Truro River, Falmouth

Chlorophyll Concentration Fig 1.4 shows that the highest chlorophyll concentration was seen in the riverine end-member, with a maximum of 16.38ug/l. In general, higher chlorophyll concentrations are seen in mid salinities ie,, between 9.8 and 26.24 salinities, with a peak at 18.9 of 14.22ug/l. The lowest chlorophyll concentrations are seen at the lower salinities, with the lowest seen at 32.33 of 6.22ug/l.

Dominant Phytoplankton Species Fig 1.5 shows that the dominant phytoplankton species found at station 9 (salinity 18.9) is Nitzschia sp. with 2.7x107 cells per litre. This is also true at station 6 (salinity 31.39), where 3.25x105 cells per litre of Nitzschia sp. were found. At stations 15 (salinity 32.33) and 17 (salinity 33.36) Rhizosolenia sp. were dominant, with 5.4x104 and 1.79x105 cells per litre respectively. At station 17 a large number of Mesodinium rubrum (a ciliate) were found, with 6.1x104 cells per litre. At the higher salinity of 33.89 at station 14, Chaetoceros sp. were more dominant with 6.7x104 cells per litre present. This shows a general pattern of Nitzschia sp. being dominant between salinities 18.9 and 31.39, Rhizosolenia between 32.33 and 33.36 and Chaetoceros at 33.89.

Dominant Zooplankton Species Fig 1.6 shows the dominant zooplankton species found during the different zooplankton trawls. As seen, at net 1 (furthest up estuary) there is a low number of both species and individuals present. Here, the dominant species are Copepoda and Cirripedia larvae with 1.68 cells per m3. At net S1, Cirripedia larvae are dominant with 84.1 cells per m3 present. This is also true at net 2, with 56.4 cells per m3 of Cirripedia larvae. The largest diversity of zooplankton is found further down the estuary at net S2. Here 12 different species were identified, with Copepoda being dominant at 33.7 cells per m3.

Fig 1.4 Change in Chlorophyll Concentration along the River Truro, Falmouth

Fig 1.5 Phytoplankton Species found at each station sampled along the Truro River, Falmouth. Due to low numbers, some species are not visible.

Fig 1.6 Zooplankton species observed along the Truro River, Falmouth.  Due to low numbers, some species are not visible.

Fig 1.7 Variation in euphotic zone depth with salinity.

Euphotic Zone Depth Fig 1.7 shows that as salinity increases, so does the euphotic zone depth. This is probably due to the higher turbidity of the Riverine end-member, which in turn is caused by increased run-off from the land. Also, as the water is shallower, any mixing will lead to sediment suspension throughout the water column, decreasing the euphotic zone depth. A third reason for increased turbidity could be through churning of the water column due to the small boats turning around that area.

Vertical Profiles Fig 1.8 below shows the change in stratification from station 10 upstream to station 14 at Turnaware Point. Further upstream the water column is more stratified, for example at station 10 there is a change in salinity of 1.65 between the surface and 6.40m. At station 15 a difference in salinity of 1.35 between the surface and 1.1m was recorded. This indicates that there is stratification at the surface, but beneath it there is a well-mixed water column. Further downstream at stations 17 and 14 the water column is more well-mixed, indicated by the smaller change in salinity and temperature between the surface and at depth.

Fig 1.8 Vertical profiles of salinity, temperature and density at four different stations along the Truro River, Falmouth.

Summary

Further upstream, it has been observed that the Truro River is slightly stratified, as seen in the vertical profiles above. Also, there is addition of both silicate and nitrate, which are likely to have been input from the Truro River. As the local geology is mainly granite, this could be a source of the high silicate concentrations observed. The main nitrate input in the area is run-off from the surrounding agricultural fields, which contain both livestock and crops. There may also be some sewage inputs as well. In contrast, there is a decrease in phosphate concentration in the upper estuary. This could explain why, following the initial riverine chlorophyll reading, there is a decline in chlorophyll present in the upper estuary despite the high levels of nitrate present.

Further downstream, at salinity 18.9, there is a sudden increase in phosphate concentration present. This is coupled with a rapid decline in nitrate concentration as well as a rapid increase in chlorophyll concentration. This indicates the presence of a high quantity of phytoplankton, which is made possible by the increase in phosphate concentration. This is confirmed by the high concentration of Nitzschia sp. phytoplankton observed at this station (2.7x107cells per litre).

As salinity increases, the concentration of all 3 nutrients decreases, becoming conservative after salinity 30. Coupled with this, the chlorophyll concentration decreases, probably due to the declining level of nutrients present. This decline in phytoplankton could also be due to the increased zooplankton species present further downstream, which graze on phytoplankton. In contrast, the euphotic zone depth increases with salinity as the water becomes less turbid. Also, the water column is more well-mixed. This leads to a change in conditions present, leading to an alteration in the dominant phytoplankton species to Chaetoceros sp.

Conway

Date: 11/7/2006
Location: Mouth of Fal estuary (50º08.609N, 005º02.451W) to King Harry Ferry, Truro River (50º13.378N, 005º01.447W).
Time: started 0931GMT, ended 1412GMT
Tides: 05:00GMT 4.92m
11:30GMT 1.04m
17:20GMT 5.31m

Introduction

Following the analysis of the upper estuary using the RIBs, the lower estuary was surveyed to establish the biological, chemical and physical mechanisms in process. We concentrated more on the physical processes because we have existing data on the biology and chemistry from Group.3's survey of the area which has been incorporated into our RIB data. We feel that data from the chemical and biological aspects of Group.3's survey is relevant to our RIB collected data because it was collected under the same weather and tidal conditions.

Aims

To further understand the biological, chemical and physical processes occurring in the Lower Estuary.

Objectives

- To collect water samples for further lab analysis to determine nutrient concentrations and biological presence

- To use the ADCP and CTD to determine the physical structure of the Lower Estuary

Results

Station.2

Fig 2.1 Vertical Profiles showing the change in temperature, salinity, fluorometry and nutrient concentrations with depth at station 2, the mouth of the Fal estuary adjacent to St Antonys Head.

Station 2 was situated near the mouth of the estuary, east of Black Rock. As seen in fig 2.1, the surface levels of chlorophyll l are high with corresponding low nutrient levels. At 8.6m, the chlorophyll concentration decreases to 1.54ug/l. At the same depth, nutrient concentrations increase. This could be due to reduced numbers of phytoplankton present to utilize the nutrients, resulting in higher concentrations.

The TS probe indicates a very thin surface layer of fresh water. This could be due to the ebb of the tide. Below this, the water column is well mixed, again due to the tide retreating as well as turbulence and increased wind speeds in the days prior to sampling. There is a distinct thermocline at 1m depth due to warm freshwater input and a second definite thermocline at 10.5m depth resulting from seasonal surface heating.

The fluorometry data somewhat contradicts our discrete chlorophyll data, showing a peak between 6m and 12m, corresponding to the lowest discrete chlorophyll reading at 8.6m. This may be because the major fluorescing pigments in the water column at these depths are not those measured in discrete analysis.

Station.3

Fig 2.2 Vertical profiles showing the change in temperature, salinity, flurometry and nutrient concentrations with depth at station 3, Smugglers Cottage.

At the surface, all parameters, including chlorophyll, are high, as seen in fig 2.2. This could indicate high riverine nutrient input - this station is the furthest that we sampled up the estuary. When the depth reaches 5.3m, chlorophyll, silicate and nitrate concentrations all decrease. However, the phosphate concentration increases slightly. This could be due to sediment resuspension or enhanced anthropogenic inputs, despite occurring at a shallow depth. However, the change in phosphate concentration with depth is actually very small, the change could be regarded as being almost negligible.

The vertical profile indicates that the water column is more stratified at this station. This is probably because it is further upstream and therefore the riverine input has greater influence and the water is shallower so surface heating has a greater effect. Furthermore, this station was sampled at the low tide stand and thus, the lack of water movement might temporarily enhance stratification.

At Station 3, phytoplankton cell counts at 1.6m depth indicate very large numbers of species in the Rhizosolenia genus, almost three times the numbers of diatoms at other stations. There were also a large number species in the Nitzschia genus, which corresponds to the bloom of Nitzschia longissimus we found above the Truro tide gates in our RIB sampling of the upper estuary.

Station.6

Fig 2.3 Vertical profiles showing the change in temperature, salinity, flurometry and nutrient concentrations with depth at station 6

As seen in fig 2.3, fluorescence at Station 6 follows changes in temperature with depth very closely. The highest fluorometry values occur at the depth of greatest temperature change, 0-2m, because here, nutrients recycled from depth or input in river water accumulate along the pycnocline. Both temperature and fluorescence then decrease with depth. Salinity increases with depth because of the progressive mixing of saline water with freshwater at the surface.

The nutrient profile shows high surface values, decreasing at mid-depth and then increasing to the depth of our lowest discrete sample for chlorophyll and all nutrients bar phosphate which decreases further with depth. The low nutrient and chlorophyll values at 5.9m depth could be because phytoplankton are nutrient limited or that their numbers have been reduced by predation following a period of high-growth.

ADCP Analysis

A transect across the main channel commencing from 50°08.609 N, 005°02.451 W to 50°08.441 N, 005°01.100 W highlights the presence of two faster flowing bodies of water in the main area of the channel, as seen in Fig 2.4. This appears to be the tidal flow as the general direction of flow is southwards towards the sea, as the tide is on the ebb this would be the expected direction. This can be seen in Fig 2.5 as this plot of the velocity direction shows the flow direction is predominantly southward, as would be expected with the predicted tidal flow. Note the small eddie flow possibly due to the higher relief topography in the immediate vicinity.

Fig 2.4 ADCP velocity magnitude plot for transect 1.

Fig 2.5 ADCP velocity direction plot for transect 1.

Tidal activity was high during the course of the survey; a transect further up river from 50°09.968N, 005°02.819W to 50°09.819N, 005°01.869W revealed the change in flow direction from ebbing to flooding (Fig 2.6). This change in tidal flow direction is visible in another transect taken at 50°13.378N, 005°01.447 W to 50°13.322 N, 005°01.672 W. As can be seen there now appears to be a strong northward flow upriver in the channel Fig 2.7.

Fig 2.6 ADCP velocity direction profile for transect 3

Fig 2.7 ADCP north velocity profile for transect 4

Conclusions

Each of the vertical profiles above indicates that at the surface chlorophyll concentrations are high and nutrients low. However, at the station furthest up the estuary there are high nutrient levels at the surface present. This could be due to the increased influence of the riverine input further up the estuary. At stations 3 and 6 the nutrient and chlorophyll concentrations decrease at mid depths. However, at station 2 the nutrient levels increase. This could be because of a lack of phytoplankton utilising the nutrients or because of mixing with deeper water increasing the concentrations.

The Vertical profiles also show the change in water column structure from well mixed at the mouth of the estuary to slightly stratified further up. This agrees with the RIBs data described above. The degree of mixing at the mouth could be due to tidal movement, whereas the slight stratification observed further upstream could be because it was during the slack water period.

The ADCP data shows the change of the tidal conditions we experienced during our sampling. The ebbing tide is shown in the transect across the mouth of the Fal, whereas the change in tide from ebb to flood is depicted later on in the day. This shows that the flooding tide enters the Fal on the west  whilst the tide is still ebbing in the east. The ADCP profiles also show that when the tide is flooding it enters first in the deepest part of the channel.

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Offshore

Primary 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

Date: 7/7/06
Location: Black Rock (50°08.548N, 005°01.629W) and inshore (49°54.892N, 005°11.954W) to offshore Lizard Point (49°52.468N, 005°11.410W).
Time: started 0901GMT, finished 1451GMT
Tides: 0756GMT 1.80m
1356GMT 4.40m
2029GMT 1.80m

Introduction

The 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.

Aims

The 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

  • To obtain physical and biological profiles of the inshore and offshore region surrounding the Lizard Peninsula region of the English Channel and how they affect each other.
  • Construct vertical profiles of temperature, salinity and fluorometery using the CTD and sample the waters accordingly.
  • Take appropriate samples for chemical analysis at depths selected using the CTD downcast, and carry out zooplankton trawls at selected depths identified from the CTD data.
  • To look for eddies and internal waves in the inshore region of the peninsula.

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

Fig 3.1 Vertical Profile showing the change in temperature, salinity, fluorometry, nutrient and chlorophyll concentrations with depth at 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).

Plankton

Station 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

Fig 3.2 Vertical profiles showing the change in temperature, salinity, flurometry and nutrient and chlorphyll concentrations with depth at 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).
However we need to question the accuracy values of phosphate concentrations below approximately 0.08µmol/L because the spectrophotometer becomes limited below these.

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.

Plankton

Larger 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

Fig 3.3 Vertical profile showing change in temperature, salinity, fluorometry and nutrient and chlorophyll concentrations with depth at station 3.


Station 3 is the intermediate offshore location, for which a vertical profiles of in situ and discrete samples down to 69.4m depth have been constructed as shown in fig 3.3.

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.

Nitrate

Nitrate 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.

Phosphate

Phosphate 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.

Silicate

Silicate 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

Fig 4.4 Vertical profile showing change in temperature, salinity, fluorometry and nutrient and chlorophyll concentrations with depth at 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.

Silicate

The 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.

Phytoplankton

The 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.

Zooplankton

Groups well represented in the samples include Hydromedusae, Copoda, Polychaeta larvae and Chaetognatha.

ADCP analysis

Several 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:

Fig 3.5 ADCP profile showing the average backscatter highlighting the presence of internal waves at station 2

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.

Fig 3.6 ADCP profile showing the up velocity. Vertical cells are highlighted in red boxes.

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.

Fig 3.7 ADCP profile showing average backscatter at station 4

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.

Fig 3.8 ADCP profile showing velocity magnitude in between stations 2 and 3

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.

Fig 3.9 ADCP profile showing the east velocity component. Slower velocity water is highlighted in a red box.

Summary

All 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.

Station 1 sits apart from the main area surveyed and so does not fit with the continuous data set obtained around Lizard Point. Black Rock shows much less vertical stratification than the other stations surveyed; this is largely due to its location at the mouth of the estuary. Its vertical profile shows the water column is relatively well mixed with regards to temperature, and the salinity structure reflects the station’s position in the estuary with low surface salinity values indicative of the riverine input to the system. The nutrient samples at this location correlate well with the phytoplankton communities observed, with diatoms dominating.

Station 2 shows a prominent seasonal thermocline between 12m and 15m, whilst salinity shows only minimal variation with depth. The ADCP data is particularly interesting here, with an internal wave being clearly visible in the data. This internal wave was likely to have been caused by the topography of the seafloor displacing deeper denser water in the water column to shallower depths.

Station 3 was found to have a fairly stable water column. There are high concentrations of toxic phytoplankton species Nitzschia longissimus at 69m depth coupled with an increase in nutrients; this is attributed to mixing both aspects.

Station 4 was the deepest survey conducted down to 80m depth, and displayed a double peak in the measured chlorophyll levels on the thermocline. This was found to be caused by variations in the plankton communities, with phytoplankton being more abundant in the peaks along with carnivorous zooplankton and the trough dominated by zooplankton species such as copepods.

Transects between stations were also conducted, as mentioned before ADCP analysis yielded interesting results; the transect undertaken between stations 3 and 4 highlighted the presence of a faster moving body of water at the shelf break. Another interesting point to note was the slower moving water behind this cell that was flowing in the opposite direction.
 

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Geophysics

Date: Started: 09:52 GMT Finished : 12:25 GMT
Location:  
Tides:

07:20GMT 5.19m

   13:40GMT 0.71m
   18 :50GMT 5.54m
Weather:

0/8 cloud cover, strong breeze (20kn), sunny

Sorting through grab samples.

Introduction

A 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.

Aims

To 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

Method

We 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.

Results

Van Veen Grabs:

Grab 1: 50°11.5383N 005°03.0280W, 11:06GMT

Sediment Description:
Thin light brown surface layer, possibly sand, which was coarser than the layer below. Thr redox layer was approximately 2 mm below the surface. Sediment below was cohesive, mostly clay and very dark brown/black with a metallic sulphur like smell. We considered this to be largely anoxic very fine mud.

Benthic Biology:
Oyster Shell fragments (old worm casts on some)
Sea lettuce - light brown, flat seaweed.
Several tube worm casts, some with inhabitants
Cockle shells (unidentified species)
Errant polychaetes approximately 3cm long
Sand gaper shells (Mya arenaria)

Grab 2: 50°11.491N, 005°02.622W, 11:47GMT

Sediment Description:
Silt layer with the redox layer about 2 mm below the surface.
Anoxic layer littered with dead calcareous maerl and shell fragments.

Benthic Biology:
Dead Maerl – cold water coral
Calcareous tube dwelling polychaetes
Sand gaper shells (Mya arenaria)
Slipper Limpet (Crepidula fornicata)
Topshells – grey and purple
Lugworms - Arenicola sp.

Grab 3: 50°11.4783N, 005°02.4490W, 12:25GMT

Sediment Description:
Coarse throughout and cohesive. 0.5mm redox layer at the surface with a gradual change. Shell fragments were approximately 1mm in diameter (smaller than station 2).

Benthic Biology:

Dead Maerl (cream/white)
Small gastropod shells
Top Shell
Sand gaper shells (Mya arenaria)
Worty Venus (Venus verrucosa)
Queens shell (Trivia monacha)
Errant polychaetes
Netted dog whelk shells (Hinia reticulata)

Conclusions

The 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.

Most of the area surveyed is largely mud, but an area with seaweed growth was identified in the north west of the survey. This is where the first grab sample was taken and a small piece of seaweed was discovered. An area with lots of shell fragments to the east of the channel was also identified, both by a darker area on the Side Scan plot, indicating increased backscatter, and in the 3rd grab where many fragments of various shells were collected. Apart from one slipper limpet in the second grab, the only live species collected were polychaetes (errant and sedentary).

This, together with the shallow redox layer, indicates a polluted environment. Polychaetes are highly tolerant pioneering species and, where a pollution gradient is identified, are indicators of disturbed/polluted areas. Restronguet Creek has been the subject of environmental studies as it is considered to be high in pollutants.

The main pollutants are Organotins (TPT and TBT) originating from Falmouth Docks, shipping, sediments and sewage discharge now throughout the entire system and are primarily a threat to Mollucs. Metals (especially Arsenic, Copper, Cadmium, Iron and Zinc) are mostly from past mining activity and discharges and mainly affect Restronguet and Mylor threatening invertebrates.

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Examples of samples from Van-veen grabs.  Left to right: Nereis sp.  Turret shell.  Maerl.  Warty Venus.  Cowrie shell. Grey top shell.

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Conclusion

The 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.

The physical structure of the estuary fluctuates depending on the location sampled, with a consistently well mixed water body being seen near the mouth of the estuary, where wave and tidal influences are more significant. Toward the landward end of the estuarine system, up the River Truro, the physical structure appeared to be partially mixed. Thermal stratification and a distinct thermocline prevail in the offshore regions around the Lizard peninsula, due to surface heating and reduced mixing during the summer months. Further landward, the River Truro and surrounding tributaries display only slight stratification, with a warmer surface layer prevailing from the recent warm weather and land runoff. Carrick Roads area is influenced by both waves and tides, therefore showing highly changeable conditions depending on the physical dynamics at that time.

Nitrate, phosphate and silicate all act conservatively throughout the Carrick roads area and into the offshore environment, as to be expected. Signs of removal and subsequent addition occur frequently at lower salinities, higher up the River Truro. Exact nutrient dynamics fluctuate depending on the Station sampled, with one station highlighting severe removal due to a huge Nitzschia bloom. Freshwater runoff and fluvial inputs are the main suppliers of nutrients to the River Fal, silicate from the surrounding geology and phosphate and nitrate from agricultural inputs and fertilisers.

Whilst there is significant variation in the phytoplankton species present through the estuary and offshore, the same species often dominate. The Nitzschia and Rhizosolenia genera are the most prolific; the former is in bloom conditions in the upper estuary and dominates many of the stations offshore. Rhizosolenia are present in all of our phytoplankton samples, but never at the numbers of Nitzschia. In the wates off Malpas and further upstream towards Truro, we found toxic dinoflagellates of the genus Alexandrium. This genus is responsible for PSP or Paralytic Shellfish Poisoning and limits the spread of aquaculture upstream. Offshore, four miles south of the Lizard, Dinoflagellates of the genus Prorocentrum and the species Karenia mikimotoi are present in the mid-water water column in high numbers. This genus prefers the stratified water offshore and thus is not present in the same numbers in our inshore samples, where diatoms tend to dominate (Miller 2003).

Zooplankton tows conducted offshore were dominated by both hydromedusae and copepods, of numbers 2971 and 1521 per m3 respectively at one station. This ties in well with the position of phytoplankton seen offshore, with larger numbers of zooplankton present just below the thermocline at most stations. Throughout the estuary, zooplankton numbers are controlled by phytoplankton abundance and toxicity. Higher up in the River Truro, zooplankton numbers showed significant reduction in comparison to estuarine and offshore counts. Dominant species included numerous larvae and copepods, alongside fish eggs and hydromedusae. Some species of zooplankton, like dinoflagellates, prefer less turbid waters.

Turbidity in the upper estuary, leading to the River Truro, is much higher than that seen offshore, with a secchi depth of 28cm against depths between 9m and 15m. With the exception of periods of high river flow, current speed in the estuary is similar to that found at our offshore stations, being almost wholly tidally induced. Increased turbidity within the estuary occurs because there is more friction between the water and the bed and channel morphology than there is between the different water bodies offshore. The variation in Secchi depth is because there is a far greater amount of suspended material within the estuary and upstream than there is offshore. Suspended sediment settles around the mouth of the estuary and at major changes in salinity; occurring at the former because flow velocity decreases as the water depth channel size increases, reducing the energy available to transport sediment. At the latter because the increased ion concentration at higher salinity increases the Van der Waals forces between the suspended particles and causes flocculation and thus, sedimentation.
 

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Esther!

 

[caption]

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References

Admiralty (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.

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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.
Copyright 'group 4' 2006. All rights reserved.