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Estuary

Introduction

The aim of the study was to investigate the chemical and physical structure of the Fal estuary and its degree of mixing by collecting water samples and measuring the concentration of nutrients at various stations down the estuary. The Fal estuary has an unusual distribution of nutrients and a changing flow dynamic throughout. Therefore another aim was to understand the phytoplankton and zooplankton community dynamics at different stations along the estuary with respects to these properties.


Methodology

At a station several factors were measured, this included:

The methods used to preserve the water samples for each parameter can be seen in the methods section of ‘Offshore’.


Physical analysis

ADCP

Data was taken from stations 1, 2, 3 and 4 on a rising tide, whilst stations 5, 6, 7 and 8 were taken on a falling tide. This tidal pattern is reflected in the average current readings from the ADCP. At station 3, where the river channel flows from east to west, the ADCP transect shows a strong eastward flow at depth, where dense sea water on the rising tide undercuts the less dense fresh water. At station 4 (fig.1), there is evidence of a landward flow at approximately 6-18m depth, and a strong seaward flow at the surface. As well as the vertical differences in flow, there are lateral variations due to coriolis. The seaward river flow at the surface is much more dominant in the western estuary, since flow is being deviated to the right. Similarly, the deeper tidal flow is being deflected to the east as it enters the estuary, shown in figure below. At this station, where data was taken at a time approaching slack water, the deeper landward flow is not as strong as it was at transects which were sampled earlier in the day when the tide was still rising, 0.1ms-1 compared to 0.3ms-1 at station 2. Increased backscatter at points in the estuary occur where sediments have been re-suspended from the sea bed. This is particularly noticeable at station 7 (fig.2), where there is a clear backscatter peak at a depth of approximately 10-12m. Where the tide is falling, re-suspension of sediments is particularly noticeable.



FLUSHING TIME CALCULATIONS

Tidal flushing of the estuary will result in a Flushing time T, of approximately:


Ttidal= 12.42 hours (tidal period)

Vestuary= 1.36×107m3 (Volume of Estuary=mean area× mean depth, the mean depth is 10m, which makes the area 1.36×106m2.)

Vprism= Area of estuary × h (tidal range) = 1.36 × 106 m2 × 3.4m

h= (average of spring and neap tidal range) =3.4m


        T= 1.5 days.


Chemical analysis


Station 1(fig. 3 & 4)

Dissolved silicon appears to be depleted (1.17µM) in the surface waters where there is an indication of diatoms, but dissolved silicon increases at 2m depth. All nutrients show a lower concentration at the base of the profile than at 2m depth, due to being utilised in overlaying water.  Dissolved oxygen shows super saturation at 111.8%, due to the mixing of the water column. Chlorophyll concentration is also higher at the middle of the water column, where light levels are higher although variation throughout the water column is lacking ultimately due to the shallow depth of this station, the small variation is reinforced within the CTD profile. The water column is separated by the thermocline, due to warmer fresh water and cooler salt water intrusion throughout.


Station 2 (fig. 5& 6)

At station 2 dissolved silicon, phosphorous and nitrate all follow similar patterns, decreasing quite dramatically in the first 5m of the profile. This is consistent with the chlorophyll patterns which remain approximately constant around 3.164mg/l for the first five metres, and decrease significantly after this. Dissolved oxygen increases with depth, due to the higher biological presence at the lower depths depleting the oxygen within the water column. The calm surface conditions mean that limited oxygen would enter the water from the surface compared on days where conditions are likely to be more volatile and surface shear is greater. The flurometer and chlorophyll data do not appear to be correlated particularly well, with the flurometer only registering a spike just deeper than 6m. The water column is vertically stratified due to warmer fresh water at the surface and cooler salty water below the mixed layer.  


Station 3 (fig. 7 & 8)

Dissolved silicon concentrations continue to decrease with depth. Chlorophyll concentrations appear to peak at the surface with values of 3.521µM, but the flurometer indicates a peak in chlorophyll between 6m and 7m. This is equitable between available nutrients of phosphate and nitrate, as well as oxygen with the surface light availability.  Dissolved oxygen follows the previous station trend.


Station 4 (fig. 9 & 10)

Dissolved silicon concentrations change similarly to other stations, with the base of the water column having a higher concentration. Phosphate and nitrate decrease with depth, as they are utilised by the phytoplankton present. Chlorophyll and flurometer data contrast each other at this station, so if it were possible, these samples should be re-run, or the flurometer checked, as it is impossible to make valid conclusions. As with other profiles salinity increases with depth and temperature decreases, a result of the mixing of the fresh and salty water.


Station 5 (fig. 11 & 12)

Dissolved silicon, phosphate and nitrate all decrease in concentration with depth alongside dissolved oxygen. There are considerable differences in dissolved silicon at this station compared to previous stations (silicon concentration at depth for station 4 is 7.69µM whilst at station 5 it is 0.42µM) this gives evidence that diatoms are present between these stations and are utilising the silicon. This is reflected in the increasing chlorophyll and flurometer readings from the surface to the seabed.


Station 6 (fig. 13 & 14)

Dissolved silicon is depleted homogenously throughout the water column, whilst phosphate and nitrate decrease rapidly in the upper 3m but increase at the seabed with a maximum 0.33µM and 2.99µM respectively. Chlorophyll and the flurometer readings indicate increased phytoplankton below the thermocline at approximately 3.3m at 2.12mg/l. This pronounced thermocline is likely to be emphasised by the sunny conditions that allow for strong stratification to occur.


Station 7 (fig. 15 & 16)

Silicon at this station decreases rapidly in the surface layer, and remains in low concentrations down to 0.252µM compared to the riverine end members with an average value of 108µM. Phosphate concentrations appear relatively unaffected by chlorophyll concentration changes. As expected, nitrate varies inversely to the flurometer readings and chlorophyll concentrations. Dissolved oxygen decreases as the depth increases, due to organisms consuming oxygen throughout the water column to facilitate metabolic respiration. The thermocline seen at 5m depth correlates to an increase in chlorophyll concentration as nutrients from the sub-surface mixed layer can be utilised whilst light is not limited down to depth of 15m before chlorophyll concentrations deplete towards the bed.


Station 8 (Black rock)(fig. 17 & 18)

Dissolved silicon increases in concentration until 9.1m depth, where dissolved silicon peaks at 3.382µM at the base of the thermocline, then decreases to surface concentrations at 19m. Phosphate is depleted rapidly from 0.42µM at 1m, down to 0.12µM at 2m, and then increases with depth to values similar to those at the surface (0.40µM at 18.45m). Nitrate follows a similar pattern but is depleted from the water at approximately 18.45m depth, and covers the same range of concentrations seen at the northern end of the estuary.


Fig.1 ADCP transect across the Fal estuary at station 4 showing velocity in a longitudinal axis

Fig.2 ADCP transect showing backscatter at station 7  longitudinal axis

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Overall

Oxygen % saturation rates are fairly constant throughout the estuary, whilst other elements are depleted. Dissolved silicon decreases significantly between station 4 and 5, which could signify a diatom bloom between these stations. Nitrate levels are at their lowest mid-estuary, with concentrations appearing to increase around Black Rock. Chlorophyll levels also appear to remain constant, with a decline at Black rock to much lower values in the surface waters. Phosphate follows the same trend as silicon and nitrate, with the lowest values found in the mid-estuary, but these only increase slightly in water around Black Rock.

Mixing Diagrams

The mixing diagrams produced for silicon (fig. 19), phosphate (fig. 20) and nitrate (fig. 21) can be used to determine whether the mixing was behaving conservatively or non-conservatively within the Fal estuary.

The silicon mixing diagram shows a theoretical dilution line supporting a higher concentration of dissolved silicon towards the riverine end member, as would be expected. However the water that was sampled only decreased to a salinity of 32.8 and therefore the mixing diagram cannot be used to decipher whether the dissolved silicon was acting conservatively or non-conservatively because there were no concentrations to represent the fresher water.  This is because the Fal estuary is a Ria and therefore the freshwater inlet is small so the estuary is dominated by higher salinity waters. The mixing diagrams produced for phosphate and nitrate face similar problems in that no samples were collected at salinities lower than 32.7. This meant that the behaviour of both constituents cannot be described in terms of the degree of mixing. However, the samples that were collected for phosphate, may suggest that the nutrient is behaving non-conservatively because the values do not plot along the theoretical dilution line, suggesting that there is an addition of phosphate towards the seawater end member,  however due to lack of lower salinity samples, the behaviour of these constituents cannot be determined accurately.

Biological analysis

Phytoplankton:

Chaetoceros increases from station 4 to 7 (fig. 22) with a peak of 1120 cells ml-1 at station 7, however, there is an exceptionally low cell count at station 8 (10ml-1), which may be due to equipment or human error, or the region may have an abnormally low Chaetoceros population relative to the surrounding area. The Nitzschia population appears to fluctuate between stations; whilst no Nitzschia were observed at stations 1 and 7, it dominates station 4 with 1590 cells ml-1. The considerable variation could be due to the sampling method which does not reflect the average distribution in the water column, as only 1 measurement was carried out per station. Coscinodiscus appears to be relatively low across all stations, with 2 stations (1 and 7) recording a cell count of 0ml-1. The maximum cell count is at station 6 with 110ml-1, suggesting Coscinodiscus is possibly less suited to estuarine conditions present in the Fal estuary than Nitzschia and Chaetoceros. The total population of “Other” species remains relatively low across all the stations, with a slight exception at station 1 where a cell count of 230ml-1 is observed. This suggests that the 3 species of Chaetoceros, Nitzschia and Coscinodiscus account for the majority of all phytoplankton recorded by our sampling in the estuary.


Zooplankton

Adult copepods and nauplii made up the vast majority of the zooplankton community at Station 1 (fig. 23) and 5 (fig. 24). Station 8 (Black rock, fig. 25) is notable in that no nauplii were recorded, which is surprising due to the high abundance of adult life stages. Polychaete larvae were found at all three stations, although the highest numbers were taken from Black rock (314 ind. m -3). The remaining zooplankton populations at each site described as ‘other’ include fish larvae, siphonophores, ctenophores, echinoderm larvae, chaetognaths, gastropod and decapod larvae and mysidacea.


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Limitations

There are several limits associated with the estuary practical that we undertook. A major limitation was that we split the sampling between groups 3 and 5. Inconsistencies with the sampling methods that were undertaken could have influenced results that we obtained, alongside a significant time delay where groups changed over. Limited bottle sampling also means that at all stations phytoplankton was sampled only once, and the zooplankton was only trawled at 3 stations - conclusions about how these changed throughout the estuary can be accumulated, but how they changed at the individual stations was not investigated. Sampling on one day, at one tide means that locations and types of organisms found may have been inconsistent with other groups as this is likely to vary with the state of the tide – and with some stations taken before and other after high water this is likely to affect our results. Our lowest salinity that we were able to sample was 32.5, meaning that our estuary mixing diagram significantly lacks a transition zone of salinities due to the fact that the Fal estuary is tidally mixed.  Like the offshore data, there are drifting considerations to do with the CTD profile, although this is to a lesser extent due to a lower depth. CTD accuracy such as a jump in the salinity profile that was noticed at station 2, is always a concern too.

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