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The silicate concentrations in the Tamar Estuary show a non-conservative pattern with the riverine endmember having the highest concentration, and the sea water endmember showing the lowest concentration of silicate.


This reduction of silicate with increased salinity is due to the river acting as the source of silicate into the ocean. Removal is occurring throughout the estuary which coincides with the chlorophyll data; showing increased chlorophyll at high salinities indicating the presence of more phytoplankton. The removal could also indicate non-biological removal in the Tamar estuary (Morris, et al 1981) caused by the re-suspension of sediments leading to down-drawing of silicate into sediments.


The two graphs are a result of two different methods at collecting data. The auto analysed silicate concentrations were collected using a Quaatro 39 analyser, whereas the blue silicate concentration graph was collected manually.

The anomaly of the 5th sample at a salinity of 8.8 shows an AA silicate sample of 75.30 compared to the manually analysed silicate concentration at the same salinity of 42.55, this could be due to a sharp increase in silicate being added into the estuary, for example by an increase in agricultural run-off. Alternatively, it could be due to human error in group 8’s data when processing the data into excel, or a technical error from the auto-analyser.



Silicate

Phosphate

.The phosphate concentrations show non-conservative behavior, with additions throughout the estuary, at increased magnitude especially in the lower estuary.


The addition seen compared to the TDL is likely caused by phosphate run off in fertilizers found to be in high use in the rural, agriculturally dominated catchment region (Uncles et al., 2002).


A peak in phosphate at around a salinity of 30 is likely to be where the river Lynher joins the estuary bringing in phosphate rich freshwater with it, again from high agricultural run-off in its catchment region.


Similar to the silicon TDLs there is an anomaly at salinity 8.8, station A3 in the upper estuary, which is likely to have been caused by an error when using the Quaatro 39 auto analyser.


Nitrogen

The TDL for nitrate in the Tamar estuary shows mostly conservative behavior throughout the estuary apart from two anomalies at around 23 and 34 salinity. There is very slight negative deviation from the TDL at low salinities but nitrate as a whole is essentially conservative in the estuary, showing simple dilution by sweater at higher salinities (Uncles et al, 2002).


Nitrite displays very strong non-conservative behavior with all data points showing addition from the TDL. The trend of the nitrite data shows a negative correlation with decreasing nitrite concentrations with increasing salinity. The addition is very large in the upper estuary; being diluted with increasing salinity. This does not fit with the endmember sample collected however, this may be due to the distance between our recorded lowest salinity and where the riverine endmember was taken. In this large gap there has likely been a large addition of nitrite to the upper estuary causing such strongly non conservative behavior. Research by Morris et al. (1985), found a seasonal increase in nitrite addition to the middle-upper estuary which was caused by increased ammonia concentrations, variably distributed by tidal conditions. This leads to high levels of ammonia oxidation by bacteria, producing high concentrations of reduced N species, including nitrite, directly into the water column.


There is also evidence of a small addition at salinities around 28-31, in a similar location to the lower estuary addition of phosphate around the river Lyhner confluence. This suggests the Lyhner acts as a small source of addition of nitrite, as well as phosphate.  The concentration of nitrites in the estuary is only a fraction of the total N species with the majority being in nitrate form.



References

A. Morris, A. Bale, R. Howland, 1981. Nutrient distributions in the estuary: Evidence of chemical precipitation of dissolved silicate and phosphate. Estuarine, Coastal and Shelf Science. Volume 12, Issue 2, pp 205-216



A. Morris, R. Howland, E. Woodward, A. Bale, R. Mantoura, 1985. Nitrite and ammonium in the Tamar estuary. Netherlands Journal of Sea Research, Volume 19, Issue 3, pp 217-222



R. Uncles, A. Fraser, D. Butterfield, P. Johnes, T.Harrod, 2002. The predictions of nutrients into estuaries and their subsequent behavior: Application to the Tamar and comparison to the Tweed, U.K. Hydrobiologia, Volume 475, Issue 1, pp239-250



The silicate,

nitrogen and phosphate

samples were measured in the

lab manually and using a Quaatro

39 auto analyser and compared to a TDL

in order to show the behaviour of the nutrients throughout the estuary.

All opinions expressed are of our own, and not of the University of Southampton

Figure 1. Silicate change with increasing salinity from water samples in the Tamar estuary to the Plymouth Sound breakwater. Samples analysed using auto-analyser and physical measurement against a known standards curve. Theoretical dilution line plotted from riverine and seawater endmember samples. N = 31.


Figure 2. Phosphate change with increasing salinity from water samples in the Tamar estuary to the Plymouth Sound breakwater. Samples analysed with an auto-analyser. Theoretical dilution line plotted from riverine and seawater endmember samples. N =31.



Figure 3. Nitrite (top) and Nitrate (bottom) change with increasing salinity from water samples in the Tamar estuary to the Plymouth Sound breakwater. Samples analysed with an auto-analyser. Theoretical dilution line plotted from riverine and seawater endmember samples. N = 31.