Group 2 Plymouth Field Course

ESTUARY

Station Locations

Salinity Gradients

Zooplankton

Phytoplankton

Fig 1 - Contour plot of Salinity with depth across a Salinity gradient in the Estuary

Fig 2 - Contour plot of Temperature with depth across a Salinity gradient in the Estuary

Fig 3 - Bar chart of different nutrient concentrations across a Salinity gradient in the Estuary overlaid with the surface chlorophyll data through the estuary.

Fig 4 - Pie charts of zooplankton diversity at different stations throughout the estuary; a=station 1 from the Winnie the Pooh, b=station 6 from the Winnie the Pooh, c=station 14 from the Winnie the Pooh, d=station D from the Falcon Spirit, e=station H from the Falcon Spirit and f=station I from the Falcon Spirit

Fig 5 - Pie charts of phytoplankton diversity through the Tamar Estuary. Stations 1, 6, 11 and 14 from the Winnie the Pooh and all stations from the Falcon Spirit. Click on the image to enlarge it.

Fig 6 A - Total Phytoplankton count at each station from the Falcon Spirit.

Fig 6 A - Total Phytoplankton count at each station from the Winnie the Pooh

For each Zooplankton sample taken, Copepods dominate the populations (fig 4). At station H30 the sample analysed found 168 Copepods and 196 Copepod naupliia per m3 of sea water (fig 4e). Station D32 had an estimated 54 Copepod individuals per m3 of sample (fig 4d). Station I38 had a much smaller density of zooplankton, with only an estimated 3 individuals per m3 of sample, 1 of which was found to be a Copepod (fig 4f.), compared to 523 individuals per m3 at Station H30. Station I 38 was by the Plymouth break water, where as H20 and D32 were further up the estuary. This difference in location could be the cause in the difference in zooplankton density. The Plymouth break water prevents mixing so surface waters are nutrient limited and with the high temperatures, stratification of the water column will be increased by strengthening the thermocline, preventing mixing. Further up the estuary the water will be more mixed, due to it nearing high tide when samples were taken, distributing nutrients in the water column. The nutrient TDL’s showed that as soon as the salinity reached 34/35 (off shore) nutrients fell out of the water column (see estuary TDLs section) , indicating there is less nutrients available so only smaller populations can be sustained. More predation could have occurred towards the sea rather than in the estuary due to only limited species being adapted to the fluctuating salinities. Species were less diverse at I38 compared to the estuary site. This area could be more susceptible to predation due to the sea and the river meeting. All had fish larvae, indicating the lower estuary could be a nursery ground for some species

Further up the estuary, Copepods still dominate the zooplankton population. Station 1 had 107 copepods per m3 of sample (fig 4a), station 6 had 847 copepods per m3 (fig 4b) and station 14 had 1675 copepods per m3 (Fig 4c). Decapod larvae were also found at each station. Species diversity was similar between stations, but zooplankton density varied greatly. Station 1, which was the highest station up the estuary, had a count of 143 individuals per m3 compared to station 14, which was further down the estuary, had a count of 1692 individuals per m3. This could be caused by predation by fresh water species. Further down the estuary, the fluctuating conditions create a harsh environment with few species enable to survive in these conditions, so perhaps less predation occurs here. The low phytoplankton levels further up the estuary will influence the number of zooplankton, due to being a food source.

Phytoplankton density varies across sample sites. The stations near the break water have higher population densities, with station J39 having 321 cells per ml compared to stations C34 having 13 cells per ml (fig 5). This is probably due to high turbidity occurring in the estuary compared to the sea, limiting the phytoplankton growth, even though there are high concentrations of nutrients in the estuary compared to near the breakwater (See TDLs). Grazing pressure by zooplankton will have also affected phytoplankton densities, with zooplankton populations greater towards the estuary than out by the break water. Diatoms dominated throughout the stations, with Chaetoceros counts reaching 162 individuals per ml of sample at station H30 (fig 5). Phytoplankton species were more diverse towards the lower estuary and break water (Fig 6) compared to the number of species found up the estuary. This may be because the pressure for nutrients is high in this area, enabling more species to dominate new niches. Previous research has found phytoplankton species richness declined with increased nutrient levels (Leibold, 1999).

Further up the estuary, phytoplankton density varied between sites once again. Station 1, the station furthest up the river, only had 4 cells per ml of sample, compared to station 14, further towards the estuary, with 628 cells per ml of sample (fig 6). This is probably due to increased turbidity up the estuary, it was noted when analysing station 1 samples, there was a lot of dirt in the sample, which would not be ideal for photosynthesis and growth of phytoplankton due to low light levels.  Diatoms dominated throughout the samples, with counts reaching as high as 140 cells per ml at station 14 (fig 5). Diatoms require less energy to form in comparison to other shell forming phytoplankton and so out-compete other species (Raven, 1983). Species diversity does not fluctuate greatly between stations indicating that competition and predation pressure does not vary from station to station.

References

Leibold., M., A., (1999). “ Biodiversity and nutrient enrichment in pond plankton communities”. Evol. Ecol., Res 1: 73-95

Raven, J. A. (1983). “The Transport and Function of Silicon in Plants”. Biol. Rev., 58. pp179-207.

Lui, H. and Chen, C. (2012). The nonlinear relationship between nutrient ratios and salinity in estuarine ecosystems: implications for management. Current Opinion in Environmental Sustainability, 4(2), p.1.

Morris, A., Bale, A. and Howland, R. (1981). Nutrient distributions in an estuary: Evidence of chemical precipitation of dissolved silicate and phosphate. Estuarine, Coastal and Shelf Science, 12(2), pp.209

Statham, P. (2012). Nutrients in estuaries — An overview and the potential impacts of climate change. Science of The Total Environment, 434, pp.213-227.

The views and opinions expressed are those of the individual and not representative of the University of Southampton or the National Oceanography Centre.

Temperature was plotted against salinity and depth. Salinity was used as a conservative measure of the river-estuary transition. As the stations were not evenly distributed, location could not be set as the x axis.  

The actual data points are indicated by squares on the contour graph. Therefore, the temperatures that fall away from these points have been generated by the software and do not represent samples, so cannot be assumed to be accurate. For example, where salinity was less than 16, the maximum depth was only 4m - but the graph has generated temperatures for up to 5m depth, and these points can be disregarded. Likewise, the graph shows intrusions of temperature layers between data points, such as the warm (23.5 ˚C) pocket at salinity 24 penetrating to 1.5m, despite having no data to prove this assumption.

Regardless, the contour graph is still indicative of the major trends, and is a useful tool in visualising this. It is clear that the warmer surface waters has been heated most extensively by the sun to up to 23.5˚C, whilst the deeper (beyond 3m), saltier (above 30 psu) water intruding from the ocean is much cooler, down to 19.5˚C

This contour graph shows the salinity changes in the vertical and horizontal directions for the Tamar estuary. Salinity is used one more time as the independent variable.

The rising graph represents a transect that describes a partially mixed estuary, where the river input is being subjugated by a tidal input. This is shown by the evenly distributed isohalines sloping towards the head of the river (where salinity is 0 psu); fresh, lighter water sits on top while salty water relies at the bottom creating a frictional gradient that turns into turbulence and eventually mixing, pushing the isohalines towards vertical profiles.

Observing a correlation with the temperature contour, the tidal input is clearly detected by its high salinity (32.6 psu) and low temperature. Likewise, the riverine endmember is spotted sitting at the first and highest station with a salinity of 6.8 psu.

Figure 3  shows surface concentrations of the nutrients silicon, phosphate, nitrite and nitrate across a transect of the Tamar Estuary, starting at a salinity of 7 near Calstock and becoming more saline up to the confluence of the rivers Tamar and Tavy.  Overall, there is generally decrease in the total nutrient concentration as salinity increases. The riverwater has a high nutrient load due to weathering of rocks in the river’s catchment, which are then removed along the estuary by processes such as biological uptake, flocculation and coagulation so by the time it reaches the estuary mouth, the water column is depleted in nutrients as seawater generally has a low nutrient concentration (Lui and Chen, 2012). In contrast, there is a sharp increase in phosphate concentration at the confluence at a salinity of 33, suggesting a new input of this nutrient (Morris, Bale and Howland, 1981). The dominant nutrient in this estuary is nitrate, which is expected as this is the more stable form of nitrogen in oxic conditions (Statham, 2012) and can often have high anthropogenic inputs due to processes such as agricultural runoff and sewage treatment ((Lui and Chen, 2012).

Figure 3 also shows surface concentration of the pigment chlorophyll with increasing salinity along the Tamar estuary. The trend shows an overall decrease, although this fluctuates greatly with a sharp increase at around salinity 16 – this corresponds with a sharp increase in nutrient concentration at this salinity shown . Chl a is often used as an indicator of presence of phytoplankton, suggesting that this increase may be due to an increase in essential nutrients available to the plankton. The overall decrease is likely to correspond to the depletion of nutrients with increasing salinity, as there are less available in the more saline water to facilitate phytoplankton blooms.