On the 21st June 2016, the Callista was taken off shore with the aim of investigating the stratification of the water column and types of plankton associated with the water column along a spatial transect. High tide was at 05:03 UTC and low tide was at 11:29 UTC, the weather was generally poor with 8/8 cloud cover and patches of rain.

The hypothesis investigated was - ‘The water column will become more stratified with distance offshore and the numbers of plankton will peak at the front between stratified and non-stratified water.’

A total of five sites were investigated, the positions of which can be seen in figure 1.

A CTD was mounted onto a rosette and used to determine the structure of the water column, in addition a fluorometer was mounted on the rosette to measure fluorescence. There were also six Niskin bottles attached to the rosette that were used to take samples from different depths. The CTD was lowered through the water column and the depths at which the Niskin bottles were to be triggered at were determined by looking at interesting features in the temp/ salinity profiles. The Niskin bottles were triggered on the upcast of the CTD remotely from the computer. Water samples were used to assess silicon, phosphate, nitrate, oxygen and phytoplankton concentrations.

A minibat was used to assess how temperature, salinity, fluorescence and density changed as distance offshore increased. The area that the minibat was towed over can be seen in figure 1 and is represented by the red line. The minibat was deployed from the back of the Callista and the laptop connected to it was used to monitor its progress throughout the water column. It was set to undulate between the surface and 40m depth.

An ADCP was used to assess the speed and direction of the flow between stations.

A zooplankton net was used to collect three zooplankton samples from different sites. The depths at which the samples were taken were determined by looking at the CTD profile for interesting areas in the water column. The zooplankton net was 0.5m in diameter and had a 100µm mesh size and was deployed vertically in the water column rather than being towed behind the boat.

For procedures used in the lab, please see here.

Methodology

The temperature and salinity depth profiles show increasing stratification with distance offshore, set up by increased irradiance from spring into summer. In shallow water tidal mixing occurs so the water column is more homogenous. The onset of stratification occurs progressively, moving from shallow to deeper water offshore, where tidal mixing dissipates.

The minibat data lends support to the theory that stratification increases as distance offshore increases. It shows an increasing divergence between the surface water and deep water. It also shows a decrease of the thermocline depth and the movement of phytoplankton (measured as fluorescence) further up the water column- with the thermocline. There is an interesting feature at 2.5km off of station 5 where the temperature and density appear to change. This may be due to localised mixing in that area.

Using the density data collected by the CTD downcasts it was possible to make simplified calculations to model the strength of stratification at each station. Potential energy anomaly was derived by assuming that the water column was split into defined segments; for instance a surface and a mixed layer. The graph below describes an increase in potential energy anomaly (J m-3) with distance offshore, this is indicative of more turbulent energy required to return the water column to a fully mixed state and therefore supports an increase in stratification moving away from the estuary mouth out to sea. It should be reiterated that these calculations are simplified and assume the CTD downcast moved and recorded at a constant speed.

The fluorometry profiles for stations 5-8 all show a peak in fluorescence at various depths in the water column. Blooms of phytoplankton happen at depth, rather than at the surface because surface waters- particularly offshore- are stripped of nutrients during the spring bloom and are not replenished due to stratification. Upwelling of nutrients occurs at the thermocline boundary between the different water masses in stratified water columns hence why phytoplankton bloom in these areas. As distance offshore increases the depth at which the peak occurs decreases. This could be due to the shallowing of the thermocline (as seen in the temp/ salinity profiles). No specific values can be taken from the fluorometry data because it has not been calibrated to in situ chlorophyll measurements.   

The nutrient profile for silicon shows that for all sites (bar site 4 since only one sample was taken and site 6 since no samples were taken) there is an overall increase in concentration with depth. All sites show low silicon concentrations in surface waters. This could be due to silicon depletion in the upper water column from the diatoms in the spring bloom; the stratification of the water column would have prevented nutrient replenishment. Silicon at all sites shows a decrease in concentration at around 20m this could be due to a small phytoplankton bloom containing diatoms occurring in that area2.

The nutrient profile for phosphate shows that for sites 7 and 8 there is a general increase in phosphate concentration with increasing depth. Site 5 shows an increase in concentration then experiences a large decrease at 30m. All sites show sudden decreases at depths corresponding with peaks in fluorescence. This is due to the phytoplankton using up the phosphate.

The nutrient profile for nitrate shows that for all sites there is a decrease in the concentration of nitrate from the surface value. The depth at which the minimum value occurs corresponds with the depth at which the peak in fluorescence occurs. This is due to the phytoplankton using up the nitrate that is supplied by the upwelling at the thermocline.    

Interestingly the nitrate and phosphate concentrations for the surface values at sites 7 and 8 are unusually high. They should be very low in concentration from being stripped from the water column during the spring bloom. The high concentrations may be due to little phytoplankton activity offshore during the spring bloom or remineralisation of nutrients by zooplankton.

The graphs of phytoplankton abundance show that abundance at sites 4, 5 and 8 are relatively low. This may be due to predation from zooplankton. Station 7 shows a very high abundance of phytoplankton although the diversity is very low, one dominant type with two other species present. Station 8 also has a low species diversity this may be due to selective grazing by copepod, while site 5 has a very high diversity of species. The zooplankton plot shows that station 8 has both the highest abundance and highest species diversity of zooplankton. This may be a result of heavy grazing on the phytoplankton populations at the station causing a zooplankton bloom. Station 5 has the lowest species diversity and lowest abundance of zooplankton. Station 7 has a moderately high population of zooplankton, which may be a result of the high phytoplankton populations at this site. The presence of the zooplankton may also be why the diversity of phytoplankton is very low at this site, they may be selectively grazing on certain species and leaving others1.  

The oxygen saturation profiles for all stations show an increase in saturation at the depth where there is a peak in fluorescence. This is due to the phytoplankton producing oxygen from photosynthesis. There tend s to be a general decrease in oxygen saturation below these depths. This is because of respiration from zooplankton or heterotrophic bacteria that are re-mineralising the phytoplankton once they die.

References

[1] Cowles, T. J., 1979, ‘The feeding response of copepods from the Peru upwelling system: food size selection.’, Journal of Marine Research, 13, 601-622.

[2] Hecky, R.E., Kilham, P., 1988, ‘Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence
on the effects of enrichment’, Limnology and Oceanography, 33, 4 (Part 2), 796-822.

Introduction

Results & Discussion