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Method and Equipment
Silicate was filtered and stored in plastic bottles rather than glass, to prevent contamination. The samples were then stored in the fridge to prevent further degradation. These samples were then analysed in the onshore labs following the method as described by Mullin and Riley[4], using a Hitachi spectrophotometer to measure the absorbance.
Both phosphate and nitrate were filtered and stored in glass bottles before being also stored in the fridge to preserve the samples. Phosphate was then analysed in the onshore labs using the method as described by Murphy and Riley[5], again using the Hitachi spectrophotometer to measure the absorbance. Nitrate was analysed in the labs using a very similar method as outlined by Johnson and Petty[6], using a Spectometer Unicam to measure absorbance.
The dissolved oxygen was sampled and immediately had 1ml manganese chloride and also of alkaline iodide added, before being stored into a container of water to keep the temperature constant and prevent an exchange of oxygen with the atmosphere. These samples were taken to the lab and analysed using the methods outlined by Grasshoff et al.[7], using a titrator device.
Results
Silicate
The main silicate sources in the ocean are riverine inputs and upwelling of nutrient rich bottom waters. Some types of phytoplankton, especially diatoms, integrate silicate in their frustules. Therefore, it often is a limiting nutrient for the growth of these organisms. All stations showed a low silicate concentration between 0.1 and 0.2 umol/L in the surface waters and increasing concentration with depth. Station 3, which is furthest offshore, had the highest increase up to a concentration of 1.4 umol/L at 60 m depth.
Nitrate
Close to agglomerations and agriculture, nitrate enters the coastal waters via runoff into rivers. Additionally, nitrogen fixing bacteria such as cyanobacteria make nitrate available to other organisms. The nitrate concentration at all stations is close to zero in the surface waters and increase with depth, showing the remineralisation processes of particulate organic matter by microorganisms. The variation with depth at the three stations are similar to the changes seen in the silicate above. Station 2, however, shows an extraordinary high nitrate concentration at 40 m depth.
Phosphate
Phosphate concentration is constantly the lowest in the surface waters at each station due to its use in photosynthetic and cellular processes by biota. There is an increase in concentration with depth at all stations, which is because of the remineralisation with depth as this releases phosphate from organisms back into the water column. The largest increases in concentration with depth occurred at stations further out to sea, which is not expected as the areas closer to land typically have more anthropogenic sources of phosphate, therefore usually resulting in higher concentrations. At station 3, the phosphate decreases below the deep chlorophyll maximum, suggesting stratification. Looking at data from the L4 dataset, phosphate is currently at its lowest concentration for the yearly cycle [8].
Dissolved Oxygen
In surface waters the oxygen concentration is in equilibrium with the atmospheric oxygen concentration and, in most cases, shows a 100% oxygen saturation according to temperature and density of the water body. In deeper waters no gas exchange can take place and the oxygen levels can vary according to the activity of primary production and microbial respiration. The oxygen saturation at all stations is close to 100% in the surface waters due to the gas exchange with the atmosphere. The saturation decreases with depth to levels of around 92% in the bottom waters indicating that the respiration is higher than the photosynthesis. Station 1 showed oversaturated surface waters with an oxygen saturation of 103%. This can be due to high photosynthesis rates or bubble entrainment.
The views expressed here are not representative of the University of Southampton.