The development and evaluation of new technologies for the study of eutrophication in coastal waters

Abstract

Increased anthropogenic input of nutrients into the marine system can lead to eutrophication and can have a significant negative impact on some coastal marine ecosystems. The complicated biogeochemical and physical processes involved in the marine system make it difficult to monitor and predict future eutrophication events. I n order to understand the processes which cause these events, we need to be able to measure key in-situ biogeochemical parameters relevant to the phytoplankton such as nutrients, at a high temporal and spatial resolution. Traditional techniques only allow limited monitoring of nutrients and are therefore likely to miss short-term transients. The NAS-2EN nitrate analyser was field tested during the PROVESS experiment and the results demonstrate that the performance of the cadmium reduction column is erratic and full data sets were not collected. However, although there were stability problems with the cadmium reduction column, the nitrate results obtained when the column was stable was within the expected range of nitrate concentrations for the two sites studied in the North Sea. Also, the data sets showed that the instrument was capable of recording short term transient events. Bench experiments investigating the reduction capability of the wire cadmium reduction column used by the instrument, found that the expected reduction efficiencies of above 97% was not being achieved and demonstrated that the column was not only reducing the nitrate to nitrite but was further reducing the nitrite. A cadmium granular column and control macro were developed and the concentration of the chemical reagents adjusted. A stable reduction efficiency of almost 100% was achieved. Therefore, the new cadmium granular reduction column, control macro and chemicals were deployed in Loch Etive as part of the REES project carried out by Dunstaffnage Marine Laboratory. The on-board standard of the surface instrument was stable and measured over 900 samples with a coefficient of variation of 3.6%. Water ingress into a seabed instrument deployed at the seabed caused it to shut down after 16 days. With these instruments it was possible to monitor the nutrient fluxes caused by the renewal events in the Loch and estimate rates of denitrification in the bottom water. The new cadmium granular column and control macro and chemicals represent a significant improvement of the instrument's performance and it has been demonstrated that the NAS-2EN used with this new method is capable of acquiring large, reliable data sets which was not previously possible.

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