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Estuaries are highly biologically active zones lying between freshwater and marine systems. The classical view is that materials such as nitrates and phosphates which run into rivers as a result of man’s activity are used by the planktonic algae, or phytoplankton, for growth – in some cases causing nuisance blooms of these organisms. The management of the reduction of these blooms is based on the classical assumption that the materials stimulating them are brought into the estuary by the river, and that effective control of the blooms can be achieved by setting limits on the initial discharge of these materials into rivers. Funded under the EU INTERREG II (Ireland-Wales) programme, two groups of marine scientists from the University of Wales, Bangor and the National University of Ireland, Galway made a co-operative study of the Waterford (Ireland) and Conwy (Wales) estuaries. It was found that whereas the source of nitrogen for the estuarine phytoplankton was from the rivers, the main source of phosphate was from the sea. Phytoplankton blooms were being encouraged within the plume zone near the mouth of the estuaries, a region poised between a nitrate-rich freshwater and, relatively, phosphate-rich seawater. The management consequences of the findings are profound. Phosphates contribute significantly to the pollution of rivers and lakes, systems where there is usually an abundance of nitrogen and algal growth is governed by the availability of phosphorus. Management of these freshwater systems is thus achieved through control of the input of phosphates. Results achieved during the present study show that this criterion does not apply to estuaries and estuarine blooms, as material (phosphate) supporting them comes from the seawater end of the system and is therefore obviously unmanageable. The requirement to control nitrogen (nitrate) levels in estuaries is therefore all the more important in order to properly manage phytoplankton blooms, and thus water quality, in estuaries., Funder: European Union

The paper examines the conventional notion of the food chain of the oceans in the light of field studies of microheterotrophic processes. In common with the earlier observations of POMEROY (1974), the conclusion is that the classical paradigm is not compatible with contemporary observations and appears to need extending in order to take them into account. The evidence would seem to point towards at least half of primary production passing through the planktonic microheterotrophs before it is mineralized. The possible routes of flow of organic material from the classical food chain (phytoplankton excretion, losses during grazing and zooplankton excretion) were examined in some detail. The conclusions were that the cumulative production of dissolved organic material from the above sources, estimated to amount to about 60% of primary production, are sufficient to sustain the anticipated rates of microheterotrophic activity. These considerations, by themelves, give no reason to seriously doubt the accuracy of contemporary measurements of primary production. It was calculated, given the present day estimates of microbial growth yields, that secondary production at the microbial level may be comparable to or greater than that of herbivorous zooplankton. When considering the sources of supply of organic material for the microheterotrophs, the events occurring prior to herbivore ingestion were found to be more important than those subsequent to ingestion. As a consequence, the overall accuracy of the estimates of the supply of organic material to the microheterotrophs was very much dependent upon the assessment of total phytoplankton exudation of organic material, i.e. the measured excretion plus that taken up by heterotrophic micro-organisms during the measurement period. The review also highlighted the need for a better understanding of the fate(s) of microbial production: to what extent it is utilized directly by metazoan hervibores as opposed to passing through a protozoan food chain.