PHYTOPLANKTON OF THE NORTH-SEA AND ITS DYNAMICS - A REVIEW

Phytoplankton is the major contributor to algal biomass and primary production of the North Sea, although crops of macroalgae can locally be up to 2000 g C.m-2 along the coast of the U.K. and Norway, and microphytobenthos dominates production in the shallow tidal flat areas bordering the coasts of England, the Netherlands, Germany and Denmark. Data collected since 1932 during the Continuous Plankton Recorder Survey show consistent patterns of geographical, seasonal and annual variation in the distribution of phytoplankton and its major taxonomic components. There is a trend of increased colouration in Recorder silks in the southern North Sea until approximately 1975 since when Colour levels (assumed to be indicative of algal biomass) have declined. In the eutrophic Dutch Wadden Sea the algal crop continued to increase; in Dutch coastal North Sea waters a trend of biomass increase reversed since 1984, apparently due to a reduction in Rhine river outflow. Long-term observations made at Helgoland since the 60's also show trends of increasing nutrients and phytoplankton biomass only to 1984. Adverse effects such as deoxygenation, foam formation and toxin production have been linked to mass concentrations of algae known as blooms. There is no evidence from existing reports for an increase in their frequency, although some years stand out with larger numbers. Occurrence of blooms can partly be explained by hydrographic conditions. More than 30 taxa are recognised as occurring in bloom proportions in the North Sea, approximately one third of which can be toxic. The crop of Bacillariophyceae (diatoms) is not likely to increase with eutrophication due to silicate limitation. An extensive subsurface maximum of armoured dinoflagellates, its abundance governed by hydrographic conditions, is the most characteristic feature of the central and northern North Sea in the summer months. Abundance, sometimes dominance, of picoplankton and of species that are not readily detected by microscopic observations has been documented by measurements of taxon-specific pigments such as chlorophyll b (green algae), alloxanthin (Cryptophyceae) and 19' - hexanoyloxyfucoxanthin (Prymnesiophyceae or Haptophyceae). Analysis of time series of satellite images is a promising way to assess in a quantitative and, more important, synoptic way the patchy distribution of phytoplankton over large regions. Growth processes of the phytoplankton respond according to variables amenable to such satellite remote sensing. Empirical and theoretical relationships that can be established between chlorophyll a, C-14 uptake, turbidity, stratification, suspended sediment type, irradiance and temperature in some well-investigated areas make remote sensing a potential tool to obtain reliable estimates of primary production in the whole North Sea. The C-14 method for estimates of the rate of algal growth processes appears to agree reasonably well with other methods, both involving incubation of samples and in situ measurements of temporal changes of oxygen and pH. The level of net primary production is 250 g C.m-2.a-1 in the central North Sea, 150 to 200 g C.m-2.a-1 in the northern North Sea, and 200 g in the South. The main metabolic processes involved in phytoplankton growth have been modelled mathematically in terms of the most important controlling environmental parameters. Such parameters comprise not only those of a chemical signature (micro- and macronutrients, both inorganic and organic) but also physical effects of vertical mixing and sinking, and biological effects including allelopathic interactions, antibiotic excretions, vertical migration, and mortality due to grazing and parasitism. The balance between primary production and consumption of organic matter appears to vary both geographically and seasonally. The process of regeneration of primary products both in the water column and in and near the bottom seems to be of major importance. Future research should center around a study of growth-controlling parameters in laboratory culture experiments. The studies should include uptake of dissolved organic compounds by all taxonomic groups, including pico- and nanophytoplankton, and all aspects of ecosystem structure and function involving the relation between algae and microheterotrophs making up the small food web. There is a need to synthesize existing information on phytoplankton in the North Sea and the factors governing its growth, such as nutrients, river input and stratification intensity. Complicated inter-relationships and successional patterns between individual species which are limited by varying physiological requirements and adaptation to differing hydrographic regimes reemphasizes the importance of species identification in phytoplankton studies. Many future problems in phytoplankton research will not be resolved without accurate identification of algal species. Taxonomic expertise takes many years to acquire; there is at present a shortage of skills in this area and more resources should be turned towards training and long-term support