Exceptional Algal Blooms in Dutch North Sea Waters

Abstract

During airborne and sea-going surveys, phytoplankton composition, and dynamics as well as relevant factors involved, were studied in Dutch North Sea waters. The aim of the study is to increase our understanding of phytoplankton dynamics, the impacts of eutrophication and the measures that are taken to reduce the risk of adverse eutrophication effects. In the coastal zone during spring (April), the relative abundance of diatoms is small when compared with the dominant scum-forming species Phaeocystis pouchetii (71-96% of total). Some potentially toxic species, like Dinophysis acuminata. were present predominantly in late summer and autumn during calm weather conditions, in the coastal zone and northern offshore waters (Oyster grounds, Dogger Bank), but in lower concentrations in 1988 and 1989 when compared with 1976, 1979 and 1981. During these three years Dinophysisacuminata blooms gave rise to mussel infection (DSP) in the Dutch Wadden Sea. The potentially toxic species Gyrodinium aureolum showed higher abundance in somewhat deeper waterlayers than at the surface. The presence of the potentially toxic species Gonyaulax(Alexandrium)tamarensis was for the first time recorded in June 1989 in concentrations of ca. 2000 cells per litre. The results further indicate an increase in occurrence of the non-toxic species Noctiluca scintillans in Dutch North Sea waters. Airborne surveys from 1979 onwards demonstrated an annual occurrence of large-scale surface algal blooms with a total annual area coverage ranging from 30 km2 (in 1982) to 1860 km2 (in 1989). Surface algal blooms are predominantly occurring during May through August, and are likely to be promoted by calm sunny weather conditions, with windforce < 5 Bft over several days. Distinct “stratified” algal layers of some dominant species within an algal patch were observed, with Noctiluca in the top layer, and rather low oxygen concentrations below the large surface algal blooms. Since nutrient availability in Dutch coastal waters has remained rather constant and high over the last 10-15 years, the risks of occurrence of nuisance algal blooms and their adverse effects have remained unchanged.