Long-term studies on Noctiluca scintillans in the German Bight population dynamics and red tide phenomena 1968–1988

The seasonal development of Noctiluca scintillans has been studied by continuous monitoring at Helgoland-Roads over a period of 20 years. These long-term studies have revealed a temporal distribution pattern of seasonal periodicity with maximal abundances in June/July, and with a low but pronounced peak in December/January. Additionally, over a 21-year term a periodicity of high abundances in three years intervals is observed. In Helgoland waters the mean near surface abundances are higher than the near sea bed values. Apart from physico-chemical influences, buoyancy in Noctiluca is also controlled by its nutritional status. Since 1981, investigations have been extended to the German Bight. Potential centres of Noctiluca growth are found within certain areas near the east-frisian Wadden Sea, persisting from April/May until late July when the centres of high cell concentrations are separated from the coastline. Simultaneously, there is a strong tendency for vertical separation, generating the well-known red-tide phenomena with large mass aggregations at the sea surface, often misleadingly called Noctiluca-blooms, although the cell masses are irreversibly damaged and die off very rapidly. The spatial distribution of Noctiluca as determined every year in July, serves as a realistic base for biomass calculations. From 1981 to 1988 the total per season produced biomass (ashfree dry-weight) of Noctiluca ranges between 30000 (1981) and 69000 tons (1982).