Gymnodinium blooms in the Helgoland bight (North Sea) during August, 1968

Abstract

1. As in other parts of the North Sea, dinoflagellate red tides occurred in Helgoland waters in August, 1968. Measurements of plankton, and physical and chemical water properties at the permanent station "Helgoland Roads" were analyzed to describe the blooms. In addition, planktological and hydrographical investigations at three areas south, southwest and northwest of Helgoland on 27 and 28 August, as well as at two drifting stations off the mouths of the Elbe and Eider rivers on 6 and 8 August, were used for this work. 2. Gymnodinium sp. was abundant at all these localities, forming blooms near Helgoland from 14 to 30 August, with a maximum of 3 to 3.25 × 106 cells/l (and 18 to 19 μg chlorophyll a/l) on 28 and 30 August at Helgoland Roads. The primary production was as high as 0.98 mg C/l in 6 hours in a suspension of 3.7 × 106Gymnodinium/l, where diatoms had been removed. This means that one million Gymnodinium produced 0.265 mg C in 6 hours. At Helgoland Roads nitrate and nitrite were depleted at times, but not phosphate. 3. About 3 × 106Gymnodinium sp./l were found in the upper 16.5 m of water investigated around Helgoland on 27 and 28 August (maximal 7.8 × 106 cells/l at 3 m). There was a marked vertical stratification of Gymnodinium with a concentration towards the surface during the day. This was particularly the case in the turbid water off the Elbe estuary, where numbers up to 0.3 × 106/l were counted. Gymnodinium sp. formed 96 to 99 % of the phytoplankton biomass during maximal development. The chlorophyll a content of one million Gymnodinium was only 3.5 μg. An extinktion of E=0.083/1 m was measured in a suspension of 106Gymnodinium/l in samples with minimal other plankton and detritus. 4. The water masses containing Gymnodinium blooms, which reached Helgoland after a change of wind direction, were characterized by lower salinity and higher temperature. This indicates that blooms developed in the coastally influenced water masses east of Helgoland. The red tides occurred during a period of minimal discharge of Elbe river water and of relatively high salinity of the coastal water in the Inner German Bight. They developed after a long period of calm winds. There were no records of reported fish or shellfish poisoning. © 1971 Biologischen Anstalt Helgoland.