Relationships between Benthic Activity and the Annual Phytopigment Cycle in Near-bottom Water and Sediments in the Southern North Sea

Abstract

In this study, the annual cycle of phytoplankton input as a food source for benthic organisms into the sediments of the southern North Sea is described. Three stations across a frontal zone in the southern North Sea were visited in six different months in 1993. Surface, near-bottom water and sediment samples were taken in order to describe pelagic phytopigment concentrations and composition and to quantify and qualify phytopigment input in sediments. Further, sediment oxygen demand and growth of a deposit-feeding bivalve were determined in order to assess the benthic reaction to this phytopigment input.Although clear differences between the stations were observed, the ‘classic’ algal succession was found: diatoms were present in late winter/early spring (February/March) followed by a typical spring bloom (May) ofPhaeocystissp., and lower chlorophyllaconcentrations during late summer and autumn. Phytopigment composition changed noticeably in summer and autumn. Algal families were dominated by prasinophytes south of the front and dinoflagellates north of the front. The highest overall input of algal matter into the sediment occurred at the station situated at the frontal zone. It is argued that the phytopigment signature originates mainly from ‘local’ primary production.While bivalve growth was highest during the spring and early summer when the concentrations of chlorophyllawere highest, bacterial activity increased with rising temperatures during mid and late summer and autumn. This temporal uncoupling of macrofaunal and bacterial processes indicates that macrobenthos rely more on ‘locally’ produced algal detritus and less on bacteria for growth, while bacterial production can be stimulated by material transported over relatively larger distances.