RELATIONSHIPS BETWEEN BENTHIC ACTIVITY AND THE ANNUAL PHYTOPIGMENT CYCLE IN NEAR-BOTTOM WATER AND SEDIMENTS IN THE SOUTHERN NORTH-SEA

In this study, the annual cycle of phytoplankton input as a food sourc e for benthic organisms into the sediments of the southern North Sea i s described. Three stations across a frontal zone in the southern Nort h Sea were visited in six different months in 1993. Surface, near-bott om water and sediment samples were taken in order to describe pelagic phytopigment concentrations and composition and to quantify and qualif y phytopigment input in sediments. Further, sediment oxygen demand and growth of a deposit-feeding bivalve were determined in order to asses s the benthic reaction to this phytopigment input. Although clear diff erences between the stations were observed, the 'classic' algal succes sion was found: diatoms were present in late winter/early spring (Febr uary/March) followed by a typical spring bloom (May) of Phaeocystis sp ., and lower chlorophyll a concentrations during late summer and autum n. Phytopigment composition changed noticeably in summer and autumn. A lgal families were dominated by prasinophytes south of the front and d inoflagellates north of the front. The highest overall input of algal matter into the sediment occurred at. the station situated at the fron tal zone. It is argued that the phytopigment signature originates main ly from 'local' primary production. While bivalve growth was highest d uring the spring and early summer when the concentrations of chlorophy ll a were highest, bacterial activity increased with rising temperatur es during mid and late summer and autumn. This temporal uncoupling of macrofaunal and bacterial processes indicates that manobenthos rely mo re on 'locally' produced algal detritus and less on bacteria for growt h, while bacterial production can be stimulated by material transporte d over relatively larger distances. (C) 1998 Academic Press Limited.