BENTHIC COMMUNITY STRUCTURES IN THE NORTH-SEA

Coherent assemblages of marine benthic species have been recognized fr om the early twentieth century, and the classical papers of Petersen ( 1914, 1918) were based on studies of limited areas in the North Sea. I n 1986, a synoptic survey of the North Sea north to 57 degrees N was u ndertaken by a group of ten laboratories from seven North Sea countrie s. The results of this survey have recently been published (Heip et al ., 1992a, b; Kunitzer et al., 1992; Huys et al., 1992), and some of th e results are summarized in this paper. The analysis of the macrofauna is based on slightly more than 700 taxa. In general, the North Sea ma crofauna consists of northern species extending south to the northern margins of the Dogger Bank, and southern species extending north to th e 100 m depth line. The central North Sea is an area of overlap of sou thern and northern species, especially around the 70 m depth contour. Consistent groupings of species are recognized that were summarized in seven faunal groupings. Macrofaunal body weight, density and diversit y increase Linearly towards the north. Macrofaunal biomass for the who le area averages 7 g adwt. m(-2) and decreases from south to north. Di stribution patterns and trends within the meiofauna were very differen t. Nematodes, which are the dominant taxon overall, are least abundant in the sandy sediments of the Southern Eight, then increase to a maxi mum around 53 degrees 30' N and slowly decrease again towards the nort h. Copepod density and diversity are highest in the Southern Eight, du e to the presence of many interstitial species. A large number of spec ies new to science were recorded by the North Sea Benthos Survey and a bout 1500 species are expected to occur. Copepods show very distinct a ssemblages according to water depth and sediment type. The contrasting patterns in latitudinal gradients of body weight and number of specie s of macro- and meiofauna can be only partially explained. Latitude an d sediment characteristics, such as grain size and content in plant pi gments, and water depth, determine part of the variance in species com position, density and biomass of the benthic fauna, but the patterns t hat are observed are different for different benthic groups, requiring careful consideration as to their use in biological monitoring proced ures. Distributions are related to current patterns in the North Sea, annual temperature variations and availability of food. However, large parts of the variance in many parameters remain unexplained.