Growth trends in three bivalve species indicate climate forcing on the benthic ecosystem in the southeastern North Sea

Abstract

Internal growth lines in shells of 3 suspension-feeding bivalves (Arctica islandica, Mya truncata and Chamelea striatula) from the SE North Sea were analysed in search of common inter- annual variations of their growth rate. The high similarity (69 to 80%) between the growth records in the 3 species suggests that a large-scale (climatic) factor is responsible for an important part of the variation in annual shell growth. The common trend extracted from the individual growth records was correlated with monthly data on phytocolour, NAO index, temperature and wind. The strength and sign of the effect of the last 3 factors varied seasonally. Effects of wind strength and direction were analysed in detail, as in situ measurements showed that wind speeds >10 m s -1 (Beaufort 6) cause resuspension of fine sediment at the site where the bivalves were collected. Resuspended sed- iment causes a drop in the quality of food for suspension feeders. During the bivalve growing season, northwesterly to easterly winds tended to depress growth, whereas winds with a westerly component mainly had a positive effect. Strong winds from a WNW direction in May, however, had a negative effect and could explain the highest proportion of variance in the common growth trend; WNW winds have a long fetch at the study area and generate (high) long waves, facilitating resuspension of fine sediment at relatively low wind speeds.