Composition and dynamics in space and time of polychaete larvae in coastal waters of the North Sea
Polychaete meroplanktonic larvae from the List tidal basin were identified, their horizontal and vertical distribution patterns described, and temporal abundance fluctuations analyzed. Tides cause marked changes in the depth of this basin, while seasonality and weather dependence cause high environmental and biological variability. The taxonomic composition was analyzed from samplings undertaken between 1996-2002, during flood and ebb periods. Spionid, phyllodocid, and polynoid larvae are representative for this basin, accounting 60% of the total number of taxa (46 taxa). The larval production is numerically dominated by Pygospio elegans, Spio martinensis, Polydora cornuta and Lanice conchilega. Since their benthic stages are well known inside the basin, a large proportion of the larval production can be considered local. The low correspondence between benthic and planktonic assemblages suggested a qualitatively important input of long-distance larval vagrants. Sites within the basin differed in species composition, suggesting that larvae are not randomly mixed by the tidal hydrodynamics. Polychaete larvae are retained inside the basin, in spite of the continuous exchange of water with the North Sea. Analyses of the spatial distribution undertaken at three spatial scales during flood and ebb periods indicated that benthic occurrence, coastal morphology, and larval behavior could enhance species-specific distribution patterns. Significant fluctuations in total and partial larval abundances occurred between 1996-2001 were probably related to variables with influence over gonadal maturation and spawning in adult stages, as well as larval feeding and performances.