Population dynamics of the spionid polychaete Pygospio elegans in Isefjord and Roskilde Fjord.

We sampled populations of Pygospio elegans at four locations (Lynæs, Lammefjord, Vellerup, Herslev) in the Isefjord - Roskilde Fjord estuary complex with approximately monthly intervals from March to November 2014. The goal of this study is to compare the population dynamics, with special focus on reproduction and development, for P. elegans populations exposed to differing environmental factors and that have, based on previous work, different genetic compositions. We wanted to examine to what extent environmental versus genetic factors determine the population dynamics and reproductive mode for this species. We determined the size distribution and the developmental and reproductive status and sex (when possible) of at least 30 individuals from each sampling. Temperature and salinity were continuously monitored by in situ loggers at the same sites. Sediment characteristics (particle size distribution, water content, organic content) were determined at 4 times (March, May, August, November). The major difference between the sites was that Herslev had notably lower (average of 12 ‰) and more variable salinity than the other 3 sites (average of 18 ‰). There were small spatial and temporal differences in the other sediment characteristics. During spring, all populations were reproductively active (Fig. 1), but to a greater extent at Vellerup and Herslev. Juveniles appeared at all sites during summer and again in the fall but numbers were reduced during late summer. All populations began to reproduce again during fall although to varying degrees. More detailed analyses of seasonal changes in worm size and reproductive mode and how this relates to environmental variables will be presented.