Diel vertical migration and tidal influences on plankton densities in dynamic coastal systems

Recent increased application of optical imaging devices have facilitated efficient capture of plankton abundance and community composition, enabling the study of plankton distribution in situ and at a high spatio-temporal resolution. In this study, we aim to investigate how the abundances and distribution patterns of plankton taxa relate over 24-h periods, covering tidal and diel cycles, in the southern North Sea using data from a WP2 net and a Video Plankton Recorder. In the highly dynamic southern North Sea, we document diel vertical migration patterns in the pelagic zone of both pelagic and hyperbenthic taxa, including Calanoida (Copepoda), Amphipoda, Annelida, and Cumacea. In addition, the densities of plankton taxa showed significant small-scale geographical variation over a 24-h period for which tidal currents played an important role, a source of considerable variation that is typically not accounted for. This study adds to the current understanding of plankton distribution and behaviour, particularly in the context of coastal areas characterised by strong tidal cycles and currents, by using in situ imaging techniques.