Connectivity and larval drift across marine protected areas in the German bight, North Sea: Necessity of stepping stones

This study investigated the transport of European flat oyster (Ostrea edulis) larvae between two Marine Protected Areas in the North Sea: Borkum Reef Ground (BRG), where oysters were recently reintroduced, and Sylt Outer Reef (SOR). Additionally, we determined the source of oyster genetic material collected during cruises in 2022 between BRG and SOR. To achieve these goals, numerical simulations focusing on surface and depth-averaged water mass transport were conducted using the FESOM-C coastal ocean model with a forward/backward Lagrangian module. Surface drifter data were also analysed to examine surface transport and validate the model output. Our results indicate that typical summer wind conditions, along with tidal residual currents, support the transport of water masses and passive tracers from BRG to SOR. Surface water masses from BRG generally approach SOR from the south and west. However, BRG and SOR are usually connected over periods exceeding two weeks, even considering the fastest surface currents. Strong and persistent south-westerly winds, which are uncommon in summer, can accelerate this connection to under two weeks. Conversely, strong and persistent easterly or south-easterly winds, also rare in summer, can prevent some passive tracers originating from BRG from ever reaching SOR or the eastern North Sea. In the case of depth-averaged transport, significantly more time is required, with a minimum duration of eleven weeks to connect the domains. This connection could be facilitated by an intermediary habitat - as a stepping stone in the transition zone, if that provides suitable habitat for settlement and subsequent larval production.