Particle exchange between coast and fjords and its biological implications

The exchange of water and particles between coastal and fjord areas is crucial for the overall health of fjord ecosystems. In this study, we use a Lagrangian ocean particle tracking model combined with zooplankton field data in the Hardangerfjord region of western Norway, to investigate advective exchange of particles between the coast and fjord and to provide insights on its biological implications. Our findings indicate that particles in the coastal region have a limited time-window to enter the fjord and an even lower chance of reaching the inner part, especially during spring compared to autumn, driven by seasonal variation of physical conditions at the coast. The low connectivity between the coastal area and the inner part of the fjord leads to the observed decreasing gradient in the abundance of drifting zooplankton with oceanic origins, such as Calanus spp., and a shift in zooplankton composition along the fjord axis. The vertical distribution of zooplankton also plays a critical role in the exchange of these organisms between the coast and fjord. Our study shows that the population of Calanus spp. in the Hardangerfjord in spring depends on advective inputs from coastal waters as well as local production inside the fjord. Despite the low chance of particles being advected into the fjord, the large number of individuals in the advective layer at the coast in spring serves as a significant source. In contrast, local production becomes more critical than advective inputs in autumn when Calanus spp. descend to deeper depths in coastal waters. Our study further suggests that the recolonization of northern shrimp in Hardangerfjord is likely to be a slow process due to the low connectivity between the coast and fjord. This finding calls for caution in coastal management regarding resident or local species in fjords which are more vulnerable and harder to restore than drifting species.