Latitudinal gradients in zooplankton communities in Norwegian fjords resolved by an integrated morphological and molecular approach

Abstract Fjords are coastal habitats that are often partially isolated from surrounding shelf waters and can contain ecologically unique and diverse ecosystems. Here, we offer a comprehensive overview of zooplankton communities at the end of the productive season across 34 fjord locations along the Norwegian west coast, ranging from 62 to 69°N latitude. We applied an integrated methodological approach that included traditional microscopy, FlowCam image analysis, community DNA metabarcoding, and bulk size-fractionated biomass measurements. Together, these combined data revealed distinct geographical patterns in zooplankton quantity and community composition. Water temperature, which was closely correlated to latitude, and bottom depth were the most important physical parameters driving zooplankton biomass, abundance, community structure, and size distribution. Multivariate analysis of species composition using both microscopy and metabarcoding-derived data identified three distinct assemblages that were strongly correlated to temperature, latitude, and bottom depth. Our comparison of the applied methodologies demonstrated differential strengths and limitations of these methods as monitoring tools for capturing zooplankton community dynamics. Our study underscores the need for continued, multifaceted biological surveys that can help inform effective ecosystem management and conservation strategies in response to climate-related and anthropogenic pressures.