Halocline as an environmental barrier for the larvae of Arctic marine benthic invertebrates

• Halocline acts as a barrier, structuring meroplankton assemblages. • Salinity and chlorophyll a explained nearly 50% of meroplankton variation. • Echinodermata larvae dominated above, Polychaeta larvae below the halocline. • Highly abundant Bivalvia veligers, indicated local stratification and accumulation. Many marine benthic invertebrates possess a pelagic larval life stage (meroplankton) that is highly sensitive to environmental changes, influencing their spatial distribution, survival, and settlement. This study was conducted on the adjacent shelf and in Kongsfjorden, a high-Arctic fjord system in Svalbard, where the Kongsfjorden Transect (KT), influenced by Arctic and Atlantic oceanic regimes, shows strong variability in physico-chemical conditions. Consequently, changes in environmental conditions, including the depth of the halocline, can act as a physical barrier for organisms, affecting local biodiversity. We, investigated meroplankton vertical distribution in relation to the halocline, and hypothesized that the salinity gradient acts as a barrier for larvae, influencing their movements, distribution and diversity. We identified 25 meroplankton morphotypes along the KT, with Bivalvia veligers as the dominant taxon. Taxonomic assemblages differed significantly between above (AH) and below (BH) the halocline. More abundant Echinodermata larvae characterized the AH assemblage, which remained compositionally similar throughout the studied transect. In contrast, Polychaeta larvae were characteristic of the BH assemblage with greater spatial variability, consistent with decreasing temperature, salinity, and chlorophyll a concentration, towards the inner part of the fjord. By integrating detailed morphotype analysis with hydrographic data, we showed that the halocline is a barrier for meroplankton larvae, defining their vertical distribution, abundance, and taxonomic composition; we also present the indicator taxa of the AH and BH assemblages. Our results emphasize the role of the salinity gradient in shaping larval communities and support their potential role as early indicators of environmental change in the Arctic fjords.