Community structure and diversity changes for fish in a temperate tidal lagoon, as a response to changes in water temperature and depth

Coastal marine ecosystems are characterized by high productivity and biodiversity supporting coastal fisheries for centuries. These ecosystems have undergone changes over the last few decades, experiencing shifts in seasonal patterns due to climate change. As a case study for responses of cold temperate coastal ecosystems to climate change, changes in fish diversity in the Sylt-Rømø Bight (SRB), northern Wadden Sea; 54°58’40”N, 8°29’45”E, were analyzed using data from the monthly monitoring from 2007 to 2019. Results showed that the diversity changes correlated to seasonal changes in water temperature. The spatial distribution of fish to intertidal areas for feeding and refuge was correlated to changes in water depth. Rank abundance curves (RACs) showed that a few species dominated the fish community and this changed per season and habitat type. General Additive Models (GAMs) showed higher species richness ( S ) at 5°C and 15°C, which are seasonal transition phases for winter/spring and summer/autumn, respectively. Evenness (J) and Shannon-Wiener Index (H) decreased with increasing water temperatures in the benthic and pelagic habitats while dominance (D) increased. Generalized linear mixed-effects models (GLMMs) showed that S decreased while J increased with water depth in benthic habitats. Similar patterns were observed in the nearshore pelagic habitats contrary to the deep tidal channels. There were no significant effects of water depth on H. The diversity changes reveal the sensitivity of fish to seasonal changes in oceanographic processes and the use of intertidal habitats. Thus, the significance of shallow coastal habitats for fish needs implementation in conservation and management measures.