Seasonality and hydroclimatic variability shape the functional and taxonomic diversity of nearshore fish communities in glacierized estuaries of Alaska

Nearshore fish communities in glacierized estuaries contend with environmental changes brought on by seasons and a shifting climate, which include alterations in freshwater runoff and environmental conditions shaped by the interplay of warming temperatures and receding glaciers. Spatial and temporal changes in environmental parameters can directly impact fish behavior and community structure, thereby affecting the dynamics of the entire ecosystem. Taxonomic diversity is commonly used to measure changes in communities, and while it offers important insights into community structure, considering the functional roles of organisms is necessary for understanding community dynamics through expressed traits and trophic interactions. Here, we evaluate the influence of environmental drivers on both taxonomic and functional diversity of fish communities at multiple sites in two glacially-influenced, high-latitude regions in the Gulf of Alaska (GoA): oceanic-influenced Kachemak Bay and the more typical estuarine Lynn Canal. Sites were analyzed monthly (April–September) for three years (2019, 2021, 2022) to address two questions: (1) Do taxonomic and functional diversity of nearshore fish communities show similar patterns of interannual and regional variation in glacially-influenced GoA estuaries? and (2) Do similar seasonal (i.e., monthly) and environmental (i.e., temperature, salinity, turbidity, freshwater discharge) drivers shape taxonomic and functional fish communities within these regions? Taxonomic and functional diversity were both significantly different between the two glacially-influenced GoA regions in all years. Environmental drivers of these patterns differed, but were weak across regional comparisons. Regional taxonomic composition was correlated to temperature, salinity, and turbidity while regional functional composition was not related to any environmental variables. Within regions, seasonality played a much stronger role in structuring Lynn Canal taxonomic and functional composition compared to Kachemak Bay where a stronger interannual signature was present. Taxonomic composition in Kachemak Bay was correlated with similar environmental variables to the regional comparison while Lynn Canal taxonomic composition was correlated to salinity and discharge. Both regions exhibited weak or non-existent relationships of functional composition to environmental drivers. In the more freshwater-influenced Lynn Canal, strong taxonomic and functional coupling across months indicates that seasonality structures communities, while in the more oceanic Kachemak Bay, weak seasonal differences and strong interannual differences indicate a system more influenced by oceanographic processes, as opposed to local changes. • Regions differed in functional and taxonomic composition between all years necessitating a closer look within region. • Kachemak Bay highlighted the significance of larger oceanic drivers on community composition compared to seasonal changes. • Lynn Canal highlighted the importance of seasonality in more freshwater-influenced estuaries. • Generally, functional composition was more robust to annual and seasonal environmental changes. • This study highlights the importance of multi-year and multi-season monitoring efforts for nearshore fish communities.