Effects of hypoxic disturbances on an estuarine nekton assemblage across multiple scales

Disturbances influence ecological communities over a wide range of scales. We investigated the effects of localized hypoxic disturbances on an estuarine fish assemblage at several spatial (m² and 10s km²) and temporal (days, seasons, years) scales in a multivariate framework (temperature, salinity, depth, dissolved oxygen). We examined whether there were consistent changes in fish and crustacean estuarine assemblage characteristics along environmental gradients and whether these relationships were altered by hypoxic disturbances. We also investigated at what scale dissolved oxygen concentration may be influencing the structure of motile estuarine assemblages and whether the size of the hypoxic zone altered its effects on the estuarine assemblage. Hypoxic disturbances altered fish and crustacean assemblages along the depth gradients that were present during well-oxygenated periods. Species diversity, richness, and catch rates were lower in hypoxic patches than in oxygenated areas. Dissolved oxygen concentration remained an important explanatory variable for patch-level assemblage dissimilarity, species richness, and diversity when data were aggregated across seasons. When we examined the data at a larger scale, by aggregating information across the study area, we did not detect influences of hypoxia on assemblage structure. Fish moved out of local hypoxic zones, but remained within the estuary even in years with extensive hypoxia. There was no effect of size of the hypoxic disturbance on whether organisms responded to hypoxia or on diversity or richness of the study site. These results suggest that these local disturbances play an important role in structuring motile species assemblages at a patch-level within an estuary, but regional factors such as recruitment and migration are important in influencing species assemblages for the entire estuary over months and years.