Exploring Gradients: Comparing Aquatic Communities Across Spatial and Environmental Scales in the Mobile Bay Estuary

Estuaries are important ecosystems that interface between fresh and marine waters, providing a number of important ecosystem services. Understanding the fish and macroinvertebrate structure of estuarine communities can provide a baseline for the health of these systems. The Mobile Bay Estuary, Alabama is a river-dominated estuary in the Northern Gulf of Mexico. Changes in river discharge as well as wind and storm-driven water movements can produce rapid changes in salinity, temperature, and dissolved oxygen levels within the system. We quantified the fish and macroinvertebrate communities at 9 sites across a physical-chemical gradient within the Mobile Bay Estuary over a 4 - year period (April 2011–February 2015). Using standardized seine hauls and electrofishing; we simultaneously measured salinity, water temperature, dissolved oxygen, and secchi depth. Ordination techniques were used to identify patterns within the fish and macroinvertebrate communities and statistical techniques were also used to determine the correspondence of the communities with environmental variables. Surface salinity, and proximity to the Gulf of Mexico were found to play a prominent role in determining the broad scale patterns of community organization. Smaller-scale patterns suggested that individual species' seasonal migrations, and variation in water temperature were important predictors of community structure.