Freshwater discharge disrupts linkages between the environment and estuarine fish community

Abstract

Freshwater diversions and damming have altered coastal ecosystems greatly, resulting in fundamental changes in salinity, eutrophication, and ensuing shifts in ecological communities. Recently, efforts to restore water delivery and water quality in coastal systems have increased. To explore the impacts of altered freshwater discharge to a subtropical estuary, here Bayesian structural equation models are used to quantify the relationships among water quality properties, nutrients, and predator (sport) and prey (forage) components of the fish community. During periods of relatively low freshwater input, predatory sport fish and lower trophic level forage fish guild dynamics were tightly coupled with water quality and one another. In this low freshwater discharge system, water quality and nutrients were the most strongly linked components of the system. Conversely, during periods of high-water discharge, there was virtually no link between water quality and sport or forage fish guilds, and the sport fish and forage fish guilds became the most tightly coupled components. Ultimately, freshwater discharge disrupted existing linkages within the broader ecosystem. Based on these results, we expect the restoration of low freshwater flow into the estuary would strengthen the relationship between the fish community and the environment. As restoration of estuaries and rivers continues globally, freshwater delivery to downstream systems will be altered. Following the approach presented here, Bayesian structural equation models can be used to generate insight regarding potential system wide shifts following alteration of freshwater delivery, thereby providing a critically important tool for evaluating management strategies.