Benthic communities restructure quickly under shifting salinities: a reciprocal transplant experiment

Salinity regimes in estuarine habitats will shift pending sediment diversions, which will reconnect Louisiana, USA, coastal wetlands to the Mississippi River. The river has been hydrologically disconnected from fringing estuaries for over a century, but high local rates of relative sea-level rise have contributed to land loss, and sediment diversions will be built to prevent future loss. A reciprocal transplant experiment of benthic cores was used to determine how macrofaunal communities may be altered pending salinity regime shifts. A total of 147 benthic cores (3142 cm 3 ) were excavated and transplanted within 3 salinity regimes in Barataria Bay, LA, for 2 or 5 mo. We observed a significant effect of salinity treatment on benthic macroinfaunal community composition in every transplant, regardless of origin or date sampled. Results show that communities recolonized in as little as 2 mo, and reveal the species assemblages responsible for the restructure. Species assemblage changes included dominance of generalist species (spionid and capitellid polychaetes) and shifts between deposit feeders and suspension feeders as freshwater flow varied. Lower-salinity treatment decreased species richness, while higher-salinity treatment increased both species richness and abundance. Benthic communities will likely restructure quickly under the sediment diversion scenarios and leave few founder effects in response to changes in salinity regimes across the estuary. Pulsed diversion operations may yield different results, but only springtime operations are likely to impact benthic recruitment. Comprehensive knowledge of how benthic communities restructure post-restoration will enable both appropriate regional planning as well as future predictions in coastal estuaries undergoing hydrologic alteration.