The role of climate variability and riverine pulsing in the community dynamics of estuarine nekton in Breton Sound, Louisiana

Abstract

Climate controls biotic community composition at multiple spatiotemporal scales through variability in environmental control mechanisms (assembly filters). This research investigated the role of climate variability in the community dynamics of estuarine nekton in Breton Sound estuary, Louisiana, and, specifically the effects of El Niño Southern Oscillation (ENSO), freshwater discharge, and a tropical cyclone. A teleconnection was found between ENSO and juvenile brown shrimp (Farfantepenaeus aztecus) abundance in Breton Sound from 1988 – 2007. ENSO affected winter weather conditions (air pressure, temperature and precipitation), and spring brown shrimp abundance in Breton Sound. Juvenile brown shrimp abundance lagged ENSO by three months; below-average abundance of juvenile brown shrimp was caught in springs following El Niño winters while above-average abundance of brown shrimp was caught in springs following La Niña winters. Salinity was the dominant ENSO-forced assembly filter that regulated brown shrimp abundance. Study of short-term freshwater pulses revealed higher nekton density and biomass in marshes receiving pulsed riverine flow (inflow) than in reference marshes, due to differences in water depth and flooding duration caused by the pulses. Communities consisted mainly of marsh resident species; individual-species examination revealed habitat preference related to water depth. Inflow marshes were capable of producing optimum growth of Gambusia affinis (0.001 g DW d-1) and energetically valuable habitat (> 6,000 cal g-1) for trophic transport. Riverine pulses may enhance secondary productivity in Breton Sound estuary. Breton Sound was directly hit by Hurricane Katrina in 2005, causing extreme physical habitat damage and a protracted period of elevated salinity in tidal freshwater marshes. Results included higher nekton densities and a nekton community shift from one dominated by Tidal freshwater / Resident (T/R) species toward one that included Brackish / Migrant species. Effects were short lived; by spring 2007, the nekton community had returned to T/R species, despite the lasting loss of vegetated marsh habitat. These findings provide greater understanding of large-scale climate effects on local estuarine nekton community dynamics and productivity.