Influence of Black Mangrove Expansion on Salt Marsh Faunal Communities of Eastern Coastal Louisiana

Abstract

The salt marshes along the Gulf of Mexico provide a critical habitat for many species, but their spatial distribution is changing, because Avicennia germinans (black mangroves) are expanding due to warmer winters. Currently, little is known about how A. germinans are used as habitat providing food and refuge for aquatic fauna. Therefore, the overall goal of this study was to determine how A.germinans expansion into the Spartina alterniflora (smooth cordgrass) marshes of Louisiana’s Mississippi River delta plain affects salt marsh-dependent organisms. This study analyzed three habitat types: Spartina-dominated, Avicennia-dominated, and a mix of the two, focusing on habitat use and food webs. The results show aquatic faunal habitat use can change by habitat type, however other factors such as location, elevation, and season affected the extent of this change. Coastal geomorphology and hydroperiod control accessibility to these habitats, changing abundance and density of nekton and epifauna. The food web results, analyzed using bulk stable isotope, show habitat type influenced primary production sources in common aquatic consumers. Consumers in all habitats typically incorporated a mixture of microphytobenthos, suspended particulate organic matter, and S. alterniflora. Microphytobenthos contributed the majority to consumers regardless of habitat type. Only Littoraria irrorata (marsh periwinkle) showed a shift in production source with habitat. A. germinans contributed to snails from the mangrove habitat, possibly due soil organic matter feeding. The soil organic matter was a mixture from a variety of sources, including the dominant vegetation of the habitat. Overall, the effects of black mangrove expansion on Louisiana’s aquatic organisms may be site- and species-specific. It is imperative to continue examining temporal and spatial variations in responses of aquatic fauna as A. germinans continues to expand into salt marshes of the northern Gulf of Mexico.