Estuarine Nekton Assemblages along a Marsh-Mangrove Ecotone

The marsh-mangrove ecotone along the southeastern US Atlantic coast occurs in northeast Florida within the Guana-Tolomato-Matanzas (GTM) estuary, where emergent vegetation transitions from marsh-dominated in the north to mangrove-dominated in the south. Dominant vegetation type has been shown to influence creek bank slope, nekton access to refuge, predation risk, and access to food. The northward distribution of mangroves in the estuarine mosaic is in flux in northeast Florida, and the effect on subtidal nekton, including commercially important species, is not known. To determine if estuarine nekton assemblages differ along the marsh-mangrove ecotone, a 60-km transition zone within GTM estuary was divided into 20 sub-zones where nearshore subtidal nekton communities were sampled monthly with trawls for 1 year. A total of 15,750 individuals consisting of 100 species were collected during the study period; 13 species made up 90% of the total catch. Subtidal nekton assemblages in marsh sites were dominated by typical salt marsh species (i.e., Leiostomus xanthurus, Anchoa spp., Bairdiella chrysoura) and had little overlap with assemblages in mixed and mangrove sites, which were dominated by structure-oriented species (i.e., Lagodon rhomboides and Eucinostomus spp.). Despite similar environmental conditions among the zones, there were clear differences in the subtidal nekton community along the marsh-mangrove ecotone, largely driven by fish species. This change in nekton community along the ecotone suggests that ecological processes such as food availability or predator/prey dynamics affected by changes in marsh surface habitats may result in differences in nekton species distribution and abundance across interconnected habitats such as in subtidal nekton that we observed in the GTM estuary.