Effects of black mangrove Avicennia germinans expansion on salt marsh nekton assemblages before and after a flood

Climate change is facilitating black mangrove Avicennia germinans (L.) encroachment into Gulf of Mexico (GOM) estuaries, where mangroves are displacing Spartina alterniflora (Loisel) and other marsh plants. Western GOM estuaries have low tidal exchange, and salinity ranges from 0 to > 50 ppt depending upon rainfall. Besides promoting expansion of tropical species into the GOM, climate change will likely affect salinity by making droughts more intense and storms and flooding more severe. We investigated the combined effects of A. germinans encroachment and salinity changes on associated nekton. In the spring and fall of 2014, nekton communities were significantly different and organisms significantly less abundant in wetlands dominated by A. germinans, even when S. alterniflora remained abundant. In spring 2015, flooding and reduced salinity obscured this trend, although in fall 2015 salinity increased, and organisms were again more abundant in areas without A. germinans. Thus, climate change can have significant effects on the distribution of wetland foundation species and associated faunal community structure, but, ultimately, precipitation and changes in salinity regimes can override the influence of foundation species on fauna. Climate change may alter the faunal composition of coastal wetlands by facilitating shifts in foundation species and by altering precipitation and salinity regimes.