Benthic metabolism across a gradient of anthropogenic impact in three shallow coastal lagoons in NW Florida

Seagrasses are being lost at alarming rates worldwide, most often due to anthropogenic effects, but few reports have examined how seagrass loss affects the metabolism of coastal ecosystems. Here, we address this question by comparing both areal and system-integrated daytime benthic metabolic rates across 3 lagoons in the North Central Gulf of Mexico that display varying levels of abundance of the shoalgrass Halodule wrightii (i.e. from 64% of the bottom covered with shoalgrass down to between 4 and 0%), partially due to contrasting anthropogenic pressures. When comparing the 2 shoalgrass-containing lagoons, shoalgrass patches featured higher areal rates of gross primary production (GPP) and respiration (R), and, to a lesser extent, higher rates of net production (NP), than did bare sediment. These results were robust despite across-lagoon differences in percentage cover and areal biomass of shoalgrass and benthic microalgae (i.e. the lagoon with less cover also had lower areal shoalgrass biomass in the shoalgrass patches and higher areal microalgal chlorophyll a concentrations in the sediment). We did not find any consistent differences in the metabolic rates of bare sediment across the 3 lagoons, despite the fact that areal microalgal chlorophyll a concentrations in bare sediment increased as shoalgrass abundance decreased across the 3 lagoons. Systemintegrated rates of benthic GPP and R were higher in the lagoon with the highest shoalgrass cover when compared with the lagoons with little or no shoalgrass; but, surprisingly, system-integrated rates of benthic NP did not differ significantly across lagoons. This result suggests that the large decrease in shoalgrass abundance across the lagoons examined does not greatly affect the lagoon’s potential capacity for accumulation and/or export of organic carbon. It also underlines the importance of deriving system-integrated estimates to properly understand how decreasing seagrass abundance can alter the daytime metabolism of coastal systems.