Environmental and Spatial Influences on Biogeography and Community Structure of Saltmarsh Benthic Diatoms

Benthic microalgae play important roles in energy flow and biogeochemistry of coastal ecosystems; however, factors influencing community composition remain largely unknown. Our purpose was to identify and compare spatial and environmental influences on benthic diatom biogeography at regional scales. In summer 2018, we sampled sediment at various spatial scales (0.0001-180 km) from five saltmarshes in South Carolina, USA, and characterized diatom assemblages using DNA metabarcoding. Twenty-three environmental variables from marsh sediments, adjacent creeks, and atmosphere were recorded. Multivariate analyses revealed that the saltmarsh communities were clearly distinct. Community dissimilarity was compared to both geographic distance and environmental differences to determine influences on community structure. Mantel tests and redundancy analysis revealed that spatial influences, in addition to several environmental factors (phosphate, grain size, sediment sorting, sediment moisture, and creek salinity), were significant. Variation partitioning revealed that 26% of community variation was explained by geographic distance alone, whereas 60% could be explained by combined spatial/environmental factors. In contrast, the independent explanatory power of individual environmental factors was negligible. Our findings suggest that spatially structured environmental variation mainly conditioned the saltmarsh diatom biogeography in this region. Dispersal limitation, as evinced by the large pure spatial effect and distance-decay pattern, was also important. Spatial effects were stronger relative to that previously observed in other microbial groups (marine bacteria and phytoplankton) and detected at finer spatial scales. Results support prior studies that suggest body size and dispersal mode are important drivers of metacommunity structure and therefore must be considered when studying aquatic microbial biogeography.