Diatoms as tools for inferring ecotone boundaries in a coastal freshwater wetland threatened by saltwater intrusion

Species sorting mechanisms often control community assembly patterns across environmentally heterogeneous landscapes, particularly within microbial communities that respond quickly to environmental variability and are not dispersal-limited on intermediate time scales. In this study, we describe the spatial and seasonal patterns of two key environmental drivers, porewater (PW) conductivity and total phosphorus (TP), in the southern Everglades, FL., USA where saltwater intrusion, caused by rising sea level and hydrologic management, are transforming the natural environmental gradients of this ecological stressor and limiting nutrient. We surveyed diatom assemblages along transects capturing PW conductivity and TP gradients and searched for spatial boundaries in diatom assemblages along each transect. We also determined diatom assemblage thresholds to PW conductivity and TP, and identified significant indicator taxa with either negative (declining) or positive (increasing) relationships to each driver and their individual thresholds. We demonstrate that the southern Everglades exhibits spatially-structured gradients of conductivity and P that are oriented in two dimensions (i.e., with distance from the coast and from west to east) and are often, but not always, positively correlated. Our results show that these gradients drive spatial patterns of compositional similarity among our sampling sites. We found the location of greatest dissimilarity in diatom assemblages for each transect coincided with the upper boundary of the “white zone” - the visible ecotone between freshwater and coastal marshes. We did not detect seasonal differences in the position of the diatom-inferred ecotone as expected, nor did we detect significant differences in PW conductivity or TP between wet and dry seasons. Diatom assemblages were highly sensitive to both PW conductivity, with freshwater indicator assemblages declining above 2 mS cm−1 and becoming replaced by a brackish water assemblage at around 20 mS cm−1, and periphyton TP, with thresholds at 82 and 285 µg g−1 for negatively- and positively responding taxa, respectively. Our study highlights that small increases in PW conductivity and TP are sufficient to cause shifts in the diatom assemblages of the Everglades. As saltwater continues to encroach into this area, compositional changes in this important primary producer assemblage are expected to cascade through the ecosystem and influence the food web. The diatom indicator taxa and assemblage thresholds presented here offer a sensitive tool that should continue to be developed and applied to management strategies for saltwater intrusion while its effects can still be mitigated.