Abstract: Primary producers in oligotrophic, shallow-water ecosystems receive periodic nutrient subsidies from rain and water inflow and from soil via inundation. Spatially heterogeneous freshwater influxes characterize the hydrologic regime across a landscape and can create favorable environments for producer interactions. Macrophytes and periphyton mats are crucial wetland engineers, and the dominance of one over the other can alter nutrient cycling and storage, habitat heterogeneity, and shift energy allocation at higher trophic levels. In the Everglades, short-hydroperiod marl prairies contain the most productive periphyton mats and species rich macrophyte communities dominated by Cladium mariscus subsp. jamaicense and Muhlenbergia capillaris var. filipes. Understanding how hydrology regulates production by macrophytes vs. mats is crucial for determining the outcome of macrophyte-periphyton interactions and producer dominance in oligotrophic ecosystems like the Everglades.
A three-year harvest experiment took place at three locations in the Everglades marl prairies to investigate how macrophytes and periphyton influence each other’s biomass, and if periphyton impacts macrophyte community structure along a hydrologic gradient. Four transects were established at each location and 24 pairs of control and treatment plots were designated for either macrophyte or periphyton removal along each transect. Macrophytes or periphyton were removed from their respective treatment plots bimonthly for one year. One pair of macrophyte and periphyton plots were then harvested bimonthly from each transect for two years to assess the effects of removal.
At the shallowest, shortest-hydroperiod location, macrophyte removal significantly increased mat biomass, while periphyton removal significantly decreased macrophyte total biomass. For both removal treatments, the shortest hydroperiod site had significantly greater macrophyte biomass and lower periphyton biomass than the deeper, longer-hydroperiod sites. Periphyton removal did not significantly impact macrophyte community structure as a whole; however, when examining the impacts of periphyton removal on each macrophyte taxa separately, C. mariscus subsp. jamaicense biomass and stem density decreased when periphyton was absent.
Overall, macrophyte and periphyton mats were found to interact with one another, with the effects of removal on biomass more pronounced at locations with lower water level and shorter hydroperiods. Periphyton mat presence on macrophyte stems and over sediment may protect macrophytes from desiccation and/or ultraviolet radiation, particularly where the environment is dry for longer periods. In contrast, higher biomass of macrophytes may be limiting space for periphyton colonization. In conclusion, hydrology may influence the intensity of macrophyte-periphyton interactions, which can impact the allocation of ecosystem biomass between producers and ultimately shift producer dominance and ecosystem function.
