Contrasting Photosynthetic Responses of Two Dominant Macrophyte Species to Seasonal Inundation in an Everglades Freshwater Prairie

Abstract

The Everglades short-hydroperiod freshwater prairies exhibit strong reductions in CO₂ uptake that coincide with inundation, but the underlying basis is not fully understood. To address one of the processes potentially underlying this decline, we measured photosynthetic capacity of the dominant species, sawgrass (Cladium jamaicense) and muhly grass (Muhlenbergia filipes), during wet and dry seasons (2009–2012 and 2016–2017). The measurements in the seasonally inundated prairie were compared to those taken on a nearby rarely inundated levee situated 29 cm above the prairie with similar species composition. During the dry seasons, muhly grass exhibited a much higher photosynthetic capacity (28.5 μmol m⁻² s⁻¹) than sawgrass (14.2 μmol m⁻² s⁻¹), and no differences were found between the prairie and levee for either species. During the wet seasons when the prairie was inundated, photosynthetic capacity declined substantially (67%) in the prairie for muhly grass while it remained similar across seasons for sawgrass. Analyses of leaf reflectance, chlorophyll fluorescence and leaf nutrient indicated photosystem impairment was the main driver of photosynthetic capacity reduction for muhly grass when the ecosystem is inundated. Our study suggests that declines in photosynthesis of macrophytes, particularly muhly grass, contribute to low wet season productivity in Everglades short-hydroperiod freshwater prairies.