Multiple studies on higher plants and metazoans demonstrated that species richness and diversity increase productivity. Field observations suggest that more diverse algal communities in lakes are more efficient in using phosphorus (P) than less diverse ones. Experimental manipulations on bacteria, algae and microscopic consumers support the notion that species richness increases productivity and resource use efficiency. On the other hand, organisms strongly competing for limiting resources and/or best adapted to environmental stresses may dominate a system’s productivity. Thus both diversity and dominance influence ecosystem processes and functioning. Knowledge of the relationships between diversity / dominance, ecosystem processes (e.g. resource use), and ecosystem functioning (e.g. productivity / biomass, and organic matter transfer to the food webs) is insufficient for periphytic algae in wetlands. Thus we assess the relationships between: i) algal dominance (% biovolume of the top five taxa) and biomass (Chlorophyll a, Chl a), and resource use efficiency (RUE= Chl a / Total P, TP); ii) algal richness and diversity (Shannon, Simpson and Shannon Evenness indices) and periphyton quality (% organic content) in the Florida Everglades. Our hypotheses are: 1) cyanobacteria dominance maximizes Chl a and RUE; 2) algal species richness and diversity increase periphyton organic content.
We analyzed a seven year data series on algal species composition and biovolume, Chl a, and TP in the Everglades along nutrient and hydroperiod gradients. The mean relative biovolume of the top five algal taxa, an indicator of dominance, was significantly higher in oligotrophic and short hydroperiod sites (often > 90%), than in nutrient-rich and long hydroperiod sites (80-85%; Kruskal-Wallis test: χ2 = 154.3; Mann-Whitney U test: p<0.001). Chl a and RUE - respectively an indicator of algal biomass and an ecosystem process - were significantly higher in shallower sites, with P scarcity and desiccation stress, than in deeper sites with higher P (Kruskal-Wallis test: χ2 = 270.9 and χ2 = 301.6; Mann-Whitney U tests: p<0.001). This is due to the dominance of cyanobacteria able to tolerate such conditions in the cohesive periphyton mats in Everglades National Park, e.g. the filamentous taxa Scytonema spp. and Schizothrix spp.. In loose mats with higher P in Water Conservation Areas and Loxahatchee, species richness and diversity, and organic content are higher, thus increasing periphyton quality. Therefore in the Everglades, species diversity enhances ecosystem functioning via increasing organic matter transfer to the aquatic food webs, but resource use efficiency is maximized by cyanobacteria dominance.