Algal richness and life‐history strategies are influenced by hydrology and phosphorus in two major subtropical wetlands

1. We explored controls of algal taxon richness (hereafter richness) in complex and hydrologically dynamic flood-pulsed wetlands by comparing results from independent studies in two globally important subtropical wetlands: the Okavango Delta (Botswana) and the Florida Everglades (U.S.A.). In both wetlands, the flood pulse hydrology is regulated by distinct wet and dry seasons, and creates floodplain landscapes with heterogeneous habitats; algal growth is limited by phosphorus (P); and water uses threaten ecosystem function. To inform future comparisons of algal richness and distribution patterns, we assessed the role of hydrology and P as key controls of richness, and identified indicator taxa of desiccation disturbance and P scarcity in these wetlands under increasing hydrological, nutrient, and habitat changes. 2. We used the intermediate disturbance hypothesis, and the species-energy theory to explain algal richness patterns, and the competitive, stress-tolerant, ruderal (CSR) framework to classify indicator taxa. We collected algal samples, environmental data and information expected to influence community structure (water depth, relative depth change, P concentrations, hydroperiod and habitat type) over several years at sites representing a broad range of environmental characteristics. To account for sample size differences, we estimated algal richness by determining the asymptote of taxon accumulation curves. Using multiple regression analysis, we assessed if and how water depth, depth change, P, hydroperiod, and habitat type influence richness within each wetland. We then compared the strength of the relationships between these controlling features and richness between wetlands. Using indicator species analysis on relative abundance data, we classified C, S and R indicator taxa associated with shorter/longer hydroperiod, and lower/higher P concentrations. 3. In either wetland, we did not observe the negative unimodal relationship between site-specific richness and water depth change that was expected following the intermediate disturbance hypothesis. It is possible that this relationship exists at more highly resolved temporal scales than the semi-annual to annual scales hypothesised here. However, as nutrient flows and algal habitats depend on these wetlands' flood pulse, maintaining the Okavango's natural pulse, and increasing freshwater flow in the Everglades would help protect these wetlands' algal diversity. Chlorophyta richness (Okavango), and total, Bacillariophyta, Chlorophyta and cyanobacteria richness (Everglades) increased with higher P concentrations, as per species-energy theory. In the Okavango, we classified 6 C and 49 R indicator taxa (e.g. many planktonic Chlorophyta), and in the Everglades, 15 C, 1 S and 9 R taxa (e.g. benthic Bacillariophyta and planktonic/benthic Chlorophyta), and one stress- and disturbance-tolerant cyanobacterium species. 4. Our results offer baseline information for future comparisons of richness, and abundance of C, S and R indicator taxa in subtropical wetlands; this can be used to quantify how algal communities may respond to potential changes in hydrology and P due to water diversion, anthropogenic nutrient loads, and climate change. Examining microhabitat heterogeneity, nitrogen and light availability, and grazing pressure in such wetlands would further illuminate patch-scale controls of richness and life-history strategy distribution in algal communities.