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

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. 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. 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\u27 flood pulse, maintaining the Okavango\u27s natural pulse, and increasing freshwater flow in the Everglades would help protect these wetlands\u27 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. 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

Spatial variability of chlorophyll-a and abiotic variables in a river–floodplain system during different hydrological phases

Chlorophyll-a (Chl-a) and abiotic variables were measured in the main channel and floodplain waterbodies of the Middle Paraná River to analyse the system dynamics and to assess their spatial variability during different hydrological phases, including an extreme flood. We wanted to test that the flood does not always have a homogenising effect in a river-floodplain system. An explanatory model for Chl-a was performed according to Akaike?s Information Criterion (AIC), and the relation of water level with the coefficient of variation (CV) among sites for each variable was explored. The model explained 64% of Chl-a variability. Water level, depth:euphotic zone ratio (Zd:Zeu) (inverse correlation) and conductivity (direct correlation) were the significant explicative variables. The CV of Chl-a decreased with flood from the main channel to the floodplain, but for turbidity, Zd:Zeu, pH, dissolved oxygen, soluble reactive phosphorus and Chl-a:pheophytin-a ratio, it increased. However, within the floodplain, CV of turbidity, Zd:Zeu and pH decreased during flood. These suggest that the homogenising effect frequently observed during inundation cannot be generalised and that the floodplain may maintain its identity even during flood. The extreme flood and its overlap with the warm season and sedimentological pulse probably contributed to the heterogenity in the spatial gradient.Fil: Mayora, Gisela Paola. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Devercelli, Melina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Giri, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentin