Can free-floating and emerged macrophytes influence the density and diversity of phytoplankton in subtropical reservoirs?

Abstract

Aquatic macrophytes and phytoplankton are important for primary production and metabolism in lentic systems. We investigated the influence of the macrophytes on the phytoplankton in a subtropical reservoir on a seasonal scale (2009–2010). Our study was one of the first to investigate the effects of free-floating and emerged macrophytes on phytoplankton assemblages in a subtropical reservoir. The concentrations of total phosphorus (25–92 μg/L) and the inflow discharge to the reservoir (6.4–54.3 m3/s) varied seasonally (p < 0.05, multivariate analysis of variance [MANOVA]) and were positively and negatively related with the macrophyte biomass, respectively (principal components analysis [PCA]). Light availability was significantly different within the sampling periods (p < 0.05) and influenced phytoplankton densities (0.6 × 103 to 21.0 × 103 ind/mL) and chlorophyll a (<0.5–51.3 μg/L). Abiotic environmental factors and macrophytes influenced the phytoplankton dynamics, and it was difficult to clearly separate these effects. Although the MANOVA did not show seasonal variation of the macrophyte biomass (p > 0.405), the PCA suggested that phytoplankton densities were inversely correlated with the biomass of most of the macrophyte species. Higher phytoplankton diversity and flagellate species were associated with greater plant biomass. Our study has management implications because (1) although not presenting significant seasonal variation, the macrophyte biomass was inversely correlated with phytoplankton densities, and (2) the presence of free-floating and emerged plants induced qualitative shifts in the phytoplankton, increasing the diversity of species. The removal of the macrophytes by management actions (e.g., mechanical removal) or natural forces (e.g., wind effect) might influence the phytoplankton community and the trophic state of subtropical reservoirs.