Interactive Effects of Nutrients and Salinity on Phytoplankton in Subtropical Plateau Lakes of Contrasting Water Depths

Eutrophication and salinization are serious global environmental problems in freshwater ecosystems, occasionally acting jointly to exert harmful effects on aquatic ecosystems. To elucidate the interactive effects of nutrients and salinity on phytoplankton assemblages, we conducted a four-season study during 2020-2021 of eight lakes from Yunnan Plateau (Southwest China) with a wide range of conductivities (Cond, reflecting degree of salinization), eutrophic states, and water depths and used General Additive Modeling (GAM) of the data. We found that: (1) species number (SN), density (D-Phyt), and biomass (B-Phyt) of phytoplankton showed stronger seasonal dynamics in shallow lakes than in deep lakes, all being, as expected, higher in the warm season; (2) annual and summer data revealed highly significant positive relationships between SN, D-Phyt, and B-Phyt with total nitrogen (TN) and total phosphorus (TP), which became weaker at high TP occurring when the N:P ratio was low, indicating N limitation; (3) SN, D-Phyt, and B-Phyt showed a unimodal relationship with salinity, peaking at 400-1000 mu S/cm (Cond); (4) the two dominant taxa (cyanobacteria and chlorophyta) showed different patterns, with chlorophyta generally dominating at low TN and cyanobacteria at high TN and Cond, suggesting the synergistic effect of nitrogen and Cond on cyanobacterial dominance.

Automated summary

Eutrophication and salinization interact to negatively affect freshwater ecosystems. In a study of eight lakes in Southwest China, we found that phytoplankton dynamics were more pronounced in shallow lakes and peaked in the warm season. Total nitrogen and total phosphorus were positively correlated with species number, density, and biomass, but this correlation weakened at high TP and low N:P ratio, indicating nitrogen limitation. Salinity showed a unimodal relationship with phytoplankton characteristics, peaking at 400-1000 mu S/cm (Cond). Different dominant taxa (cyanobacteria and chlorophyta) responded differently to nitrogen and salinity, with chlorophyta dominating at low TN and cyanobacteria dominating at high TN and Cond.