Primary productivity and seasonal dynamics of planktonic algae species composition in karst surface waters under different land uses

Human activities have altered land use patterns and, thus, the hydrochemical features (e.g., CO2(aq), NO3-, and PO43-) of watersheds, influencing both the C cycle and the quality of water (e.g., eutrophication) within them. The ways in which primary productivity and the dynamics of planktonic algae species composition are related to these hydrochemical changes remains unclear, especially in karst waters with high pH and high concentrations of Ca2+ and dissolved inorganic C. In this study, we used the Shawan Karst Test Site, Puding, SW China to analyze the influences of the NO3-, PO43-, and CO2(aq) concentrations on the primary productivity and dynamics of planktonic algae species composition in five spring-fed ponds under different land use patterns at the site during 2018. Our results show the following. (1) Agricultural farming increased the input of NO3-, PO43-, and CO2(aq) from springs to the ponds (i.e., surface waters); meanwhile, shrub and grass lands reduced the input of NO3- and PO43-, but significantly increased the input of CO2(aq) to the water. (2) The total number of planktonic algal cells was mainly related to CO2(aq) in January (i.e., the dormant period), but mainly related to NO3- and PO(4)(3- )in July (i.e., the growth period); this possibly due to the seasonal succession of phytoplanktonic algae species sensitivity of different nutrient elements. (3) In the dormant period, the system is dominated by the phytoplanktonic algal cells sensitive toCO(2(aq)), especially in grass and shrub ponds. In the growth period, the system is dominated by the planktonic algal cells sensitive to NO3- and PO43-; however, they are far less than others in grass and shrub ponds. Thus, we predict that groundwaters with relatively high CO(2(aq) )and low NO3- and PO43- concentrations (resulting from natural afforestation in karst areas) may help alleviate eutrophication and increase C sequestration in the corresponding surface waters, indicating the importance of land-use adjustment in the management of water quality and carbon cycle.