Short-term effects of Ca(OH)2 additions on phytoplankton biomass: A comparison of laboratory and in situ experiments

To identify mechanisms by which Ca(OH)2 additions reduce phytoplankton biomass in the short term (<20 d), we investigated changes in concentrations of chlorophyll a (chl a), total phosphorus (TP) and calcium (Ca), and pH and turbidity in four eutrophic hardwater lakes, two dugouts and limnocorrals. We also tested the potential effects of high pH and turbidity on biomass of three representative species of planktonic cyanobacteria, green algae and diatoms in the laboratory. When chl a concentrations were higher than 10 μg•l−1, Ca(OH)2 additions with dosages ranging from 25 to 87 mg•l−1 decreased chl a concentrations by 80% in lakes. Chl a concentrations were also reduced by adding 50, 75 or 100 mg•l−1 Ca(OH)2 to limnocorrals and 250 mg•l−1 Ca(OH)2 to dugouts. Transitory high pH and turbidity did not reduce phytoplanton biomass in the laboratory. The pH in limnocorrals and dugouts increased to > 10, which may have affected phytoplankton biomass. After treatment, TP concentrations declined in limnocorrals and dugouts, but not always in lakes. In lakes and dugouts, Ca concentrations > 30 mg•l−1 or pH levels > 10 were related to < 10 μg•l−1 chl a. In lakes, pH was always < 10 due to the inherently high buffering capacity, and the reduced phytoplankton biomass coincided with decreased Ca concentrations; therefore, precipitation of phytoplankton cells likely caused the short-term reduction in chl a.