Year: 1996
Abstract:
The aim of my study was three-fold: (1) to document natural year-to-year variability in the phytoplankton communities of untreated eutrophic hardwater lakes with relatively low runoffs, (2) to investigate the mechanisms by which Ca(OH)$\sb2$ additions reduce phytoplankton biomass over the short term ($<$20 d), and (3) to identify the effects of treatment on phytoplankton communities in the long term ($>$20 d).
The short-term changes in chlorophyll a (chl a), TP and calcium (Ca) concentrations, and pH and turbidity were investigated in four lakes, two dugouts and six limnocorrals after treatment with various Ca(OH)$\sb2$ dosages. Treatments with 250 mg$\rm{\cdot}L\sp{-1}$ of Ca(OH)$\sb2$ in dugouts reduced chl a and TP concentrations in the water by $>$90% and 60%, respectively. Treatment with dosages $\rm{>}75 mg{\cdot}L\sp{-1}$ of Ca(OH)$\sb2$ depressed more chl a and TP concentrations than did dosages $\rm{\le}50 mg{\cdot}L\sp{-1}$ in the limnocorrals. Transitory high pH and turbidity did not reduce phytoplankton biomass in the laboratory. In lakes, treatment with dosages ranging from 25 to 87 mg$\rm{\cdot}L\sp{-1}$ of Ca(OH)$\sb2$ decreased chl a concentrations by 80%, but did not always reduce TP concentration in the water.
Effects of lime treatment on phytoplankton communities in the long term were identified by comparing phytoplankton communities in three lime-treated lakes with pre-treatment communities and with communities in three untreated lakes, for the ice-covered period (November to April) and open-water season. Under ice cover, both phytoplankton species composition and biomass fluctuated irregularly from winter-to-winter in all lakes. Treatment effects, if any, on phytoplankton communities were not identified. During the open-water season, mean total phytoplankton biomass and TP concentrations remained unchanged in the year after a single treatment with 72 or 87 mg$\rm\cdot L\sp{-1}\ Ca(OH)\sb2$ in two lakes. In contrast, multiple treatments with dosages ranging from 25 to 75 mg$\rm\cdot L\sp{-1}\ Ca(OH)\sb2$ in Halfmoon Lake reduced mean chl a and TP concentrations by 50% after the first treatment in 1988, and chl a and TP concentrations remained low from 1989 to 1993. Similarly, multiple treatments in Figure Eight Lake, with Ca(OH)$\sb2$ and/or CaCO$\sb3$ at dosages ranging from 5 to 24 mg$\rm\cdot L\sp{-1}$ from 1986 to 1992, reduced mean chl a and TP concentrations from a pre-treatment average (1985 to 1986) of 61 and 161 $\rm\mu g{\cdot}L\sp{-1}$ to 21 and 80 $\rm\mu g{\cdot}L\sp{-1}$ during the repeated treatment years (1987 to 1992). Species diversity decreased in Lofty Lake for up to one year after treatment but was unchanged in N. Halfmoon Lake. Three species of phytoplankton (Gomphosphaeria naegeliana, Lyngbya birgei and Asterionella formosa), which were dominant before treatment, were not recorded for up to two years after lime treatment. More chlorophyte species were found in post- than pre-treatment samples in the three treated lakes.
My results suggest that multiple treatments at low to moderate dosages or a single treatment at high dosages are likely required to reduce phytoplankton biomass effectively.
