Perennial domination of phytoplankton by Botryococcus and Peridinium in a discontinuously polymictic reservoir (tropical Australia)

Abstract

The effect of holomixis and wet season inflow on the phytoplankton composition of Darwin River Reservoir (DRR) is examined. Both phenomena constitute potential disturbances to the pelagic environment of this shallow (average depth 6 m) tropical reservoir and its phytoplankton community. Over the two year study period, the phytoplankton concentration of DRR remained much the same (0.48-2.4 mm(3)/L). At least half the biovolume comprised two species, Botryococcus braunii and Peridinium gutwinskii. These two taxa, together with other Peridinium species and desmids (predominantly Staurastrum and Cosmarium) constituted 68-98% of the reservoir's total phytoplankton biovolume. Potential disturbance by wet season inflow and 13 holomictic events had minimal affect on the domination of the Botryococcus-Peridinium-Desmidaceae assemblage, owing mainly to their short duration (4-5 days) and minor influence on light intensities in the mixed layer. Some holomictic events, however, favoured the growth of the diatom Aulacoseira granulata which reached a maximum of 17 % of the reservoir's total phytoplankton biovolume. Fast growing, small r-selected phytoplankton, which are favoured by nutrient rich waters, constituted a minor component (<1 % total biovolume) of the reservoir's phytoplankton, probably owing to the phosphorus impoverishment of the mixed layer. B. braunii colonies and P. gutwinskii have many characteristics of K-selected (slow growing) S-species, which mark the later stages of autogenic succession. There was no evidence of phytoplankton autogenic succession with the onset of thermal stratification in DRR. The perennial domination of the reservoir's phytoplankton by a single assemblage, and notably by two species, suggests the reservoir's phytoplankton community is in dynamic equilibrium (or steady-state) with the environment, supporting the contention that climax communities are self-sustaining.