Occurrence of Hypolimnetic blooms of the Purple Sulfur Bacterium, Thiopedia rosea, and the Green Sulfur Bacterium, Chlorobium limicola, in an Australian Reservoir

Abstract

Seasonal blooms of green and purple sulfur bacteria were found in the anoxic hypolimnion of Puddledock Reservoir, a small, eutrophic water-supply storage in north-eastern N.S.W. These blooms were dominated by the green sulfur bacterium C. limicola, and were sometimes overlain by a bloom of the purple sulfur bacterium T. rosea. They generally formed a plate just below the oxycline at a depth of between 4 and 6 m in the shallow, sulfuretted, nutrient-rich hypolimnion. The blooms increased in density from about mid summer until holomixis, when the return to oxygenated conditions killed the sulfur bacteria. Maximum bacterial chlorophyll levels varied with the light regime, but typically reached 250µg L^-1, although the maximum recorded value was 410µg L^-1. The Secchi transparency during summer stratification was variable, but was typically around 1.2 m, with extremes ranging from 4.5 m on one occasion to as low as 0.8 m for an extended period, as a result of epilimnetic algal levels of 40µg L^-1 chlorophyll. Gilvin (g₄₄₀) levels during these bacterial blooms were always less than 2.5 m^-1. Sulfur bacteria were absent during one summer; this was attributed to a dramatic change in the light regime as a result of a major inflow event. The latter resulted in a Secchi depth of 0.75 m associated with very high gilvin levels of 7.5-11.0 m^-1 and high inorganic turbidity levels, which would have resulted in a very dim red light regime in the hypolimnion.

This study is the first report of a bloom of purple sulfur bacteria in Australian fresh waters, and the first to detail the occurrence of blooms of green and purple sulfur bacteria in a seasonally stratifying water body.