Influence of added nutrient on the seasonal variation of algal growth potential of Mt Bold reservoir, South Australia

Abstract

Bioassay experiments with the natural phytoplankton assemblage showed that the major nutrient input to Mt Bold reservoir, South Australia, occurred with the onset of winter rains. Growth potential within the euphotic zone generally increased from June to November but was undetectable from December to March when the autumn overturn circulated nutrient-rich water from the hypolimnion to cause an increase in growth. Subsequently, the growth potential fell and did not rise until the onset of winter rains. Routine enrichment experiments with phosphorus, nitrogen and Na-EDTA substantially increased the growth potential above the control during periods of suspected nutrient depletion. During periods of nutrient depletion, the influence of 10 single and 45 dual nutrient additions showed that only the combined addition of nitrogen and phosphorus caused a significant increase in growth potential. To investigate further the influence of nitrogen and phosphorus on algal growth, factorial nutrient-enrichment experiments using five levels of both nitrogen and phosphorus and three of Na-EDTA at two temperatures (11 and 20ºC) were conducted. The work is discussed in relation to the routine nutrient analyses done by the Engineering and Water Supply Department of South Australia.