Temporal trends in total phosphorus, temperature, oxygen, chlorophyll a and phytoplankton populations in Hartbeespoort Dam and Roodeplaat Dam, South Africa, between 1980 and 2000

Eutrophication, phytoplankton blooms and changes to the phytoplankton composition are some of the problems facing the managers of Hartbeespoort Dam and Roodeplaat Dam. Trends, both decreasing and increasing, are visible in the phosphorus concentrations, temperatures, dissolved oxygen concentrations and chlorophyll a concentrations in the water of the two impoundments. In order to investigate a decrease in phosphorus concentration (averaging 25μg l−1 per year) with an increase in chlorophyll a concentration (averaging 6.8μg l−1 per year) in Hartbeespoort Dam, the authors assessed trends in temperature and dissolved oxygen concentrations in this impoundment. From 1980–2000, surface water temperatures in both Hartbeespoort Dam and Roodeplaat Dam increased at an average rate of 0.14°C/year. These increases may be the result of a number of external factors, including climate. In Roodeplaat Dam the total phosphorus concentration first decreased by an average rate of 19μg l−1 per year from 1980– 1990, then increased by 27μg l−1 per year from 1990–2000. The average increase between 1980 and 2000 was 5μg l−1. These increases in total phosphorus concentration were accompanied by increases in chlorophyll a concentration in Roodeplaat Dam, averaging 0.8μg l−1 per year. The dissolved oxygen concentration displayed no measurable trends. Between 1997 and 2000, changes in dominant phytoplankton species occurred in both reservoirs. In Hartbeespoort Dam, Cyanophyta used to predominate in the summer period each year; however, since 1999, Ceratium hirundinella (Pyrrhophyta) have become more dominant for extensive periods. Similarly, in Roodeplaat Dam, Cyanophyta used to be the dominant group annually, but from 1997 to 2000, a Scenedesmus sp. (Chlorophyta) became increasingly important in the phytoplankton composition.