How drought affects range and variability of environmental conditions in subtropical South African estuaries

Globally estuaries are under increasing pressure from human development impacts as well as the growing effects of climate change that is already, and increasingly so, causing major changes in events such as droughts. Although droughts are regular occurrences and are projected to become progressively more significant in many parts of the world, their potential impacts on estuaries requires better understanding and quantification. This study aims to quantify environmental and phytoplankton parameter changes in two contrasting subtropical estuary types in South Africa; the temporarily closed uMdloti Estuary and the predominantly open to the sea uMlalazi Estuary during a hydrological drought event and subsequent non-drought period. Drought caused lower salinities, almost exclusively freshwater, throughout the temporarily closed estuarine type uMdloti. The marine-estuarine salinity environment of the predominantly open uMlalazi during drought expanded to include lower salinities in the non-drought period. Dissolved oxygen levels were more variable during drought conditions including periods of hypoxia, but more stable at elevated levels in both estuaries during the non- drought period. Temperature measurements included higher values during drought, especially in the dry season. Chl-a concentrations were overall higher and more variable during drought in uMdloti, with periods of bloom formation as well as decay. Drought-induced conditions can span several generations for estuarine and estuarine associated organisms. The results from the study provide quantitative support for the hypothesis that extended periods of drought conditions lead to a decrease in niche availability to estuarine organisms in a range of estuary types.