Not so pristine – characterising the physico-chemical conditions of an undescribed temporarily open/closed estuary

Abstract

rapid ecological water requirement study of the Department of Water Affairs in 2009 classified the Goukamma Estuary as largely natural with few modifications, i.e., in an A/B state. However this detailed 13-month study showed that the estuary is impacted. Salinity stratification and hypoxic conditions (dissolved oxygen (DO) < 3 mg·l-1) were measured during both open and closed mouth states. The Goukamma Estuary is a blackwater system and in its natural state is expected to be nutrient poor. However high nutrient concentrations were measured in the middle and upper reaches of the estuary, adjacent to cattle farms. Nutrient concentrations represented mesotrophic (DIN > 500 µg·l-1) to eutrophic conditions (DIP > 25 µg·l-1). Nutrient input stimulated microalgae to attain a high biomass, ranging between 0.3 and 112 µg·l-1 (average 7.7 ± 1.3 µg·l-1) and 0.8 and 289 µg·l-1 (average 21.1 ± 4.4 µg·l-1) during the open and closed mouth states, respectively. Exacerbated by natural salinity stratification which effectively limits oxygenation of the water column, unnaturally high nutrient concentrations, microalgal blooms and coinciding organic loads all indicate poor water quality of the Goukamma Estuary. This research showed that detailed studies involving field measurements of water quality are required before the present health status of an estuary can be assessed.

Keywords: Temporarily open/closed estuaries, blackwater system, water quality, stratification, hypoxia, DIN, DIP, microalgae biomass