The benthic regeneration of N and P in the Great Brak estuary, South Africa

Abstract

The Great Brak is a temporarily open/closed estuary (TOCE) located on the south coast of South Africa. The construction of the Wolwedans Dam in 1989 reduced baseflow to the estuary by 56%, decreasing the intensity of flushing events and causing the mouth to breach less often. The aim of this study was to investigate the flux of inorganic nutrients (NH 4, SRP) as well as total N and P across the sediment-water interface in the estuary. There have been very few studies on nutrient cycling and benthic pelagic coupling in South African estuaries. This study showed that the sediment had a net efflux of NH 4 + , SRP, TN and TP while TOxN was taken up or converted to other forms of N. The estuary acted as a source of N and P during both summer and winter. If the estuary remains closed for a prolonged period (12 months), with an increased organic load present on the benthos, the associated rates of efflux of N and P would increase. In order to reduce the organic load to the system better flushing methods or, more importantly, an increase in base flow, is needed to reduce residence times of water in the estuary.

Nutrient enrichment as a threat to the ecological resilience and health of South African microtidal estuaries

“…Upon decomposition, oxygen is removed from the water, creating anoxic conditions rich in hydrogen sulphide. In the Groot Brak Estuary, regeneration of N and P from anoxic sediments ensures a tight cycle of nutrients between the sediment and primary producers (Human et al 2015b). Despite low in situ nutrient concentrations, macroalgal blooms, such as Cladophora glomerata (Linnaeus) Kützing, indicate a deterioration in water quality.…”

“…Gamtoos and Sundays) and well-flushed estuarine bays (e.g. Knysna) (Snow et al 2000;Human et al 2015b;2016b;Lemley et al 2018b).…”