A paper published in Water SA has highlighted the shortcomings of rapid Ecological Water Requirement studies that are based on ‘educated guesses’ rather than reliable data. EWR studies are conducted for estuaries to determine the Ecological Reserve, enshrined in the National Water Act, and are more typically referred to as environmental flow requirement studies in the case of rivers. Either way, the studies yield estimates of both the quantity and quality of water prevailing in the system’s natural, pristine state – its so-called reference condition – versus its present-day state, and describe the respective physical processes and habitat as well as the associated plant, invertebrate, fish and bird life. The ecological consequences of a range of flow scenarios are evaluated, and recommendations made.

A team of specialists is assembled for such studies, but funding and time constraints mean that few estuaries are afforded the benefit of comprehensive EWR studies, which allow for the collection of new data during high- and low-flow periods. Most are rapid, desktop studies that rely on available information – and if no recent monitoring has taken place on any particular component, the relevant specialists must use their knowledge of other systems to provide an expert opinion.

The rapid EWR study conducted for the Goukamma Estuary in 2008 as part of a broader Outeniqua Reserve Study (DWAF 2009) recognised that nutrients seeping from farmlands in the upper reaches of the estuary caused some degradation of water quality, but still considered the estuary to be in excellent condition. Subsequently, however, Tanja Kaselowski measured physico-chemical parameters and sampled microalgae in the estuary for 13 consecutive months for her Masters degree at the Nelson Mandela Metropolitan University, and found the situation was much bleaker than expected.

In a paper* co-authored with her supervisor, Prof Janine Adams, she reports that nutrient concentrations were significantly elevated, particularly in the middle and upper reaches of the estuary (the upstream boundary of which is considered to be some 3 km inland of the N2 bridge). The nutrient enrichment resulted in high microalgal biomass, especially species of blue-green algae or cyanobacteria - many of which are toxic - in the fresher areas. The deeper waters were severely oxygen-depleted, which can be attributed to the high organic loading and its bacterial decomposition, exacerbated by the salinity-dependent stratification that naturally occurs where wind, waves and currents are insufficient to mix the water column. Inflowing freshwater forms a distinct layer above the denser seawater, separating bottom waters from the surface waters that are well aerated through photosynthesis and atmospheric exchange.

The authors concluded that detailed studies involving water quality measurements are necessary before an estuary’s health status can be assessed. Clearly, one or two sampling trips are better than no data at all, but they may not provide reliable information on aspects such as the initial pulse of nutrients entering an estuary with the first winter rains, occasional mouth openings, short-lived microalgal blooms or the persistence of water column stratification or low-oxygen conditions.

* Kaselowski T & Adams JB 2013. Not so pristine – characterising the physico-chemical conditions of an undescribed temporarily open/closed estuary. Water SA Vol. 39 No. 5 October 2013. Available on www.wrc.org.za