Physico-chemical and microalgal characteristics of the Goukamma Estuary

Estuaries are intrinsically complex and dynamic ecosystems that display marked spatial and temporal variability. Because estuaries are situated at the receiving end of catchment activities, they are at particular risk of alterations to their natural complexity. The overarching objective of this study was to gain an overview of the abiotic conditions and biotic response of the Goukamma Estuary, a small temporarily open/closed estuary (TOCE) which is situated in a relatively undisturbed catchment in the Southern Cape. Physico-chemical properties drive estuarine ecology, and together with biological indicators, are commonly assessed to determine the present status of an estuary. During the study, physico-chemical parameters reflected great spatial and temporal variability in response to the mouth state over a 13 month period. Parameters ranged within expected limits, as proposed by the conceptual model for water quality of TOCE’s (Snow and Taljaard 2007). Of particular importance was the prominent occurrence of salinity stratification and hypoxic conditions (dissolved oxygen [DO] < 3 mg l-1) during both open and closed mouth states. Data indicated that in the wide and shallow lower reaches, weak stratification gradients were present and oxygenated conditions (DO > 6 mg l-1) were maintained throughout the water column mainly by wind and tidal action. However, stratification increased towards the deeper, channel-like middle and upper reaches of the estuary, followed by a significant reduction in bottom DO concentrations and development of hypoxia and anoxia. Bottom water hypoxia commonly occurs in microtidal estuaries due to the limited influence of mixing forces, mainly by wind and tidal action. The Goukamma Estuary is a channel-like microtidal estuary where stratification effectively limited oxygenation of the bottom water which resulted in frequent occurrence of bottom water hypoxia. During June 2010 when the highest local rainfall (75 mm) was recorded for the region, salinity and DO data showed that this amount of rainfall was insignificant as it did not replenish the water column of oxygen. Only the surface 0.5 m layer was fresh and oxygenated while below this, the water column was completely hypoxic. In an unimpacted state, the Goukamma Estuary is a blackwater system and is expected to be nutrient poor; however, farming activities in the catchment have resulted in elevated nutrient concentrations. This study showed that significantly higher nutrient concentrations were measured in the middle and upper reaches of the estuary, adjacent to cattle farms situated in the floodplain of these reaches. Nutrient concentrations represented mesotrophic (dissolved inorganic nitrogen [DIN] > 500 μg l-1) to eutrophic conditions (dissolved inorganic phosphorus [DIP] > 25 μg l-1). Nutrient input stimulated phytoplankton to attain a significantly high biomass, ranging between 0.3 – 112 μg l-1 (~ 7.7 ± 1.3 μg l-1; n = 128) and 0.8 – 289 μg l-1 (~ 21.1 ± 4.4 μg l-1; n = 80) during the open and closed states, respectively. High organic loads are associated with high oxygen demands which consequently result in hypoxia following decomposition. Exacerbated by natural salinity stratification which effectively limits oxygenation of the water column, unnaturally high nutrient concentrations and coinciding organic loads place the estuary at particular risk of degradation. This study captured key patterns and processes by quantifying salinity, oxygen and nutrient concentrations in addition to biological indicators (phytoplankton biomass and community composition). Considering possible budget constraints, it is recommended that monthly salinity and oxygen concentrations should be monitored as well as seasonal nutrient concentrations. It is also recommended that riparian buffer zones should be established in the middle and upper reaches of the estuary, as these vegetation buffers have been well documented to contribute to nutrient attenuation and improved water quality from agricultural run-off