Tidal exchanges of organic carbon, nitrogen and phosphorus between a south temperate Sarcocornia marsh and its associated estuary are examined. Subterranean water flow was small, and the hydraulic exchange between the two systems largely surficial. The dominant tidal signal was semi-diurnal, and the extent of inundation of the marsh varied considerably as a consequence of interactions of semi-lunar tidal cycles with changes in daily mean sea level. Annual net fluxes of organic carbon were directed from the marsh to the estuary, but amounted to less than 2% of marsh aerial net primary productivity. This indicates the incompatibility of E.P. Odum's outwelling hypothesis to this marsh-estuarine system. The direction of net flux of organic carbon switched on a time-scale of days. These directions were largely correlated with mesoscale oceanic events, which materially altered the extent of marsh inundation, and which provided evidence of the mutual exclusivity of outwelling of DOC from the marsh and oceanic upwelling. Laboratory mesocosm experiments using intact marsh blocks of sediment from the marsh were conducted to identify the proximate processes and interactions at the marsh-water interface responsible for the variability of marsh-estuarine exchanges. Patterns of fluxes of organic carbon, total nitrogen and phosphorus were markedly different in the structurally contrasted tidal creek and Sarcocornia Zone regions of the marsh. Both regions exported these components, but the fluxes of organic carbon and total phosphorus were significantly larger from the tidal creek than from the Sarcocornia zone, and the opposite applied to nitrogen. The presence of brachyuran crabs . the most numerous macrofauna on the marsh enhanced the flux of carbon, nitrogen and phosphorus from the marsh biocoenosis, largely as a result of the effect of their bioturbation. Evidence is examined which suggests that differential mobilization of nutrients in the two zones by crabs is responsible for biogeochemical coupling of these two regions , which may account for the elevated productivity of salt- marsh systems

The nutrient dynamics in two contrasting estuaries and in the adjacent nearshore environment along the south-east coast of South Africa was investigated seasonally. Due to an inter-basin transfer of water from the Gariep Dam to the Great Fish River, the Great Fish estuary is a fresh water dominated, terrestrially driven system with an annual fresh water inflow of 250 – 650 x 10⁶ m³ per year. In contrast, the Kariega estuary is a fresh water deprived, marine dominated system with a fresh water inflow estimated at 2.5 – 35 x 10⁶ m³per year. The reduced fresh water inflow into the estuary is attributed to regular impoundments along the Kariega River. Water samples were collected from surface and subsurface layers along the length of the estuaries as well as from a series of transects occupied in the nearshore environment. Samples were analysed for nitrate, nitrite, ammonium, phosphate and silicate. Temperature and salinity were recorded at each station. A Land-Ocean Interactions in the Coastal Zone (LOICZ) budget was constructed for each estuary to describe the role of ecosystem-level metabolism as either a sink or a source of phosphorus, nitrogen and carbon. Seasonal variation in physico-chemical properties and nutrient concentrations in the Kariega estuary was minimal due to constant low inflow, while in the Great Fish estuary, concentrations varied in response to changes in flow rate. Nutrient concentrations were consistently higher in the Great Fish estuary than in the Kariega estuary, largely reflecting differences in fresh water inflow. During periods of high flow (32.92 m³.s⁻¹in the Great Fish River) dissolved inorganic nitrogen (DIN) concentrations in the Great Fish estuary were an order of magnitude higher than those recorded in the Kariega estuary. Results of the LOICZ budgeting procedures revealed that in spite of the contrasting hydrodynamic features, the estuaries behave in largely the same manner – both predominantly sources of nutrients with heterotrophic processes dominating over autotrophic actions and both were net denitrifyers during all surveys. This was, however, due to different sets of processes operating in the two estuaries, namely low nutrient concentrations resulting in microbial activity in the Kariega estuary, and riverine influx of nutrients and phytoplankton combined with a short residence time of the water in the Great Fish estuary. In the marine nearshore environment, higher nutrient concentrations were recorded adjacent to the Great Fish estuary than offshore of the Kariega estuary. This was due to a surface plume of less saline water leaving the Great Fish estuary, which acted as an ‘outweller’ of nutrients. Offshore of the Kariega estuary, on the other hand, the nutrient concentrations were characteristic of marine waters due to a lack of fresh water outflow from the estuary. Nutrient concentrations in the marine environment adjacent to the Kariega estuary were, at times, higher than those recorded within the estuary. This observation supports previous statements which suggest that the Kariega estuary is not an ‘outweller’ of dissolved nutrients and particulate material, but rather an extension of the marine environment.

Many South African estuaries are currently believed to be generating lower levels of services than they used to in the past due to substantially reduced inflow of river water, among other reasons. The basis by which river water is allocated in South Africa has had to be re-examined. Local authorities are now required to integrate into their development planning sensitivity to the ways estuaries work; the relevant legislation being the Municipal Systems Act No. 32 of 2000. Sound water resource management requires that the benefits and costs of different water allocations be compared and an optimum determined. The Contingent Valuation Method (CVM) is used in this study to estimate the benefits of changing allocations of river water into estuaries. This study builds on a CVM pilot project done at the Keurbooms Estuary in the Southern Cape in year 2000 (Du Preez, 2002). Further CVM studies were conducted at the Knysna, Groot Brak and Klein Brak estuaries (Dimopolous, 2004). The CVM is a valuation technique based on answers given to carefully formulated questions on what people are willing to pay for specified changes of freshwater inflows into estuaries. The CVM depends on there being a close correspondence between expressed answers given to hypothetical questions and voluntary exchanges in competitive markets that would be entered into if money did actually change hands. The fact that it has proved very difficult to establish this correspondence has led to CVM being subject to criticism. However, many aspects of this criticism have been addressed in the form of methods to reduce biases, and the application of the technique has grown steadily in popularity during the past 25 years. Four estuaries, the Swartkops, Kariega, Mngazi and Mngazana, were surveyed as part of this study in order to determine users’ willingness to pay (WTP) for changes in freshwater inflows. Considerable research time was devoted at the estuaries getting to know how things worked around and in the estuaries. The Swartkops estuary is a permanently open system within the Nelson Mandela Bay metropolitan area. The estuary has the third largest salt marsh in South Africa. Its banks are highly developed with residential and industrial property and it is heavily used for both recreation and subsistence fishing by locals. The Kariega estuary is located near the semi-rural town of Kenton-on-sea, between Port Elizabeth and East London. Although it is permanently open, the Kariega estuary has very low inflows of river water. It is mainly used by retired pensioners living in holiday houses at Kenton-on-sea. The Kariega is not heavily used for recreation and subsistence fishing, except during holidays and the festive season because of its proximity to other estuaries such as the Bushmans and the Kleinemond. The Mngazi and the Mngazana estuaries are located in the Wild Coast area of the Eastern Cape, in the Port St Johns Municipal district. The Mngazi is a temporarily open/closed system which does not have high botanical ratings, although it is heavily used by visitors to the well known Mngazi River Bungalows, a highly rated hotel near the mouth of the Mngazi River. The Mngazana estuary is a permanently open system renowned for its Mangrove forests and excellent fishing spots. Both the Mngazi and Mngazana estuaries are located in rural areas and are heavily used by local village residents for subsistence purposes.