Abstract
There are currently no South African nutrient standards for estuarine waters since the concentration of nutrients in the water column will not necessarily be predictive of the response by aquatic biota. In this paper a more suitable approach to assessing nutrients in relation to ecosystem integrity is proposed through compiling a nutrient mass balance for an estuary, which can identify major sources and sinks of nutrients. Recently there has been a move to develop budgets that link several variables using known stoichiometric relationships using limited data availability to infer underlying fluxes. The LOICZ biogeochemical budgeting protocol was applied to data from the Mhlathuze, Mvoti, Nhlabane and Thukela Estuaries in KwaZulu-Natal. Results indicated that these estuaries are all net sources of dissolved inorganic nitrogen and phosphates. The application of the flux-derived parameters used in the LOICZ modelling approach was used to demonstrate how nutrient assessment categories can be assigned to estuaries. As the LOICZ-derived fluxes represent steady-state conditions these were regarded as the reference/benchmark or present ecological state conditions. Alterations in the nutrient assessment category are demonstrated using hypothetical conditions in the Mhlathuze Estuary.
Highlights
The chemistry of estuaries is dependent on a combination of tidal pulses, riverine flow, hydrodynamic and autochthonous biological processes (Allanson and Winter, 1999)
Grobbelaar (1992) argued that over-simplified models of nutrient loads are inadequate for estuaries and other ecosystems where hydrodynamic factors and high turbidity can mediate the effects of nutrients, as is the case in most South African estuaries
This paper reports on the interpretation of chemical data from four sub-tropical estuaries in KwaZulu-Natal using the United Nations Environmental Program (UNEP) biogeochemical modelling framework
Summary
The chemistry of estuaries is dependent on a combination of tidal pulses, riverine flow, hydrodynamic and autochthonous biological processes (Allanson and Winter, 1999). Only measuring the concentration of nutrients in the water column does not take into account the fact that polluted water bodies will have significant stores of N and P in the sediments and associated with suspended particulate matter. Plants can derive their nutrients from sources other than in the water column, e.g. sea-grass beds meet their high nutrient demands by trapping nutrients and by uptake and recycling in the beds, not in the water column (Erftemeijer and Middleburg, 1995). Grobbelaar (1992) argued that over-simplified models of nutrient loads are inadequate for estuaries and other ecosystems where hydrodynamic factors and high turbidity can mediate the effects of nutrients, as is the case in most South African estuaries
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