Macrobenthic community and ecotoxicological status of the Nhlabane Estuary
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Date
2010
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Abstract
The Nhlabane Estuary is a small temporary open/closed estuary along the Northern KwaZulu-Natal coastline, 40 km north of Richards Bay. Historically, the Nhlabane Estuary formed the tidally influenced lower reaches of the Nhlabane coastal lake system, but in 1978, its connection with the coastal lake was permanently severed by the construction of a weir across the upper end of the estuary. The weir altered the natural flushing regime of the estuary, resulting in the estuary mouth often remaining closed for a number of years. Dune mining operations around the estuary raised concerns about trace metal concentrations in the estuary. This study was initiated following concerns about the ecological integrity of the estuary in relation to reduced freshwater input into the system and its effect on the macrobenthic community, as well as the potential trace metal contamination of the estuary water, sediment and biota due to mining activities. The study aimed to describe the macrobenthic community of the Nhlabane Estuary during the post-drought period 1996-1999, to assess the ecotoxicological status of the Nhlabane Estuary in relation to dune mining activities and to determine the suitability of two resident amphipod species, Corophium triaenonyx and Grandidierella lignorum, as potential test organisms for sediment toxicity testing in estuarine environments. Quarterly samples of the benthos, water and sediment were collected at six sites in the estuary over the period 1996-1999. Physical water quality parameters were measured in situ at each sampling station. At the start of the study, the estuary showed a highly stressed benthic community characterized by low species richness and densities, caused by a combination of a naturally occurring drought event exacerbated by anthropogenically induced lack of freshwater input into the system. The number of taxa and benthic densities increased significantly after the mouth was breached. The 32 macrobenthic taxa recorded during the four year study period were dominated by estuarine crustaceans, notably the two amphipods amphipods, C. triaenonyx and G. lignorum, and the polychaetes Ceratonereis keiskamma, Desdemona ornata and Prionospio sexoculata and the tanaid Apseudes digitalis. Multivariate analysis showed a marked temporal change in the benthic community, reflected in significant changes in the number of taxa, density, species richness and species diversity during the 1996-1996 study period. This represented a recovery in the Nhlabane Estuary benthic following the extended period of mouth closure during the 1992-1995 drought. Trace metal concentrations in the Nhlabane Estuary water, sediment and invertebrate tissues were consistently lower than in regional anthropogenically influenced estuaries. Normalization of Nhlabane Estuary sediment metal concentrations to background concentrations for uncontaminated KwaZulu- Natal coastal areas showed the system to be uncontaminated, with dune mining not being the cause of pollution in the system. Trace metal concentrations in the two amphipod species, G. lignorum and C. triaenonyx, also compared favorably with that found in similar species, providing further evidence of the relatively uncontaminated conditions in the system. Amphipods are commonly utilized throughout the world to test the toxicity of potentially contaminated sediments in marine and estuarine environments, because they are ecologically relevant, are sensitive to sediment contaminants and are suitable for laboratory experimentation. Laboratory studies with cadmium, zinc and copper to test the suitability of the two local amphipod species, Grandidierella lignorum and Corophium triaenonyx as test organisms in sediment toxicity tests, showed that both species accumulated Cu and Zn in their bodies in relation to increasing concentrations of the metals in the sediment. The ability to accumulate trace metals during exposure to contaminated sediments is one of the primary requirements in sediment bioassay test organisms, and results from this study have clearly shown both species to be sensitive to and accumulate bioavailable Cu and Zn in sediment. Lack of estuarine management means that estuaries in South Africa have been subjected to increasing pressure, both indirectly from the effects of catchment utilization, which affect their water supply, and directly from the increasingly industrial and residential development along the coastal zone. As a result, many South African estuaries have become functionally degraded, which has resulted in a loss of species. The Nhlabane system is located in a forestry dominated catchment, so the system is subjected to limited industrial and residential encroachment and organic and inorganic chemical contamination. Results from this study showed that trace metal concentrations in the water and sediment were low and within environmentally acceptable levels. Construction of the Nhlabane weir, however, deprived the Nhlabane Estuary of adequate freshwater and altered the natural flushing regime. The functioning of the estuary and the state of its biotic communities are largely dependent on the maintenance of the estuarine-marine link, which in turn are directly related to freshwater input from the lake. In an Estuarine Flow Requirement (EFR) study, which forms part of the RDM based Ecological Reserve Determination process for aquatic ecosystems in South Africa, water allocations were determined to maintain the ecological integrity of the Nhlabane Estuary. The EFR study recommended that during droughts the system be permitted to close for a year, but that this was only acceptable 1 year in 3. The recommended environmental flows for the estuary have not been implemented by DWAF and no allocation of water is allowed for the Reserve for the estuary. As such, abstraction of water from the lake is still causing water levels in the lake to be consistently below overflow levels. Results from this study showed that the Nhlabane Estuary, once regarded as one of the most pristine estuaries along our coastline, has been adversely affected by impoundment of the lake to the point where it ceases to be a functional estuary for periods of time extending over a number of years. Although this study has shown that the ecotoxicological condition of the estuary is generally good, the system is hydrologically stressed due to freshwater deprivation. There is currently no management plan for the estuary that can address these issues. Based on this, the following key issues should be addressed if the future ecological functioning of the Nhlabane Estuary is to be ensured: It is imperative that the Ecological Reserve for the estuary be implemented, as this will ensure that freshwater allocations for the estuary be adhered to. A management plan should be designed and implemented for the estuary by MCM in which all stakeholders are represented. Management of the estuary should incorporate not only the estuarine environment, but also be directed at a catchment level. A monitoring program consisting of key ecological components, which should complement the existing database and investigate long-term temporal and spatial changes in important biotic communities in the estuary, should be regarded as a priority. The Nhlabane Estuary should be considered to be of high conservation importance as it is one of the few estuaries along the coastline that is not adversely affected by industrial development, other than dune mining, in its catchment. Motivation for the Nhlabane Estuary to be made a Estuarine Conservation Areas (ECAs) should be considered.
Description
Submitted to the Faculty of Science in fulfilment of the requirements for the degree of Doctor of Philosophy in the Department of Zoology at the University of Zululand, South Africa, 2010.
Keywords
Macrobenthic community, Nhlabane Estuary, Estuaries -- South Africa