The macrobenthos of the Mlalazi estuary: KwaZulu-Natal
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Date
2002
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Abstract
The Mlalazi estuary situated in the Mlalazi Nature Reserve is regarded as one of the least spoiled estuaries on the KwaZuIu-Natal coast, with the relatively unspoilt character being enhanced by the location of the estuary in a marine protected area (MPA). Available information of the biota of the estuary is, however, limited to one ecological study undertaken by Hill (1966). A survey, conducted by the Coastal Research Unit of Zululand for the purpose of a masters study, of the benthic community was initiated during the period 1989-1991 but unfortunately was never written up or published. Additional sampling in the Mlalazi estuary was carried out from August 1999 to July 2000 at the same six sites where the 1989-1991 samples were collected, with the addition of site 7 in the mouth region.
A prawn farm was established in 1992 approximately 500 m from and adjacent to the middle reaches of the estuary. Concern has been expressed about water being discharged into the estuary from the prawn farm since this represents a source of anthropogenic disturbance.
This study focused on describing the water quality and macrobenthos of the Mlalazi estuary during the 1989-1991 and 1999-2000 sampling periods. The community structure, temporal and spatial abundance patterns of the two sampling periods were examined A relationship between the observed community patterns and environmental variables was also determined to find the possible causes for the observed abundance patterns. Since no study has been done on the potential impact of the prawn farm on the biotic community of the estuary, comparison of the 1989-1991 (pre-prawn farm period) and 1999-2000 (post prawn farm period) sampling periods attempted to correlate the prawn farm activities to changes in community patterns of the two periods.
The physico-chemical results indicated high concentrations of nutrients (nitrates, nitrites, orthophosphates, total phosphates) during the two sampling periods. Since there were no major industries and the prawn farm was not yet established during the
1989-1991 sampling period, high nutrient concentrations were suspected to be due to agricultural runoff. The high nutrient load during the 1999-2000 sampling period indicated the prawn farm as the source. This is because high concentration of these nutrients were recorded at the prawn farm outlet and at Site 3, which is the site closest to the prawn farm outlet, compared to the rest of the sites.
The results of the biotic analysis indicated that the biotic community of both periods was disturbed to some degree with the 1989-1991 period indicating that it was more disturbed than the 1999-2000 sampling period. The 1989-1991 period was characterised by a reduced number of taxa (28) and was dominated by highly abundant opportunistic species such as Prionospio and capitellid polychaetes. The 1999-2000 period recorded 36 taxa but was also dominated by highly abundant Prionospio spp
Multivariate analysis (classification and ordination) indicated the importance of environmental factors in determining the structure of biotic communities for both the sampling periods. The significant variable accounting for the observed distributional patterns of the biotic community in 1989-1991 was the median particle diameter (pw = 0.505) whereas in the 1999-2000 sampling period it was salinity (pw - 0.292). However a combination of environmental factors played an important role in determining the observed biotic patterns. A combination of salinity, oxygen, turbidity, depth and median particle size was responsible for distribution of the benthic fauna during the 1989-1991 period. During the 1999-2000 sampling period, salinity and organic content formed a combination most responsible for the biotic patterns observed.
Even though the above results indicated that organism abundance and distribution were largely determined by physical factors, the chemical results did indicate that large amounts of nutrients in the estuary are being discharged from the prawn farm. The impact on the estuary and the organisms inhabiting it may not be apparent at this stage but there is a concern that continuous discharge of such a high nutrient load may affect the estuary in future. A monitoring program should therefore be initiated with this study being used as a baseline from which references can be made.
Description
Submitted to the Faculty of Science in fulfilment of the requirement for the degree of Master of Science in the Department of Zoology at the University of Zululand, 2002.
Keywords
Estuaries--South Africa--Mlalazi estuary, Estuaries--South Africa--KwaZulu-Natal, Benthos--Mlalazi estuary--South Africa