pollution on the natal coast

Discussion in 'General Discussions and Advice' started by SaltyReef, 26 Jul 2010.

  1. SaltyReef


    20 Apr 2010
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    The Durban Metropolitan Area's (DMA) coastline is heavily modified by urban development which has destroyed much of the primary dune habitat. It does, however, host numerous tropical and sub-tropical animals and plants, many of which occur only in this area. Most fish and invertebrate species targeted by recreational and commercial fishers are either fully exploited or over-exploited. In particular, several species of reef fishes e.g. seventy four and red steenbras are now endangered. Water quality along the DMA's coast is generally good except for areas around the Umlass and Reunion canals. Plastic pollution remains a problem along the entire coast.
    1) Coastline
    The Durban Metropolitan Area has 83 km of coastline bordering the Indian Ocean The coastline comprises both sandy and rocky shores. This coastline is punctuated by 13 estuaries as well as the port of Durban into which several canalized rivers flow. Urban development extends into much of the DMA's coastline, with housing being located close to the sea along almost the entire coastline. Much of the primary dune habitat has been destroyed by this development. In some cases this has impacted on the profile of beaches.
    2) Marine resources
    </B>The warm coastal waters of Durban (19-26 oC) are strongly influenced by the southward-flowing Agulhas current. This has resulted in Durban's coastline hosting numerous tropical and sub-tropical animals and plants. Most of the marine biota is of Indo-Pacific origin. There is also a high percentage of species which only occur in the area (endemic). Most of the fish and invertebrate species targeted by recreational and commercial fishers are either fully exploited or over-exploited.
    The DMA's rocky shores are inhabited by species such as mussels, oysters, octopus, crayfish and crabs. Between the rocky outcrops there are sandy beaches where species such as mole crabs and ghost crabs are found. The principal invertebrate species collected are crayfish, mussels, oysters, red bait, prawns and sea lice. Their status is generally fair to good. This is because of regulations which have been applied to their harvesting.
    There are approximately 125 sand prawns/m2 on some of the sand banks in the Durban harbour, and red bait densities on some intertidal and subtidal reefs along the KwaZulu-Natal coast may exceed 2 kg wet flesh/m2(Tomalin, 1995, 1996 & Fielding, 1992). Crayfish densities on reefs between Richards Bay and Port Edward are in the region of 7 crayfish per 100 m2 of reef (Fielding, 1997).
    Mussel stocks have declined somewhat since the 1970s because large-scale stripping of the rocky shore has resulted in major declines in recruitment of juvenile mussels. Despite this decline, mussels and most of the other species collected by recreational harvesters are still readily available along the DMA's coastline. Mussel cover is patchy, but in heavily harvested areas such as that between Umhlanga and Mdloti, there is about 40% mussel cover in the lower intertidal rocky shore.
    Below the DMA's tidal zone there are sandy expanses that are interspersed with reefs where a high diversity of marine fish species occur. In the past, reef fishes such as the seventy four, poenskop and red steenbras made up 60-80% of the catch by weight. Today, these species make up less than 1% of the catch. Seventy four are now specially protected, and poenskop and red steenbras are both on the critical list (Marine Living Resources Act 18 of 1998).
    In the 1970s, the shad (or elf as it is known in the Cape) fishery experienced a serious decline in catch rates and total catches. Subsequent legislation has resulted in a stabilisation of this fishery and it is currently sustainably harvested. The status of pelagic game fishes such as king mackerel and queen mackerel is generally good, but the management of these resources is complicated because the fish migrate over long distances and the stocks are shared with neighbouring Mozambique.
    Mammals and seabirds
    Marine mammals such as dolphins and whales, and a variety of seabirds, are also regularly sighted along the DMA coast. The presence of dolphins along Durban's coast is a marketable tourist attraction and companies now offer tours for people wanting to view marine mammals in their natural environment.
    3) Water quality
    The quality of water along Durban's coast is generally good except for areas around the Umlaas and Reunion canals. During the summer, heavy rainfall washes large quantities of soil into the rivers which is carried out to sea discolouring seawater close to the coast. Plastic pollution remains a problem along the entire coast.
    Because of the mixing and dispersion resulting from the Agulhas current, inshore currents and wave action, the impact of effluent disposal into the marine environment along the Metropolitan coastline has been limited.

    Industrial outfalls
    Offshore water sampling in the vicinity of the industrial outfalls indicates that the effluent is sufficiently diluted to be non-hazardous. The concentration of mercury in water samples collected close to the AECI pipeline have on occasions been above the maximum accepted level (0.3 mg/l), but sediment mercury levels in the region of the outfall are low and do not appear to be increasing. The effluent occasionally discolours the sea in the vicinity of the outfall, but is otherwise relatively benign.
    The effluent from the SA Tioxide pipeline is extremely acidic and the ferrous sulphate component flocculates on contact with sea water. This has, in the past, resulted in stained sediments and impoverished benthic communities within a radius of 750 m of the outfall. Trace metal concentrations in the sediment are within acceptable limits. Surface discoloration is also present on occasion.
    The SAPPI-SAICCOR pipeline is probably the most controversial of the marine outfalls affecting the DMA marine environment. The effluent is buoyant and frequently discolors the sea. It has little adverse effect on benthic fauna except for a possible mild organic enrichment of the sediment close to the outfall. The discharge is, however, close to Aliwal Shoal which is an important focus for SCUBA divers, commercial and recreational fishers and spearfishers. They believe the discoloration is harmful to the Aliwal Shoal environment. In addition, algal blooms associated with persistent foam in the surf occurs on beaches both north and south of Umkomaas.
    Sewage outfalls
    The main sewage and domestic effluent outfalls in the DMA appear to have little negative impact on water quality. Continuous monitoring has shown that the environment around the outfalls is satisfactory. There is no build-up of sewage organisms in the sediment stations closest to the outfalls, trace metal concentrations are low, rich and diverse benthic communities are present close to the outfalls and the effluent falls within the accepted toxicity range (CSIR, 1999). Two stations near the southern outfall had impoverished benthic communities in the middle 1990s, but this was not considered significant. Elevated levels of trace metals (copper, lead and zinc) have been found in sediments at one monitoring station near the southern outfall, but generally levels are low. Trace metals in beach sediments, and mussel and oyster flesh from the shore adjacent to the pipelines, are generally low. In the area of the Umlaas and Reunion canals there are slightly elevated levels of mercury, chrome, copper and lead.
    Annual mean heavy metal concentrations in the effluent of the Southern outfall
    (From CSIR, 1999) ​

    An historical record of surf zone pollution along the coast between the Umgeni and Isipingo Rivers is shown in this figure . The horizontal axis shows the stations approximately distanced and the vertical axis shows years. Red shaded Class IV areas are heavily polluted, while lightly shaded blue areas indicated as Class I are not seriously affected. The level of pollution is determined from the sum of a number of indicator tests such as levels of E. coli, Salmonellae, Shigellae, Staphylocci and parasites. Clearly, there was a major reduction in inshore levels of pollution after the commissioning of the offshore pipelines in 1968 and 1969. Today, water quality along the coast is generally very good except for the areas around the Mlaas and Reunion canals.

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  3. Pistolshrimp


    3 Aug 2009
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    interesting article dude, i still need to get me some sand shrimp, can never see the buggers

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