Physical processes contributing to harmful algal blooms in Saldanha Bay, South Africa

The study synthesises current understanding of the predominant physical pro cesses responsible for the seasonality of harmful algal blooms, notably Ale xandrium catenella and Dinophysis spp., in the nearshore environment of Sal danha Bay on the west coast of South Africa. Saldanha Bay is one of the few naturally sheltered areas on the South African coastline suitable for in s itu shellfish farming and is the major site for the production of black mus sel Mytilus galloprovincialis in South Africa. Mussel farming started there in 1985 and the present level of production is some 2 700 tons per annum. Since 1994, disruption of harvesting as a result of the presence of harmful algal species has been a regular late-summer phenomenon. Toxic blooms that are ultimately advected into the bay develop on the continental shelf to t he north between 32 degreesS and St Helena Bay, a region characterized by f avourable conditions for dinoflagellate growth and circulation patterns tha t facilitate build-up of intense blooms during late summer, Offshore dinofl agellate populations are advected shorewards and polewards in response to r elaxation of upwelling at the Namaqua cell to the north. Dinoflagellate blo oms are advected south from the southern Namaqua shelf during upwelling rel axation. Under such conditions, the gyre south of Elands Bay moves offshore and a barotropic flow past Cape Columbine is established. Evidence suggest s that the near-surface component of the flow occurs as a sudden "flood" ev ent. These dinoflagellate-containing shelf waters are in turn advected into Saldanha Bay when upwelling relaxes, when the density gradient between the bay and the shelf drives surface inflow and bottom water outflow. These fl ows are reversed with the resumption of upwelling over the shelf, resulting in intrusion and entrainment of bottom water and surface outflow. Entrainm ent dictates that the bay acts as a net importer of bottom water and net ex porter of surface waters over a synoptic cycle. This system of exchange bet ween Saldanha Bay and the shelf curtails the duration and severity of toxic episodes in the bay relative to the shelf.