Biological effects of salinity gradient reversals in a southeast African estuarine lake

Abstract

St Lucia Lake on the north coast of Natal, South Africa, has an area of 325 km 2 and is the largest estuarine complex in Africa. It consists of a 20 km tidal channel, averaging ca. 400 m in width, linking the sea with the non-tidal lake which is H-shaped with a maximum length of ca. 40 km and width of ca. 20 kin. Except during flood periods the depth of the lake does not exceed 2 m. The salinity gradient depends on evaporation, the configuration of the mouth and on the input of fresh water from four rivers which discharge into the northern and western areas of the lake. If fresh water input is high, the lake and much of the channel may be fresh. An intermediate stage features a normal salinity gradient while a third stage shows a reversed salinity gradient with salinities in excess of 100%o in the upper reaches of the system. Changing salinities have marked effects on the biota. Aquatic macrophytes show cycles of appearance and disappearance depending on salinity tolerance and the presence of dormant stages. The resident benthic faunal species go through cycles of range expansion and contraction depending on prevailing salinities and recolonisation by dispersal phases. To date salinities in the southern part of the lake have approached, but not exceeded, lethal levels and this has therefore acted as a reservoir area. Catchment degradation and water abstraction are anticipated to exacerbate future salinity extremes. This has resulted in concern for the long term viability of this Ramsar site which has major southern African populations of hippopotamus and crocodile, provides breeding sites for South African Red Data water bird species and plays an important nursery role for marine fish and penaeid prawns.

Fish assemblage in a temperate estuary on the uruguayan coast: seasonal variation and environmental influence

“…As many fishes spend most of their life cycle and have migratory routes there, the environmental integrity of in estuarine habitats is threatened by development of urban communities on their shores (BECK et al, 2001;ABLE, 2005;ELLIOT et al, 2007;FRANCO et al, 2008a;SELLESLAGH;AMARA, 2008). Environmental assessment studies have declared the Uruguayan coastal zone an area of high ecological value and biodiversity but subject to the increasingly adverse effects of anthropogenic activities (EcoPLATA, 2000;FREPLATA, 2005).…”

Persistent Phytoplankton Bloom in Lake St. Lucia (iSimangaliso Wetland Park, South Africa) Caused by a Cyanobacterium Closely Associated with the Genus Cyanothece (Synechococcaceae, Chroococcales)

“…Lake St. Lucia forms part of the iSimangaliso Wetland Park, a World Heritage Site, and is the largest estuarine lake in Africa (19,20). It has been subject to at least a century of indirect anthropogenic alteration, which has dramatically reduced the freshwater inflow to it.…”

“…The current 10-year-long drought is one of the longest on record, however, and has produced profound changes in the ecological conditions of the lake (4,9,20,37,39,40,52). Since 1992 the mouth has opened to the Indian Ocean on only two occasions: briefly in January 2004 and then for a 6-month period in 2007, when waves generated by Cyclone Gamede, combined with high equinoctial spring tides, breached the berm from the ocean side (9).…”

“…This infrequent opening of the estuary mouth, coupled with very low freshwater inflow, has resulted in low water levels throughout the system and progressively rising salinities, particularly in the shallow North Lake and False Bay, where high insolation rates, shallow water levels, and high temperatures result in rapid evaporation (58). As a result, hypersalinity, at times in excess of 200 (on the practical salinity scale) has become the norm for much of the northern reaches, and a reverse salinity gradient has been established, with the lowest salinities near the mouth and hypersaline conditions to the north (20).…”

“…Under normal conditions, the whole St. Lucia system supports a diverse biotic community (20,39,40). However, as a result of rising salinity, many species have been eliminated from large areas of the lake, and elsewhere only the most resistant eukaryotes appear to survive (9,37,39,40).…”