An aquatic ecotone and its significance in conservation

Abstract

Aquatic invertebrates were sampled throughout an area of transition between a well-established reservoir and a perennial stream in the KwaZulu-Natal Midlands of South Africa. The visibly obvious separation of stream and reservoir was not reflected by the invertebrates. Communities overlapped in species, ranging from fast-flowing stream and moderate-flowing stream, through a transitional ecotone, which ran along the exposed reservoir edge and stream edges, to sheltered edges of reservoir and stream then to open reservoir. These features reflected the degree of water movement (whether from waves or riffles) and type of substratum, rather than visibly lotic or lentic conditions. The main aquatic ecotone between the open reservoir and the main stream was therefore not at the mouth of the stream but along the edges of both systems. The centre of the reservoir and centre of the stream, although physically connected, were quite different in their invertebrates and were separate ‘patches’ with a sharp boundary. They were faunistically connected through the communities of the edge ecotone. The ecotone acted like a semi- permeable membrane to the drifting stream fauna with movement back again apparently mostly in the air, suggesting a ‘patchy metapopulation’ model. Both the river continuum and boundary concepts were applicable to this multi-system. The ecotone did not show any edge effect and accords more with the ‘variegation’ than ‘fragmentation’ model. The great difference in patterns of pelagic, littoral and terrestrial communities has important implications for zoning of water bodies for different conservation and recreation activities.