Abstract
An overview of prospects and limitations for the application of ‘classical' top-down biomanipulation as a management tool to ameliorate the consequences of eutrophication under the conditions applicable to reservoirs in South Africa is presented. This is structured by considering successive stages in reservoir food-web structure and function as far as can be generalised for South African biophysical conditions. Features and conditions that influence the potential vulnerability of local reservoirs to the effects of eutrophication, and prospects for its amelioration by biomanipulation intervention are examined. Physical factors linked to latitude (irradiation pattern and water-column stability) enhance the potential severity of eutrophication consequences in local reservoirs, although conversely, these are offset by suspended-clay turbidity. The predominance of Microcystis in local eutrophic waters is perceived as a primary major constraint in implementing ‘classical' food-web manipulation. Intrinsic limitations on the ability of zooplankton grazers to control this cyanobacterium, and subsequent food-web linkages widely applicable in local reservoirs are discussed and evaluated accordingly. On balance, available evidence indicates a range of limitations that are likely to apply in respect of familiar contemporary approaches to biomanipulation in use today, although the uniqueness of each reservoir ecosystem is recognised. Some novel alternative management approaches are suggested, and a range of associated research requirements necessary to advance locally relevant scientific comprehension of ‘biomanipulation' and its application are itemised. National revitalisation of reservoir limnology remains paramount. Water SA Vol.32 (4) 2006: pp.567-575
Highlights
Eutrophication – the enrichment of aquatic ecosystems by nitrogen and phosphorus – is recognised as a serious and growing global threat to lakes, rivers and estuaries (OECD, 1982; Klapper, 1991; Moss, 1998; Straškraba and Tundisi, 1999; Holdren et al, 2001)
This manifests in blooms of generally undesirable planktonic ‘algae’ and/or prolific stands of rooted or floating vascular hydrophytes. The former commonly include ‘harmful’ toxic representatives, encapsulated as HABs. (I use ‘algae’ as a collective term to include both conventional algae, as well as photosynthetic prokaryote bacteria). This elevated primary production is a major concern to the water quality of inland waters (OECD, 1982; Klapper, 1991; Moss, 1998; Straškraba and Tundisi, 1999; Holdren et al, 2001) in view of its profound and generally ‘disruptive’ impacts on ecosystem structure and functioning
Emphasis was initially directed at reducing nutrient loading rates (OEDC, 1982), an approach that was (e.g. Walmsley and Butty, 1980; Grobler and Silberbauer, 1984), and remains prominent in the South African context (e.g. Walmsley, 2000; Anon., 2004a, b)
Summary
Eutrophication – the enrichment of aquatic ecosystems by nitrogen and phosphorus – is recognised as a serious and growing global threat to lakes, rivers and estuaries (OECD, 1982; Klapper, 1991; Moss, 1998; Straškraba and Tundisi, 1999; Holdren et al, 2001). The direct consequence of this nutrient ‘pollution’ is the excessive growth of autotrophic organisms in affected water-bodies In standing waters, this manifests in blooms of generally undesirable planktonic ‘algae’ and/or prolific stands of rooted or floating vascular hydrophytes. (I use ‘algae’ as a collective term to include both conventional algae (photosynthetic eukaryote Protista), as well as photosynthetic prokaryote bacteria) This elevated primary production is a major concern to the water quality of inland waters (OECD, 1982; Klapper, 1991; Moss, 1998; Straškraba and Tundisi, 1999; Holdren et al, 2001) in view of its profound and generally ‘disruptive’ impacts on ecosystem structure and functioning. Perhaps partly from desperation at the severity of eutrophication evident in various South African
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