Relationships Between the Biomass of Waterfowl and Submerged Macrophytes in a South African Estuarine Lake System

Relationships between the biomass of waterfowl and submerged macrophytes in a South African estuarine lake system

IA Russell1*, RM Randall1, BM Randall2 and N Hanekom1 1 South African National Parks, PO Box 176, Sedgefield 6573, South Africa 2 PO Box 853, Sedgefield 6573, South Africa * Corresponding author, e-mail: [email protected]

The Wilderness Lakes system, comprising three estuarine lakes (Eilandvlei, Langvlei and Rondevlei), supports a diverse waterbird community, which includes 12 duck species and the abundant Red-knobbed Coot Fulica cristata. Biannual counts of waterfowl (ducks and Red-knobbed Coot) and assessments of submerged macrophyte standing crop, undertaken between 1992 and 2005, were used to investigate relationships between the biomass of macrophytes and waterfowl. Large changes in macrophyte biomass occurred during most of the study period in Eilandvlei (decreases) and Rondevlei (increases), whereas changes in Langvlei were small with no apparent long-term trends. Significant positive correlations existed between duck biomass and macrophyte biomass in Rondevlei and Eilandvlei, and between Red-knobbed Coot biomass and macrophyte biomass in Eilandvlei. Trends in the biomass of Red-knobbed Coot and macrophyte biomass were similar in Rondevlei and Langvlei, although statistically significant correlations could not be demonstrated. Management of the lakes should include provisions to maintain or restore water quality, which is adequate to sustain submerged macrophytes and the abundant waterfowl that they support.

Introduction

The Wilderness Lakes, one of only a few coastal lake Aquatic invertebrates form the major component of the diet systems in South Africa, is a Ramsar site (Cowan and of Cape Shoveler, Fulvous Duck Dendrocygna bicolor and Marneweck 1996), supporting a diverse (at times 65 Cape Teal (Siegfried 1981, Brickell 1988). In the Wilderness species) and large (at times 7 000 individuals) waterbird and adjacent Swartvlei lake systems there is a strong community (Boshoff and Palmer 1991). Six of the 13 species correlation between aquatic plant biomass and invertebrate studied (Red-knobbed Coot Fulica cristata, Yellow-billed standing stocks (Parker and Wucher 1978, Davies 1982). Duck Anas undulata, South African Shelduck Tadorna cana, Most of the zoobenthos, including more than 80% of the Egyptian Goose Alopochen aegyptiaca, Cape Teal Anas bivalve Brachidontes virgiliae population, which itself typically capensis and Cape Shoveler Anas smithii) are found at most constitutes between 95 and 99% of the benthic standing ( 65%) of the major (supporting 5 000 birds) wetlands in stock (Allanson 1981, Davies 1982), occur on submerged the Western Cape province, while the others have a more aquatic plants (Parker and Wucher 1978, Davies 1982). restricted distribution (Ryan et al. 1988). The most abundant The close link between zoobenthos and submerged aquatic of these waterbirds in the Wilderness Lakes system is the plants has been demonstrated in the adjacent Swartvlei Red-knobbed Coot (Boshoff et al. 1991). Here it feeds almost system where the senescence of submerged macrophyte exclusively on sago pondweed Potamogeton pectinatus and beds over a three-year period, characterised by a 60% stoneworts Chara spp. (Fairall 1981). decline in primary production (Taylor 1983), resulted in a Twelve principally nomadic duck species in the families 74% decline in littoral invertebrate biomass (Davies 1982). Dendrocygnidae (two spp.) and Anatidae (10 spp.) have Cape Shoveler feed predominantly on insect larvae, crusta- been recorded, of which Yellow-billed Duck, Red-billed ceans and molluscs, along with plant material agitated from Teal Anas erythrorhyncha, Hottentot Teal Anas hottentota, subsurface substrata (Brickell 1988). Thus, the presence Southern Pochard Netta erythropthalma, White-backed and biomass of stands of submerged aquatic plants is likely Duck Thalassornis leuconotus, Maccoa Duck Oxyura to be of considerable importance in determining food availa- maccoa, South African Shelduck, Egyptian Goose and bility for waterfowl (ducks and Red-knobbed Coot). Spur-winged Goose Plectropterus gambensis are primarily Both long- and short-term changes have occurred in the herbivorous (Siegfried 1981, Brickell 1988). The latter three biomass and distribution of submerged aquatic plants in species feed predominantly on terrestrial vegetation, though the Wilderness Lakes system (Davies 1982, Weisser and of these only the Egyptian Goose is abundant (Boshoff et al. Howard-Williams 1982, Whitfield 1982, Weisser et al. 1992), 1991, Table 1) with most individuals recorded on the lakes which in turn would be expected significantly to influence being in moult, during which time they feed predominantly food availability for waterfowl. The objective of this study on aquatic plants (Halse 1984). was to investigate the relationship between changes in the