Abstract
Large fluxes in the physico-chemical characteristics of estuarine lakes can have profound effects on biota and processes. Where salinity in Swartvlei Lake usually ranges between 5 and 12, extended open conditions post–2007 floods, coupled with reduced freshwater inflows due to drought, resulted in salinity exceeding the upper tolerance of dominant submerged macrophytes. A resulting die-back of macrophytes equated to a 99% decline in standing crop, and was followed by a 95% decline in the biomass of waterbirds. Significant positive correlations exist between the biomass of macrophytes and both piscivorous and herbivorous waterbirds. Whereas Swartvlei Lake is expected to, in the short term, revert to its former (pre–2007 flood) state, inevitable environmental changes such as global warming and resulting changes in local climatic and marine conditions, along with increased freshwater abstraction from feeder rivers, could cause the observed large fluctuation in the abundance of aquatic biota to become more frequent.Keywords: Swartvlei Lake, estuarine lake, drought, flood, macrophyte senescence, biotic interactions, environmental change, global warming
Highlights
Estuarine submerged macrophyte communities can be highly variable at both spatial and temporal scales, with many studies from around the world describing community changes along spatial and temporal salinity gradients (Verhoeven, 1975; Bayley et al, 1978; Verhoeven and van Vierssen, 1978a, b; Vaquer and Heurteaux, 1989; Lazar and Dawes, 1991; Quammen and Onuf, 1993)
In South African estuarine systems, substantial community changes occurring in conjunction with elevated salinity have been recorded, mostly in estuarine lakes (Whitfield, 1984; Cyrus et al, 2011)
8 estuarine lake systems occur in South Africa (Whitfield, 1995), which, considering that there are in excess of 250 estuaries in the country (Van Niekerk and Turpie, 2012), makes them relatively uncommon ecosystems
Summary
Estuarine submerged macrophyte communities can be highly variable at both spatial and temporal scales, with many studies from around the world describing community changes along spatial and temporal salinity gradients (Verhoeven, 1975; Bayley et al, 1978; Verhoeven and van Vierssen, 1978a, b; Vaquer and Heurteaux, 1989; Lazar and Dawes, 1991; Quammen and Onuf, 1993). In South African estuarine systems, substantial community changes occurring in conjunction with elevated salinity have been recorded, mostly in estuarine lakes (Whitfield, 1984; Cyrus et al, 2011). 8 estuarine lake systems occur in South Africa (Whitfield, 1995), which, considering that there are in excess of 250 estuaries in the country (Van Niekerk and Turpie, 2012), makes them relatively uncommon ecosystems. What estuarine lakes lack in number, they make up for in size, collectively comprising more than 62% of estuarine area in the country (Van Niekerk and Turpie, 2012). Estuarine lakes vary in the permanency of their marine connection, from permanent (e.g. Kosi) to seldom (e.g. St. Lucia), with the Swartvlei system lying between these two extremes, being open 68% of the time (1969–2016) and more frequently in summer (Russell, 2015). Salinity in estuarine lakes is determined primarily by the balance between marine and freshwater inflows (volume and timing), as well as evaporation
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