Major and trace element geochemistry in Zeekoevlei, South Africa: A lacustrine record of present and past processes
Introduction
Trace and major elements have a dynamic distribution and behaviour in lakes. Some elements are necessary micronutrients for living organisms although many of them (e.g., Cd, Cu, Pb and Zn) are toxic over a certain concentration and can adversely affect aquatic organisms and human health (Bowen, 1966, Nor, 1987, Timmermans, 1992, Silva et al., 2000). Their sources in lakes can be natural or anthropogenic. Urban lakes, in particular, are more vulnerable to trace element pollution because of widespread cultural activities surrounding the lake.
There is a distinct lack of data on seasonal variations in trace and major element concentrations in urban lakes (Balistrieri et al., 1992, Murray, 1987). Moreover, most trace metal studies have been done in deep lakes with a seasonal or permanent anoxic deep layer (e.g., Seyler and Martin, 1989, Michard et al., 1994, Viollier et al., 1995, Mogollon et al., 1996, Widerlund et al., 2002, Granina et al., 2004). In contrast, shallow lakes remain oxic throughout the year (e.g., Elbaz-Poulichet et al., 1997). These lakes are generally characterised by high primary productivity, and their relatively shallow depth enables considerable sediment resuspension (e.g., Schallenberg and Burns, 2003). Metal enrichment in shallow lake sediments is controlled by several factors, for example, pH, phytoplankton abundance and organic matter (see Bilali et al., 2002). Any minor changes in these controlling factors can initiate a series of geochemical processes, which affect metal accumulation in sediments. More importantly, a decipherable sedimentary record of these geochemical changes can be used to interpret palaeoenvironmental processes (e.g., Sternbeck et al., 2000, Routh et al., 2004, Routh et al., 2007).
The rich ecological importance (for diverse avifaunal composition) of Zeekoevlei (vlei meaning lake in Afrikaans), and its proximity to Cape Town and the coast (2 km) has prompted this study on trace and major element deposition and cycling. The lake is hyper-eutrophic and is believed to be widely affected by anthropogenic inputs, which have resulted in enhanced productivity in recent years (Southern Waters Ecological Research and Consulting, 2000). To the best of the authors’ knowledge, there has been no published work on element cycling in Zeekoevlei. Moreover, the use of trace elements as geochemical proxies (e.g., Sternbeck et al., 2000) to interpret different biogeochemical processes in the lake sediments is rare. Hence, the main objectives of the study are to: (1) use sedimentary organic matter and element records as proxies to interpret recent historical changes in the lake and its catchment, (2) investigate the seasonal variation in concentrations of selected elements and their sources in the water column and sediments, and (3) study the sources of dissolved major inorganic ions. The existence of any possible correlation between organic matter sources and trace and major element input with particular emphasis on the lake’s primary productivity is also investigated. This study demonstrates the use of both organic and inorganic geochemical proxies in exploring the historical changes and environmental perturbations in Zeekoevlei. Many aspects of this study can thus be extended to interpret analogous observations in other lacustrine systems.
Section snippets
Study area
Zeekoevlei is a well-mixed, alkaline (pH 9) freshwater (salinity 0.3‰) coastal lake. The lake is situated on the sandy coastal plains of Cape Flats near Cape Town in the Western Cape Province, and is the largest of the shallow (maximum depth 5 m) urban lakes in South Africa (surface area = 2.56 km2, longest dimension = 2.3 km; Harding, 1997). Zeekoevlei is a warm water lake and temperature varies between 10.3 °C and 28.5 °C (mean 18 °C) in the water column. The prevailing climate is sub-tropical
Water sampling and chemistry
Surface water samples were collected in January and August 2004 (Austral summer and winter, respectively) from an inflatable boat. Divers collected water samples from the lake bottom in winter. All sample containers and laboratory glassware were pre-cleaned with detergent, soaked in HNO3 (10%) for 24 h and rinsed with deionised distilled water before use. Acids and standard solutions used during the digestion and analyses were of suprapure grade. All bottles and equipment used to collect and
Water quality characteristics and major ions
The Zeekoevlei waters had high pH, BOD and DO levels, moderate to low concentrations of dissolved major ions, low EC and TDS (Table 2). DOC concentrations in the different seasons varied within a narrow range. The calculated PL was 0.0005 ton of oil equivalent m−3.
Trace and major elements in the water column
Seasonal and depth wise variation in dissolved and particulate element concentrations are presented in Table 3. Dissolved element concentrations in surface water showed an increase in winter versus the summer samples. Dissolved
Sources of major ions and processes affecting water quality
The results of the study reveal that several ongoing physical (e.g., rain and wind) and chemical processes (e.g., surface weathering) influence the water quality in Zeekoevlei. The dominance of Na+ and Cl− ions in the lake water results in high Na+/(Na+ + Ca2+) and Cl−/Na+ (0.8 and 1.05, respectively) mass ratios. Sea salts, which are supplied by mist, salt-laden on-shore winds and coastal rainfall, are the major source of Na+ and Cl− (Gibbs, 1970, Mphepya et al., 2004). This finding is supported
Conclusions
Zeekoevlei demonstrates the affect of cultural activities over the natural in-lake processes in shallow lake environment. Although, the greater fraction of trace and major elements supplied in this lake have natural sources (e.g., chemical weathering and sea salt), an increase in anthropogenic inputs (e.g., fertilizers, agricultural wastes, sewage and road runoff) in recent years is observed from the higher accumulation of As, Cd, Cu, Pb, Zn and P in surface sediments. Moreover, the dissolved
Acknowledgements
The authors gratefully acknowledge the financial support of SIDA–NRF and SMF for this study. Heather Sessions, Geof Bailey of Marine and Coastal Management, South Africa and Roger Herbert of Uppsala University are acknowledged for their technical support. Dalton Gibbs of Rondevlei Nature Reserve is acknowledged for providing information on Zeekoevlei. Eric H. De Carlo and Kevin Taylor are acknowledged for reviewing the manuscript. This is an AEON contribution #50.
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