A glimpse into the littoral nutrient dynamics of a lake system connected to the sea

S Taljaard,Da Lemley,L Van Niekerk

doi:10.4314/wsa.v44i1.08

S Taljaard, Da Lemley + Show 1 more

Open Access

https://doi.org/10.4314/wsa.v44i1.08

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Abstract

In South Africa more than 80% of estuaries are small, dynamic and predominantly linear systems. Nutrient characteristics in these systems are mostly influenced by external catchment processes, except during extended periods of closure. However, a small percentage of the country’s estuaries (< 3%) comprise larger estuarine lakes mostly evolved from drowned river valleys. The physical properties of these systems suggest relatively low flushing rates, and the potentially stronger influence of in-situ processes on nutrient characteristics. This study investigates dissolved inorganic nutrient dynamics in the littoral zones of these estuarine lake systems, and potential influencing factors, using the Wilderness Lake System as a case study. Comparison of inter-annual and seasonal trends in dissolved inorganic nutrients (NOx-N, NH4-N and PO4-P) in river inflow and in the three lakes confirmed that external catchment fluxes are most likely not the sole, or even the dominant drivers of average long-term or seasonal dissolved inorganic nutrient patterns in the lake littoral zones. Relatively low NOx:NH4 ratios (mostly less than 1) in the lakes (especially in Eilandvlei and Langvlei) indicated a stronger influence of in-situ processes, such as bacteriological remineralisation, similar to observations in other lake type systems with low flushing rates. Low NO3:NH4 ratios are known to stimulate cyanobacteria, having the metabolic ability to potentially produce microcystins. Therefore, incremental dissolved inorganic nutrient and organic matter enrichment may cause these systems to tip into eutrophic, possibly toxic, aquatic states that will be difficult, if not impossible, to reverse given their weak flushing mechanisms. The findings from this study highlight the importance of appropriate water resource management, both in the catchments and within the floodplains of estuarine lake systems.

Highlights

Nutrient characteristics in lakes and estuaries are influenced by numerous factors, both externally and internally
While average NH4-N concentrations in the Duiwe River were lower compared with concentrations in the lakes, average NOx-N concentrations showed the contrary, i.e., concentrations in the river were higher compared with the three lakes
Across-lake comparisons show that average electrical conductivity (EC) levels in the littoral zones of the three lakes increased incrementally moving from Eilandvlei to Langvlei and into Rondevlei, echoing the reverse salinity gradient explained in previous studies (e.g. Allanson and Whitfield, 1983; Russell, 2009; Russell, 2013)

Summary

Introduction

Nutrient characteristics in lakes and estuaries are influenced by numerous factors, both externally and internally. Key external factors derive from the catchment (e.g. surface and sub-surface flow) and the sea (e.g. tidal intrusion), where the influence is mainly determined by the proportional volume contribution from the sources, and mixing processes (Nixon et al, 1996). Key activities influencing dissolved inorganic nutrient input from catchments include wastewater discharges (point sources), as well as urban runoff and agricultural return flow (non-point sources) (Fraterrigo and Downing, 2008; Poe et al, 2003). The rate of nutrient loading into receiving water bodies (e.g. lakes and estuaries) depends on a catchment’s transport capacity, i.e., primarily defined by the compositing physical effect of hydrology, geology and topography (García-Garizábal and Causapé, 2006; Fraterrigo and Downing, 2008). Irrigation practices affect a watershed’s transport capacity by reducing or changing the hydrological patterns (García-Garizábal and Causapé, 2006)

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