Nitrogen, phosphorus and silicon in riparian ecosystems along the Berg River (South Africa): The effect of increasing human land use

E Struyf,Kd Bal,A Casteleyn,E De Deckere,H Backx,D Vrebos,Lm Raitt,L Brendonck,P Meire,J Schoelynck

doi:10.4314/wsa.v38i4.15

Abstract

The Berg River is one of the main rivers in the Cape Region; it is essential for the local economy and ecology, as it supplies water to agriculture and industries, provides drinking water for the greater Cape Town region, and supports rich aquatic ecosystems. The Berg River is impacted by both diffuse pollution from agricultural run-off and point-source pollution from urban and industrial wastewater. Construction of a dam on the headwaters of the Berg River in 2007 has changed the hydrology of the upper catchment. Pelagic nutrient dynamics in the Berg River are well documented. The opposite is however true for riparian nutrient dynamics. We studied changes in riparian nutrient storage over a gradient in elevation (a proxy for flooding frequency and drought) and human influence (the Berg River dam and lateral nutrient and pollutant input). Our results show that nutrient concentrations in the riparian sediments reflect nutrient concentrations in the river. N concentrations in the sediment increased up to 1 000%, while P concentrations rose up to 200% with increasing human influence. For biogenic Si, we found generally low concentrations throughout the whole gradient sampled (all < 0.5 mg BSi g-1 sediment). Sediments closer to the river appear to have more efficient recycling and export of nutrients into the river. Overall, we conclude that the observed patterns indicate the necessity of incorporating nutrient status and management of riparian habitats in the Berg River monitoring strategy.

Highlights

The Cape Region (South Africa) is a biodiversity hotspot on the Southern African continent (Goldblatt and Manning, 2002)
De Villiers (2007) indicated up to 1 000% increases in dissolved N and P concentrations compared to pristine conditions in the Berg River water, while South African water quality guidelines stipulate that inorganic N and P concentrations should not change by more than 15% from unimpacted conditions at any time of the year
For biogenic Si, we found generally low concentrations throughout the whole sampled gradient; these concentrations are low in comparison to previous studies on Biogenic silica (BSi) in wetland sediments

Summary

Introduction

The Cape Region (South Africa) is a biodiversity hotspot on the Southern African continent (Goldblatt and Manning, 2002). One of the main rivers in the Cape Region is the Berg River. It is essential for the local economy and ecology, as it supplies water to agriculture and industries, provides drinking water for the greater Cape Town region, and supports rich aquatic ecosystems (De Villiers, 2007). The construction of a dam on the headwaters of the Berg River in 2007 and other impoundments have changed the hydrology of the upper catchment. This combination of pollutant input and hydrological effects may impact on the ecological functioning of the entire river. A combination of short unpredictable flood pulses and relatively long stable periods results in a cascade of effects on the aquatic organisms (Stanford et al, 1996; Ward et al, 2001)

Results

Discussion

Conclusion