Abstract
Abstract. Turbidity is often monitored continuously as a proxy for suspended sediment in catchment sediment load studies, but is less often applied to measuring optical ‘loads’ as they affect water quality in downstream waters. We added measurements of visual clarity, from which light (beam) attenuation can be estimated, to auto-sampler monitoring over storm events in tributary rivers of the Kaipara Harbour, a large barrier enclosed estuary complex in northern New Zealand. This paper presents, for the first time, evidence of the mutual relationships between turbidity, total suspended sediment (TSS), and visual clarity, from water samples collected under event flow conditions. The mutual relationships between turbidity, TSS and visual clarity for our monitoring sites were fairly close over about three orders of magnitude (TSS ranging from about 1–1000 mg L−1). Our results show that visual clarity (and hence light attenuation) can be predicted from turbidity, at least as precisely as more traditional predictions of TSS from turbidity. The estimation of light attenuation and corresponding load estimates from visual clarity measurements, for relatively little marginal extra effort, extends the environmental relevance and application of suspended sediment monitoring.
Highlights
Historical land clearance and the establishment of agricultural systems have increased erosion rates and the delivery of fine sediment to New Zealand rivers and receiving estuaries
In this paper we have presented data indicative of the mutual relationships between visual clarity, nephelometric turbidity and total suspended sediment (TSS) for six sites within the two largest rivers draining to the Kaipara Harbour in northern New Zealand
Our results show that visual clarity can be predicted from turbidity, at least as precisely as more traditional predictions of TSS from turbidity
Summary
Historical land clearance and the establishment of agricultural systems have increased erosion rates and the delivery of fine sediment to New Zealand rivers and receiving estuaries. Elevated sediment mass concentrations in rivers can adversely affect ecosystem health by the process of infilling and shoaling, and smothering downstream biota (Thrush et al, 2004). The light-attenuating effects of suspended sediment causing reduced visual range and light penetration, are probably of more ecological significance. While suspended sediment is the main driver of increased light attenuation and reduced light penetration, coloured dissolved organic matter (CDOM, leached from decaying plant material in soils) affects light penetration in water. Elevated suspended sediment concentrations (SSC) and CDOM are transported from catchments during flood events and dispersed in estuaries by buoyant river plumes. The greater the attenuation of light by suspended sediment and CDOM, the lower the water clarity
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