Multiple effects of fine suspended sediment from the Kaipara Harbour catchment

Fine sediment is a major concern for water and land managers, and scientists alike. Fine sediment loads result from the transport of eroded soil from land to rivers and estuaries. Our research aims to link measurements of fine sediment and water quality to advance our understanding of the multiple environmental effects of fine sediment.

We make the distinction here between "fine" sediment, and sediment generally, because it is sediment finer than sand sizes (say less than the sand/silt boundary at 63 micrometres) that stays suspended for long periods or indefinitely in (turbulent) natural water such as rivers and estuaries, and so affects their water quality.

It is usually assumed that the environmental effects of fine suspended sediment - smothering of biota and shoaling of estuaries once the sediment is deposited, are related to its mass concentration. However, there is considerable evidence that severe impacts of fine sediment occur while it is still suspended due to its interference with light through water, referred to as 'light attenuation'.

An obvious feature of fine suspended sediment is the turbidity or cloudiness created in water (Photos 1 and 2). This cloudiness results from light scattering by fine sediment particles, which reduces visual range through water for human recreationalists, fish and birds. Light attenuation by fine suspended sediment also results in reduced penetration of sunlight to benthic – bottom dwelling – plants such as sea grasses in estuaries.

Fine suspended sediment carries pollutants, such as nitrogen and phosphorous from agricultural land, and toxic materials such as trace metals and organic biocides from urban land. The bulk of these materials are typically part of the sediment or attached to sediment surfaces. The surface concentration of fine sediment in water (units: m2 of cross-sectional area per m3 of water volume) has the same dimensions as optical coefficients (units: m-1). Considerations such as this have led to our working hypothesis: that light attenuation of fine sediment in waters is a better predictor of sediment-associated effects than its mass concentration.

The main test bed and focus for our research is the Kaipara Harbour and its catchment. We are collaborating with both Auckland and Northland Regional Councils.

Mass loads and light-attenuating loads of fine sediment are being measured at key catchment sites with emphasis on storm flows that deliver the bulk of loads and create turbid river plumes in the harbour. Two instrument platforms are being installed to continuously measure optical and physical oceanographic variables in the harbour. Surveys of light attenuation by observation and instrumental measurements, and light penetration from sensor profiling, are being carried out to define spatial patterns in the harbour and develop a model of light penetration into estuarine waters.

Fine sediment, particularly layer clay minerals, causes strong light attenuation and a 'muddy' appearance in Kaipara tributary rivers (Photo 1). This increases greatly during major floods – which inject large masses of turbid water into estuaries (Photo 2) and the harbour itself. Continuous turbidity sensors (Photo 1A) have been installed at tributary river sites, together with automatic samplers that obtain water samples during floods (Photo 3) for measurement of suspended sediment mass and light attenuation loads.

Early findings

The turbidity peak during floods in Kaipara tributaries usually arrives ahead of the flow peak suggesting that fine clay-rich sediment is coming predominantly from nearby channel sources such as river bank erosion. (Sediment from sources remote from the river monitoring site would be expected to arrive later – after the flow peak.)

Pastoral areas of the catchment are generally larger sources of clay sediment than forested areas. In the Kaipara Harbour this fine clay sediment appears to interact with coloured dissolved organic matter (CDOM); which is comparatively high in Kaipara tributaries, to reduce light penetration.

We are measuring CDOM as well as light attenuation at tributary river sites, and have found that CDOM increases (but not so markedly as turbidity) during floods – so flood waters entering the Kaipara are both much more turbid and more coloured than normal flows.

Synoptic surveys of salinity in the harbour and its estuaries show that CDOM correlates strongly with salinity in an inverse pattern: that is, as salinity rises, CDOM decreases. So it should be possible to predict light at the harbour bed and define the light climate of seagrass from (modelled) salinity and measured turbidity.

This research is on-going. 

River monitoring site at a weir on the Waiwhiu Stream, a tributary of the Hoteo River which flows into the Kaipara Harbour. The white PVC tube contains a Hach Solitax turbidity sensor (the next photo shows a close-up of the turbidity sensor). Photo: Jason Julian (University of Oklahoma).
Hach Solitax turbidity sensor. Photo: Andrew Hughes.
Bridge crossing the Hoteo River (tidal creek) on the Kaipara Coast Highway (SH 16). Note the very turbid (muddy) water. Photo: Rob Davies-Colley
Automatic sampler (on left) and instrument housing at a river site where loads of sediment mass, light attenuation, and coloured dissolved organic matter (CDOM) are being measured in the Kaipara study. (This site, on the West Hoe Stream, is a native forested reference site for comparison with – generally more turbid – pastoral subcatchments). Photo: Andrew Hughes.