Abstract
Turbidity is a widely-used water quality indicator that is used to infer the volume of suspended sediment transported through riverine systems. In New Zealand, regional limits on turbidity are a component of water plans, where excessive turbidity is often used to indicate land disturbance. Turbidity of river water is a function of both organic and inorganic constituents. Measurement of inorganic suspended sediment is common; however, to date there has been little work determining how much of the total suspended material in rivers may be organic material, and how this affects relationships between suspended particulates and turbidity. The objective of this study was to determine what portion of total suspended material occurs as particulate organic matter and what effect this has on the relationship between suspended sediment and turbidity. Particulate organic matter was determined by a loss on ignition method that supplemented traditional methods for determining suspended sediment concentration. In the Glendhu Experimental Catchments, Otago, New Zealand, particulate organic matter contributed 45% of the total suspended material from a tussock catchment and 60% of the total suspended material from a forested catchment, although concentrations were highly variable: 0.6-20.3 mg L^sup -1^ (equivalent to 10-80%) in the tussock catchment and 0.7-39.7 mg L^sup -1^ (equivalent to 23-95%) in the forested catchment. These data suggest that particulate organic matter can represent a large portion of total suspended material in these catchments. The presence of dissolved organic material may also interfere with turbidity measurements, and contribute to uncertainty in deriving turbidity-suspended sediment relationships in organic-rich rivers.