Longitudinal patterns of organic matter transport and turnover along a New Zealand grassland river

1. We determined the longitudinal pattern of organic matter concentration, quality, size composition and spiralling length along a 310-km grassland river system (Taieri River, New Zealand). 2. Organic seston concentration (0.24-4.05 mg ash-free dry mass (AFDM) l(-1)) and dissolved organic carbon (DOC) concentration (2.3-5.7 mg C l(-1)) showed no obvious longitudinal patterns. In contrast, there was a significant downstream increase in inorganic seston concentration (0.13-13.73 mg ash l(-1)), presumably because of a downstream increase in the proportion of the catchment developed for agriculture. 3. Although there was a trend toward increasing particle size in the first 25 km of the river continuum, organic seston was primarily composed of ultrafine particles (0.6-30 mu m) at all study sites. The ratio of coarse (> 250 mu m) to ultrafine organic seston decreased logarithmically down the continuum. Organic content generally decreased with particle size. Ultrafine particles, however, had significantly higher organic fractions than fine (60-100 mu m) and very fine (30-60 mu m) particles. 4. Daily organic carbon turnover length ranged from 10 to 98 km and increased downstream. This is consistent with other studies along river continua and suggests that organic carbon turnover length is largely controlled by the relationship between channel dimensions and discharge, rather than the presence of specific retention devices. 5. Concentrations and nutritional quality of organic seston and concentrations of DOC were highest in an unconstrained floodplain reach in the upper river. These data suggest that new material enters the river channel in this reach, potentially providing an important energy source for the river community downstream. The effect of this reach on the longitudinal pattern of organic matter concentration and quality emphasizes how changes in channel form can alter river ecosystem structure and function.