LONGITUDINAL PATTERNS OF ORGANIC-MATTER TRANSPORT AND TURNOVER ALONG A NEW-ZEALAND GRASSLAND RIVER

1. We determined the longitudinal pattern of organic matter concentrat ion, quality, size composition and spiralling length along a 310-km gr assland river system (Taieri River, New Zealand). 2. Organic seston co ncentration (0.24-4.05 mg ash-free dry mass (AFDM) l(-1)) and dissolve d organic carbon (DOC) concentration (2.3-5.7 mg C l(-1)) showed no ob vious longitudinal patterns. In contrast, there was a significant down stream increase in inorganic seston concentration (0.13-13.73 mg ash l (-1)), presumably because of a downstream increase in the proportion o f the catchment developed for agriculture. 3. Although there was a tre nd toward increasing particle size in the first 25 km of the river con tinuum, 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 u ltrafine organic seston decreased logarithmically down the continuum. Organic content generally decreased with particle size. Ultrafine part icles, however, had significantly higher organic fractions than fine ( 60-100 mu m) and very fine (30-60 mu m) particles. 4. Daily organic ca rbon turnover length ranged from 10 to 98 km and increased downstream. This is consistent with other studies along river continua and sugges ts that organic carbon turnover length is largely controlled by the re lationship between channel dimensions and discharge, rather than the p resence of specific retention devices. 5. Concentrations and nutrition al quality of organic seston and concentrations of DOC were highest in an unconstrained floodplain reach in the upper river. These data sugg est that new material enters the river channel in this reach, potentia lly providing an important energy source for the river community downs tream. The effect of this reach on the longitudinal pattern of organic matter concentration and quality emphasizes how changes in channel fo rm can alter river ecosystem structure and function.