Assessment of particulate organic matter in river water

Particulate organic matter in a downriver riffle of the Grand River, the largest Canadian Great Lakes tributary, was studied between June 1970 and April 1972. In winter and spring, concentrations of particulate organic matter (1.0–26.2 mg/l) varied with river flow. High summer levels (3.4–12.7 mg/l) were attributable to high autochthonous primary production. Mean chlorophyll a concentration in summer (29.8 mg/m3) was nearly 15 times higher than in winter, and 8 times the spring mean level. High algal cell counts (15,000–19,000 cells/l) also occurred in summer. Autochthonous and allochthonous contributions to the total particulate organic carbon input to the river in summer were estimated by daily organic input and river flow relationships, carbon to chlorophyll a and to pheopigments ratios. The allochthonous source accounted for 21.5% of the total organic carbon while the autochthonous contributed the remaining 78.5%. The latter included living algae (23.0%), senescent plant material (30.3%) and detritus (25.2% — including microbes). The study establishes a new approach whereby the various components of particulate organic matter in river water can be indirectly partitioned and their biomass estimated by using quantitative relationships among readily obtainable parameters of river flow, standing biomass, chlorophyll a and pheopigments.