Community Metabolism in Natural and Agriculturally Disturbed Riffle Sections of the Chippewa River, Isabella County, Michigan

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Abstract
Estimates of aquatic community metabolism were made seasonally (1983–1984) at three sites representing an agricultural gradient along the Chippewa River. Annual mean photosynthesis/respiration ratios (P/R), monitored with an in situ chamber method, demonstrated an autotrophic-heterotrophic shift consistent with increased farming intensity (P/R=1.31 vs 1.08 vs 0.64). Other productive and consumptive parameters supported those trends summarized in the P/R values. Within-site seasonal measurements indicated summer maxima and winter minima. Particle-sized organic content (AFDW) was determined for chamber sediments in order to locate the source of oxygen metabolism. While productive sections maintained populations of epilithic diatoms and filamentous green algae on relatively large substrates, the most disturbed site was heterotrophically dominated by very small particles.