Benthic organic matter as a function of stream order in Oregon

The quantity, size composition and presumed food quality of organic de-tritus is compared in four coniferous forest streams of different order in theCascade Mountains of Oregon, U.S.A. The amount of detritus is greatest in smallheadwater streams, and decreases as stream order increases. Over 90 °/o of detritusin all streams is woody debris; however, there is also a considerable quantity offine particulate organic matter (FPOM: 0.45 urn to 1 mm) in small streams. Re-lative to the headwaters, downstream reaches have less coarse particulate organicmatter (CPOM: > 1 mm) and FPOM. The CPOM : FPOM ratio appears to in-crease from 1st to 3rd-5th order streams, and decreases thereafter. The standingcrop of benthic detritus and annual load of drifting particulate organic matterare used to evaluate the resistance and resilience of each stream order to perturba-tion. The results indicate that the pathway for attaining asymptotic ecosystemstability changes from one of resistance to resilience as stream order increases.Presumed food quality of the benthic detritus, as judged by the carbon-to-nitro-gen (C : N) ratio, chlorophyll a associated with detritus, and respiration rate, ispoor for CPOM and shows no trend by stream order. Presumed food quality ofFPOM is somewhat better since, in larger streams, more chlorophyll is associatedwith the detritus and the highest respiration rates are associated with smallerparticles.