AUTOCHTHONOUS VS. ALLOCHTHONOUS ORGANIC-MATTER INGESTED BY A MACROINVERTEBRATE IN HEADWATER STREAMS - GAMMARUS SP. AS A BIOLOGICAL PROBE

In headwater streams, the quantity of settling organic matter alone ca nnot explain the biological productivity that develops in sediments. I ts use is partly determined by its quality, which in turn is related t o its origin, either autochthonous or allochthonous. The aim of the st udy was to find a way to estimate the origin (autochthonous or allocht honous) of the organic matter consumed by benthic macroinvertebrates i n small streams (Stralher order 1). Preliminary results led to the cho ice of the genus Gammarus as a biological probe. The comparison of two streams with different trophic budgets during two seasons and the ana lysis of 700 intestinal contents show that 1) the diatom/lignin ratio is the most meaningful parameter related to the origin of settling org anic matter which can be measured both in sediments and in Gammarus in testinal contents, 2) large (<10 mm) seem to selectively feed on organ ic matter with a low C/N ratio, 3) Gammarus preferred the autochthonou s part of the sediment organic pool over the allochthonous one. This s election seemed to be similar in the two streams though benthic organi c matter budgets were different.