Scale-dependent relations between bacteria, organic matter and invertebrates in a headwater stream

Microbial activity, bacterial and invertebrate densities and particulate organic matter (POM) content were assessed using a modified freeze-coring technique in a Welsh mountain stream. Bacterial cell densities ranged between 0.22 x 10(8) and 4.47 x 10(8) cells cm(-3) and increased with sediment depth reaching maxima in depths between 30 and 40-cm. In contrast, the highest microbial activity (ETS) was found at depths between 5 and 15 cm, sharply declining with depths > 20 cm. POM content was substantially higher in the upper sediment layer (0-5 cm) and declined with increasing sediment depth. Higher nitrogen and organic carbon contents were recorded for particles < 63 mu m than for larger size fractions. Meio- and macroinvertebrate densities were significantly and negatively correlated with bacterial number. The results of a multiple regression analysis suggest that bacterial densities, porewater volume and particulate organic carbon (POC) accounted for more than 50 % of the density variations of harpacticoids and larval chironomids. The results underline a strong link between physical habitat properties in a scale-dependent fashion.