The quality and availability of fine particulate organic matter for collector species in headwater streams

Fine particulate organic matter (FPOM) is abundant in rivers, but the quality and quantity of FPOM in low-order streams have been investigated less frequently than the dynamics of coarse particulate organic matter (CPOM). We (i) assessed the quantity and quality of FPOM in several low-order mountain streams and the relationship between the quantity of FPOM and invertebrate abundance and (ii) evaluated the importance of microbial colonization to the growth of the FPOM consumer Chironomus riparius. FPOM availability ranged from 0.3 to 25.9g ash-free dry mass (AFDM)/m2 and was significantly higher during summer than during autumn. The density of invertebrates (10-13,500 individuals/m2) and FPOM were correlated (R=0.74). Chironomids were the most abundant organisms in the depositional zones where FPOM accumulated. Typically, FPOM nitrogen content was <1%, C:N ranged from 21 to 30%, and lignin ranged from 44 to 66%. Nitrogen content was generally lower during the autumn than summer. The ergosterol content of the FPOM was significantly lower (18 mu g/g AFDM) than that determined for leaves, from either water or soil. Chironomids fed with FPOM obtained from milled oak leaves, natural FPOM and sterile FPOM had 80, 45, and 0% emergence, respectively. We concluded that FPOM varies seasonally in quantity and quality in low-order streams and is a poor food resource relative to CPOM. Microbial colonization of FPOM could be important for the trophic ecology of collector species in low-order streams.