We have investigated trace metal accumulation in sediment and in insect larvae in a small lowland river, which is an ecosystem with a large load of organic allochthonous matter (leaf packs) due to its first stream order section running across forest as well as along the edge of a large city (anthropogenic enrichment). Two study sites of this reach are separated by recreation ponds. Mean annual discharge at the upstream site (BZA) was below 0.010 m3 s-1, while at downstream one (BZB) it increased to 0.037 m3 s-1, the latter flow highly fluctuating. Riverbed at BZA consisted of sand and cobbles, while at BZB of different fractions of sand (substrate inorganic index, SI, significantly lower). Beside benthic samples obtained at monthly intervals over a year, additional samples were collected three times over the year: in winter, late spring, and autumn from each site to get organisms (insect larvae) and sediment for metal analysis. Two species representing chironomids (Chironomus riparius and Prodiamesa olivacea) and one ephemeropterans (Ephemera danica) were selected for detailed analysis. Despite these three taxa being different in terms of morphology and/or physiology they belonged to the same functional feeding group, i.e. sediment feeders. We avoided species occurring in food chains in which sediment indirectly affected organisms. Correlations between metal concentrations in these aquatic organisms and their environments were recorded at the analysed sites. Higher index values of metal bioaccumulation (bioconcentration factor, BCF) by insects were obtained at the downstream site (BZB), with relatively low levels of toxicants. For example, 8.5 for Zn (in spring), 23.0 for Pb, 3.3 for Ni and 39.5 for Cd (last three values in autumn); each of them was characteristic for P. olivacea. In turn, for the ephemeropteran species E. danica the respective highest BCF values were: 4.8 for Cu (in spring) and 1.6 for Mn (in autumn). On the other hand, lower BCF indices we gained at the BZA site, where higher concentrations of metals in sediment were recorded; for example, 2.7 for Zn (in autumn, P. olivacea), 4.3 for Pb, 6.8 for Cd, 4.3 for Cu (each value in spring, C. riparius). An exception of this relation was 5.6 for Ni (C. riparius, in autumn), and 1.8 for Mn (C. riparius, in spring) where BCF reached higher values at BZA than at BZB. Note that C. riparius did not appear at BZA during the studied period. Despite this species’ absence the above mentioned relation may testify to the ability of sediment feeders to regulate metal contaminant content in their tissues. It seems that natural organic load rather than low levels of metal toxicants decided about macroinvertebrate assemblages in the study river.
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