Suspension feeders transform massive amounts of seston in large northern rivers

The concentration and transport of faecal pellets (FPs) produced by blackfl y (Diptera: Simuliidae) larvae were estimated in a large, free-flowing rive r in the north of Sweden during 1997-1999. FPs were abundant from May throu gh August and FP loads in transport peaked at 429 t dry mass d(-1) at a sit e in the lower part of the river in 1997. Daily transport at the same site, averaged over each study period (16 Jun.-18 Sep. 1997, 6 Apr.-10 Sep. 1998 , and 21 Apr.-5 Aug. 1999). was estimated to be 93.7 t dry mass (3.7 t carb on), 47.5 t dry mass (1.9 t carbon and) and 69.2 t dry mass (2.7 t carbon), respectively. On a large scale, there was a downstream trend of increasing FP concentration and, during periods with greater discharge, sedimentation was reduced so that more material was exported from the river. Samples fro m six sites in a regulated river (into which our focal river flows) paralle l to six sites in the unregulated tributary showed considerably lower FP co ncentrations in the regulated river, presumably because of much smaller bla ckfly populations as a consequence of habitat loss through damming. A surve y of two other large, unregulated rivers in northern Sweden confirmed that these also carry large amounts of FPs. We conclude that the transformation of small suspended food particles into considerably larger FPs by huge popu lations of filter-feeding blackfly larvae is a major process in large north ern rivers.