EFFECTS OF CHIRONOMID (INSECTA: DIPTERA) TUBE‐BUILDING ACTIVITIES ON STREAM DIATOM COMMUNITIES1,2

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Abstract
Chironomid retreats, constructed out of sand grains upon submerged wood debris, increase the surface area available for diatom colonization. The three dimensional substratum afforded by chironomid tubes supports up to twelve times the diatom biavolume found upon adjacent, unmodified substratum in a northern Michigan stream. Diatom enumeration within scrapings from small defined areas on artificial substrata, combined with examination of intact natural substrata through scanning electron microscopy (SEM), reveals distinct, microdistribution patterns. The larval retreats of two major taxa of tube‐dwelling chironomid's(Micropsectrasp. andPseudodiamesacf.pertinaxGarrett) display significantly different diatom communities relative to adjacent masonite substratum. Substratum without chironomid tubes is primarily colonized byAchnanthes minutissimaKütz. andCocconeis placentulaEhr., exhibiting the lowest species diversity of microhabitats examined. The diatom flora upon sand tubes ofMicropsectrasp. is dominated byOpephora martyiHerib., as is the flora of sand grains collected from the stream sediment load. These two micro‐habitats exhibit a high community similarity (SIMI). The SIMI index also suggests that the flora ofP. pertinaxtubes is highly similar to that of sand grains. Diversity, however, is almost three times greater onP. pertinaxtubes and SEM observations reveal that this microhabitat is characterized by a more spatially complex flora; NitzschiaandNaviculaspp. dominate the upperstory, andO. martyiis located on underlying sand grains. Results indicate that tube‐building chironomids in Carp Creek affect diatom microdistribution by: (1) stabilizing sand grains and associated flora within their retreats, (2) providing a ‘refugium’ for upperstory diatom taxa from the mayfly grazer,Baetis vagansMcDunnough (Insecta: Ephemeroptera), and (3) through local nutrient enrichment.

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