MACROINVERTEBRATE COMMUNITIES AND BIOFILM CHLOROPHYLL ON WOODY DEBRISIN 2 CANADIAN OLIGOTROPHIC LAKES

Wood-associated macroinvertebrates and biofilm chlorophyll-a were inve stigated in Scott Lake and Mykiss Lake in Algonquin Provincial Park, O ntario, Canada. Invertebrate communities on littoral submerged coarse woody debris (CWD) were compared with those on experimentally introduc ed substrates consisting of fresh branches, which were monitored for o ne year. The most abundant invertebrates were Chironomidae (Diptera), followed by Naididae (Oligochaeta), Polycentropidae (Trichoptera), and Heptageniidae (Ephemeroptera). Collector and scraper taxa dominated, highlighting the importance of the surface biofilm as a food source. I nvertebrate community composition changed slightly as the wood decompo sed, with Trichoptera, Elmidae (Coleoptera) and xylophagous chironomid s such as Stenochironomus spp. becoming more abundant on microbially c onditioned CWD. Mean invertebrate densities on the introduced substrat es were 2,827 and 4,827 . m(-2) in Scott and Mykiss, respectively, whi le corresponding values on natural CWD were only 922 and 1,249 . m(-2) . Mean invertebrate biomass on the introduced substrates were 387 mg . m(-2) in Scott and 698 in Mykiss, while corresponding values for natu ral CWD were 134 and 302 mg . m(-2). Invertebrate density and taxa ric hness on the introduced wood was highest on white cedar (Thuja occiden talis), intermediate on eastern hemlock (Tsuga canadensis) and lowest on sugar maple (Acer saccharum), probably because of differences in ba rk microhabitat and availability of biofilm. Chlorophyll-a also tended to be highest on cedar substrates, intermediate on hemlock and lowest on maple, with mean values of 10.3, 8.3 and 4.7 mg . m(-2), respectiv ely. The higher mean chlorophyll-a content on the natural CWD (22.5 mg . m(-2)) may have resulted from longer submergence time, lower invert ebrate grazing pressure, or differences in methodology.