We conducted experimental studies of growth for two common genera of snag-dwelling mayflies in outdoor artificial air-lift streams over a 12 month period at a field site on the Ogeechee River, a blackwater river in the southeastern U.S.A. Coastal Plain. Temperature was varied both naturally and artificially to examine its effect on mayfly growth. Growth rates of Stenonema spp. in artificial streams were correlated with temperature (r2 = 0.71) and were as high as 0.12 mg mg-1 d-1 in the summer. Baetis spp. growth rates were also temperature-dependent (r2 = 0.84) and summer values were even higher (0.35 d-1) than those of Stenonema. Experimental treatments at natural temperatures, but reduced algal food (no light) or reduced detrital/bacterial food (20-mu-m filtered vs. 0.5-mu-m filtered river water) showed little effect on Stenonema growth rates. Algae were a small component of the biofilm that accumulates on solid substrata (snags) and Stenonema were able to feed and grow very well on only non-algal microfine particulate organic matter and microbes. The high densities of sestonic bacteria and tendency for dissolved organic carbon to flocculate on substrata in this Coastal Plain river make it unlikely that food limits Stenonema growth rates through most of the year. These results are consistent with independent feeding experiments using radio-labelled bacteria showing that bacteria can comprise a major component of Stenonema diets and serve as an important trophic link between organic matter and macroconsumers. Thus, natural fluctuation in temperature appears to play a more significant role than variation in food quality or quantity in controlling mayfly growth rates in this blackwater river.
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