BACTERIAL ALGAL RELATIONSHIPS IN STREAMS OF THE HUBBARD BROOK EXPERIMENTAL FOREST

Epilithic bacteria appear to be an important trophic component in many stream ecosystems, but it is not clear whether bacterial abundance an d growth covary with epilithic algae, one potential carbon source for bacteria. Epilithic bacterial abundances and growth rates were measure d for 2 yr in two streams with contrasting trophic bases at the Hubbar d Brook Experimental Forest. The reference stream (Bear Brook) has a c losed canopy, low epilithic primary production, and a high standing st ock of detrital organic matter. A nearby stream in a clear-cut catchme nt had experienced algal blooms in the early years following cutting, and rates of primary production measured during our study period were 5 times greater than in Bear Brook. Chlorophyll a concentrations on ti les did not differ between streams, indicating that algal abundance wa s limited by factors other than light availability. Bacteria were not consistently correlated with algal abundances in either stream. Bacter ial abundance and growth did not differ statistically between streams despite the large differences in rates of primary production. A shadin g experiment conducted in both streams was successful in reducing chlo rophyll levels but did not reduce bacterial growth rates. A laboratory experiment involving nutrient additions and high light levels greatly stimulated algal abundance and carbon fixation, but there was no para llel increase in bacterial abundance or growth. Apparently, in contras t to planktonic systems, there is not a tight trophic connection betwe en algal and bacterial productivity in these streams.