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 and growth covary with epilithic algae, one potential carbon source for bacteria. Epilithic bacterial abundances and growth rates were measured for 2 yr in two streams with contrasting trophic bases at the Hubbard Brook Experimental Forest. The reference stream (Bear Brook) has a closed canopy, low epilithic primary production, and a high standing stock of detrital organic matter. A nearby stream in a clear-cut catchment 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 tiles did not differ between streams, indicating that algal abundance was limited by factors other than light availability. Bacteria were not consistently correlated with algal abundances in either stream. Bacterial abundance and growth did not differ statistically between streams despite the large differences in rates of primary production. A shading experiment conducted in both streams was successful in reducing chlorophyll 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 parallel increase in bacterial abundance or growth. Apparently, in contrast to planktonic systems, there is not a tight trophic connection between algal and bacterial productivity in these streams.