PERIPHYTON REACTIONS TO DIFFERENT LIGHT AND NUTRIENT LEVELS AND THE RESPONSE OF BACTERIA TO THESE MANIPULATIONS

We determined the extent of seasonal light and nutrient limitation of algae in open and closed canopy sections of a second order stream. By artificially shading portions of the open and closed canopied sections, we were able to manipulate algal densities and thus observe the effects of these algal manipulations on bacterial abundance. Furthermore, by using clay diffusion pots, we also determined the extent of nutrient limitation of both attached algae and bacteria. Algal abundance was reduced by 40 % or more in natural and artificially shaded sections suggesting that light was the primary limiting resource in shaded sections. Limitation of algae by nitrogen and phosphorus was observed only during October in the open and closed canopy sections (after leaf abscission). Bacterial abundance was positively correlated with algal abundance and thus suggests a possible interaction between the two. Bacteria also were nutrient-limited but only when algal abundance was very low (artificially shaded site under a naturally closed canopy). Our results suggest that bacteria within the biofilm may rely on algal exudates for an energy source and possibly inorganic nutrients as well for growth.