Periphytic bacterial and algal response to a hydraulic gradient under different light levels: Test of algal-bacterial coupling in the laboratory

Abstract

A laboratory experiment was conducted to examine the simultaneous response of periphytic bacteria and algae to a gradient in hydraulic conditions under high and low irradiance. Different current regimes were maintained within laboratory channels to produce a range in algal biomass. This allowed us to test for the potential coupling between periphytic bacteria and algae. In each laboratory channel, two light treatments were performed by shading of blocs with artificial substrata and by pairing each of these blocs with an adjacent, unshaded bloc. Shading significantly reduced algal biomass, bacterial abundance and production. Illuminated periphyton was characterized by a significant lower content of N and P (higher C: P and C: N ratios) than shaded periphyton, indicating a reduced availability of inorganic nutrients from the water column within a high-biomass unshaded periphyton. Under high light, algal biomass tended to decrease with increasing current. In contrast, under low light algal biomass was higher in running than in still water. In illuminated periphyton, significant relationships between algal biomass and bacterial number as well as production were found, while these relationships were very weak in shaded periphyton. Higher availability of external organic carbon in thinner shaded periphyton most likely caused a weak algal-bacterial coupling under low light. When data from both light regimes were combined, carbon-specific bacterial production significantly increased with decreasing periphyton density as a result of an improved supply of labile organic carbon and inorganic nutrients from the overlying water. Based on these results, we conclude that light-exposed periphyton preferentially used local resources, whereas shaded periphyton was mainly dependent upon external resources from the overlying water column.