Does nutrient enrichment decouple algal–bacterial production in periphyton?

Coupled production between algae and bacteria in stream epilithon was assessed along a nutrient-enrichment gradient in 8 Texas streams with open canopies. Photosynthesis (PS) and bacterial biomass production (BBP) were measured simultaneously using a dual-label radioassay (C-HCO3 – uptake and H-L-leucine incorporation into protein) on multiple samples within a stream reach. PS and BBP were measured after light (1200–1500 lmol m 2 s ) and dark incubations. The degree of coupled production between algae and bacteria within a stream was estimated as the covariation (i.e., correlation or covariance) between PS and BBP derived from unshaded replicates in each stream. Streamwater nutrients ranged from 0.18 to 8.1 mg/L total N and 0.009 to 2.0 mg/L total P. Epilithon N and P content (as % dry mass) and C:N:P ratios varied widely among streams and were positively correlated with streamwater nutrient concentrations. Mean BBP measured in light incubations (BBPL) was greater than mean BBP measured in dark incubations (BBPD), and the difference between the 2 means (BBPL – BBPD) was positively correlated with mean PS among streams (R 1⁄4 0.53). Covariance between PS and BBPL within streams (COVPS–BBP) decreased as epilithon nutrient content increased. COVPS–BBP was positively correlated with both epilithon C:N (R1⁄4 0.78) and C:P (R1⁄4 0.77) among streams. These results suggest that algal and bacterial production are decoupled by nutrient enrichment, and that algae might rely more heavily on bacterial-regenerated nutrients than on streamwater nutrients to support production in nutrient-poor streams.