The quality of organic matter mediates the response of heterotrophic biofilms to phosphorus enrichment of the water column and substratum

1. Heterotrophic biofilms are important drivers of community respiration, nutrient cycling and decomposition of organic matter in stream ecosystems. Both organic matter quality and nutrient levels have been shown to affect biofilm biomass and activity individually, but both factors have rarely been manipulated simultaneously. 2. To experimentally manipulate the organic matter quality and phosphorus (P) levels of both the substratum and water column, we first used cellulose cloth as a low-quality organic material and enhanced its quality and P-content by amending the underlying agar with maltose and P, respectively (Experiment I). To manipulate water column P, artificial substrata were incubated in low- and high-P sites of a whole-stream P-enrichment in lowland Costa Rica. 3. Results from Experiment I suggest that heterotrophic biofilm respiration on cellulose cloth is co-limited by carbon (C) and P. Biofilm respiration responded in an additive manner to combined effects of maltose and P-enrichment of water column and synergistically to maltose and high-P in substrata. 4. As decomposing organic matter that supports heterotrophic biofilms varies naturally in its labile C content along with other physical and chemical properties, we conducted a second experiment (Experiment II) in which we amended leaf discs from two species (Trema integerrima, a labile C source and Zygia longifolia, a recalcitrant C source) with maltose. We incubated the substrata in low- and high-P sites of the P-enrichment stream. 5. Results from Experiment II indicate that biofilm respiration on a labile C source (Trema) was not C-limited, while biofilm respiration on a recalcitrant C source (Zygia) was C-limited. Phosphorus stimulated the biofilm respiration and breakdown rate on Trema, but not on Zygia, supporting the hypothesis that the stimulatory effect of P-enrichment is dependent on the availability of labile C in decomposing leaves. 6. Our results suggest that the interactive effects of organic matter quality and nutrient loading of streams can significantly increase microbial biofilm activity, potentially altering the trophic base of stream food webs. Researchers should consider both the organic matter quality and the enrichment of both water column and substrata to better predict the effects of anthropogenic nutrient loading to stream the ecosystems.