Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

1. Boreal headwater streams typically receive large contributions of dissolved organic matter (DOM) from wetland and forest soils and can exhibit high CO2 effluxes, but little is known about how stream biofilm respiration of DOM responds to carbon and nutrient substrates in boreal catchments. 2. The aim of this study was to assess (i) the response of heterotrophic biofilms to increased availability of labile carbon (C), nitrogen (N) and phosphorus (P), and (ii) how this response varies across sites differing in the source of catchment organic matter. 3. Normalised biofilm respiration rates (R-Biofilm) of dissolved organic carbon (DOC) of DOM sources derived from wetland, deciduous forest and ponds were up to 10 times greater than for coniferous forest DOM. Experimental additions of bog- and pond-derived DOM stimulated R-Biofilm in coniferous forest sites (1.5-2.5 times), adding further weight to the evidence for differing responses to stream DOM sources in this boreal catchment. 4. Mineralisation of added glucose by the biofilms was only increased by added N and P, coinciding with a reduction in mineralisation of the extant stream DOM. These findings suggest that increases in labile C, N and P can reduce biofilm mineralisation of stream dissolved organic nitrogen and phosphorus, perhaps due to the greater energy required to access the more complex DOM. 5. In 15 of 19 experiments, the addition of glucose had no effect on biofilm mineralisation of stream DOC (R-DOC). In the presence of added N and P, however, R-DOC decreased by 30 to 38% with the addition of glucose at sites with the most active biofilms in summer. When glucose was added alone, conversely, R-DOC was stimulated (40-50%) at these same sites in autumn, when biofilm respiration was lower and DOM more aromatic and less carbohydrate-rich. Combined, these results suggest labile C sources, such as algal exudates, may (i) compete as a source of energy and/or stimulate the incorporation rather than mineralisation of the more N- and P-rich stream DOM or (ii) stimulate the mineralisation of stream DOM, potentially depending upon nutrient availability and the composition of stream DOM.