Ammonium and glucose amendments stimulate dissolved organic matter mineralization in a Lake Superior tributary

Dissolved organic matter (DOM) plays an integral role in the biogeochemical cycles of aquatic ecosystems, and rates of DOM mineralization can be affected by nitrogen (N) and carbon (C) availability in the water column. To determine if DOM mineralization rates were limited by availability of N or C in a small tributary of Lake Superior, we conducted 28-day laboratory experiments where we amended stream water with a microbial inoculum and ammonium (NH4+) and/or glucose in a full factorial design. We repeated these experiments on four dates (July, October, January, April) because we expected nutrient limitation to vary with temporal changes in sources of DOM and nutrients. DOM mineralization rates were stimulated by additions of NH4+ in three of the four experiments and by glucose in all four experiments, even in October when carbon inputs to the stream via leaf litter were greatest. Mean biodegradable dissolved organic C was greater in July (40.6% ± 5.4) than October (10.2% ± 0.9) or January (9.6% ± 10.9) but did not differ from April (18.4% ± 0.7), which suggests the composition of DOM varies temporally. Our results suggest that biodegradable DOM is exported from this forested stream to nearshore Lake Superior throughout the year, but DOM mineralization can be stimulated by additions of both carbon and nitrogen. Nutrient enrichment may alter in-stream processing of DOM in small forested tributary streams, in turn reducing export of biodegradable DOM to nearshore regions of the Great Lakes where it fuels microbial production.