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Abstract. Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12 %–25 % of the total carbon flux from soils and bedrock. This CO2 is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to CO2. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance – SUVA320; spectral slope ratio – Sr), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May–August 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that, in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, ∼ 30 % of the DOC was processed by sunlight, and a minimum of 1 % was photomineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs.

Abstract. Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12–25 % of the total carbon flux from soils and bedrock. This CO2 is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to CO2. We measured changes in DOC concentration, the optical characteristics of the DOC (SUVA320 and Sr), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May–August, 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, ~ 30 % of the DOC was processed by sunlight and ~ 2 % was photo mineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs.

Dissolved organic carbon (DOC) provides an important source of energy in lotic systems that can regulate food chain dynamics. It is thought that inorganic nutrient addition to stream culture samples will increase microbial respiration and bacterial growth efficiency (BGE). Nutrient limitation may prevent microbial communities from reaching their full potential in terms of biomass and efficiency, which could limit organisms at higher trophic levels. This study utilized batch cultures to assess changes across a variety of water quality and nutrient parameters in an effort to assess the role of inorganic nutrient addition in regulating microbial activity in headwater streams. Treatments of phosphorus addition (þP) and nitrogen plus phosphorus addition (þNþP) were used to assess changes in microbial activity in samples collected from forested and agricultural headwater streams. Phosphorus limitation was observed only at a single site, suggesting that a secondary limiting nutrient (e.g. nitrogen) may affect microbial activity at the other sites. Combined phosphorous and nitrogen additions demonstrated that nitrogen did limit microbial respiration at many of the study sites. BGE was highly variable and did not seem to be controlled by inorganic nutrient concentration. We did observe increased carbon production with nutrient addition at some of the study sites. The results of this study indicate that small streams could export more carbon through respiration rather than by incorporating available DOC into biomass.

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