Primary productivity is a fundamental ecosystem process that shapes the structure and function of lake ecosystems with important implications for the ecosystem services lakes provide. The conventional paradigm that nutrient availability is the main driver of primary production in lake ecosystems has been challenged with recent evidence suggesting light availability is more important in regulating primary production in unproductive lakes. Additionally, long-term increases in lake dissolved organic carbon (DOC) concentration have been observed in northern-hemisphere lakes. Diverse responses of primary productivity to DOC have been observed prompting the recent development of theory. This theory has generated predictions that suggest an interaction between the alleviation of nutrient limitation and increasing light limitation caused by DOC associated nutrients and its light absorbing nature can generate a hump-shaped response of primary productivity to DOC. To test predictions of these modeling efforts, a two-by-two, full factorial manipulation of DOC chromophoricity (light absorbing characteristics) and nutrient stoichiometry was imposed in conjunction with a temporal increase of DOC using flow-through in situ mesocosms. Daily rates of gross primary production (GPP) were estimated as our primary response variable.
Results/Conclusions
Our treatments were successfully imposed with an average increase in DOC from 6.5 mg C L-1 to 41.8 mg C L-1 in addition to an increase and decrease in total phosphorus and light climate with time, respectively. We observed a hump-shaped response in GPP to increased DOC with significant treatment effects resulting in four distinct humps. We estimated the DOC concentration at which maximum GPP occurred (critical DOC; x component of the vertex) and the magnitude of the GPP maximum (max GPP; y component of the vertex) using the parameterized quadratic model. Max GPP ranged from 135 to 295 mg C m-2 day-1 and critical DOC values ranged from 16.5 to 26.3 mg C L-1 in a manner consistent with theory. These results suggest our experiment served as a robust test of the hypothesized hump-shaped relationship between primary production and DOC. Given evidence that northern hemisphere lakes are subject to increased exports of DOC (‘browning’), future research should focus on spatiotemporal drivers of browning. A particular focus on how these drivers impact DOC’s chemical attributes (chromophoricity and stoichiometry) will be key given these variables appear to be responsible for the trajectory of change in primary production with increases in DOC.