Physical characteristics of the stream – lake transitional zone affect littoral lake metabolism

Abstract

Lake ecosystems are sentinels of change as integrators of upland and upstream stressors. Lake metabolism, i.e., gross primary production (GPP) and respiration ®, in stream – lake transitional zones may be key to understanding how lakes integrate terrestrial-derived inputs. Therefore, examining site differences in lake metabolism in stream – lake transitional zones may help identify drivers of within-ecosystem variability and provide insights to landscape-scale freshwater ecosystem function. In this study, we used high-frequency dissolved oxygen data and inverse modeling to estimate daily rates of GPP and R in an oligotrophic lake at a pelagic site and two littoral sites located near major inflows. We examined the relative importance of stream predictor variables in comparison to meteorological and in-lake predictors of GPP and R. One of the inflow streams was substantially warmer than the other and primarily entered the lake’s epilimnion, whereas the colder stream primarily mixed into the metalimnion or hypolimnion. Overall, rates of GPP and R were higher at littoral sites than the pelagic site. An ensemble machine learning analysis revealed that >30% of variability in littoral zone GPP and R was attributable to stream and stream – lake transitional zone predictors. The warm-stream inflow likely stimulated littoral GPP and R, while the relationship between the cold-stream inflow and littoral zone GPP and R depended on stream – lake transitional zone physical characteristics. Our study demonstrated that littoral metabolism near inflows can be an important indicator of the connections between upstream inputs and downstream lake processing and provide insights into lake ecosystem function.