Experimental mixing of a north-temperate lake: Effects on ecosystem metabolism
In lake ecosystems, thermal stratification is a dominant control over factors affecting metabolism. Altered patterns of stratification from lake management strategies, extreme storm events, or global climate change, may alter the production of basal resources of food webs or shift a lake from a carbon sink to a source. Understanding the relationship between metabolism and stratification provides fundamental insights to predicting lake response to changing stratification patterns. To determine the relationship between gross primary production (GPP), respiration (R), net ecosystem production (NEP), and stratification, we experimentally mixed a small oligotrophic lake in northern Wisconsin, USA. We applied a BACI design with two untreated years (2011 and 2014), two treatment years (2012 and 2013), and continuous monitoring of a nearby reference lake.
Mixing gradually reduced thermal stratification during the open water period of treatment years. GPP and R correlated significantly with Schmidt stability (p <0.05; 2012GPP: r = -0.77, 2012 R: r = -0.72; 2013 GPP: r = -0.43, 2013 R: r = -0.43). On average, GPP and R increased nearly an order of magnitude above the reference lake (2012: 12 mmol O2/m3/d; 2013: 14 O2/m3/d) with peaks occurring at early turnover. Increased GPP and R were primarily driven by elevated concentrations of epilimnetic phosphorus (2012: r = 0.86, p <0.05; 2013: r = 0.75, p <0.05) and coincided with higher epilimnetic chlorophyll concentrations, decreased photic depth, and a displacement of the deep chlorophyll maximum towards the lake surface. We found R to be tightly coupled to GPP, and NEP to be independent of stratification throughout all seasons and years of the experiment. These results indicate stratification may regulate metabolism through impeding nutrient flux. Disturbance to stratification can substantially alter the basal energy flows of food webs but had little effect on the carbon sequestration capacity of this oligotrophic lake.