Metabolic and physiochemical responses to a whole-lake experimental increase in dissolved organic carbon in a north-temperate lake

Abstract

Over the last several decades, many lakes globally have increased in dissolved organic carbon (DOC), calling into question how lake functions may respond to increasing DOC. Unfortunately, our basis for making predictions is limited to spatial surveys, modeling, and laboratory experiments, which may not accurately capture important whole-ecosystem processes. In this article, we present data on metabolic and physiochemical responses of a multiyear experimental whole-lake increase in DOC concentration. Unexpectedly, we observed an increase in pelagic gross primary production, likely due to a small increase in phosphorus as well as a surprising lack of change in epilimnetic light climate. We also speculate on the importance of lake size modifying the relationship between light climate and elevated DOC. A larger increase in ecosystem respiration resulted in an increased heterotrophy for the treatment basin. The magnitude of the increase in heterotrophy was extremely close to the excess DOC load to the treatment basin, indicating that changes in heterotrophy may be predictable if allochthonous carbon loads are well-constrained. Elevated DOC concentration also reduced thermocline and mixed layer depth and reduced whole-lake temperature. Results from this experiment were quantitatively different, and sometimes even in the opposite direction, from expectations based on cross-system surveys and bottle experiments, emphasizing the importance of whole-ecosystem experiments in understanding ecosystem response to environmental change.Many northern hemisphere lakes have experienced a gradual increase in dissolved organic carbon (DOC) concentration over the past several decades, a phenomenon termed "global browning" (Evans et al. 2006;Roulet and Moore 2006;Monteith et al. 2007). The increase in DOC concentration has been attributed to a recovery from acidification (Evans et al. 2006;Monteith et al. 2007), increased catchment terrestrial primary production (Freeman et al. 2004), high nitrogen loads affecting soil decomposition (Findlay 2005), ecosystem effects of climate change (Urban et al. 2011), and changes in catchment hydrology (Evans et al. 2005). Although the mechanism for global browning is important to understand and currently still debated, the ecological consequences of increased DOC concentration on lake processes are poorly understood. DOC has both abiotic and biotic effects on lake ecosystems, and comparative studies suggest DOC as a master variable in structuring aquatic ecosystems . Abiotic effects of DOC on lake ecosystems are expressed through its light attenuating properties, as the absorption of solar radiation affects the vertical distribution of light and heat, and in turn, affects a host of other lake ecosystem functions. For example, highly colored north-temperate lakes had reduced epilimnetic depth, temperature, and irradiance compared with clearer lakes (Houser 2006). Additionally, modeling of a north-temperate bog lake showed that a 50% reduction in DOC concentration caused a deepening of the mi...

Periphyton support for littoral secondary production in a highly humic boreal lake

“…Nutrients increase phytoplankton biomass and result in reduced benthic PP because of shading (Vadeboncoeur et al 2003(Vadeboncoeur et al , 2008. Investigators often assume that energy mobilization shifts from benthic and autotrophic toward pelagic and heterotrophic as increasing allochthonous C reduces light availability (Ask et al 2009, Zwart et al 2015. This assumption has led to the prevalent notion that littoral pri-mary producers are of minor importance in the energy budgets of humic lakes.…”

Bridging between litterbags and whole-ecosystem experiments: a new approach for studying lake sediments

“…For example, we found that we could recreate natural DOC concentrations in sediment pore water. This finding is important because whole-ecosystem experiments have differed from smaller lab-based incubations in estimates of DOC dynamics (Zwart et al, 2016). Our method may therefore bridge contrasting approaches to studying lake sediments, i.e.…”

Model‐Data Fusion to Test Hypothesized Drivers of Lake Carbon Cycling Reveals Importance of Physical Controls

“…We focused on modeling C dynamics in the treatment portion of the lake (East Long Lake; area = 3.2 ha), because the surface inlet to the lake enters that basin and therefore we were able to constrain DOC inputs to that basin well. East Long Lake is a dimictic, mesotrophic (total phosphorus: 15.9 μg L −1 ; chlorophyll‐a: 7.9 μg L −1 ) lake with a water residence time of 296 days (Zwart et al, , ). Light is attenuated fairly quickly in the water column of East Long Lake (light extinction coefficient: 2.86 m −1 ) due to the high concentration of DOC (Table ).…”

“…Rainfall was measured hourly with a tipping bucket rain gauge (TE525MM‐L; Campbell Scientific, Inc.), whereas the DOC and CO 2 concentrations in rainwater were measured twice over the sampling period; therefore, the average concentrations were used to estimate DOC and CO 2 input with rain (Zwart et al, ). Monthly measurements of groundwater discharge showed that the lake perpetually loses water to the regional aquifer; we used average groundwater discharge values to estimate DOC and CO 2 flux to groundwater.…”