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

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 (Solomon et al. 2015). 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 mixed layer depth by 44% and a warmer whole-lake water temperature (Read and Rose 2013). These changes in temperature and light regimes have strong implications for lake food webs as DOC concentration has been shown to limit primary productivity via light attenuation (del Giorgio and Peters 1994; Ask et al. 2009; Godwin et al. 2014), consequently reducing invertebrate and fish productivity (Karlsson et al. 2009; Finstad et al. 2013; Kelly et al. 2014). Additionally, *Correspondence: [email protected] Additional Supporting Information may be found in the online version of this article. 723 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 61, 2016, 723–734 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10248