Assessing carbon dynamics in natural and perturbed boreal aquatic systems

Abstract

[1] Most natural freshwater lakes are net greenhouse gas (GHG) emitters. Compared to natural systems, human perturbations such as watershed wood harvesting and long-term reservoir impoundment lead to profound alterations of biogeochemical processes involved in the aquatic cycle of carbon (C). We exploited these anthropogenic alterations to describe the C dynamics in five lakes and two reservoirs from the boreal forest through the analysis of dissolved carbon dioxide (CO 2 ), methane (CH 4 ), oxygen (O 2 ), and organic carbon (DOC), as well as total nitrogen and phosphorus. Dissolved and particulate organic matter, forest soil/litter and leachates, as well as dissolved inorganic carbon were analyzed for elemental and stable isotopic compositions (atomic C:N ratios, d 13 C org , d 13 C inorg and d 15 N tot ). We found links between the export of terrestrial organic matter (OM) to these systems and the dissolved CO 2 and O 2 concentrations in the water column, as well as CO 2 fluxes to the atmosphere. All systems were GHG emitters, with greater emissions measured for systems with larger inputs of terrestrial OM. The differences in CO 2 concentrations and fluxes appear controlled by bacterial activity in the water column and the sediment. Although we clearly observed differences in the aquatic C cycle between natural and perturbed systems, more work on a larger number of water bodies and encompassing all four seasons should be undertaken to better understand the controls, rates, and spatial as well as temporal variability of GHG emissions, and to make quantitatively meaningful comparisons of GHG emissions (and other key variables) from natural and perturbed systems.

Pelagic food webs of humic lakes show low short‐term response to forest harvesting

“…Given that forest harvest did not affect nutrient and DOC concentrations it is not surprising that neither basal food quantities of phytoplankton or bacteria nor stoichiometry of seston were affected. We hypothesized earlier that, in humic lakes, heterotrophic bacteria should benefit most from increased carbon and P availability, resulting in increased bacterial production (Huttunen et al 2003, Ouellet et al 2012, Lapierre et al 2013. However, in our experiment, bacterial production did not increase, suggesting that, besides food quantity, food chain length was also not affected by forest clearcutting.…”

Higher late summer methane emission from central than northern European lakes

“…These variables include lake morphology, relative volume of the hypoxic layer, temperature, dissolved organic carbon (DOC) concentration, and nutrient concentrations (Michmerhuizen et al, 1996;Bastviken et al, 2004;Juutinen et al, 2009;Schrier-Uijl et al, 2011;Ouellet et al, 2012;Kankaala et al, 2013;Rasilo et al, 2014).…”

Greenhouse gas emissions from boreal inland waters unchanged after forest harvesting

“…However, the absence of initial effects does not necessarily imply absence of longer-term effects. On decadal time scales, forestry may change soil carbon cycling (Diochon 25 et al 2009), leading to enhanced terrestrial organic matter exports and lake CO2 emissions (Ouellet et al 2012). Clearly, future work should explore how universal our results are across different hydrological conditions, other types of systems and longer time scales.…”

“…and net CO2 uptake (Ask et al 2012;Lapierre et al 2013). Therefore, any elevated terrestrial carbon inputs due to forest clearcutting may further increase net heterotrophy and CO2 emissions (Ouellet et al, 2012) or stimulate methanogenic bacterial activity in lakes . Forest clear-cuts also often enhances nutrient exports, with less pronounced changes for phosphorous, but large increases for nitrogen (N), especially for nitrate (Nieminen 2004;Palviainen et al 2014;Schelker 5 et al 2016).…”

“…Even though spatial surveys indicate that changes in vegetation (Maberly et al, 2013;Urabe et al, 2011), forest fires 20 (Marchand et al 2009) and forestry activities (Ouellet et al, 2012) affect the greenhouse gas balance of inland waters, we are lacking mechanistic evidence from whole-catchment forest manipulation experiments. Here, we experimentally assess the impact of forest clear-cuts on CO2, CH4 and N2O emissions from lakes and streams in four boreal headwater catchments.…”