Controls and significance of priming effects in lake sediments.

Warming and eutrophication influence carbon (C) processing in sediments, with implications for the global greenhouse-gas budget. Temperature effects on sedimentary C loss are well understood, but the mechanism of change in turnover through priming with labile organic matter (OM) is not. Evaluating changes in the magnitude of priming as a function of warming, eutrophication, and OM stoichiometry, we incubated sediments with 13 C-labeled fresh organic matter (FOM, algal/cyanobacterial) and simulated future climate scenarios (+4°C and +8°C). We investigated FOM-induced production of CH4 and microbial community changes. C loss was primed by up to 17% in dominantly allochthonous sediments (ranging from 5% to 17%), compared to up to 6% in autochthonous sediments (-9% to 6%), suggesting that refractory OM is more susceptible to priming. The magnitude of priming was dependent on sediment OM stoichiometry (C/N ratio), the ratio of fresh labile OM to microbial biomass (FOM/MB), and temperature. Priming was strongest at 4°C when FOM/MB was below 50%. Addition of FOM was associated with activation and growth of bacterial decomposers, including for example, Firmicutes, Bacteroidetes, or Fibrobacteres, known for their potential to degrade insoluble and complex structural biopolymers. Using sedimentary C/N > 15 as a threshold, we show that in up to 35% of global lakes, sedimentation is dominated by allochthonous rather than autochthonous material. We then provide first-order estimates showing that, upon increase in phytoplankton biomass in these lakes, priming-enabled degradation of recalcitrant OM will release up to 2.1 Tg C annually, which would otherwise be buried for geological times.

Effects of phytoplankton blooms on fluxes and emissions of greenhouse gases in a eutrophic lake.

Lakes are important sources of greenhouse gases (GHGs) to the atmosphere. Factors controlling CO2, CH4 and N2O fluxes include eutrophication and warming, but the integrated influence of climate-warming-driven stratification, oxygen loss and resultant changes in bloom characteristics on GHGs are not well understood. Here we assessed the influence of contrasting meteorological conditions on stratification and phytoplankton bloom composition in a eutrophic lake, and tested for associated changes in GHGs inventories in both the shallow and deep waters, over three seasons (2010-2012). Atmospheric heatwaves had one of the most dramatic effects on GHGs. Indeed, cyanobacterial blooms that developed in response to heatwave events in 2012 enhanced both sedimentary CH4 concentrations (reaching up to 1mM) and emissions to the atmosphere (up to 8 mmol m-2 d-1). That summer, CH4 contributed 52% of the integrated warming potential of GHGs produced in the lake (in CO2 equivalents) as compared to between 34 and 39% in years without cyanobacterial blooms. High CH4 accumulation and subsequent emission in 2012 were preceded by CO2 and N2O consumption and under-saturation at the lake surface (uptakes at -30 mmol m-2 d-1 and -1.6 µmol m-2 d-1, respectively). Fall overturn presented a large efflux of N2O and CH4, particularly from the littoral zone after the cyanobacterial bloom. We provide evidence that, despite cooling observed at depth during hot summers, CH4 emissions increased via stronger stratification and surface warming, resulting in enhanced cyanobacterial biomass deposition and intensified bottom water anoxia. Our results, supported by recent literature reports, suggests a novel interplay between climate change effects on lake hydrodynamics that impacts both bloom characteristics and GHGs production in shallow eutrophic lakes. Given global trends of warming and enrichment, these interactive effects should be considered to more accurately predict the future global role of lakes in GHG emissions.

Effects of climate change and episodic heat events on cyanobacteria in a eutrophic polymictic lake.

Mixing regime and CO2 availability may control cyanobacterial blooms in polymictic lakes, but the underlying mechanisms still remain unclear. We integrated detailed results from a natural experiment comprising an average-wet year (2011) and one with heat waves (2012), a long-term meteorological dataset (1960-2010), historical phosphorus concentrations and sedimentary pigment records, to determine the mechanistic controls of cyanobacterial blooms in a eutrophic polymictic lake. Intense warming in 2012 was associated with: 1) increased stability of the water column with buoyancy frequencies exceeding 40 cph at the surface, 2) high phytoplankton biomass in spring (up to 125 mg WW L-1), 3) reduced downward transport of heat and 4) depleted epilimnetic CO2 concentrations. CO2 depletion was maintained by intense uptake by phytoplankton (influx up to 30 mmol m-2 d-1) in combination with reduced, internal and external, carbon inputs during dry, stratified periods. These synergistic effects triggered bloom of buoyant cyanobacteria (up to 300 mg WW L-1) in the hot year. Complementary evidence from polynomial regression modelling using historical data and pigment record revealed that warming explains 78% of the observed trends in cyanobacterial biomass, whereas historical phosphorus concentration only 10% thereof. Together the results from the natural experiment and the long-term record indicate that effects of hotter and drier climate are likely to increase water column stratification and decrease CO2 availability in eutrophic polymictic lakes. This combination will catalyze blooms of buoyant cyanobacteria.

Climate Effects on High Latitude Daphnia via Food Quality and Thresholds.

Climate change is proceeding rapidly at high northern latitudes and may have a variety of direct and indirect effects on aquatic food webs. One predicted effect is the potential shift in phytoplankton community structure towards increased cyanobacterial abundance. Given that cyanobacteria are known to be a nutritionally poor food source, we hypothesized that such a shift would reduce the efficiency of feeding and growth of northern zooplankton. To test this hypothesis, we first isolated a clone of Daphnia pulex from a permafrost thaw pond in subarctic Québec, and confirmed that it was triploid but otherwise genetically similar to a diploid, reference clone of the same species isolated from a freshwater pond in southern Québec. We used a controlled flow-through system to investigate the direct effect of temperature and indirect effect of subarctic picocyanobacteria (Synechococcus) on threshold food concentrations and growth rate of the high latitude clone. We also compared the direct effect of temperature on both Daphnia clones feeding on eukaryotic picoplankton (Nannochloropsis). The high latitude clone had a significantly lower food threshold for growth than the temperate clone at both 18 and 26°C, implying adaptation to lower food availability even under warmer conditions. Polyunsaturated fatty acids were present in the picoeukaryote but not the cyanobacterium, confirming the large difference in food quality. The food threshold for growth of the high latitude Daphnia was 3.7 (18°C) to 4.2 (26°C) times higher when fed Synechococcus versus Nannochloropsis, and there was also a significant negative effect of increased temperature and cyanobacterial food on zooplankton fatty acid content and composition. The combined effect of temperature and food quality on the performance of the high latitude Daphnia was greater than their effects added separately, further indicating the potentially strong indirect effects of climate warming on aquatic food web processes.