Decrease in CO2 efflux from northern hardwater lakes with increasing atmospheric warming.

Boreal lakes are biogeochemical hotspots that alter carbon fluxes by sequestering particulate organic carbon in sediments and by oxidizing terrestrial dissolved organic matter to carbon dioxide (CO2) or methane through microbial processes. At present, such dilute lakes release ∼1.4 petagrams of carbon annually to the atmosphere, and this carbon efflux may increase in the future in response to elevated temperatures and increased hydrological delivery of mineralizable dissolved organic matter to lakes. Much less is known about the potential effects of climate changes on carbon fluxes from carbonate-rich hardwater and saline lakes that account for about 20 per cent of inland water surface area. Here we show that atmospheric warming may reduce CO2 emissions from hardwater lakes. We analyse decadal records of meteorological variability, CO2 fluxes and water chemistry to investigate the processes affecting variations in pH and carbon exchange in hydrologically diverse lakes of central North America. We find that the lakes have shifted progressively from being substantial CO2 sources in the mid-1990s to sequestering CO2 by 2010, with a steady increase in annual mean pH. We attribute the observed changes in pH and CO2 uptake to an atmospheric-warming-induced decline in ice cover in spring that decreases CO2 accumulation under ice, increases spring and summer pH, and enhances the chemical uptake of CO2 in hardwater lakes. Our study suggests that rising temperatures do not invariably increase CO2 emissions from aquatic ecosystems.

Unabated Nitrogen Pollution Favors Growth of Toxic Cyanobacteria over Chlorophytes in Most Hypereutrophic Lakes.

Human release of reactive nitrogen (N) to the environment has increased 10-fold since 1860 and is expected to increase by a further ∼75% by 2050. Much of this N enters phosphorus (P)-rich, eutrophic lakes in agricultural and urbanized watersheds. While N pollution of eutrophic lakes can promote toxic cyanobacterial growth, some cases of extreme N loading have led to the dominance of chlorophytes (green algae). As N loads required to shift communities from cyanobacterial to chlorophyte dominance are unclear, we experimentally tested phytoplankton responses to a gradient of N loading in a P-rich lake. Low-to-moderate doses (1-3 mg N L-1 week-1) promoted toxic cyanobacterial dominance and elevated concentrations of the hepatotoxin microcystin. Conversely, loads characteristic of pure urban or agricultural effluents (up to 18 mg N L-1 week-1) led to the dominance of chlorophytes over cyanobacteria and lower microcystin content. This indicates that N loads needed to sustain chlorophyte dominance are uncommon, likely restricted to select shallow lakes directly exposed to urban or agricultural effluents. As most N pollution regimes in P-rich lakes will favor toxic cyanobacterial dominance, restricting future N pollution will help curb further cyanobacterial dominance in lakes both directly and by constraining the capacity for future P loading and climate warming to drive cyanobacterial growth.

Surface water CO2 concentration influences phytoplankton production but not community composition across boreal lakes.

Recent experimental evidence suggests that changes in the partial pressure of CO2 (pCO2 ), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO2 environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO2 , and where phytoplankton structure and activity are under complex control of diverse local and regional factors. Here, we use a large-scale comparative study of 69 boreal lakes to explore the influence of existing CO2 gradients (c. 50-2300 µatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO2 gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO2 , demonstrating that environmental context is key in determining pCO2 -phytoplankton interactions. We further argue that increased atmospheric CO2 is unlikely to influence phytoplanktonic composition and production in northern lakes.

Differential effects of energy and mass influx on the landscape synchrony of lake ecosystems.

Interannual variation of 45 annually resolved time series of environmental, limnological, and biotic parameters was quantified (1994-2009) in six lakes within 52,000 km2 to test the hypothesis that influx of energy (E; as irradiance, heat, wind) varies synchronously among sites and induces temporal coherence in lakes and their food webs, whereas influx of mass (m; as water, solutes, particles) reduces synchrony because local catchments uniquely modify hydrologic inputs. Overall, 82% of parameters exhibited significant (P < 0.05) synchrony (S) estimated as mean pair-wise correlation of Z-transformed time series. Influx of E as atmospheric heat and irradiance was both more highly synchronous and less temporally variable (months-to-decades) than influx of m as summer precipitation, snow, or river discharge. Similarly, S of limnological parameters varied from 0.08 to 0.85, with variables known to be regulated by E influx (ice melt, gas solubility) up to twofold more coherent than those regulated by m inputs (organic solutes). Pairs of variables linked by simple direct mechanisms exhibited similar S values (air temperature and ice melt, nutrients and algae), whereas the coherence of other parameters (water temperature, mixing) was intermediate to that of multiple regulatory agents. Overall, aggregate measures of plankton density varied more coherently among lakes than did constituent taxa. These findings suggest that environmental variability is transmitted to most levels of aquatic ecosystems, but that the precise effects depend on whether E or m fluxes predominate, the coherence of each forcing mechanism, and the strength of linkages between exogenous forcing and lake response.

Maintenance of positive diversity-stability relations along a gradient of environmental stress.

BACKGROUND: Environmental stress is widely considered to be an important factor in regulating whether changes in diversity will affect the functioning and stability of ecological communities. METHODOLOGY/PRINCIPAL FINDINGS: We investigated the effects of a major environmental stressor (a decrease in water volume) on diversity-abundance and diversity-stability relations in laboratory microcosms composed of temperate multi-trophic rock pool communities to identify differences in community and functional group responses to increasing functional group richness along a gradient of environmental stress (low, medium, and high water volume). When a greater number of functional groups were present, communities were less temporally variable and achieved higher abundances. The stabilizing effect of increased functional group richness was observed regardless of the level of environmental stress the community was subjected too. Despite the strong consistent stabilizing effect of increased functional group richness on abundance, the way that individual functional groups were affected by functional group richness differed along the stress gradient. Under low stress, communities with more functional groups present were more productive and showed evidence of strong facilitative interactions. As stress increased, the positive effect of functional group richness on community abundance was no longer observed and compensatory responses became more common. Responses of individual functional groups to functional group richness became increasing heterogeneous are stress increased, prompting shifts from linear diversity-variability/abundance relations under low stress to a mix of linear and non-linear responses under medium and high stress. The strength of relations between functional group richness and both the abundances and temporal variability of functional groups also increased as stress increased. CONCLUSIONS/SIGNIFICANCE: While stress did not affect the relation between functional group richness and stability per se, the way in which functional groups responded to changes in functional group richness differed as stress increased. These differences, which include increases in the heterogeneity of responses of individual functional groups, increases in compensatory dynamics, and increases in the strength of richness-abundance and richness-variability relations, may be critical to maintaining stability under increasingly stressful environmental conditions.