Unimodal response of fish yield to dissolved organic carbon

Here, we demonstrate a contrasting effect of terrestrial coloured dissolved organic material on the secondary production of boreal nutrient poor lakes. Using fish yield from standardised brown trout gill‐net catches as a proxy, we show a unimodal response of lake secondary productivity to dissolved organic carbon (DOC). This suggests a trade‐off between positive and negative effects, where the initial increase may hinge upon several factors such as energy subsidising, screening of UV‐radiation or P and N load being associated with organic carbon. The subsequent decline in production with further increase in DOC is likely associated with light limitations of primary production. We also show that shallow lakes switch from positive to negative effects at higher carbon loads than deeper lakes. These results underpin the major role of organic carbon for structuring productivity of boreal lake ecosystems.     

土壤溶解性有机碳在陆地生态系统碳循环中的作用

土壤溶解性有机碳(DOC)是有机碳库的活跃组分,在陆地生态系统碳循环中发挥重要作用.本文从碳循环重要性着手,综述了土壤DOC在土壤碳固持与温室气体排放中的作用;结合我国的现实情况(如土壤酸化、气候变暖等),探讨了土壤DOC的相关影响因素如土壤性质、环境因素、人为活动对土壤DOC的影响及作用机制,对进一步理解土壤DOC在陆地生态系统碳循环与温室气体减排中的作用具有重要意义.     

Linking forest fires to lake metabolism and carbon dioxide emissions in the boreal region of Northern Québec

Natural fires annually decimate up to 1% of the forested area in the boreal region of Québec, and represent a major structuring force in the region, creating a mosaic of watersheds characterized by large variations in vegetation structure and composition. Here, we investigate the possible connections between this fire‐induced watershed heterogeneity and lake metabolism and CO₂ dynamics. Plankton respiration, and water–air CO₂ fluxes were measured in the epilimnia of 50 lakes, selected to lie within distinct watershed types in terms of postfire terrestrial succession in the boreal region of Northern Québec. Plankton respiration varied widely among lakes (from 21 to 211 μg C L^?1 day^?1), was negatively related to lake area, and positively related to dissolved organic carbon (DOC). All lakes were supersaturated in CO₂ and the resulting carbon (C) flux to the atmosphere (150 to over 3000 mg C m² day^?1) was negatively related to lake area and positively to DOC concentration. CO₂ fluxes were positively related to integrated water column respiration, suggesting a biological component in this flux. Both respiration and CO₂ fluxes were strongly negatively related to years after the last fire in the basin, such that lakes in recently burnt basins had significantly higher C emissions, even after the influence of lake size was removed. No significant differences were found in nutrients, chlorophyll, and DOC between lakes in different basin types, suggesting that the fire‐induced watershed features influence other, more subtle aspects, such as the quality of the organic C reaching lakes. The fire‐induced enhancement of lake organic C mineralization and C emissions represents a long‐term impact that increases the overall C loss from the landscape as the result of fire, but which has never been included in current regional C budgets and future projections. The need to account for this additional fire‐induced C loss becomes critical in the face of predictions of increasing incidence of fire in the circumboreal landscape.     

森林生态系统可溶性碳和颗粒碳通量

可溶性碳(Dissolved carbon,DC)和颗粒碳(particulate carbon,PC)通量作为森林生态系统碳收支的重要组分,在森林固碳功能的评价和模型预测中具有重要意义,但常因认识不足、测定困难等而在森林碳汇研究中被忽略。综述了森林生态系统DC和PC的组成、作用、相关生态过程及其影响因子,并展望了该领域应该优先考虑的研究问题。森林生态系统DC和PC主要包括可溶性有机碳、可溶性无机碳和颗粒有机碳,主要来源于生态系统的净初级生产量。DC和PC是森林土壤的活性碳库,主要以大气沉降、穿透雨和凋落物的形式输入森林土壤系统,并通过土壤呼吸、侧向运输及渗透流失的方式输出生态系统。从局域尺度看,DC和PC通量受根系分泌、细根分解、微生物周转等生物过程的影响较大;从区域尺度看,它们受土壤和植被特性、生态过程耦联关系、气候因子以及全球变化的综合影响。该领域应该优先考虑:(1)探索不同时空尺度下森林生态系统DC和PC通量的控制因子及其耦联关系,揭示其中的驱动机理;(2)探索DC和PC与其它森林生态系统碳组分的相互关系及转化,阐明DC和PC通量与其它养分之间潜在的生态化学计量关系;(3)探索全球变化,特别是人类活动(如森林经营)和极端干扰事件(如林火、旱涝、冰冻、冻融交替等)对森林生态系统DC和PC通量的影响。     

Experimental whole‐lake increase of dissolved organic carbon concentration produces unexpected increase in crustacean zooplankton density

The observed pattern of lake browning, or increased terrestrial dissolved organic carbon (DOC) concentration, across the northern hemisphere has amplified the importance of understanding how consumer productivity varies with DOC concentration. Results from comparative studies suggest these increased DOC concentrations may reduce crustacean zooplankton productivity due to reductions in resource quality and volume of suitable habitat. Although these spatial comparisons provide an expectation for the response of zooplankton productivity as DOC concentration increases, we still have an incomplete understanding of how zooplankton respond to temporal increases in DOC concentration within a single system. As such, we used a whole‐lake manipulation, in which DOC concentration was increased from 8 to 11 mg L^?1 in one basin of a manipulated lake, to test the hypothesis that crustacean zooplankton production should subsequently decrease. In contrast to the spatially derived expectation of sharp DOC‐mediated decline, we observed a small increase in zooplankton densities in response to our experimental increase in DOC concentration of the treatment basin. This was due to significant increases in gross primary production and resource quality (lower seston carbon‐to‐phosphorus ratio; C:P). These results demonstrate that temporal changes in lake characteristics due to increased DOC may impact zooplankton in ways that differ from those observed in spatial surveys. We also identified significant interannual variability across our study region, which highlights potential difficulty in detecting temporal responses of organism abundances to gradual environmental change (e.g., browning).     

Interactions among shrub cover and the soil microclimate may determine future Arctic carbon budgets

Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO₂ flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one‐half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO₂ exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (> 10 °C) were net sources of CO₂, whereas woody sites with cold soils (< 10 °C) were strong sinks. Our results indicate that transition to a shrub‐dominated Arctic will increase the rate of C cycling, and may lead to net C loss if soil temperatures rise.     

Links between Terrestrial Primary Production and Bacterial Production and Respiration in Lakes in a Climate Gradient in Subarctic Sweden

We compared terrestrial net primary production (NPP) and terrestrial export of dissolved organic carbon (DOC) with lake water heterotrophic bacterial activity in 12 headwater lake catchments along an altitude gradient in subarctic Sweden. Modelled NPP declined strongly with altitude and annual air temperature decreases along the altitude gradient (6°C between the warmest and the coldest catchment). Estimated terrestrial DOC export to the lakes was closely correlated to NPP. Heterotrophic bacterial production (BP) and respiration (BR) were mainly based on terrestrial organic carbon and strongly correlated with the terrestrial DOC export. Excess respiration over PP of the pelagic system was similar to net emission of CO₂ in the lakes. BR and CO₂ emission made up considerably higher shares of the terrestrial DOC input in warm lakes than in cold lakes, implying that respiration and the degree of net heterotrophy in the lakes were dependant not only on terrestrial export of DOC, but also on characteristics in the lakes which changed along the gradient and affected the bacterial metabolization of allochthonous DOC. The study showed close links between terrestrial primary production, terrestrial DOC export and bacterial activity in lakes and how these relationships were dependant on air temperature. Increases in air temperature in high latitude unproductive systems might have considerable consequences for lake water productivity and release of CO₂ to the atmosphere, which are ultimately determined by terrestrial primary production.     

A big‐microsite framework for soil carbon modeling

Soil carbon cycling processes potentially play a large role in biotic feedbacks to climate change, but little agreement exists at present on what the core of numerical soil C cycling models should look like. In contrast, most canopy models of photosynthesis and leaf gas exchange share a common ‘Farquhaur‐model’ core structure. Here, we explore why a similar core model structure for heterotrophic soil respiration remains elusive and how a pathway to that goal might be envisioned. The spatial and temporal variation in soil microsite conditions greatly complicates modeling efforts, but we believe it is possible to develop a tractable number of parameterizable equations that are organized into a coherent, modular, numerical model structure. First, we show parallels in insights gleaned from linking Arrhenius and Michaelis–Menten kinetics for both photosynthesis and soil respiration. Additional equations and layers of complexity are then added to simulate substrate supply. For soils, model modules that simulate carbon stabilization processes will be key to estimating the fraction of soil C that is accessible to enzymes. Potential modules for dynamic photosynthate input, wetting‐event inputs, freeze–thaw impacts on substrate diffusion, aggregate turnover, soluble‐C sorption, gas transport, methane respiration, and microbial dynamics are described for conceptually and numerically linking our understanding of fast‐response processes of soil gas exchange with longer‐term dynamics of soil carbon and nitrogen stocks.     

Sources and fates of dissolved organic carbon in lakes as determined by whole-lake carbon isotope additions

Four whole-lake inorganic ¹³C addition experiments were conducted in lakes of differing trophic status. Inorganic ¹³C addition enriched algal carbon in ¹³C and changed the C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.     

火烧对森林土壤有机碳的影响研究进展

对国内外火烧影响森林土壤有机碳动态的研究成果进行了综合述评。较多研究表明低强度火烧不会造成土壤有机碳贮量的明显变化,但火烧非常强烈而彻底,土壤有机碳明显减少。有限研究表明火烧对森林土壤呼吸的影响结果有增加、降低或无影响,因火烧强度、火后观测时间、森林类型、火烧迹地上植被恢复进程和气候条件等而异。同时,火烧对土壤有机碳组分(活性有机碳和黑碳)也具有不同程度的影响。随着全球变化研究的深入,火烧作为森林主要管理措施对大气CO2浓度影响亦愈来愈受重视,今后应着重开展以下几方面研究:(1)扩大气候和经营管理的变化对森林土壤有机碳贮量时空动态影响研究;(2)深入探讨火烧影响土壤CO2释放的过程及机理;(3)加强火烧历史和频率对黑碳影响的研究;(4)从广度和深度上加强火烧等经营措施对亚热带森林土壤碳动态影响的研究。     

Permafrost collapse alters soil carbon stocks,respiration, CH4, and N2O in upland tundra

Release of greenhouse gases from thawing permafrost is potentially the largest terrestrial feedback to climate change and one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Some of this uncertainty stems from abrupt thaw processes known as thermokarst (permafrost collapse due to ground ice melt), which alter controls on carbon and nitrogen cycling and expose organic matter from meters below the surface. Thermokarst may affect 20–50% of tundra uplands by the end of the century; however, little is known about the effect of different thermokarst morphologies on carbon and nitrogen release. We measured soil organic matter displacement, ecosystem respiration, and soil gas concentrations at 26 upland thermokarst features on the North Slope of Alaska. Features included the three most common upland thermokarst morphologies: active‐layer detachment slides, thermo‐erosion gullies, and retrogressive thaw slumps. We found that thermokarst morphology interacted with landscape parameters to determine both the initial displacement of organic matter and subsequent carbon and nitrogen cycling. The large proportion of ecosystem carbon exported off‐site by slumps and slides resulted in decreased ecosystem respiration postfailure, while gullies removed a smaller portion of ecosystem carbon but strongly increased respiration and N₂O concentration. Elevated N₂O in gully soils persisted through most of the growing season, indicating sustained nitrification and denitrification in disturbed soils, representing a potential noncarbon permafrost climate feedback. While upland thermokarst formation did not substantially alter redox conditions within features, it redistributed organic matter into both oxic and anoxic environments. Across morphologies, residual organic matter cover, and predisturbance respiration explained 83% of the variation in respiration response. Consistent differences between upland thermokarst types may contribute to the incorporation of this nonlinear process into projections of carbon and nitrogen release from degrading permafrost.     

Temperature and precipitation drive temporal variability in aquatic carbon and GHG concentrations and fluxes in a peatland catchment

The aquatic pathway is increasingly being recognized as an important component of catchment carbon and greenhouse gas (GHG) budgets, particularly in peatland systems due to their large carbon store and strong hydrological connectivity. In this study, we present a complete 5‐year data set of all aquatic carbon and GHG species from an ombrotrophic Scottish peatland. Measured species include particulate and dissolved forms of organic carbon (POC, DOC), dissolved inorganic carbon (DIC), CO₂, CH₄ and N₂O. We show that short‐term variability in concentrations exists across all species and this is strongly linked to discharge. Seasonal cyclicity was only evident in DOC, CO₂ and CH₄ concentration; however, temperature correlated with monthly means in all species except DIC. Although the temperature correlation with monthly DOC and POC concentrations appeared to be related to biological productivity in the terrestrial system, we suggest the temperature correlation with CO₂ and CH₄ was primarily due to in‐stream temperature‐dependent solubility. Interannual variability in total aquatic carbon concentration was strongly correlated with catchment gross primary productivity (GPP) indicating a strong potential terrestrial aquatic linkage. DOC represented the largest aquatic carbon flux term (19.3 ± 4.59 g C m^?2 yr^?1), followed by CO₂ evasion (10.0 g C m^?2 yr^?1). Despite an estimated contribution to the total aquatic carbon flux of between 8 and 48%, evasion estimates had the greatest uncertainty. Interannual variability in total aquatic carbon export was low in comparison with variability in terrestrial biosphere–atmosphere exchange, and could be explained primarily by temperature and precipitation. Our results therefore suggest that climatic change is likely to have a significant impact on annual carbon losses through the aquatic pathway, and as such, aquatic exports are fundamental to the understanding of whole catchment responses to climate change.     

Integrating aquatic and terrestrial components to construct a complete carbon budget for a north temperate lake district

The development of complete regional carbon (C) budgets for different biomes is an integral step in the effort to predict global response and potential feedbacks to a changing climate regime. Wetland and lake contributions to regional C cycling remain relatively uncertain despite recent research highlighting their importance. Using a combination of field surveys and tower‐based carbon dioxide (CO₂) flux measurements, modeling, and published literature, we constructed a complete C budget for the Northern Highlands Lake District in northern Wisconsin/Michigan, a ～6400 km² region rich in lakes and wetlands. This is one of the first regional C budgets to incorporate aquatic and terrestrial C cycling under the same framework. We divided the landscape into three major compartments (forests, wetlands, and surface waters) and quantified all major C fluxes into and out of those compartments, with a particular focus on atmospheric exchange but also including sedimentation in lakes and hydrologic fluxes. Landscape C storage was dominated by peat‐containing wetlands and lake sediments, which make up only 20% and 13% of the landscape area, respectively, but contain >80% of the total fixed C pool (ca. 400 Tg). We estimated a current regional C accumulation of 1.1±0.1 Tg yr^?1, and the largest regional flux was forest net ecosystem exchange (NEE) into aggrading forests for a total of 1.0±0.1 Tg yr^?1. Mean wetland NEE (0.12±0.06 Tg yr^?1 into wetlands), lake CO₂ emissions and riverine efflux (each ca. 0.03±0.01 Tg yr^?1) were smaller but of consequence to the overall budget. Hydrologic transport from uplands/wetlands to surface waters within the region was an important vector of terrestrial C. Regional C fluxes and pools would be misrepresented without inclusion of surface waters and wetlands, and C budgets in heterogeneous landscapes open opportunities to examine the sensitivities of important fluxes to changes in climate and land use/land cover.     

Direct and indirect climatic drivers of biotic interactions: ice-cover and carbon runoff shaping Arctic char Salvelinus alpinus and brown trout Salmo trutta competitive asymmetries

One of the major challenges in ecological climate change impact science is to untangle the climatic effects on biological interactions and indirect cascading effects through different ecosystems. Here, we test for direct and indirect climatic drivers on competitive impact of Arctic char (Salvelinus alpinus L.) on brown trout (Salmo trutta L.) along a climate gradient in central Scandinavia, spanning from coastal to high-alpine environments. As a measure of competitive impact, trout food consumption was measured using ¹³⁷Cs tracer methodology both during the ice-covered and ice-free periods, and contrasted between lakes with or without char coexistence along the climate gradient. Variation in food consumption between lakes was best described by a linear mixed effect model including a three-way interaction between the presence/absence of Arctic char, season and Secchi depth. The latter is proxy for terrestrial dissolved organic carbon run-off, strongly governed by climatic properties of the catchment. The presence of Arctic char had a negative impact on trout food consumption. However, this effect was stronger during ice-cover and in lakes receiving high carbon load from the catchment, whereas no effect of water temperature was evident. In conclusion, the length of the ice-covered period and the export of allochthonous material from the catchment are likely major, but contrasting, climatic drivers of the competitive interaction between two freshwater lake top predators. While future climatic scenarios predict shorter ice-cover duration, they also predict increased carbon run-off. The present study therefore emphasizes the complexity of cascading ecosystem effects in future effects of climate change on freshwater ecosystems.     

Acid rain recovery may help to mitigate the impacts of climate change on thermally sensitive fish in lakes across eastern North America

From the 1970s to 1990s, more stringent air quality regulations were implemented across North America and Europe to reduce chemical emissions that contribute to acid rain. Surface water pH slowly increased during the following decades, but biological recovery lagged behind chemical recovery. Fortunately, this situation is changing. In the past few years, northeastern US fish populations have begun to recover in lakes that were historically incapable of sustaining wild fish due to acidic conditions. As lake ecosystems across the eastern United States recover from acid deposition, the stress to the most susceptible populations of native coldwater fish appears to be shifting from acidification effects to thermal impacts associated with changing climate. Extreme summer temperature events – which are expected to occur with increasing frequency in the coming century – can stress and ultimately kill native coldwater fish in lakes where thermal stratification is absent or highly limited. Based on data from northeastern North America, we argue that recovery from acid deposition has the potential to improve the resilience of coldwater fish populations in some lakes to impacts of climate change. This will occur as the amount of dissolved organic carbon (DOC) in the water increases with increasing lake pH. Increased DOC will reduce water clarity and lead to shallower and more persistent lake thermoclines that can provide larger areas of coldwater thermal refuge habitat. Recovery from acidification will not eliminate the threat of climate change to coldwater fish, but secondary effects of acid recovery may improve the resistance of coldwater fish populations in lakes to the effects of elevated summer temperatures in historically acidified ecosystems. This analysis highlights the importance of considering the legacy of past ecosystem impacts and how recovery or persistence of those effects may interact with climate change impacts on biota in the coming decades.     

Permafrost conditions in peatlands regulate magnitude,timing, and chemical composition of catchment dissolved organic carbon export

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m^?2) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m^?2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.     

Cross‐scale controls on carbon emissions from boreal forest megafires

Climate warming and drying is associated with increased wildfire disturbance and the emergence of megafires in North American boreal forests. Changes to the fire regime are expected to strongly increase combustion emissions of carbon (C) which could alter regional C balance and positively feedback to climate warming. In order to accurately estimate C emissions and thereby better predict future climate feedbacks, there is a need to understand the major sources of heterogeneity that impact C emissions at different scales. Here, we examined 211 field plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories (NWT), Canada after an unprecedentedly large area burned in 2014. We assessed both aboveground and soil organic layer (SOL) combustion, with the goal of determining the major drivers in total C emissions, as well as to develop a high spatial resolution model to scale emissions in a relatively understudied region of the boreal forest. On average, 3.35 kg C m^?2 was combusted and almost 90% of this was from SOL combustion. Our results indicate that black spruce stands located at landscape positions with intermediate drainage contribute the most to C emissions. Indices associated with fire weather and date of burn did not impact emissions, which we attribute to the extreme fire weather over a short period of time. Using these results, we estimated a total of 94.3 Tg C emitted from 2.85 Mha of burned area across the entire 2014 NWT fire complex, which offsets almost 50% of mean annual net ecosystem production in terrestrial ecosystems of Canada. Our study also highlights the need for fine‐scale estimates of burned area that represent small water bodies and regionally specific calibrations of combustion that account for spatial heterogeneity in order to accurately model emissions at the continental scale.     

Allometric constraints on,and trade‐offs in,belowground carbon allocation and their control of soil respiration across global forest ecosystems

To fully understand how soil respiration is partitioned among its component fluxes and responds to climate, it is essential to relate it to belowground carbon allocation, the ultimate carbon source for soil respiration. This remains one of the largest gaps in knowledge of terrestrial carbon cycling. Here, we synthesize data on gross and net primary production and their components, and soil respiration and its components, from a global forest database, to determine mechanisms governing belowground carbon allocation and their relationship with soil respiration partitioning and soil respiration responses to climatic factors across global forest ecosystems. Our results revealed that there are three independent mechanisms controlling belowground carbon allocation and which influence soil respiration and its partitioning: an allometric constraint; a fine‐root production vs. root respiration trade‐off; and an above‐ vs. belowground trade‐off in plant carbon. Global patterns in soil respiration and its partitioning are constrained primarily by the allometric allocation, which explains some of the previously ambiguous results reported in the literature. Responses of soil respiration and its components to mean annual temperature, precipitation, and nitrogen deposition can be mediated by changes in belowground carbon allocation. Soil respiration responds to mean annual temperature overwhelmingly through an increasing belowground carbon input as a result of extending total day length of growing season, but not by temperature‐driven acceleration of soil carbon decomposition, which argues against the possibility of a strong positive feedback between global warming and soil carbon loss. Different nitrogen loads can trigger distinct belowground carbon allocation mechanisms, which are responsible for different responses of soil respiration to nitrogen addition that have been observed. These results provide new insights into belowground carbon allocation, partitioning of soil respiration, and its responses to climate in forest ecosystems and are, therefore, valuable for terrestrial carbon simulations and projections.     

Browning affects pelagic productivity in northern lakes by surface water warming and carbon fertilization

Global change impacts important environmental drivers for pelagic gross primary production (GPP) in northern lakes, such as temperature, light, nutrient, and inorganic carbon availability. Separate and/or synergistic impacts of these environmental drivers on pelagic GPP remain largely unresolved. Here, we assess key drivers of pelagic GPP by combining detailed depth profiles of summer pelagic GPP with environmental and climatic data across 45 small and shallow lakes across northern Sweden (20 boreal, 6 subarctic, and 19 arctic lakes). We found that across lakes summer pelagic GPP was strongest associated with lake water temperatures, lake carbon dioxide (CO₂) concentrations impacted by lake water pH, and further moderated by dissolved organic carbon (DOC) concentrations influencing light and nutrient conditions. We further used this dataset to assess the extent of additional DOC-induced warming of epilimnia (here named internal warming), which was especially pronounced in shallow lakes (decreasing 0.96°C for every decreasing m in average lake depth) and increased with higher concentrations of DOC. Additionally, the total pools and relative proportion of dissolved inorganic carbon and DOC, further influenced pelagic GPP with drivers differing slightly among the boreal, subarctic and Arctic biomes. Our study provides novel insights in that global change affects pelagic GPP in northern lakes not only by modifying the organic carbon cycle and light and nutrient conditions, but also through modifications of inorganic carbon supply and temperature. Considering the large-scale impacts and similarities of global warming, browning and recovery from acidification of lakes at higher latitudes throughout the northern hemisphere, these changes are likely to operate on a global scale.     

Influence of soil temperature and moisture on the dissolved carbon,nitrogen, and phosphorus in organic matter entering lake ecosystems

Concentrations of terrestrially derived dissolved organic matter (DOM) have been increasing in many north temperate and boreal lakes for over two decades. The concentration of DOM in lakes is influenced by a number of environmental factors, but there is still considerable debate about how the availability of terrestrial DOM, and associated dissolved nitrogen and phosphorus, may be affected by drivers of climatic change. Using experimental and observational methods, we considered how changes in soil temperature and moisture affected the composition of carbon, nitrogen, and phosphorus entering freshwater lakes. In our experiment, organic soil cores were collected from the wetland shoreline of a darkly-stained seepage lake in northern Wisconsin, USA and manipulated in laboratory with temperature and moisture treatments. During the 28-day study, soil leachate was sampled and analyzed for optical properties of DOM via UV/Vis absorbance, as well as concentrations of dissolved organic carbon (DOC), total dissolved nitrogen, and total dissolved phosphorus (TDP). DOM optical properties were particularly sensitive to moisture, with drier scenarios resulting in DOM of lower molecular weight and aromaticity. Warmer temperatures led to lower DOC and TDP concentrations. To consider long-term relationships between climate and lake chemical properties, we analyzed long-term water chemistry data from two additional Wisconsin lakes from the long term ecological research (LTER) project in a cross correlation analysis with Palmer drought severity index data. Analysis of the LTER data supported our experimental results that soil moisture has a significant effect on the quality of DOM entering lakes and that climate may significantly affect lake chemical properties. Although unexpected in terms of DOM loading for climate change scenarios, these results are consistent with patterns of decomposition in organic soils and may be attributed to an increase in soil DOM processing.