Photodegradation in lakes is a major sink for the toxin methylmercury (MeHg) in forest-wetland-lake ecosystems. Previous attempts to estimate annual rates of MeHg photodegradation in lakes have followed a "black-box approach", where the process has been related to incident sunlight rather than photon absorption. Here we use experimental data from three contrasting dark and clear boreal lakes to develop the first apparent quantum yield (AQY) model for spectral MeHg photodegradation rates in lakes. The model was proven universal by its ability to predict experimental data from 22 lakes representing five global regions, covering wide ranges in dissolved organic carbon (DOC), specific UV absorbance at 254 nm, and pH (1.8-39.5 mg C L-1, 1.7-5.7 L mg-1 C m-1 and 4.8-8.5, respectively). The AQY model manifests a dual role played by dissolved organic matter (DOM) as a sensitizer, by producing reactive transient species (RTS) upon photon absorbance, and as an inhibitor, by scavenging of RTS by antioxidants associated with aromatic structures in DOM. Using site-specific data on direct and diffuse solar irradiance, MeHg concentrations, and spectral light absorption properties in 1033 lakes, we estimate an annual MeHg photodegradation rate of 12.1 kg y-1 in the total volume of Swedish lakes. This value corresponds to 24% of the estimated 51 kg of MeHg that annually is transported with runoff from soil into the same lakes. By the AQY model, we calculate the first regional estimates of MeHg photodegradation in lakes of boreal and temperate Europe, temperate North America, subtropical North America, and tropical South America, providing a basis for the establishment of regional MeHg cycling models.

· Bedömningen baseras på litteratur om laxens biologi och om påverkansfaktorer från havsbaserad vindkraft. Det finns i dagsläget inga vindparker där det förekommer lax i svenska hav så att man kan studera faktiska effekter, och inte heller några undersökningar från vindparker i andra länder. · Enligt nuvarande kunskapsläge bedöms risken som låg att vandrande lax påverkas negativt, om vindparker anläggs med bottenfasta fundament med ett långt avstånd mellan tornen och lokaliseras i utsjön på ett inte alltför grunt vattendjup (mer än cirka 30 meter). · Risken för negativ påverkan på vandrande lax bedöms som låg även vid användning av flytande fundament, men osäkerheten är något högre. De kablar som transporterar el från flytande fundament kommer att ligga i vattenmassan och skulle komma närmare laxen än vid användning av bottenfasta fundament. Det magnetiska fältet från kablarna har dock en mycket begränsad spridning, i storleksordningen högst enstaka meter. · Även om risken bedöms som låg så är det viktigt med uppföljande studier i och omkring de vindparker som anläggs, med fokus på att klarlägga laxens beteende vid anläggningarna. Om flera vindparker byggs är en samordnad övervakning viktig för att kunna följa eventuella kumulativa effekter. Syftet med undersökningarna skulle vara att klarlägga kunskapsläget och ge möjlighet att identifiera behov av anpassningar för att lindra eventuella oförutsedda negativa effekter på större rumslig skala, till exempel vandringsmönster.

Havsbaserad vindkraft är en viktig komponent i omställningen till förnybar energi för att bemöta den globala klimatkrisen. För att bättre förstå förutsättningarna för utbyggnad av havsbaserad vindkraft behövs kunskap om möjligheter, hinder och åtgärder för samexistens mellan vindkraft och andra behov till havs. Den här rapporten presenterar resultat från en litteraturanalys för att belysa nuvarande kunskapsläge om samexistens mellan havsbaserad vindkraft och yrkesfiske, vattenbruk respektive naturvård. Angående samexistens med yrkesfiske är huvudsakliga möjligheter som diskuteras i litteraturen att utforma fiskeredskap och -metoder som är kompatibla med vindparker, och designa vindparker så att det kan finnas förutsättningar för att även utöva fiske. Huvudsakliga potentiella hinder som diskuteras i litteraturen är låg acceptans för havsbaserad vindkraft inom fiskesektorn som kan försvåra möjligheter till utvecklingsprojekt, och säkerhetsaspekter, som risk för olyckor, skada på vindparkens installationer, samt skada eller förlust av fiskeredskap, och även tillhörande osäkerheter kring försäkringsaspekter. Åtgärder som undersöks enligt litteraturen inkluderar dels förebyggande åtgärder, som noggrant samarbete med aktörer och intressegrupper vid platsval, lokala eller regionala samrådforum, och styrning på högre politisk nivå, dels strategiska åtgärder, som satsningar på utveckling av teknik som medför minskad risk för skada, utveckling av fiskeredskap som kan användas i vindparker och småskaliga pilotprojekt kring god praxis för fiskevänliga vindparker. Bland planeringsmässiga åtgärder betonas framförallt proaktiv havsplanering. Angående samexistens med vattenbruk diskuteras i litteraturen framförallt fördelar med att kombinera vattenbruk med vindparker, som kan leda till mer effektiv platsanvändning och ökade möjligheter för att etablera vattenbruk längre bort från kusten. Som hinder diskuteras främst att utvecklingen av sådan fleranvändning fortfarande är i ett tidigt skede och inte kommersiellt gångbar, så att det fortfarande behövs kunskapsutveckling i form av pilotstudier, teknikutveckling, risk- och konfliktanalyser, men även att lagstiftningen idag kan vara försvårande för samexistens. Centrala åtgärder som diskuteras i litteraturen är till exempel att stimulera forskning, innovation och utveckling, inkludera fleranvändning i havsplaneringen och utveckla ett gemensamt ramverk för aktörer inom fleranvändning inklusive ett tydligt regelverk för förvaltning, tillståndsprocesser och övervakning av verksamheterna. Angående samexistens med naturvård fokuserar rapporten på olika aspekter kring om, och i så fall hur, havsbaserad vindkraft kan vara förenlig med naturvårdens syften. Möjligheter för samexistens mellan vindparker och lagstadgade skyddade områden är svåra att fastställa på en generell nivå, då det beror på om vindparken medför en risk för det skyddade områdets målsättning eller inte. Förutsättningarna påverkas även hurdana förflyttningseffekter som skulle kunna uppstå inom fiske och andra marina användningar, och vilka miljöeffekter dessa kan leda till. Bland sätt på vilka havsbaserade vindparker skulle kunna gynna naturvården, på en mer generell nivå, belyser litteraturen till exempel att 1) artificiella reveffekter kan främja vissa arter, vilket skulle vara gynnsamt om det stärker hotade eller sårbara arter, eller arter som fyller en önskad funktion i ekosystemet, som filtrering eller bioreglering, och 2) indirekta skyddseffekter kan uppstå om fiske utesluts helt eller delvis i parken, och ge möjlighet till återhämtning för arter som dör i fisket, samt havsbottnar (om området tidigare har påverkats av bottentrålning). Litteraturen, och olika pågående pilotprojekt, belyser även möjligheter att integrera naturinkluderande designer i vindparkernas utformning, till exempel hur vindkraftverkens fundament och erosionsskydd skulle kunna utformas för att främja vissa, önskade arter. Bland potentiella hinder identifieras till exempel risken att en samlokalisering med skyddade områden skulle innebära ömsesidiga kompromisslösningar, så att nätverket av skyddade områden blir suboptimalt. Litteraturen diskuterar även en risk att reveffekter vid vindkraftverken kan motverka syftet med det skyddade området, till exempel att oönskade arter gynnas eller att den nya artificiella livsmiljön skadar naturligt förekommande livsmiljöer, samt osäkerheter kring hur fiskemönster kommer att utvecklas i området och dess närhet, inklusive förflyttningseffekter. Möjligheten att integrera naturbaserade lösningar, till exempel att utforma vindkraftverkens fundament så att de kan främja vissa arter, diskuteras allt mer. En farhåga som lyfts i detta sammanhang är att den forskningsbaserade utvecklingen går långsamt framåt, då det fortfarande finns osäkerheter kring ekologisk effektivitet, effektstorlek eller möjliga risker med sådana lösningar. Möjliga åtgärder som diskuteras i litteraturen för att stärka naturvården är till exempel att strategiskt använda havsplaneringen för att lokalisera områden för vindkraft på ett sätt som kan gynna sådana arter och livsmiljöer som behöver stärkas eller rehabiliteras från fysisk påverkan, samt att testa och vidareutveckla naturbaserade lösningar. En övergripande aspekt som lyfts i litteraturen är vikten av tillräcklig och kontinuerlig kommunikation, och av riktade insatser för att öka förutsättningarna för samexistens och acceptans. Exempel är att stärka möjligheter till engagemang från olika aktörer, konsultera en bredd av sektorer och intressegrupper, säkerställa information till allmänheten, samt att påbörja samråd tidigt i processen och på ett sätt så att det blir tydligt vilken typ av inflytande som är möjlig i vilket skede. Stimulering av forskning och innovation, transparenta och strukturerade processer för havsplanering, kunskapsutbyte mellan länder samt offentlig tillgång till data är andra centrala insatser som betonas.

Abstract. Coastal ecosystems are biologically productive and their diversity underlies various ecosystem services to humans. However, large-scale species richness (SR) and its regulating factors remain uncertain for many organism groups, owing not least to the fact that observed SR (SRobs) is strongly dependent on sample size and inventory completeness (IC). We estimated changes in SR across a natural geographical gradient using statistical rarefaction and extrapolation methods, based on a large fish species incidence dataset compiled from Swedish fish survey databases. The data covered nearly five decades (1975–2020), a 1,300 km north-south distance and a 10-fold salinity gradient along sub-basins of the Baltic Sea plus Skagerrak. Focusing on shallow coastal and offshore areas (< 30 m depth), we calculated standardized SR (SRstd) and estimated SR (SRest), and related these to sub-basin annual mean salinity and water temperature. IC was high, 98.5 %–99.9 %, in the 10 sub-basins with sufficient data for analysis. The recorded fish species were of 75 % marine and 25 % freshwater origin. Total fish SRobs was 144 for shallow coastal areas, and 110 for shallow offshore areas. Sub-basin specific SRest for coastal areas varied between 35 ± 7 (SE) and 109 ± 6 fish species, and was ca. three times higher in the most saline (salinity 29-32) compared to the least saline sub-basins (salinity 2.7). Completing information on functional attributes showed that differences along the salinity gradient reflected an increased share of coastal resident fish species in lower salinities, and a higher share of migratory fish at higher salinities. The proportion of benthic and demersal fish species was also lower in the least saline sub-basins, and increased with increasing salinity. If climate change lowers the salinity regime of the Baltic Sea in the future this may hence influence the SR and community composition of fish.

Solar radiation initiates photochemical oxidation of dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in inland waters, contributing to their carbon dioxide emissions to the atmosphere. Models can determine photochemical DIC production over large spatiotemporal scales and assess its role in aquatic C cycling. The apparent quantum yield (AQY) spectrum for photochemical DIC production, defined as mol DIC produced per mol chromophoric dissolved organic matter-absorbed photons, is a critical model parameter. In previous studies, the principle for the determination of AQY spectra is the same but methodological specifics differ, and the extent to which these differences influence AQY spectra and simulated aquatic DIC photoproduction is unclear. Here, four laboratories determined AQY spectra from water samples of eight inland waters that are situated in Alaska, Finland, and Sweden and span a nearly 10-fold range in DOM absorption coefficients. All AQY values fell within the range previously reported for inland waters. The inter-laboratory coefficient of variation (CV) for wavelength-integrated AQY spectra (300-450 nm) averaged 38% +/- 3% SE, and the inter-water CV averaged 63% +/- 1%. The inter-laboratory CV for simulated photochemical DIC production (conducted for the five Swedish lakes) averaged 49% +/- 12%, and the inter-water CV averaged 77% +/- 10%. This uncertainty is not surprising given the complexities and methodological choices involved in determining DIC AQY spectra and needs to be considered when applying photochemical rate modeling. Thus, we also highlight current methodological limitations and suggest future improvements for DIC AQY determination to reduce inter-laboratory uncertainty.

Ekosystembaserad havsforvaltning anges som ett viktigt verktyg for att na Sveriges miljomal. Denna rapport tar ett forsta steg i riktning mot ett vetenskapligt underlag for att stodja ekosystembaserad havsforvaltning i ett pilotomrade i sodra Bottenhavet. Ekosystemkomponenter (dvs. arter och livsmiljoer) som ar viktiga for modellering av ekosystemet identifieras och deras status samt faktorer som paverkar dem redovisas. Aven kunskapsluckor kopplade till paverkansfaktorer diskuteras, samt hur dessa paverkansfaktorer integreras med ekosystemkomponenterna, liksom vilka ekosystemtjanster som ekosystemkomponenterna bidrar till. Manga av ekosystemkomponenterna har inte god miljostatus, sarskilt grunda bottnar som har ett hogt exploateringstryck. Orovackande nog saknas det overvakning av bade grunda kustnara mjukbottnar och utsjobankar, fastan dessa omraden ar av intresse for exploatering samtidigt som de har hog biodiversitet och ar kopplade till manga ekosystemtjanster. Dock finns det en del data tillgangligt i omradet som kan anvandas vid modellering for att ta fram kartor over ekosystemkomponenter och aven ekosystemtjanster, som kan vara viktiga underlag for ekosystembaserad forvaltning i sodra Bottenhavet. I flera fall ar kunskapen om belastningar i sodra Bottenhavet och hur de kopplar till statusen av ekosystemkomponenter relativt god, men det saknas information om kumulativa effekter av paverkansfaktorer. Manga av de marina arter som finns langst in i Ostersjon lever har vid sin nordliga utbredningsgrans, vilket kan innebara att de ar extra kansliga for manskliga belastningar och klimatforandring. Storskaligt fiske efter stromming i utsjon och dess effekter pa strommingsbestanden kan paverka ekosystemets funktion. Strommingen ar talrik och spelar en stor roll i sodra Bottenhavets ekosystem. Eftersom stromming vandrar mellan utsjon och kusten kan den koppla samman naringsvavar i kust och utsjo. I Bottenhavets omrade kan man se tydliga intressekonflikter gallande resursforvaltning. Traditionella lokala naringar baserar sig mycket pa fiske av stromming och laxfisk, men vikande fangster av den mer storvuxna stromming som fiskas for humankonsumtion, liksom av laxfisk, skapar problem for det kustnara yrkesfisket. Har finns en uppenbar konkurrenssituation bade med det storskaliga pelagiska fisket i utsjon och med naturliga predatorer. Dessa konflikter ar svara att losa med de forvaltningsmetoder som anvands idag. Sodra Bottenhavets ekosystem skulle sannolikt gynnas av en mer helhetsbaserad forvaltning av fiskbestanden och livsmiljoer, utifran samtliga faktorer som paverkar dem. I kustomradet galler detta aven, inte minst, de omraden dar gosens och sikens status ar mycket svag, liksom viktiga omraden for rekrytering av gadda. En sadan mer helhetsbaserad forvaltning innefattar en samplanering av fiskeregleringar, skyddade omraden och atgarder for att restaurera och skydda diverse livsmiljoer. Forbattring av livsmiljoer for fisk forvantas aven gynna andra delar av den biologiska mangfalden och ekosystemtjanster, inklusive olika arters motstandskraft och formaga att anpassa sig till pagaende klimatforandringar.

Many boreal lakes are experiencing an increase in concentrations of terrestrially derived dissolved organic matter (DOM)-a process commonly labeled "browning." Browning affects microbial and photochemical mineralization of DOM, and causes increased light attenuation and hence reduced photosynthesis. Consequently, browning regulates lake heterotrophy and net CO2-efflux to the atmosphere. Climate and environmental change makes ecological forecasting and global carbon cycle modeling increasingly important. A proper understanding of the magnitude and relative contribution from CO2-generating processes for lakes ranging in dissolve organic carbon (DOC) concentrations is therefore crucial for constraining models and forecasts. Here, we aim to study the relative contribution of photomineralization to total CO(2)production in 70 Scandinavian lakes along an ecosystem gradient of DOC concentration. We combined spectral data from the lakes with regression estimates between optical parameters and wavelength specific photochemical reactivity to estimate rates of photochemical DOC mineralization. Further, we estimated total in-lake CO2-production and efflux from lake chemical and physical data. Photochemical mineralization corresponded on average to 9% +/- 1% of the total CO2-evasion, with the highest contribution in clear lakes. The calculated relative contribution of photochemical mineralization to total in-lake CO2-production was about 3% +/- 0.2% in all lakes. Although lakes differed substantially in color, depth-integrated photomineralization estimates were similar in all lakes, regardless of DOC concentrations. DOC concentrations were positively related to CO2-efflux and total in-lake CO2-production but negatively related to primary production. We conclude that enhanced rates of photochemical mineralization will be a minor contributor to increased heterotrophy under increased browning.

Xenobiotic compounds are commonly detected in inland waters. Sunlight-induced photochemical reactions contribute to xenobiotic degradation, but the role of different photoreactions on large geographic scales remains poorly understood. Here, we used a combination of photochemical modelling and large-scale field data from 1020 lakes across Sweden to elucidate the photodegradation kinetics of the commonly used antibiotic sulfadiazine (SDZ) in organic matter-rich lakes. SDZ occurs in two forms, namely acidic HSDZ (pK(a) = 6.5) and basic/deprotonated SDZ . Both species are oxidised fast by the photogenerated triplet states of natural organic matter ((NOM)-N-3*). However, they also undergo efficient back reactions because the partially oxidised HSDZ (and SDZ(-) to a larger extent) can be reduced back to the initial compounds by the phenolic moieties contained in NOM. Typical lakes in Sweden are rich in NOM and have low pH, with the consequence that SDZ photochemistry would be dominated by HSDZ. Our simulation results showed that SDZ photodegradation kinetics in Swedish lakes would become significantly slower with increasing water depth and pH, while it depended little on latitude, which affects irradiance, or on organic matter content. As a consequence, SDZ would be particularly persistent in lakewater in some densely populated areas with relatively deep and high-pH lakes such as, most notably, the Stockholm region. Here the surface waters could be more heavily contaminated by pharmaceuticals compared to the scarcely populated regions in the centre-north of the country, where lakewater could otherwise promote an efficient photodegradation of SDZ. (C) 2018 Elsevier B.V. All rights reserved.

Lake water constituents, such as chromophoric dissolved organic matter (CDOM) and nitrate, absorb sunlight which induces an array of photochemical reactions. Although these reactions are a substantial driver of pollutant degradation in lakes they are insufficiently understood, in particular on large scales. Here, we provide for the first time comprehensive photochemical maps covering a large geographic region. Using photochemical kinetics modeling for 1048 lakes across Sweden we simulated the steady-state concentrations of four photoreactive transient species, which are continuously produced and consumed in sunlit lake waters. We then simulated the transient-induced photochemical transformation of organic pollutants, to gain insight into the relevance of the different photoreaction pathways. We found that boreal lakes were often unfavorable environments for photoreactions mediated by hydroxyl radicals (OH) and carbonate radical anions (CO3−), while photoreactions mediated by CDOM triplet states (3CDOM*) and, to a lesser extent, singlet oxygen (1O2) were the most prevalent. These conditions promote the photodegradation of phenols, which are used as plastic, medical drug and herbicide precursors. When CDOM concentrations increase, as is currently commonly the case in boreal areas such as Sweden, 3CDOM* will also increase, promoting its importance in photochemical pathways even more.

Lake sediments constitute an important compartment in the carbon cycle of lakes, by burying carbon over geological timescales and by production and emission of greenhouse gases. The degradation of organic carbon (OC) in lake sediments is linked to both temperature and oxygen (O 2 ), but the interactive nature of this regulation has not been studied in lake sediments in a quantitative way. We present the first systematic investigation of the effects of temperature on the apparent respiratory quotient (RQ, i.e., the molar ratio between carbon dioxide (CO 2 ) production and O 2 consumption) in two contrasting lake sediments. Laboratory incubations of sediment cores of a humic lake and an eutrophic lake across a 1–21°C temperature gradient over 157 days revealed that both CO 2 production and O 2 consumption were positively, exponentially, and similarly dependent on temperature. The apparent RQ differed significantly between the lake sediments (0.63 ± 0.26 and 0.99 ± 0.28 in the humic and the eutrophic lake, respectively; mean ± SD) and was significantly and positively related to temperature. The O 2 penetration depth into the sediment varied by a factor of 2 over the 1–21°C temperature range and was significantly, negatively, and similarly related to temperature in both lake sediments. Accordingly, increasing temperature may influence the overall extent of OC degradation in lake sediments by limiting O 2 supply to aerobic microbial respiration to the topmost sediment layer, resulting in a concomitant shift to less effective anaerobic degradation pathways. This suggests that temperature may represent a key controlling factor of the OC burial efficiency in lake sediments.

The reactivity continuum (RC) model is a powerful statistical approach for describing the apparent kinetics of bulk organic matter (OM) decomposition. Here, we used ultrahigh resolution mass spectrometry data to evaluate the main premise of the RC model, namely that there is a continuous spectrum of reactivity within bulk OM, where each individual reactive type undergoes exponential decay. We performed a 120 day OM decomposition experiment on lake water, with an untreated control and a treatment preexposed to UV light, and described the loss of bulk dissolved organic carbon with RC modeling. The behavior of individual molecular formulas was described by fitting the single exponential model to the change in peak intensities over time. The range of the empirically derived apparent exponential decay coefficients (kexp) was indeed continuous. The character of the corresponding distribution, however, differed from the conceptual expectations, due to the effects of intrinsic averaging, overlaps in formula-specific loss and formation rates, and the limitation of the RC model to include apparently accumulating compounds in the analysis. Despite these limitations, both the RC model-simulated and empirical (mass spectrometry-derived) distributions of kexp captured the effects of preexposure to UV light. Overall, we present experimental evidence that the reactivity continuum within bulk OM emerges from a range of reactivity of numerous individual components. This constitutes direct empirical support for the major assumption behind the RC model of the natural OM decomposition.

Sunlight induces photochemical mineralisation of chromophoric dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) in inland waters, resulting in carbon dioxide (CO2) emissions to the atmosphere. Photochemical rate modelling is used to determine sunlight-induced CO2 emissions on large spatial and temporal scales. A sensitive model parameter is the wavelength-specific photochemical CDOM reactivity, the apparent quantum yield (AQY). However, the temporal variability of AQY spectra within inland waters remains poorly constrained. Here, we studied a boreal brown water lake in Sweden. We measured AQY spectra for photochemical DIC production monthly between June and November 2014 and parameterised a photochemical rate model. The total AQY between 280 and 600 nm increased about 3-fold during the open-water period, likely due to a high rainfall event with consecutive mixing in autumn that increased availability of highly photoreactive CDOM. However, the variability in AQY spectra over time was much smaller than previously reported variability in AQY spectra between lakes. Yet, using either the AQY spectrum from the least or from the most photoreactive water sample resulted in a 5-fold difference in simulated annual DIC photoproduction (2012–2014), with 2.0 ± 0.1 and 10.3 ± 0.7 g C m−2 yr−1, respectively. This corresponded to 1 and 8 % of the mean CO2 emissions measured from this lake. We conclude that (1) it may be recommendable to conduct repeated AQY measurements throughout the season for more accurate simulation of annual photochemical DIC production in lakes and (2), in agreement with previous studies, direct CDOM photomineralisation makes only a minor contribution to mean CO2 emissions from Swedish brown water lakes.

Up to one tenth of the carbon dioxide (CO2) emissions from inland waters worldwide are directly induced by the photochemical mineralization of dissolved organic matter (DOM). The photochemical production of dissolved inorganic carbon (DIC) per photon absorbed by chromophoric DOM (CDOM) decreases exponentially with increasing irradiance wavelength, and is commonly described by an apparent quantum yield (AQY) spectrum. Although an essential model parameter to simulate photochemical mineralization the AQY remains poorly constrained. Here, the AQY of photochemical DIC production for 25 lakes located in boreal, polar, temperate, and tropical areas, including four saline lagoons, was measured. The wavelength-integrated AQY (300-500 nm; mol DIC mol CDOM-absorbed photons(-1)) ranged from 0.05 in an Antarctic lake to 0.61 in a humic boreal lake, averaging 0.24 +/- 0.03 SE. AQY was positively linearly correlated with the absorption coefficient at 420 nm (a(420)) as a proxy for CDOM content (R-2 of 0.64 at 300 nm and 0.26 at 400 nm), with specific UV absorption coefficients as a proxy for DOM aromaticity (R-2 of 0.56 at 300 nm and 0.38 at 400 nm), and with the humification index (R-2 of 0.41 at 300 nm and 0.42 at 400 nm). Hence, a considerable fraction of the AQY variability was explained by water optical properties in inland waters. The correlation of AQY with a(420) opens up the possibility to improve large-scale model estimates of sunlight-induced CO2 emissions from inland waters based on water color information derived by satellite remote sensing.

We investigated the role of lake sediments as carbon (C) source and sink in the annual C budget of a small (0.07 km(2)) and shallow (mean depth, 3.4 m), humic lake in boreal Sweden. Organic carbon (OC) burial and mineralization in the sediments were quantified from Pb-210-dated sediment and laboratory sediment incubation experiments, respectively. Burial and mineralization rates were then upscaled to the entire basin and to one whole year using sediment thickness derived from sub-bottom profiling, basin morphometry, and water column monitoring data of temperature and oxygen concentration. Furthermore, catchment C import, open water metabolism, photochemical mineralization as well as carbon dioxide (CO2) and methane (CH4) emissions to the atmosphere were quantified to relate sediment processes to other lake C fluxes. We found that on a whole-basin and annual scale, sediment OC mineralization was three times larger than OC burial, and contributed about 16% to the annual CO2 emission. Other contributions to CO2 emission were water column metabolism (31%), photochemical mineralization (6%), and catchment imports via inlet streams and inflow of shallow groundwater (22%). The remainder (25%) could not be explained by our flux calculations, but was most likely attributed to an underestimation in groundwater inflow. We conclude that on an annual and whole-basin scale (1) sediment OC mineralization dominated over OC burial, (2) water column OC mineralization contributed more to lake CO2 emission than sediment OC mineralization, and (3) catchment import of C to the lake was greater than lake-internal C cycling.

To address the link between the composition and decomposition of freshwater dissolved organic matter (DOM), we manipulated the DOM from three boreal lakes using preincubations with UV light to cleave large aromatic molecules and polyvinylpyrrolidone (PVP) to remove colored phenolic compounds. Subsequently, we monitored the dissolved organic carbon (DOC) loss over 4months of microbial degradation in the dark to assess how compositional changes in DOM affected different aspects of the reactivity continuum, including the distribution of the apparent decay coefficients. We observed profound effects on decomposition kinetics, with pronounced shifts in the relative share of rapidly and more slowly decomposing fractions of the DOM. In the UV-exposed treatment initial apparent decay coefficient k(0) was almost threefold higher than in the control. Significantly higher relative DOC loss in the UV-exposed treatment was sustained for 2months of incubation, after which decay coefficients converged with those in the control. The PVP removed compounds with absorbance and fluorescence characteristics representative of aromatic compounds, which led to slower decomposition, compared to that in the control. Our results demonstrate the reactivity continuum underlying the decomposition of DOM in freshwaters and highlight the importance of intrinsic properties of DOM in determining its decomposition kinetics.

Large carbon dioxide amounts are released to the atmosphere during organic matter decomposition. Yet the large‐scale and long‐term regulation of this critical process in global carbon cycling by litter chemistry and climate remains poorly understood. We used reactivity continuum (RC) modeling to analyze the decadal data set of the “Long‐term Intersite Decomposition Experiment,” in which fine litter and wood decomposition was studied in eight biome types (224 time series). In 32 and 46% of all sites the litter content of the acid‐unhydrolyzable residue (AUR, formerly referred to as lignin) and the AUR/nitrogen ratio, respectively, retarded initial decomposition rates. This initial rate‐retarding effect generally disappeared within the first year of decomposition, and rate‐stimulating effects of nutrients and a rate‐retarding effect of the carbon/nitrogen ratio became more prevalent. For needles and leaves/grasses, the influence of climate on decomposition decreased over time. For fine roots, the climatic influence was initially smaller but increased toward later‐stage decomposition. The climate decomposition index was the strongest climatic predictor of decomposition. The similar variability in initial decomposition rates across litter categories as across biome types suggested that future changes in decomposition may be dominated by warming‐induced changes in plant community composition. In general, the RC model parameters successfully predicted independent decomposition data for the different litter‐biome combinations (196 time series). We argue that parameterization of large‐scale decomposition models with RC model parameters, as opposed to the currently common discrete multiexponential models, could significantly improve their mechanistic foundation and predictive accuracy across climate zones and litter categories.

Temperature alone explains a great amount of variation in sediment organic carbon (OC) mineralization. Studies on decomposition of soil OC suggest that (1) temperature sensitivity differs between the fast and slowly decomposition OC and (2) over time, decreasing soil respiration is coupled with increase in temperature sensitivity. In lakes, autochthonous and allochthonous OC sources are generally regarded as fast and slowly decomposing OC, respectively. Lake sediments with different contributions of allochthonous and autochthonous components, however, showed similar temperature sensitivity in short-term incubation experiments. Whether the mineralization of OC in lake sediments dominated by allochthonous or autochthonous OC has different temperature sensitivity in the longer term has not been addressed. We incubated sediments from two boreal lakes that had contrasting OC origin (allochthonous versus autochthonous), and OC characteristics (C/N ratios of 21 and 10) at 1, 3, 5, 8, 13, and 21 degrees C for five months. Compared to soil and litter mineralization, sediment OC mineralization rates were low in spite of low apparent activation energy (E-a). The fraction of the total OC pool that was lost during five months varied between 0.4 and 14.8%. We estimate that the sediment OC pool not becoming long-term preserved was degraded with average apparent turnover times between 3 and 32years. While OC mineralization was strongly dependent on temperature as well as on OC composition and origin, temperature sensitivity was similar across lakes and over time. We suggest that the temperature sensitivity of OC mineralization in lake sediments is similar across systems within the relevant seasonal scales of OC supply and degradation.

The short-term (hourly and daily) variation in chromophoric dissolved organic matter (CDOM) in lakes is largely unknown. We assessed the spectral characteristics of light absorption by CDOM in a eutrophic, humic shallow mixed lake of temperate Sweden at a high-frequency (30min) interval and during a full growing season (May to October). Physical time series, such as solar radiation, temperature, wind, and partial pressures of carbon dioxide in water and air, were measured synchronously. We identified a strong radiation-induced summer CDOM loss (25 to 50%) that developed over 4months, which was accompanied by strong changes in CDOM absorption spectral shape. The magnitude of the CDOM loss exceeded subhourly to daily variability by an order of magnitude. Applying Fourier analysis, we demonstrate that variation in CDOM remained largely unaffected by rapid shifts in weather, and no apparent response to in-lake dissolved organic carbon production was found. In autumn, CDOM occasionally showed variation at hourly to daily time scales, reaching a maximum daily coefficient of variation of 15%. We suggest that lake-internal effects on CDOM are quenched in humic lake waters by dominating effects associated with imported CDOM and solar exposure. Since humic lake waters belong to one of the most abundant lake types on Earth, our results have important implications for the understanding of global CDOM cycling.

The emissions of carbon dioxide (CO2) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55 degrees N to 69 degrees N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 +/- 0.04 g C m(-2) yr(-1), which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 +/- 1 kt C yr(-1) was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO2 emissions from Swedish lakes. With a median depth of 3.2m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength-and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr(-1) under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO2 emissions contribute up to about one tenth to the global CO2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.

It is estimated that tropical forest soils contribute 6.2 Tg yr−1 (28%) to global methane (CH4) uptake, which is large enough to alter CH4 accumulation in the atmosphere if significant changes would occur to this sink. Elevated deposition of inorganic nitrogen (N) to temperate forest ecosystems has been shown to reduce CH4 uptake in forest soils, but almost no information exists from tropical forest soils even though projections show that N deposition will increase substantially in tropical regions. Here we report the results from two long-term, ecosystem-scale experiments in which we assessed the impact of chronic N addition on soil CH4 fluxes from two old-growth forests in Panama: (1) a lowland, moist (2.7 m yr−1 rainfall) forest on clayey Cambisol and Nitisol soils with controls and N-addition plots for 9–12 yr, and (2) a montane, wet (5.5 m yr−1 rainfall) forest on a sandy loam Andosol soil with controls and N-addition plots for 1–4 yr. We measured soil CH4 fluxes for 4 yr (2006–2009) in four replicate plots (40 m × 40 m each) per treatment using vented static chambers (four chambers per plot). CH4 fluxes from the lowland control plots and the montane control plots did not differ from their respective N-addition plots. In the lowland forest, chronic N addition did not lead to inhibition of CH4 uptake; instead, a negative correlation of CH4 fluxes with nitrate (NO3–) concentrations in the mineral soil suggests that increased NO3– levels in N-addition plots had stimulated CH4 consumption and/or reduced CH4 production. In the montane forest, chronic N addition also showed negative correlation of CH4 fluxes with ammonium concentrations in the organic layer, which suggests that CH4 consumption was N limited. We propose the following reasons why such N-stimulated CH4 consumption did not lead to statistically significant CH4 uptake: (1) for the lowland forest, this was caused by limitation of CH4 diffusion from the atmosphere into the clayey soils, particularly during the wet season, as indicated by the strong positive correlations between CH4 fluxes and water-filled pore space (WFPS); (2) for the montane forest, this was caused by the high WFPS in the mineral soil throughout the year, which may not only limit CH4 diffusion from the atmosphere into the soil but also favour CH4 production; and (3) both forest soils showed large spatial and temporal variations of CH4 fluxes. We conclude that in these extremely different tropical forest ecosystems there were indications of N limitation on CH4 uptake. Based on these findings, it is unlikely that elevated N deposition on tropical forest soils will lead to a rapid reduction of CH4 uptake.