Higher temperature reduces the effects of litter quality on decomposition by aquatic fungi

1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss. 2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms. 3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant. 4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species. 5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves. 6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams. 7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams.     

Synergistic effects of water temperature and dissolved nutrients on litter decomposition and associated fungi

In woodland streams, the decomposition of allochthonous organic matter constitutes a fundamental ecosystem process, where aquatic hyphomycetes play a pivotal role. It is therefore greatly affected by water temperature and nutrient concentrations. The individual effects of these factors on the decomposition of litter have been studied previously. However, in the climate warming scenario predicted for this century, water temperature and nutrient concentrations are expected to increase simultaneously, and their combined effects on litter decomposition and associated biological activity remains unevaluated. In this study, we addressed the individual and combined effects of water temperature (three levels) and nutrient concentrations (two levels) on the decomposition of alder leaves and associated aquatic hyphomycetes in microcosms. Decomposition rates across treatments varied between 0.0041 day^?1 at 5 °C and low nutrient level and 0.0100 day^?1 at 15 °C and high nutrient level. The stimulation of biological variables at high nutrients and temperatures indicates that nutrient enrichment of streams might have a higher stimulatory effect on fungal performance and decomposition rates under a warming scenario than at present. The stimulation of fungal biomass and sporulation with increasing temperature at both nutrient levels shows that increases in water temperature might enhance fungal growth and reproduction in both oligotrophic and eutrophic streams. The stimulation of fungal respiration and litter decomposition with increasing temperature at high nutrients indicates that stimulation of carbon mineralization will probably occur at eutrophied streams, while oligotrophic conditions seem to be ‘protected’ from warming. All biological variables were stimulated when both factors increased, as a result of synergistic interactions between factors. Increased water temperature and nutrient level also affected the structure of aquatic hyphomycete assemblages. It is plausible that if water quality of presently eutrophied streams is improved, the potential stimulatory effects of future increases in water temperature on aquatic biota and processes might be mitigated.     

Interactions between atmospheric CO2 enrichment and soil fauna

We have reviewed the responses of soil fauna to increased concentrations of atmospheric CO₂ and the consequent climate change. These will affect several attributes of animal populations and communities including their density, biomass, diversity, activity, rates of consumption, life history parameters and migration ability. Changes in the quality and quantity of litter and global warming are the main factors which are expected to modify soil fauna. Although changes have been observed in several attributes of the soil fauna as a consequence of increased concentrations of atmospheric CO₂, no general trend which might allow to the prediction of a general pattern of response has been identified. Because of the complexity of the biological mechanisms and the synergetic action of several factors, the few resulting responses reported in the literature are inconclusive. However, some aspects of the situation deserve more attention. These include the consequences of (1) changes in the food resources for soil fauna in the litter layer and in the rhizosphere, (2) the consumption of low quality litter by the macrofauna, (3) the change in life span in response to temperature elevation, (4) the enhancement of earthworm burrowing activity and (5) the changes in community composition arising because of specific differential resistance to adverse conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of warming and grazing on litter mass loss and temperature sensitivity of litter and dung mass loss on the Tibetan plateau

Knowledge about the role of litter and dung decomposition in nutrient cycling and response to climate change and grazing in alpine ecosystems is still rudimentary. We conducted two separate studies to assess the relative role of warming and grazing on litter mass loss and on the temperature sensitivity of litter and dung mass loss. Experiments were conducted for 1–2 years under a controlled warming–grazing system and along an elevation gradient from 3200 to 3800 m. A free‐air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures (average 0.5–1.6 °C) from 0 to 40 cm depth, but neither warming nor grazing affected soil moisture except early in the growing seasons at 30 cm soil depth. Heaters caused greater soil warming at night‐time compared with daytime, but grazing resulted in greater soil warming during daytime compared with night‐time. Annual average values of the soil temperature at 5 cm were 3.2, 2.4 and 0.3 °C at 3200, 3600 and 3800 m, respectively. Neither warming nor grazing caused changes of litter quality for the first year of the controlled warming–grazing experiment. The effects of warming and grazing on litter mass losses were additive, increasing litter mass losses by about 19.3% and 8.3%, respectively, for the 2‐year decomposition periods. The temperature sensitivity of litter mass losses was approximately 11% °C^?1 based on the controlled warming–grazing experiment. The annual cumulative litter mass loss was approximately 2.5 times that of dung along the elevation gradient. However, the temperature sensitivity (about 18% °C^?1) of the dung mass loss was about three times that of the litter mass loss. These results suggest greater warming at night‐time compared with daytime may accelerate litter mass loss, and grazing will enhance carbon loss to atmosphere in the region through a decrease of litter biomass and an increase of dung production with an increase of stocking rate in future warmer conditions.     

CO2浓度和温度升高对红桦根际微生物的影响

应用自控、封闭、独立的生长室系统,研究升高的大气CO2浓度(环境CO2浓度 350(±25)μmol.mol-1,EC)和温度(环境温度 2.0(±0.5)℃,ET)及其交互作用(ECT)对不同栽植密度条件下红桦根际土壤可培养微生物数量的影响。结果表明:(1)EC显著增加了高密度条件下根际细菌数量;在整个生长季中,最大的根际细菌数量增加出现在7月份;而EC对低密度处理的根际细菌数量影响不显著。除了5月和6月份,ET在其余月份均显著增加了根际细菌数量,但是与密度处理没有有意义的相关;ECT对高低密度处理的根际细菌数量均未产生有统计意义的影响。(2)EC对低密度条件下的根际放线菌数量有显著增加,而对高密度条件下的根际放线菌数量无显著影响;ET和ECT对高低密度条件下的根际放线菌数量均未产生有统计意义的影响。(3)EC和ET对高低密度条件下的根际真菌数量无显著增加,而ECT显著增加了根际真菌数量。     

三江平原草甸湿地土壤呼吸和枯落物分解的CO2释放

利用静态箱-碱液吸收法研究了三江平原草甸湿地土壤呼吸和枯落物分解的CO2释放速率,讨论了影响CO2释放的环境因素,估算了枯落物分解的CO2释放对于总释放的贡献.结果表明,生长季,小叶章沼泽化草甸和小叶章湿草甸各部分CO2释放均具有明显的时间变化特征,温度和水分是重要制约因素.两类草甸湿地的平均土壤呼吸速率分别为4.33g·m-2·d-1和6.15g·m-2·d-1,枯落物分解的CO2平均释放速率分别为1.76g·m-2·d-1和3.10g·m-2·d-1,枯落物分解的CO2释放占总释放量的31%和35%,说明在碳素由地上植物碳库转移到地下土壤碳库的过程中,湿地枯落物是一个不可忽略的碳损失源.     

Impacts of elevated atmospheric CO2 on litter quality, litter decomposability and nitrogen turnover rate of two oak species in a Mediterranean forest ecosystem

Elevated CO₂ may affect litter quality of plants, and subsequently C and N cycling in terrestrial ecosystems, but changes in litter quality associated with elevated CO₂ are poorly known. Abscised leaf litter of two oak species (Quercus cerris L. and Q. pubescens Willd.) exposed to long-term elevated CO₂ around a natural CO₂ spring in Tuscany (Italy) was used to study the impact of increasing concentration of atmospheric CO₂ on litter quality and C and N turnover rates in a Mediterranean-type ecosystem. Litter samples were collected in an area with elevated CO₂ (>500 ppm) and in an area with ambient CO₂ concentration (360 ppm). Leaf samples were analysed for concentrations of total C, N, lignin, cellulose, acid detergent residue (ADR) and polyphenol. The decomposition rate of litter was studied using a litter bag experiment (12 months) and laboratory incubations (3 months). In the laboratory incubations, N mineralization in litter samples was measured as well (125 days). Litter quality was expressed in terms of chemical composition and element ratios. None of the litter quality parameters was affected by elevated CO₂ for the two Quercus species. Remaining mass in Q. cerris and Q. pubescens litter from elevated CO₂ was similar to that from ambient conditions. C mineralization in Q. pubescens litter from elevated CO₂ was lower than that from ambient CO₂, but the difference was insignificant. This effect was not observed for Q. cerris. N mineralization was higher from litter grown at elevated CO₂, but this difference disappeared at the end of the incubation. Litter of Q. pubescens had a higher quality than Q. cerris, and indeed mineralized more rapidly in the laboratory, but not under field conditions.     

Effect at the ecosystem level of elevated atmospheric CO2in an aquatic microcosm

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO₂concentration. We used microcosms, consisting of Escherichia coli(bacteria), Tetrahymena thermophila(protozoa) and Euglena gracilis(algae), in salt solution with 50 and 500 mg l^–1of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO₂(1550±100 l l^–1) conditions. The density of E. gracilisincreased significantly under elevated CO₂in both eutrophic and hypereutrophic microcosms. In the eutrophic microcosm, the other elements were not affected by elevated CO₂. In the hypereutrophic microcosm, however, the concentrations of ammonium and phosphate decreased significantly under elevated CO₂. Furthermore, the density of T. thermophilawas maintained in higher level than that in the microcosm with ambient CO₂and the density of E. coliwas decreased by CO₂enrichment. Calculating the carbon biomasses of T. thermophilaand E. colifrom their densities, the changes in their biomasses by CO₂enrichment were little as compared with large increase of E. graciliscarbon biomass converted from chlorophyll a. From the responses to elevated CO₂in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. graciliswas a direct effect of elevated CO₂, whereas the changes in the density of E. coliand T. thermophilaand the decreases in the concentration of ammonium and phosphate are considered to be indirect effects rather than direct effects of elevated CO₂. The indirect effects of elevated CO₂were prominent in the hypereutrophic microcosm.     

Consequences of detritus type in an aquatic microsystem: effects on water quality, micro-organisms and performance of the dominant consumer

1. Variation in detritus quality and quantity can have significant effects on aquatic invertebrate food webs. Allochthonous inputs of detritus are the principal energy source for organisms in aquatic tree hole microsystems. We compared the effects of two major detritus types found in tree holes, senescent leaves (Sugar Maple and White Oak) and invertebrate carcasses (dead adult fruit flies and crickets), on several water quality characteristics of laboratory microcosms as well as on mass, survival and population performance of the dominant tree hole consumer, Ochlerotatus triseriatus (Diptera: Culicidae). To date, no study has documented the effects of animal detritus in tree hole microsystems or on resident consumers.2. Aquatic environments receiving invertebrate carcasses had significantly greater total nitrogen, total reactive phosphorus and higher pH, than leaf-based environments. Decay rate of invertebrate carcasses was greater compared to leaf material. Consumption of O(2) by micro-organisms increased with increasing detritus amounts, but we detected no difference between detritus types.3. Ochlerotatus triseriatus larvae grew faster in animal-based treatments, and mean mass of larvae was significantly greater when more animal detritus was used. The effect of animal-based treatments on larvae translated into higher performance for adults, which were three times heavier than counterparts from plant-based containers. Survivorship and estimated population growth rates were significantly greater for O. triseriatus reared on animal-based versus plant-based detritus.4. We hypothesise two mechanisms for the pronounced effect of invertebrate carcasses on mosquito performance relative to that associated with leaf detritus: (i) invertebrate carcasses decompose more quickly and release nutrients more effectively into the aquatic environment; or (ii) O. triseriatus larvae may directly ingest nutrient-rich components of invertebrate carcasses. Because even relatively small animal detritus additions can have strong effects on O. triseriatus populations, studies need to be conducted to explore the overall role of animal detritus in tree holes in nature.     

Northern pike (Esox lucius L.) and aquatic vegetation,tools in the management of fisheries and water quality in shallow waters

Species and size composition of fish communities in shallow stagnant waters appear to be associated with the type, abundance and pattern of the vegetation. Man-induced impacts as eutrophication, and suppression of vegetation for reasons of water quantity management or angling pleasure may induce irreversible changes in the aquatic ecosystem. Water quality management should aim at restoring former pike habitat. Submerged weeds are important to that effect. These habitats are characterized by relatively low densities of fish stocks. Stocking of bream and carp interferes strongly with these objectives. With respect to fish stocks, interests of water quality and fishery management may be opposed.     

Assessing the influence of water and substratum quality on benthic macroinvertebrate communities in a metal-polluted stream: an experimental approach

SUMMARY 1. Field and laboratory experiments were conducted to assess the relative influence of water quality and substratum quality on benthic macroinvertebrate communities in the Animas River, a metal-polluted stream in south-western Colorado (U.S.A.).
2. A community-level in situ toxicity test measured direct effects of Animas River water on benthic invertebrates collected from a reference stream (Elk Creek). The effects of metal-contaminated biofilm were examined by comparing macroinvertebrate colonisation of clean and contaminated substrata placed in Elk Creek. A feeding experiment with the mayfly Baetis tricaudatus Dodds (Ephemeroptera: Baetidae) examined metal bioaccumulation and effects of metal-contaminated biofilm on growth and survival.
3. Animas River water was acutely toxic to most taxa, with greatest effects observed on mayflies (Heptageniidae, Ephemerellidae) and stoneflies (Taeniopterygidae and Capniidae).
4. Although Animas River biofilm was characterised by high concentrations of metals and low algal biomass, most taxa colonised substratum from the reference stream and the Animas River equally. The exceptions were Ephemerellidae, Taeniopterygidae and Simuliidae, which were less abundant on Animas River substratum. Mayflies grazing Animas River biofilm accumulated significantly more metals and showed reduced growth compared with organisms feeding on Elk Creek biofilm.
5. Results of our experiments demonstrated that effects of heavy metals on benthic community structure in the Animas River were complex, and that responses to metals in water and contaminated substratum were species-specific. Predicting recovery of benthic communities following remediation requires an understanding of these species-specific responses.     

Effects of elevated [CO2] on forest growth and carbon storage: a modelling analysis of the consequences of changes in litter quality/quantity and root exudation

Many researchers have proposed that the stimulus of plant growth under elevated [CO₂] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO₂ responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the absence of changes in litter quality and root exudation, the short-term CO₂ stimulus of litter quantity leads to only a minimal CO₂ stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes little impact on these results; (3) a significant CO₂ response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation as soil organic matter, although the long-term CO₂ response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on the long-term CO₂-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases may be required. This revised version was published online in June 2006 with corrections to the Cover Date.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.     

Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO₂ efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO₂ (ambient + 200 mol mol^–1) and elevated temperature (ambient + 4 °C) treatments during the first year. Rhizosphere respiration increased most under elevated CO₂, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO₂ efflux increased significantly under elevated CO₂ and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO₂, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO₂ in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO₂ production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.     

Decomposition of litter produced under elevated CO2: Dependence on plant species and nutrient supply

We investigated the effect of CO₂ concentration and soilnutrient availability during growth on the subsequent decomposition andnitrogen (N) release from litter of four annual grasses that differ inresource requirements and native habitat. Vulpia microstachys isa native grass found on California serpentine soils, whereas Avenafatua, Bromus hordaceus, and Lolium multiflorum areintroduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO₂ altered litter C:N ratio,decomposition, and N release, but the direction and magnitude of thechanges differed among plant species and nutrient treatments. ElevatedCO₂ had relatively modest effects on C:N ratio of litter,increasing this ratio in Lolium roots (and shoots at high nutrients),but decreasing C:N ratio in Avena shoots. Growth of plants underelevated CO₂ decreased the decomposition rate of Vulpialitter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO₂ on N loss fromlitter also differed among species, with Vulpia litter from high-CO₂ plants releasing N more slowly than ambient-CO₂litter, whereas growth under elevated CO₂ caused increased Nloss from Avena litter. CO₂ effects on N release in Lolium and Bromus depended on the nutrient regime in whichplants were grown. There was no overall relationship between litter C:Nratio and decomposition rate or N release across species and treatments.Based on our study and the literature, we conclude that the effects ofelevated CO₂ on decomposition and N release from litter arehighly species-specific. These results do not support the hypothesis thatCO₂ effects on litter quality consistently lead to decreasednutrient availability in nutrient-limited ecosystems exposed to elevatedCO₂.     

The impact of extensive planting of Miscanthus as an energy crop on future CO2 atmospheric concentrations

A process‐based model of the energy crop Miscanthus×giganteus is integrated into the global climate impact model IMOGEN, simulating the potential of large‐scale Miscanthus plantation to offset fossil fuel emissions during the 21st century. This simulation produces spatially explicit, annual projections of Miscanthus yields from the present day to the year 2100 under an SRES A2 anthropogenic emissions scenario and includes the effects of climate change. IMOGEN also simulates natural vegetation and soil carbon storage throughout the 21st century. The benefit of Miscanthus cultivation (avoiding fossil fuel emissions of CO₂) is then compared with the cost of displacing natural vegetation (carbon emissions from vegetation and soil). The time taken for these effects to cancel out, the pay‐back time, is calculated regionally. The effects of large‐scale Miscanthus plantation are then integrated globally to produce an estimate of atmospheric CO₂ concentrations throughout the 21st century. Our best estimate of the pay‐back time for Miscanthus plantation is 30 years. We project a maximum possible reduction in atmospheric CO₂ of 323 ppmv by the end of 21st century, with a reduction of 162 ppmv corresponding to the best estimate scenario.     

Effects of stormwater sewer discharges on the aquatic community in urban canals in Lelystad

In urban areas with a separate sewerage system, the stormwater runoff is discharged into surface waters. A study on the effects of stormwater sewer discharges on the composition of the aquatic community in urban waters was carried out in two areas with a primarily residential use in the new town of Lelystad. The aquatic organisms considered included hydrophytes, epiphytic diatoms, filamentous algae and macroinvertebrates.The results indicate that the stormwater sewer discharges cause a slight change in the composition of the aquatic community. The water in the urban surface waters can be characterized as eutrophic and - toa-mesosaprobic. Near stormwater sewer outfalls the water tends to the more polluteda-mesosaprobic state. The shifts in the composition of the aquatic community could be traced primarily on the basis of a number of epiphytic diatoms and macroinvertebrates (in particular some Diptera and Trichoptera). Most hydrophytes, filamentous algae and the remaining epiphytic diatoms and macroinvertebrates (in particular water beetles and water mites) turned out to be of little or no use in this respect.Because the investigation was carried out in an unusually dry summer, the results probably underestimate the effects of stormwater discharges on the aquatic community.     

CO2 supersaturation along the aquatic conduit in Swedish watersheds as constrained by terrestrial respiration,aquatic respiration and weathering

We tested the hypothesis that CO₂ supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized‐additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO₂ in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg²⁺ had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg²⁺ for streams compared with TOC. Both DSi and TOC explained 22–46% of the pCO₂ variability in various lake classes (0.01–>100 km²) and Mg²⁺ and TOC explained 11–41% of the pCO₂ variability in the various stream orders. This suggests that aquatic pCO₂ has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO₂ efflux. pCO₂ and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO₂ with the atmosphere and in‐lake mineralization of TOC. For streams, we observed a decreasing trend in pCO₂ with stream orders between 3 and 6. We calculated the total CO₂ efflux from all Swedish lakes and streams to be 2.58 Tg C yr^?1. Our analyses also demonstrated that 0.70 Tg C yr^?1 are exported to the ocean by Swedish watersheds as HCO₃^? and CO₃^2? of which about 0.56 Tg C yr^?1 is also a residual from terrestrial respiration and constitute a long‐term sink for atmospheric CO₂. Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr^?1, which corresponds to 10% of the NEE in Sweden.     

Effect of water alkalinity on gill CO2 exchange and internal PCO2 in aquatic animals

In addition to metabolic CO2 production and gill ventilatory flow rate, expired water PCO2 is very dependent on water acid-base balance in a complex way. This is particularly true in carbonated waters at low ambient PCO2 and high pH, where CO2 excreted in the gill water may be buffered by carbonate ions, leading to an increased CO2 capacitance coefficient. The higher the carbonate alkalinity (CA) and the lower the inspired PCO2 (i.e., the higher the inspired water pH), the stronger the carbonate buffering and the smaller the increase of PCO2 in the gill water during respiratory CO2 exchanges. As a consequence, as shown by a number of reported data, increasing the CA leads to blood hypocapnia and respiratory alkalosis at constant low, but not at high, inspired PCO2. In the low range of inspired PCO2, internal PCO2 becomes very sensitive to even small changes of water PCO2, which may explain at least in part the large variability of reported blood PCO2 values in gill breathers. Water CA also influences the amplitude of respiratory acid-base disturbances caused by changes of the gill ventilatory flow rate. Carbonate buffering of excreted CO2 and thus dependence of blood PCO2 on water alkalinity requires catalysis of CO2 hydration by carbonic anhydrase, that must be available from the water side of the gill epithelium.