Acid-induced changes in DOC quality in an experimental whole-lake manipulation. (original) (raw)
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Limnology and Oceanography, 2003
We evaluated the significance of photochemical and biological degradation of allochthonous dissolved organic carbon (DOC) on in-lake H ϩ budgets by laboratory experiments and with a mass budget study for major ions in three atmospherically acidified forest lakes in the Bohemian Forest. In the experiments, photodegradation of DOC from a lake tributary resulted in (1) a liberation of organically bound Al and Fe, which consumed an equivalent amount of H ϩ , (2) a minor decrease in concentrations of organic acid anions (A Ϫ ) despite a major decrease in DOC concentrations, and (3) the production of biologically available DOC. Biological degradation of the photochemically transformed DOC resulted in a lesser decrease in DOC concentrations than during photodegradation (28-45% of the total decline) but in a pronounced decrease in A Ϫ concentrations (64-85% of the total decline), leading to a significant pH increase. Hydrolysis of photoliberated metals under increasing pH partly reduced net H ϩ consumption within the whole process. Watersheds of the lakes studied exported more SO , NO , and H ϩ than they received 2Ϫ Ϫ 4 3 by throughfall, and the lakes were the dominant acidity-consuming parts of the whole ecosystems, neutralizing 50-58% of H ϩ input. In-lake photochemical, biological, and chemical changes in A Ϫ fluxes consumed 56-190 meq m Ϫ2 yr Ϫ1 of H ϩ and were the third major internal alkalinity-producing mechanism after the biochemical reduction of NO and SO (333-396 and 143-214 meq m Ϫ2 yr Ϫ1 , respectively). In contrast, the hydrolysis of inorganic Al
Increased Photoreactivity of DOC by Acidification: Implications for the Carbon Cycle In Humic Lakes
Limnology and Oceanography, 2003
Effects of ultraviolet (UV)-B radiation and acidification on pelagic carbon flux in a humic lake (dissolved organic carbon [DOC] ϳ15 mg C L Ϫ1 ) were studied in a mesocosm experiment during the summer of 2000. Triplicate tanks (107 liters volume, 1 m high) were exposed to natural solar radiation, a daily extra dose of UV-B radiation, or kept dark. One set of tanks was submitted to a decrease in pH (from 5.7 to 4.7), and one set was kept at the natural pH level. During 70 d, water samples were taken regularly from the mesocosms for measurements of DOC, absorbance, dissolved inorganic carbon (DIC), and pH. Additionally, we regularly incubated samples to measure photooxidation rates, primary production, and community respiration. We found an increase in the photooxidation rates in the acidified mesocosms relative to ambient pH. The greater abiotic production of DIC (i.e., photooxidation) in acidified conditions could explain ϳ27% of the decline in DOC in the same conditions. Laboratory experiments were done to test the effects of pH on the dissolved organic matter (DOM) photoreactivity. At lower pH values, we found both higher abiotic DIC production and specific absorbance fading, relative to neutral pH values in water from a humic lake. In a separate experiment, samples were exposed to prolonged irradiation under laboratory conditions, resulting in complete loss of absorptivity in the wavelengths between 290 and 400 nm. Decreases in DOC in the long-term exposure caused by photochemical mineralization were ϳ45 and 55% of the initial pool for natural pH and acidified samples, respectively, at the end of the experiment. An increase in the dissolved organic matter photoreactivity by acidification could be an important mechanism to explain the increased water transparency and in-lake DOC removal in acid lakes found in several previous studies.
Enhanced photochemical loss of organic carbon in acidic waters
Biogeochemistry, 2001
Previous studies have shown that (a) a large portion of the annual total organic carbon (TOC) inputs to central Ontario lakes is either lost to sediments or degraded and lost via evasion to the atmosphere, (b) the partitioning of organic carbon between sediments and the atmosphere appears to be a function of acidity and (c) UV irradiation can account for observed long-term loss of TOC from the water column. These findings were extended by examining whether acidity enhances photo-oxidative losses of TOC. Stream waters (initial alkalinities between-97 and 233 µeq l −1) were incubated in UV-transparent containers under incident solar radiation for periods ranging from 14 to 23 days. The highest photo-oxidation rates occurred when alkalinity was negative. Additions of acid and base to stream waters increased and decreased photo-oxidation rates, respectively. The exceptional clarity of atmospherically acidified lakes is usually attributed to increased precipitation of Al-organic carbon complexes but may instead be due to higher photo-oxidation rates of allocthonous organic carbon leading to higher evasion rates of CO 2 .
Scientific Reports, 2019
Dissolved organic carbon (DOC) concentrations and water colour are increasing in many inland waters across northern Europe and northeastern North America. This inland-water “browning” has profound physical, chemical and biological repercussions for aquatic ecosystems affecting water quality, biological community structures and aquatic productivity. Potential drivers of this “browning” trend are complex and include reductions in atmospheric acid deposition, changes in land use/cover, increased nitrogen deposition and climate change. However, because of the overlapping impacts of these stressors, their relative contributions to DOC dynamics remain unclear, and without appropriate long-term monitoring data, it has not been possible to determine whether the ongoing “browning” is unprecedented or simply a “re-browning” to pre-industrial DOC levels. Here, we demonstrate the long-term impacts of acid deposition and climate change on lake-water DOC concentrations in low and high acid-deposi...
Limnology and Oceanography, 1999
Fossil pigment analyses and 19 year-long historical records were used to quantify whole-lake algal response to changes in optical and chemical properties following experimental acidification of Lake 302 with H 2 SO 4 (south basin, 302S; 1981-1989) or HNO 3 (north basin, 302N; 1982-1986) and HCl (1987-1989). Undisturbed sediments were collected by freeze-coring, sectioned in approximately annual intervals, and analyzed for fossil carotenoids, chlorophylls, and derivatives by high performance liquid chromatography. Concentrations of fucoxanthin (diatoms, chrysophytes, some dinoflagellates) were correlated with algal standing crop (r 2 ϭ 0.67, P Ͻ 0.05; 1978-1989) and increased 6-fold following acidification of Lake 302S with H 2 SO 4 from pH 6.6 to 5.0, consistent with observed reductions in dissolved organic carbon (DOC) from 7 to 4.5 mg liter Ϫ1 , improved water clarity, and increased biomass of deep-water chrysophytes. However, fucoxanthin concentrations declined to baseline values in sediments from 1988 to 1990, concomitant with severe acidification to pH 4.5, continued DOC loss (Ͻ1.5 mg liter Ϫ1) and an estimated 8-fold increase in the penetration of UVb radiation (UVR-b). Increased penetration of ultraviolet radiation (UVR) was recorded also by increased relative abundance of pigments characteristic of UVR-transparent environments. In contrast, pigments from green algae (Chl b, pheophytin b, lutein-zeaxanthin) doubled during acidification with H 2 SO 4 , while those from cryptophytes (alloxanthin) were unaffected and diatoxanthin from diatoms declined. Patterns of ubiquitous -carotene, Chl a, and pheophytin a suggested that total algal biomass increased ϳ200-400% by the mid-1980s, but declined to near-baseline under severe acidification. Variance partitioning using redundancy analysis captured 80-83% of variation in fossil chlorophylls and carotenoids and suggested that the direct effects of pH were greater (ϳ50% of total variance) than those of irradiance (ϳ12%), but that ϳ20% of variance was attributable to factor interactions. Fossil concentrations of pigments from green algae and diatoms increased ϳ100% following acidification of Lake 302N to pH 6.1, but there were few signals of deep-water blooms, possibly because DOC remained 3.5-5.0 mg liter Ϫ1. Such complex interactions between pH, DOC, and light may help explain the high variability of algal biomass response to lake acidification.
Journal of Geophysical Research, 2010
1] In this study, we examine the impact of increasing concentrations of dissolved organic carbon (DOC) on the recovery from acidification for 66 lakes in Southern Sweden. The lakes are small, weakly buffered, and have all been affected by acidifying deposition. A majority of the lakes (∼75%) showed an increase in DOC concentrations between 1990 and 2008. The method used in this study was to model pH in 2008 from DOC, acid neutralizing capacity, pCO 2 (partial carbon dioxide pressure), and Al speciation, using both the DOC observed in 2008 and the "unelevated" DOC of 1990. Furthermore, we consider the indirect effects of increasing DOC on acidity, i.e., the ancillary effects from DOC on pCO 2 , Al transport and speciation, and release of base cations (BCs). Our results indicate that the DOC increase in the latest decades has retarded the recovery from acidification by 0.13 pH units (median for all lakes) and by more than 1 unit for individual lakes. The effects of elevated pCO 2 and BC concentrations accompanying the DOC increase compensated for each other for the average lake, whereas the effects of higher Al transport on pH were minor. The estimate of the amount of BCs released with the organic anions is however uncertain, and further studies on this topic are needed.
Photolytic regulation of dissolved organic carbon in northern lakes
Global Biogeochemical Cycles, 1997
We examined the extent to which photolytic and nonphotolytic decomposition rates of dissolved organic carbon (DOC) could account for the annual retention or loss of DOC inputs in lakes (retention is equal to stream inputs plus atmospheric inputs minus stream discharge which is equivalent to storage in sediments plus degassed to atmosphere). Losses of DOC inputs to sediments and the atmosphere were large, averaging 38 to 70% of total inputs in seven study lakes between 1980 and 1992. Up to 50% of stream DOC was lost as inorganic C when exposed to solar radiation during 6 to 11 day surface exposures in bottles whereas lake DOC concentration was unaffected by solar radiation. Stream DOC loss was significantly less in the dark suggesting a low microbial consumption rate. Photodecay constants, extrapolated to each of the study lakes after correction for in situ mixing conditions and extinction of UVA and UVB, were similar to corresponding mass balance rate constants representing sediment storage and losses to the atmosphere. This suggests that photodecay is potentially large enough in situ to account for all of the DOC losses to the atmosphere and sediments in the low DOC lakes (<4 mg L 'l) but cannot account for all of the DOC lost in the high DOC lakes (>4 mg L'l). The mass balance and photodecay approaches employed in the study of carbon budgets show that UV degradation is probably an important mechanism in transfer of stream DOC to the sediment particulate C pool and to the atmosphere. Several in-lake mechanisms may be responsible for conversion of stream DOC to DIC and particulate organic C (POC).
Journal of Geophysical Research, 2010
The spectral characteristics of whole water dissolved organic matter (DOM) and fulvic acid were studied in samples collected from an alpine lake, a subalpine lake, and a subalpine stream during snowmelt and the summer growing season. Excitation-emission matrices of whole water DOM and fulvic acid were analyzed by parallel factor analysis (PARAFAC). Allochthonous inputs of terrestrially derived fulvic acid DOM were dominant during snowmelt at the alpine lake, and during both snowmelt and summer at the subalpine sites. At the alpine lake, autochthonous inputs of DOM dominated during the summer phytoplankton bloom, and the spectral characteristics of the whole water DOM diverged from those of the fulvic acid. For example, the quinone-like fluorophores in whole water DOM at the alpine lake were more oxidized and microbially derived than the fulvic acid fraction during the summer. At the subalpine sites, the seasonal changes in the source and redox state of the quinone-like fluorophores of the whole water DOM tracked those of the fulvic acid pool. However, at both lake sites there was a greater contribution of amino acid-like fluorophores in the whole water DOM than the fulvic acid fraction. This trend was not observed at the subalpine stream site. Principal components analysis (PCA) of the PARAFAC components suggests that during snowmelt, the chemical quality of the DOM at the alpine lake was similar to that of the subalpine stream; whereas the alpine site was more similar to the subalpine lake during the summer. Spectral characterization and PCA of the PARAFAC components suggest that nonhumic quinone-like and amino acid-like fluorophores were produced in the alpine lake during the summer phytoplankton bloom. Our results show that different types of water bodies produce different seasonal patterns in whole water DOM and fulvic acid quantity and quality.
Lability of organic carbon in lakes of different trophic status
Freshwater Biology, 2009
1. We used first-order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BOD ult ), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 lg L )1 , labile organic carbon (OC L = 0.3BOD ult ) from 0.75 to 2.95 mg C L )1 and k from 0.044 to 0.14 day )1 . 3. Our data showed that there were greater concentrations of OC L but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OC L was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27-55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long-term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications.