Structure and grazing of stone surface organic layers in some acid streams of southern England

SUMMARY.

• 1 Stone surface organic layers were investigated at five sites on small, acid streams in the Ashdown Forest, southern England. Sites differed in stream water pH (means 4.3–6.6) and some other physicochemical features.
• 2 Organic layers at the stream bed surface differed between sites in structure and in the amount of organic carbon present. Algae were abundant at the sites with higher conductivity and pH, iron bacteria (mainly Leptothrix sp.) predominated in the iron-rich Broadstone stream, whereas the surface layer at the most acid site was predominantly a rather structure less organic film with few living organisms.
• 3 Amounts of organic carbon on stones buried within the stream bed for 3 months were almost identical at all sites. When viewed with the scanning electron microscope (SEM), organic layers on buried stones were rather structure less with some amorphous, floceulent material. The paucity of microflora suggests that these layers may have been formed primarily by abiotic mechanisms.
• 4 X-ray microanalysis of organic layers indicated that complexes of organic matter with the metals iron, aluminium and manganese were present.
• 5 Laboratory experiments with seven species of invertebrates showed that all were able to remove and at least partially ingest organic layers and underlying sandstone. The weight of material removed was highest on microbially impoverished layers from the most acid site, and was substantially lower where algae were abundant. 6. The role of stone surface organic layers in stream systems is discussed. They may be important sites of dissolved organic matter (DOM) uptake and transfer to the benthos. A major impact of acidity on running water could be through its effect on the structure and function of organic layers.

     

Acclimation of stream-bed heterotrophic microflora: metabolic responses to dissolved organic matter

SUMMARY.

• 1 Studies were performed to assess the acclimation of the stream-bed heterotrophic microflora to sources of dissolved organic matter (DOM) typical of its environment and microfloral responses to pulses of DOM.
• 2 Microcosm measurements of dissolved organic carbon (DOC) uptake, dissolved oxygen uptake. ATP concentration and epitluorescence microscopic counts (EMC) were performed using stream-bed sediments and heterogeneous dissolved organic matter (DOM) sources.
• 3 Three study sites included an undisturbed woodlot spring seep, a small stream traversing a cattle pasture and a larger stream draining a catchment used for silage crops, pastures and woodlands.
• 4 The DOM sources were cold water extracts of forest floor leaf litter, bovine manure, the green alga Ulothrix and jewel weed (Impaliens capensis L.) leaves.
• 5 DOC uptake occurring in 2.5–5.0 h incubations indicated an acclimation of the microflora at each site to DOM sources generated by surrounding land use.
• 6 The sediment microflora from the larger stream did not readily metabolize bovine manure DOM and the latter was used in an acclimation experiment.
• 7 A minimum of 48 h of cumulative exposure to bovine manure DOM at 15–20°C were required to yield measurable changes in sediment microbial activity of sediment microbial biomass.
• 8 The same microflora retained an ability to readily metabolize the added DOM source after 72 h of exposure to unamended stream water.
• 9 The time frame of microfloral responses during acclimation indicated that changes leading to the metabolism of a DOM source were initially enzymatic and eventually involved growth and selection for specific decomposers within the microbial community.
• 10 We conclude that in order to utilize naturally occurring pulses of carbon and energy, stream-bed heterotrophs must be already enzymatically prepared, induced, when the pulse occurs.

     

Characterisation and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

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Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

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Microbial heterotrophic utilization of dissolved organic matter in a piedmont stream

SUMMARY.

• 1 The microbial heterotrophic utilization of dissolved organic matter (DOM) was determined experimentally in microcosms using stream water and stream-bed sediments from a third order reach of White Clay Creek (Pennsylvania, U.S.A.).
• 2 Sources of DOM for the experiments included White Clay Creek water at baseflow and stormflow and cold water extracts of jewel weed (Impatiens capensis L.) and spicebush (Linderu henzoin (L.) Blume).
• 3 The heterotrophic activity of the sediments was measured as uptake of the following: dissolved organic carbon (DOC), molecular weight fractions within the DOC pool, carbohydrates, amino acids and peptides, phenolics, and dissolved oxygen (DO), all in the overlying water.
• 4 Concentrations of adenosine triphosphate (ATP), and direct microscopic counts of bacteria were used to estimate bacterial biomass in the surface sediments.
• 5 The microcosm experiments showed that specific DOC molecular size classes and DOM functional groups were selectively removed from solution, exposure to one DOM source affected responses to a different DOM source and certain DOM sources were more readily utilized than others.
• 6 Continued exposure to a DOM source increased microbial heterotrophic activity (a condition which persisted even after removal of the DOM source for several days).
• 7 Rates of biotic DOC uptake ranged from 3.6 to 242.8 mg Cm^-2h^-1.
• 8 Indirect estimates of biosynthesis calculated from DOC and DO data ranged from 1.6 at baseflow and 2.6–61.2 at stormflow to as high as 192.6 mg C m^-2 h^-1 when the community was repeatedly exposed to enriched DOM sources.
• 9 The mean generation times of bacteria in sediments, determined from direct microscopy data, ranged from 12.5 to 46.2 h at 15°C.

     

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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Land use controls stream ecosystem metabolism by shifting dissolved organic matter and nutrient regimes

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Biogeochemical processes in the groundwater discharge zone of urban streams

The influence of biogeochemical processes on nitrogen and organic matter transformation and transport was investigated for two urban streams receiving groundwater discharge during the dry summer baseflow period. A multiple lines of evidence approach involving catchment-, and stream reach-scale investigations were undertaken to describe the factors that influence pore water biogeochemical processes. At the catchment-scale gaining stream reaches were identified from water table mapping and groundwater discharge estimated to be between 0.1 and 0.8 m³ m^?2 d^?1 from baseflow analysis. Sediment temperature profiles also suggested that the high groundwater discharge limited stream water infiltration into the sediments. At the stream reach-scale, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations were higher in stream water than in groundwater. However, DOC and DON concentrations were greatest in sediment pore water. This suggests that biodegradation of sediment organic matter contributes dissolved organic matter (DOM) to the streams along with that delivered with groundwater flow. Pore water ammonium (NH₄ ⁺) was closely associated with areas of high pore water DOM concentrations and evidence of sulfate (SO₄ ^2?) reduction (low concentration and SO₄:Cl ratio). This indicates that anoxic DOM mineralization was occurring associated with SO₄ ^2? reduction. However the distribution of anoxic mineralization was limited to the center of the streambed, and was not constrained by the distribution of sediment organic matter which was higher along the banks. Lower sediment temperatures measured along the banks compared to the center suggests, at least qualitatively, that groundwater discharge is higher along the banks. Based on this evidence anoxic mineralization is influenced by groundwater residence time, and is only measurable along the center of the stream where groundwater flux rates are lower. This study therefore shows that the distribution of biogeochemical processes in stream sediments, such as anoxic mineralization, is strongly influenced by both the biogeochemical conditions and pore water residence time.     

Organic Carbon Composition and Oxygen Metabolism across a Gradient of Seasonally Inundated Limesink and Riparian Wetlands in the Southeast Coastal Plain,USA

A set of three relatively pristine seasonally inundated limesink wetlands and one riparian wetland was studied over a 4–6 month long inundation period in 2001. Patterns in organic matter properties and oxygen consumption in the water column followed a previously documented ecological gradient based on soil composition, vegetation type, and canopy cover. The full canopy, cypress-gum swamp had the highest mean concentrations of dissolved organic carbon (DOC; 26.2 mg/l) and dissolved lignin (sum 6; 299 μg/l) with lower concentrations observed in the partial canopy, cypress savanna (22.0 mg/l DOC; 252 μg/l sum 6) and the open marsh savanna (20.6 mg/l DOC; 135 μg/l sum 6), respectively. During the inundation period, DOC increased in concentration, dissolved lignin decreased, and δ¹³C shifted to more positive values which collectively indicate a large reduction in the percentage of aromatic carbon during the inundation period. All wetlands had very high concentrations of organic matter, yet microbial oxygen consumption was almost always stimulated by the addition of glucose rather than inorganic nutrients. Stimulation by glucose suggests that there were very small pools of highly bioavailable forms of DOC in the wetlands. A larger pool of moderately bioavailable organic matter had the capacity to sustain microbial oxygen consumption rates under dark conditions for at least 15 d. During the inundation period, the cypress-gum swamp had the lowest average rates of whole water oxygen consumption (1.0 μM/h) with increasing rates observed in the cypress savanna (1.3 μM/h), marsh savanna (1.6 μM/h), and riparian wetland (1.9 μM/h), respectively. The lignin compositional fingerprint varied across the gradient of limesink wetlands, and was useful for identifying different sources of vascular plant-derived DOM. Vascular plant production, algal production, microbial respiration, and UV degradation are all important drivers of DOM cycling, and the consistencies observed in this initial assessment of seasonally inundated limesink wetlands suggest they vary in predictable ways across the ecological gradient.     

The Siak, a tropical black water river in central Sumatra on the verge of anoxia

The Siak is a black water river in central Sumatra, Indonesia, which owes its brown color to dissolved organic matter (DOM) leached from surrounding, heavily disturbed peat soils. The dissolved organic carbon (DOC) concentrations measured during five expeditions in the Siak between 2004 and 2006 are among the highest reported world wide. The DOM decomposition appeared to be a main factor influencing the oxygen concentration in the Siak which showed values down to 12 μmol l⁻¹. Results derived from a box-diffusion model indicated that in addition to the DOC concentration and the associated DOM decomposition the water-depth also plays a crucial role in regulating the oxygen levels in the river because of its impact on the turbulence in the aquatic boundary layer and the surface/volume ratio of water in the river. Model results imply furthermore that a reduced water-depth could counteract an increased oxygen consumption caused by an enhanced DOM leaching during the transition from dry to wet periods. This buffer mechanism seems to be close to its limits as indicated by sensitivity studies which showed in line with measured data that an increase of the DOC concentrations by ~15% could already lead to anoxic conditions in the Siak. This emphasizes the sensitivity of the Siak against further peat soil degradation, which is assumed to increase DOC concentrations in the rivers.     

Watershed Glacier Coverage Influences Dissolved Organic Matter Biogeochemistry in Coastal Watersheds of Southeast Alaska

The Coast Mountains of southeast Alaska are currently experiencing some of the highest rates of glacier volume loss on Earth, with unknown implications for proglacial stream biogeochemistry. We analyzed streamwater for δ¹⁸O and dissolved organic matter (DOM) biogeochemistry (concentration, δ¹³C-dissolved organic carbon (DOC), and fluorescence characterization) during the 2012 glacial runoff season from three coastal watersheds in southeast Alaska that ranged in glacier coverage from 0 to 49% and a glacier outflow stream. Our goal was to assess how DOM biogeochemistry may change as receding glaciers are replaced by forests and glaciers contribute less meltwater to streamflow. Discharge and streamwater δ¹⁸O varied seasonally reflecting varying contributions of rainfall and snow/icemelt to streamflow over the runoff season. Mean DOC concentrations were lowest in the glacial outflow and highest in the non-glacial stream reflecting an increasing contribution of vascular plant-derived carbon with decreasing watershed glaciation. Fluorescence and δ¹³C-DOC signatures indicated that DOM shifted from vascular plant-derived, humic-like material in the non-glacial stream toward more δ¹³C-DOC enriched, glacier-derived DOM in the glacial outflow. Streamwater δ¹⁸O was significantly correlated to DOC concentration, δ¹³C-DOC, and protein-like fluorescence of streamwater DOM (all P < 0.05), demonstrating that changes in the source of streamwater across the glacial watershed continuum have important implications for the amount and quality of stream DOM export. Overall, our findings show that continued glacial recession and subsequent changes in glacial runoff could substantially influence the biogeochemistry of coastal temperature watersheds by altering the timing, magnitude, and chemical signature of DOM delivered to streams.     

Characterization and origin of polar dissolved organic matter from the Great Salt Lake

Polar dissolved organic matter (DOM) was isolated from a surface-water sample from the Great Salt Lake by separating it from colloidal organic matter by membrane dialysis, from less-polar DOM fractions by resin sorbents, and from inorganic salts by a combination of sodium cation exchange followed by precipitation of sodium salts by acetic acid during evaporative concentration. Polar DOM was the most abundant DOM fraction, accounting for 56% of the isolated DOM. Colloidal organic matter was ¹⁴C-age dated to be about 100% modern carbon and all of the DOM fractions were ¹⁴C-age dated to be between 94 and 95% modern carbon. Average structural models of each DOM fraction were derived that incorporated quantitative elemental and infrared, ¹³C-NMR, and electrospray/mass spectrometric data. The polar DOM model consisted of open-chain N-acetyl hydroxy carboxylic acids likely derived from N-acetyl heteropolysaccharides that constituted the colloidal organic matter. The less polar DOM fraction models consisted of aliphatic alicyclic ring structures substituted with carboxyl, hydroxyl, ether, ester, and methyl groups. These ring structures had characteristics similar to terpenoid precursors. All DOM fractions in the Great Salt Lake are derived from algae and bacteria that dominate DOM inputs in this lake.     

Land‐use effects on resource net flux rates and oxygen demand in stream sediments

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Hydrologic Drivers and Seasonality of Dissolved Organic Carbon Concentration, Nitrogen Content, Bioavailability, and Export in a Forested New England Stream

We present the results of a full year of high-resolution monitoring of hydrologic event-driven export of stream dissolved organic matter (DOM) from the forested Bigelow Brook watershed in Harvard Forest, Massachusetts, USA. A combination of in situ fluorescent dissolved organic matter (FDOM) measurement, grab samples, and bioassays was utilized. FDOM was identified as a strong indicator of concentration for dissolved organic carbon (DOC, r ² = 0.96), dissolved organic nitrogen (DON, r ² = 0.81), and bioavailable DOC (BDOC, r ² = 0.81). Relationships between FDOM and concentration were utilized to improve characterization of patterns of hydrological event-driven export and the quantification of annual export. This characterization was possible because DOM composition remained relatively consistent seasonally; however, a subtle shift to increased fluorescence per unit absorbance was observed for summer and fall seasons and percent BDOC did increase slightly with increasing concentrations. The majority of export occurred during pulsed hydrological events, so the greatest impact of bioavailable exports may be on downstream aquatic ecosystems. Export from individual events was highly seasonal in nature with the highest flow weighted mean concentrations (DOC_FW) being observed in late summer and fall months, but the highest total export being observed for larger winter storms. Seasonal trends in DOC export coincide with weather driven changes in surface and subsurface flow paths, potential for depletion and rebuilding of a flushable soil organic matter pool, and the availability of terrestrial carbon sources such as leaf litter. Our approach and findings demonstrate the utility of high frequency FDOM measurement to improve estimates of intra-annual temporal trends of DOM export.     

Influence of Litter Diversity on Dissolved Organic Matter Release and Soil Carbon Formation in a Mixed Beech Forest

We investigated the effect of leaf litter on below ground carbon export and soil carbon formation in order to understand how litter diversity affects carbon cycling in forest ecosystems. ¹³C labeled and unlabeled leaf litter of beech (Fagus sylvatica) and ash (Fraxinus excelsior), characterized by low and high decomposability, were used in a litter exchange experiment in the Hainich National Park (Thuringia, Germany). Litter was added in pure and mixed treatments with either beech or ash labeled with ¹³C. We collected soil water in 5 cm mineral soil depth below each treatment biweekly and determined dissolved organic carbon (DOC), δ¹³C values and anion contents. In addition, we measured carbon concentrations and δ¹³C values in the organic and mineral soil (collected in 1 cm increments) up to 5 cm soil depth at the end of the experiment. Litter-derived C contributes less than 1% to dissolved organic matter (DOM) collected in 5 cm mineral soil depth. Better decomposable ash litter released significantly more (0.50±0.17%) litter carbon than beech litter (0.17±0.07%). All soil layers held in total around 30% of litter-derived carbon, indicating the large retention potential of litter-derived C in the top soil. Interestingly, in mixed (ash and beech litter) treatments we did not find a higher contribution of better decomposable ash-derived carbon in DOM, O horizon or mineral soil. This suggest that the known selective decomposition of better decomposable litter by soil fauna has no or only minor effects on the release and formation of litter-derived DOM and soil organic matter. Overall our experiment showed that 1) litter-derived carbon is of low importance for dissolved organic carbon release and 2) litter of higher decomposability is faster decomposed, but litter diversity does not influence the carbon flow.     

氮沉降对杉木人工林土壤可溶性有机质数量和结构的影响

氮沉降作为现在乃至未来气候变化的趋势之一,其可能深刻影响土壤可溶性有机质的数量和结构。选取我国中亚热带杉木人工林不同深度土壤（0-10 cm和10-20 cm）进行氮沉降试验,利用光谱技术研究氮沉降对土壤可溶性有机质数量和结构的影响。试验设对照（CT,0 kg hm^-2 a^-1）、高氮（HN：80 kg hm^-2 a^-1）、低氮（LN：40 kg hm^-2 a^-1）3种处理。结果表明：（1）在0-10 cm和10-20 cm土层,HN、LN处理的土壤可溶性有机碳和可溶性有机氮含量显著高于CT。（2）在0-10 cm和10-20 cm土层,1月时HN、LN处理的芳香性指数和腐殖化程度都显著高于CT,而4月时HN、LN处理的芳香性指数和腐殖化程度都显著低于CT。除了氮含量的直接影响外,RDA （冗余分析）表明,两土层中土壤含水量、pH和土壤有机碳是驱动氮沉降对土壤DOM数量和结构的重要环境因子。因此,氮沉降对土壤DOM的影响是复杂的,未来尤其应该注重氮沉降对生态系统影响的季节模式。     

马尾松与芒萁鲜叶及凋落物水溶性有机物的溶解特征和光谱学特征

为探讨福建省三明市马尾松人工林淋溶液中水溶性有机物(DOM)的溶解特征和光谱学特征,以马尾松(Pinus massoniana)和芒萁(Dicranopteris dichotoma)鲜叶、未分解层(L层)和半分解层(F层)凋落物为研究对象进行室内淋溶,对淋溶液中的水溶性有机碳(DOC)、水溶性有机氮(DON)与水溶性有机磷(DOP)含量和紫外光谱(SUVA)、荧光光谱指标和红外光谱(FTIR)等特征进行研究。结果表明,在24 h内,随着淋溶时间的延长,DOM含量呈现有波动的上升趋势;芒萁除鲜叶的DOC含量显著高于马尾松外(P0.01),其余DOM含量均小于马尾松。两植物F层DOM的SUVA值和腐殖化指标(HIX)都显著高于鲜叶和L层(P0.05),表明F层的芳香性化合物含量和腐殖化程度越来越高。DOM的同步荧光峰值显示淋溶液中含有类蛋白和类富里酸荧光团。两植物的红外光谱显示有5个相似的吸收谱带,强度最大的吸收来自于H键键合的-OH的伸缩振动,同一植物3种样品之间红外吸收的差异证明从鲜叶-L层-F层凋落物的共轭体系逐渐增大,结构更复杂。因此,随分解进程两植被DOM的化学结构越来越复杂;马尾松较芒萁含有更多的DON和DOP,这一方面为微生物提供了更多的养分,另一方面又增加了底物分解的难度。     

Differences in temperature, organic carbon and oxygen consumption among lowland streams

1. Temperature, organic carbon and oxygen consumption were measured over a year at 13 sites in four lowlands streams within the same region in North Zealand, Denmark with the objectives of determining: (i) spatial and seasonal differences between open streams, forest streams and streams with or without lakes, (ii) factors influencing the temperature dependence of oxygen consumption rate, (iii) consequences of higher temperature and organic content in lake outlets on oxygen consumption rate, and (iv) possible consequences of forecasted global warming on degradation of organic matter. 2. High concentrations of easily degradable dissolved (DOC) and particulate organic carbon (POC) were found in open streams downstream of plankton‐rich lakes, while high concentrations of recalcitrant DOC were found in a forest brook draining a forest swamp. Concentrations of predominantly recalcitrant POC and DOC were low in a groundwater‐fed forest spring. Overall, DOC concentration was two to 18 times higher than POC concentrations. 3. Oxygen consumption rate at 20 °C was higher during summer than winter, higher in open than shaded streams and higher in lake outlets than inlets. Rate was closely related to concentrations of chlorophyll and POC but not to DOC. The ratio of oxygen consumption rate to total organic concentrations (DOC + POC), serving as a measure of organic degradability, was highest downstream of lakes, intermediate in open streams and lowest in forest streams. 4. Temperature coefficients describing the exponential increase of oxygen consumption rate between 4 and 20 °C averaged 0.121 °C^?1 (Q₁₀ of 3.35) in 70 measurements and showed no significant variations between seasons and stream sites or correlations with ambient temperature and organic content. 5. Oxygen consumption rate was enhanced downstream of lakes during summer because of higher temperature and, more significantly, greater concentrations of degradable organic carbon. Oxygen consumption rates were up to seven times higher in the stream with three impoundments than in a neighbouring unshaded stream and 21 times higher than in the groundwater‐fed forest spring. 6. A regional climate model has calculated a dramatic 4–5 °C rise in air temperature over Denmark by 2070–2100. If this is realised, unshaded streams are estimated to become 2–3 °C warmer in summer and winter and 5–7 °C warmer in spring and, thereby, increase oxygen consumption rates at ambient temperature by 30–40% and 80–130%, respectively. Faster consumption of organic matter and dissolved oxygen downstream of point sources should increase the likelihood of oxygen stress of the stream biota and lead to the export of less organic matter but more mineralised nutrients to the coastal waters.     

Bioclimatic influence on water chemistry and dissolved organic matter in shallow temperate lakes of Andean Patagonia: A gradient approach

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