Salmon‐derived nutrient and organic matter fluxes from a coastal catchment in southeast Alaska

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Soil organic matter and litter chemistry response to experimental N deposition in northern temperate deciduous forest ecosystems

The effects of atmospheric nitrogen (N) deposition on organic matter decomposition vary with the biochemical characteristics of plant litter. At the ecosystem‐scale, net effects are difficult to predict because various soil organic matter (SOM) fractions may respond differentially. We investigated the relationship between SOM chemistry and microbial activity in three northern deciduous forest ecosystems that have been subjected to experimental N addition for 2 years. Extractable dissolved organic carbon (DOC), DOC aromaticity, C : N ratio, and functional group distribution, measured by Fourier transform infrared spectra (FTIR), were analyzed for litter and SOM. The largest biochemical changes were found in the sugar maple–basswood (SMBW) and black oak–white oak (BOWO) ecosystems. SMBW litter from the N addition treatment had less aromaticity, higher C : N ratios, and lower saturated carbon, lower carbonyl carbon, and higher carboxylates than controls; BOWO litter showed opposite trends, except for carbonyl and carboxylate contents. Litter from the sugar maple–red oak (SMRO) ecosystem had a lower C : N ratio, but no change in DOC aromaticity. For SOM, the C : N ratio increased with N addition in SMBW and SMRO ecosystems, but decreased in BOWO; N addition did not affect the aromaticity of DOC extracted from mineral soil. All ecosystems showed increases in extractable DOC from both litter and soil in response to N treatment. The biochemical changes are consistent with the divergent microbial responses observed in these systems. Extracellular oxidative enzyme activity has declined in the BOWO and SMRO ecosystems while activity in the SMBW ecosystem, particularly in the litter horizon, has increased. In all systems, enzyme activities associated with the hydrolysis and oxidation of polysaccharides have increased. At the ecosystem scale, the biochemical characteristics of the dominant litter appear to modulate the effects of N deposition on organic matter dynamics.     

How do temperature,dissolved organic matter and nutrients influence the response of Leptodiaptomus ashlandi to UV radiation in a subalpine lake?

1. Ultraviolet radiation (UV) is an important stressor for zooplankton in alpine lake ecosystems. Multiple environmental variables such as dissolved organic matter (DOM), temperature and nutrient availability may alter how UV affects zooplankton. 2. We conducted a week‐long experiment manipulating UV, nutrients and DOM in enclosures suspended at the surface of cold and warm alpine lakes to determine the interactive effects of these variables on ovigerous Leptodiaptomus ashlandi (Marsh, 1893), a calanoid copepod. 3. UV had a negative effect on nauplii and gravid females at the colder temperature and at low, ambient DOM levels, but had no effect at the warmer temperature or when DOM was added. At the warmer temperature, fewer nauplii were produced in the +nutrient compared to ?nutrient treatment. Adult survival was not affected by UV or any other experimental variable. 4. These results demonstrate that the extent of the impact of UV radiation on zooplankton in alpine systems is altered by other environmental variables, and that these effects may not be apparent from experiments that look only at the survival of adult organisms that are better defended against UV.     

Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO₂ concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C‐inputs to soil (e.g. plant‐derived) can significantly affect mineralization of SOM (‘priming effects’) complicates prediction of environmental and land‐use change effects on SOM dynamics and soil C‐balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3 atm% ¹³C) addition (in the range 0–1 mg glucose g^?1 soil day^?1) were applied for 8 days. Soil CO₂ efflux was partitioned into SOM‐ and glucose‐derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass (‘apparent priming’) were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5 mg glucose g^?1 soil), priming was a soil‐specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C‐models. However, these response functions are soil‐specific and may be affected by several interacting factors at lower addition rates.     

Conceptualizing soil organic matter into particulate and mineral‐associated forms to address global change in the 21st century

Managing soil organic matter (SOM) stocks to address global change challenges requires well‐substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross‐study comparisons difficult and hindering broad‐scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral‐associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO₂ enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well‐supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad‐scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.     

The role of C, N and P in dissolved and particulate organic matter as a nutrient source for phytoplankton growth, including toxic species

Phytoplankton have traditionally been regarded as strictly phototrophic, with a well defined position at the base of pelagic food webs. However, recently we have learned that the nutritional demands of a growing number of phytoplankton species can be met, at least partially, or under specific environmental conditions, through heterotrophy. Mixotrophy is the ability of an organism to be both phototrophic and heterotrophic, in the latter case utilizing either organic particles (phagotrophy) or dissolved organic substances (osmotrophy). This finding has direct implications for our view on algal survival strategies, particularly for harmful species, and energy- and nutrient flow in pelagic food webs. Mixotrophic species may outcompete strict autotrophs, e.g. in waters poor in inorganic nutrients or under low light. In the traditional view of the ‘microbial loop’ DOC is thought to be channeled from algal photosynthesis to bacteria and then up the food chain through heterotrophic flagellates, ciliates and mesozooplankton. Are mixotrophic phytoplankton that feed on bacteria also significantly contributing to this transport of photosynthetic carbon up the food chain? How can we estimate the fluxes of carbon and nutrients between different trophic levels in the plankton food web involving phagotrophic algae? These questions largely remain unanswered. In this review we treat evidence for both osmotrophy and phagotrophy in phytoplankton, especially toxic marine species, and some ecological implications of mixotrophy.     

Drought‐resistant fungi control soil organic matter decomposition and its response to temperature

Microbial‐mediated decomposition of soil organic matter (SOM) ultimately makes a considerable contribution to soil respiration, which is typically the main source of CO₂ arising from terrestrial ecosystems. Despite this central role in the decomposition of SOM, few studies have been conducted on how climate change may affect the soil microbial community and, furthermore, on how possible climate‐change induced alterations in the ecology of microbial communities may affect soil CO₂ emissions. Here we present the results of a seasonal study on soil microbial community structure, SOM decomposition and its temperature sensitivity in two representative Mediterranean ecosystems where precipitation/throughfall exclusion has taken place during the last 10 years. Bacterial and fungal diversity was estimated using the terminal restriction fragment length polymorphism technique. Our results show that fungal diversity was less sensitive to seasonal changes in moisture, temperature and plant activity than bacterial diversity. On the other hand, fungal communities showed the ability to dynamically adapt throughout the seasons. Fungi also coped better with the 10 years of precipitation/throughfall exclusion compared with bacteria. The high resistance of fungal diversity to changes with respect to bacteria may open the controversy as to whether future ‘drier conditions’ for Mediterranean regions might favor fungal dominated microbial communities. Finally, our results indicate that the fungal community exerted a strong influence over the temporal and spatial variability of SOM decomposition and its sensitivity to temperature. The results, therefore, highlight the important role of fungi in the decomposition of terrestrial SOM, especially under the harsh environmental conditions of Mediterranean ecosystems, for which models predict even drier conditions in the future.     

Relationship of trophic and chemical conditions to photobleaching of dissolved organic matter in lake ecosystems

Dissolved organic matter (DOM) is a major light-absorbing substance, responsible for much of the color in water bodies. When sunlight energy is absorbed by DOM, some color can be lost by the process of photobleaching. We measured rates of DOM photobleaching in thirty lakes that varied greatly in color, trophic status and ionic composition. Loss of color (measured as absorbance at 440 nm and expressed as absorption coefficients) was a first order function of sunlight dose, and rates were nearly identical for 0.2m- and GF/F-filtered samples suggesting that the process was predominantly abiotic. Photobleaching rates were rapid (color loss of 1–19% d^–1) and varied about seven-fold among lakes. Our method under-estimated the actual rate by 15–20% based on comparisons between the glass bottles we used in the survey and quartz containers. The large variation in photobleaching rates was examined in relation to lake trophy and chemical conditions. The best predictor of this variability was acid-neutralizing capacity (ANC) (r ²=0.94;p<0.001) such that photobleaching was most rapid in the most alkaline lakes. The relationship between ANC and photobleaching suggests that differences in ionic conditions among lakes may influence the solubility and configuration of humic and fulvic acids and hence their susceptibility to photobleaching.     

Controls of dissolved organic matter quality: evidence from a large‐scale boreal lake survey

Inland waters transport large amounts of dissolved organic matter (DOM) from terrestrial environments to the oceans, but DOM also reacts en route, with substantial water column losses by mineralization and sedimentation. For DOM transformations along the aquatic continuum, lakes play an important role as they retain waters in the landscape allowing for more time to alter DOM. We know DOM losses are significant at the global scale, yet little is known about how the reactivity of DOM varies across landscapes and climates. DOM reactivity is inherently linked to its chemical composition. We used fluorescence spectroscopy to explore DOM quality from 560 lakes distributed across Sweden and encompassed a wide climatic gradient typical of the boreal ecozone. Six fluorescence components were identified using parallel factor analysis (PARAFAC). The intensity and relative abundance of these components were analyzed in relation to lake chemistry, catchment, and climate characteristics. Land cover, particularly the percentage of water in the catchment, was a primary factor explaining variability in PARAFAC components. Likewise, lake water retention time influenced DOM quality. These results suggest that processes occurring in upstream water bodies, in addition to the lake itself, have a dominant influence on DOM quality. PARAFAC components with longer emission wavelengths, or red‐shifted components, were most reactive. In contrast, protein‐like components were most persistent within lakes. Generalized characteristics of PARAFAC components based on emission wavelength could ease future interpretation of fluorescence spectra. An important secondary influence on DOM quality was mean annual temperature, which ranged between ?6.2 and +7.5 °C. These results suggest that DOM reactivity depends more heavily on the duration of time taken to pass through the landscape, rather than temperature. Projected increases in runoff in the boreal region may force lake DOM toward a higher overall amount and proportion of humic‐like substances.     

Variations in the composition and adsorption behavior of dissolved organic matter at a small, forested watershed

This study investigated the properties and sorption by goethite of bulk (unfractionated) dissolved organic matter (DOM) from surface and shallow groundwaters at McDonalds Branch, a small freshwater fen in the New Jersey Pine Barrens (USA). Water samples were collected in the spring and fall seasons from two surface-water sampling sites, an upstream potential recharge area and a downstream discharge area, as well as from a set of in-stream nested wells in the upstream potential recharge area. Changes in DOM concentration, molecular weight distribution, and molar absorptivity at 280 nm were measured. Surface and shallow (1.6 m below land surface) groundwater samples collected in spring 1997 in the potential recharge zone (actual recharge impeded by an extensive clay lens) were found to be very similar in terms of DOM concentrations and physicochemical properties and is believe to originate from a common source. Samples taken in fall 1997 yielded no surface water because of drought conditions, and the shallow groundwater DOM collected from the recharge well contained significantly less and chemically altered DOM. This change in chemical properties is believed to be caused in part by fractionation resulting from sorption to mineral phases. Batch isotherm experiments show that sorption by goethite of the DOM from both spring surface and shallow groundwaters in the potential recharge area were similar, whereas the fall groundwater possessed a much lower affinity for the sorbent. This study demonstrated that shallow groundwaters collected under different climatic and hydrologic conditions (spring, high flow versus fall, drought conditions) resulted in different physicochemical properties and adsorption affinities.     

Diversity of catechol 2,3-dioxygenase genes of bacteria responding to dissolved organic matter derived from different sources in a eutrophic lake

Catechol 2,3-dioxygenase (C23O) is an extradiol dioxygenase that plays an important role in degrading aromatic compounds such as those found at polluted sites. However, little is known about the diversity of C23O genes in unpolluted environments. In such environments, various factors, including the quality and quantity of dissolved organic matter (DOM), could influence the composition and behaviour of bacterial community possessing C230 genes. We investigated C23O genes in bacteria responding to DOM from various sources in a eutrophic lake by PCR and cloning. Six microcosms filled with lake water containing indigenous bacteria and DOM from different sources were incubated for 10 days. After 1 or 2 days of incubation, C23O genes were detected in the microcosms enriched with DOM recovered from inflow river water and humus from reed grass. The sequences were very diverse but had features conserved in extradiol dioxygenases. The clone libraries generated on day 2 showed distinctive compositions among microcosms, indicating that bacteria possessing a variety of C23O genes responded differently to DOM from different sources. After 10 days of incubation, C23O genes in a previously unidentified gene cluster, 'Cluster X', became dominant in the libraries.     

Light absorption by phytoplankton and chromophoric dissolved organic matter in the drainage basin and estuary of the Neuse River, North Carolina (U.S.A.)

1. We examined the absorption of solar radiation by phytoplankton and chromophoric dissolved organic matter (CDOM) taking into account riparian shading in the rivers, reservoirs, swamps of the Neuse River Estuary and its drainage basin. 2. In the streams, CDOM typically absorbed 55 and 64% of photons in the spectral range of 400–700 nm (photosynthetically active radiation, PAR) and 500–600 nm, respectively. The large proportion of photons absorbed by CDOM indicates high potential for abiotic photochemial reactions in the 500–600 nm region. 3. Despite the high concentration of nutrients, phytoplankton contributed little (2%) to the total absorption of PAR in the streams. Small (<30 m wide) streams typically received only 7% of incident PAR that impinged onto the more exposed reservoirs and estuary. Riparian shading and the low contribution of phytoplankton to the total absorption resulted in conditions where phytoplankton absorbed nearly two orders of magnitude less PAR in the streams than in the estuary and reservoirs. 4. The results indicated that riparian shading and non‐algal absorbing components can significantly restrict phytoplankton production in nutrient‐rich streams with a high concentration of CDOM flowing throughout forested catchments.     

Direct and indirect effects of additions of chromophoric dissolved organic matter on zooplankton during large‐scale mesocosm experiments in an oligotrophic lake

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Light availability regulates the response of algae and heterotrophic bacteria to elevated nutrient levels and warming in a northern boreal peatland

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Impacts of elevated terrestrial nutrient loads and temperature on pelagic food‐web efficiency and fish production

Both temperature and terrestrial organic matter have strong impacts on aquatic food‐web dynamics and production. Temperature affects vital rates of all organisms, and terrestrial organic matter can act both as an energy source for lower trophic levels, while simultaneously reducing light availability for autotrophic production. As climate change predictions for the Baltic Sea and elsewhere suggest increases in both terrestrial matter runoff and increases in temperature, we studied the effects on pelagic food‐web dynamics and food‐web efficiency in a plausible future scenario with respect to these abiotic variables in a large‐scale mesocosm experiment. Total basal (phytoplankton plus bacterial) production was slightly reduced when only increasing temperatures, but was otherwise similar across all other treatments. Separate increases in nutrient loads and temperature decreased the ratio of autotrophic:heterotrophic production, but the combined treatment of elevated temperature and terrestrial nutrient loads increased both fish production and food‐web efficiency. CDOM: Chl a ratios strongly indicated that terrestrial and not autotrophic carbon was the main energy source in these food webs and our results also showed that zooplankton biomass was positively correlated with increased bacterial production. Concomitantly, biomass of the dominant calanoid copepod Acartia sp. increased as an effect of increased temperature. As the combined effects of increased temperature and terrestrial organic nutrient loads were required to increase zooplankton abundance and fish production, conclusions about effects of climate change on food‐web dynamics and fish production must be based on realistic combinations of several abiotic factors. Moreover, our results question established notions on the net inefficiency of heterotrophic carbon transfer to the top of the food web.     

Reduced tillage,but not organic matter input,increased nematode diversity and food web stability in European long‐term field experiments

Soil nematode communities and food web indices can inform about the complexity, nutrient flows and decomposition pathways of soil food webs, reflecting soil quality. Relative abundance of nematode feeding and life‐history groups are used for calculating food web indices, i.e., maturity index (MI), enrichment index (EI), structure index (SI) and channel index (CI). Molecular methods to study nematode communities potentially offer advantages compared to traditional methods in terms of resolution, throughput, cost and time. In spite of such advantages, molecular data have not often been adopted so far to assess the effects of soil management on nematode communities and to calculate these food web indices. Here, we used high‐throughput amplicon sequencing to investigate the effects of tillage (conventional vs. reduced) and organic matter addition (low vs. high) on nematode communities and food web indices in 10 European long‐term field experiments and we assessed the relationship between nematode communities and soil parameters. We found that nematode communities were more strongly affected by tillage than by organic matter addition. Compared to conventional tillage, reduced tillage increased nematode diversity (23% higher Shannon diversity index), nematode community stability (12% higher MI), structure (24% higher SI), and the fungal decomposition channel (59% higher CI), and also the number of herbivorous nematodes (70% higher). Total and labile organic carbon, available K and microbial parameters explained nematode community structure. Our findings show that nematode communities are sensitive indicators of soil quality and that molecular profiling of nematode communities has the potential to reveal the effects of soil management on soil quality.     

Limnology of the Green Lakes Valley: phytoplankton ecology and dissolved organic matter biogeochemistry at a long-term ecological research site

ABSTRACT

Background: Surface waters are the lowest points in the landscape, and therefore serve as excellent integrators and indicators of changes taking place in the surrounding terrestrial and atmospheric environment.

Aims: Here we synthesise the findings of limnological studies conducted during the past 15 years in streams and lakes in the Green Lakes Valley, which is part of the Niwot Ridge Long Term Ecological Research (LTER) Site.

Methods: The importance of these studies is discussed in the context of aquatic ecosystems as indicators, integrators, and regulators of environmental change. Specifically, investigations into climatic, hydrologic, and nutrient controls on present-day phytoplankton, and historical diatom, community composition in the alpine lake, Green Lake 4, are reviewed. In addition, studies of spatial and temporal patterns in dissolved organic matter (DOM) biogeochemistry and reactive transport modelling that have taken place in the Green Lakes Valley are highlighted.

Results and conclusions: The findings of these studies identify specific shifts in algal community composition and DOM biogeochemistry that are indicative of changing environmental conditions and provide a framework for detecting future environmental change in the Green Lakes Valley and in other alpine watersheds. Moreover, the studies summarised here demonstrate the importance of long-term monitoring programmes such as the LTER programme.     

Concentrations and controls of dissolved organic matter in a constricted-channel region of the Ohio River

A 12-month study was conducted to measure the concentrations ofdissolved organic matter (DOC, TDN, TDP) in four sites within a119 km long reach of the Ohio River, near Louisville, KY. In thisstudy we test whether specific geomorphological and biologicalfactors influenced variations in dissolved organic matter.Concentrations of DOC in the river averaged 1200mol/L, and varied by nearly two orders of magnitudeseasonally (mean DOC during base flow 620 mol/L).Peak periods for DOC at all sites were during April–May. Thesite nearest a navigation dam (deeper, lower current velocities)had significantly lower concentrations of TDN and greater C:Nratios than upstream sites. The largest tributary entering thisreach (Kentucky River) had no significant effect on levels of DOMin the main river, despite having significantly greaterconcentrations of TDN and lower levels of DOC during most monthsof the year. Concentrations of DOC, TDN, and TDP were notsignificantly different in littoral and pelagic habitats at allsites studied, suggesting little floodplain influence on DOM inthis constricted-channel section of the Ohio River. C:N ratios ofDOM in the Ohio were significantly different among seasons; C:Nexceeded or equaled Redfield ratios in summer and fall (6 to 10),but were below Redfield (1.8 to 3.0) during winter and spring.Regression models suggest that total phytoplankton densities andflow conditions are the two most important factors regulating DOMin this very large river.     

The impact of Chironomus plumosus larvae on organic matter decay and nutrient (N, P) exchange in a shallow eutrophic lake sediment following a phytoplankton sedimentation

The importance of Chironomus plumosus larvae onbenthic metabolism and nutrient exchange across thesediment–water interface was evaluated in a shalloweutrophic lake (Lake Arreskov, Denmark) following aphytoplankton sedimentation. Chironomus plumosuslarvae were added to laboratory sediment microcosms,corresponding to a density of 2825 larvae m⁻².Non-inhabited microcosms served as controls. Asedimentation pulse of organic matter was simulated byadding fresh algal material (Chlamydomonasreinhardii) to sediment cores (36 g dryweight m⁻²). The mineralization was followed bymeasuring fluxes of O₂, CO₂, dissolvedinorganic nitrogen and phosphate. A rapid clearance ofalgae from the water column in faunated microcosmssuggested that chironomids may be of major importancein controlling phytoplankton concentrations in shalloweutrophic lakes. Chironomids increased the sedimentO₂ uptake ≈ 3 times more than what wouldbe expected from their own respiration, indicating astimulation of microbial activity and decomposition oforganic matter in the sediment. Addition of algaeenhanced the release of CO₂, NH⁺ ₄ ando-P. The excess inorganic C, N and P released inamended non-inhabited sediment after 36 dayscorresponded to 65, 31 and 58% of the C, N and P inthe added algae. In sediment inhabited by Chironomus plumosus the corresponding numbers were147, 45 and 73%, indicating that mineralization oforganic matter also from the indigenous sediment poolwas stimulated by chironomids. This revised version was published online in July 2006 with corrections to the Cover Date.     

典型喀斯特河流汛期溶解性有机质空间变化及其光学参数的耦合关系

内陆水体溶解性有机质（DOM）在全球及区域碳循环中扮演至关重要的作用。然而,具有特殊生物地球化学过程的喀斯特河流DOM成分和来源空间格局却鲜有报道。以我国典型喀斯特河流-芙蓉江为对象,探索了光学参数（SUVA₂₅₄、SUVA₂₈₀、E2：E3、S_290-350、BIX、FI、Frl和HIX）的空间变化及其相互耦合关系,并利用三维荧光矩阵平行因子分析（EEM-PARAFAC）解析了该河流DOM主要构成,同时分析了各光学参数之间的相关性。结果显示,芙蓉江标准化吸光系数（SUVA₂₅₄和SUVA₂₈₀）较低而E2：E3和S_290-350值较高,说明汛期DOM同时受生物和水文过程影响。EEM-PARAFAC表明该喀斯特河流DOM包括3项主要成分,即C1（陆源腐殖质）、C2（色氨酸）和C3（络氨酸）。河流BIX和FI分别在（0.77-1.11）和（2.14-2.39）范围,表明生物内源是DOM的主要来源。Frl值（0.75-0.94）显示河流水体以新生DOM主导,而HIX值（0.14-0.79）阐明其腐殖化程度较低。光学参数之间存在显著的相关关系,揭示了河流DOM成分、分子大小和来源具有一致的生态水文学意义。