Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.     

Uptake of Allochthonous Dissolved Organic Matter from Soil and Salmon in Coastal Temperate Rainforest Streams

Dissolved organic matter (DOM) is an important component of aquatic food webs. We compare the uptake kinetics for NH₄–N and different fractions of DOM during soil and salmon leachate additions by evaluating the uptake of organic forms of carbon (DOC) and nitrogen (DON), and proteinaceous DOM, as measured by parallel factor (PARAFAC) modeling of DOM fluorescence. Seasonal DOM slug additions were conducted in three headwater streams draining a bog, forested wetland, and upland forest using DOM collected by leaching watershed soils. We also used DOM collected from bog soil and salmon carcasses to perform additions in the upland forest stream. DOC uptake velocity ranged from 0.010 to 0.063 mm s⁻¹ and DON uptake velocity ranged from 0.015 to 0.086 mm s⁻¹, which provides evidence for the whole-stream uptake of allochthonous DOM. These findings imply that wetlands could potentially be an important source of DOM to support stream heterotrophic production. There was no significant difference in the uptake of DOC and DON across the soil leachate additions (P > 0.05), although differential uptake of DOM fractions was observed as protein-like fluorescence was removed from the water column more efficiently than bulk DOC and DON (P < 0.05). Moreover, PARAFAC analysis of DOM fluorescence showed that protein-like fluorescence decreased downstream during all DOM additions, whereas humic-like fluorescence did not change. This differential processing in added DOM suggests slow and fast turnover pools exist for aquatic DOM. Taken together, our findings argue that DON could potentially fill a larger role in satisfying biotic N demand in oligotrophic headwater streams than previously thought. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions  J.B.F. conceived of or designed study, performed research, analyzed data, contributed new methods or models, and wrote the paper. E.H. conceived of or designed study and analyzed data. R.T.E. conceived of or designed study and analyzed data. J.B.J. contributed new methods or models and analyzed data.     

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L^-1 across the ponds with an average value of 5.3 mg C L^-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.     

Scavenging of dissolved organic matter (DOM) by amorphous iron hydroxide particles Fe(OH)3(s)

Scavenging of dissolved organic matter (DOM) by particulate metal oxides like Fe(OH)_3(s) is one of three processes that can influence the concentration and composition of DOM in aquatic systems. The other two possible processes include photodegradation and biodegradation. In combination, these processes alter the concentration and composition of DOM systematically with increasing time, measured as hydrologic residence time (HRT). The objective of this research was to determine the change in Fe(OH)_3(s)-scavengable dissolved organic carbon (DOC) with increasing HRT (0–80 yr). In addition, DOC from allochthonous and autochthonous sources were included in this study. The susceptibility of DOC from surface waters to scavenging by Fe(OH)_3(s) was found to decrease as a function of HRT, from approximately 90% to 79%. The lowest HRT system was operationally considered equivalent to allochthonous DOC, while autochthonous DOC was scavenged similarly to DOC from the 80 yr HRT system. These results indicate that scavenging of bulk DOC may be limited by metal loading in aquatic systems, and that the bulk of Fe(OH)_3(s)-reactive DOC is from allochthonous sources. In addition, all surface waters treated with Fe(OH)_3(s) contained approximately 1 mg l^–1 of DOC that was resistant to scavenging (SD = 0.50, n = 5), which suggests that a refractory fraction of DOC persists in surface waters.     

Human activities cause distinct dissolved organic matter composition across freshwater ecosystems

Dissolved organic matter (DOM) composition in freshwater ecosystems is influenced by the interactions among physical, chemical, and biological processes that are controlled, at one level, by watershed landscape, hydrology, and their connections. Against this environmental template, humans may strongly influence DOM composition. Yet, we lack a comprehensive understanding of DOM composition variation across freshwater ecosystems differentially affected by human activity. Using optical properties, we described DOM variation across five ecosystem groups of the Laurentian Great Lakes region: large lakes, Kawartha Lakes, Experimental Lakes Area, urban stormwater ponds, and rivers (n = 184 sites). We determined how between ecosystem variation in DOM composition related to watershed size, land use and cover, water quality measures (conductivity, dissolved organic carbon (DOC), nutrient concentration, chlorophyll a), and human population density. The five freshwater ecosystem groups had distinctive DOM composition from each other. These significant differences were not explained completely through differences in watershed size nor spatial autocorrelation. Instead, multivariate partial least squares regression showed that DOM composition was related to differences in human impact across freshwater ecosystems. In particular, urban/developed watersheds with higher human population densities had a unique DOM composition with a clear anthropogenic influence that was distinct from DOM composition in natural land cover and/or agricultural watersheds. This nonagricultural, human developed impact on aquatic DOM was most evident through increased levels of a microbial, humic‐like parallel factor analysis component (C6). Lotic and lentic ecosystems with low human population densities had DOM compositions more typical of clear water to humic‐rich freshwater ecosystems but C6 was only present at trace to background levels. Consequently, humans are strongly altering the quality of DOM in waters nearby or flowing through highly populated areas, which may alter carbon cycles in anthropogenically disturbed ecosystems at broad scales.     

Land use and hydrologic flowpaths interact to affect dissolved organic matter and nitrate dynamics

The transport and transformation of dissolved organic matter (DOM) and dissolved inorganic nitrogen (DIN) through the soil profile impact down-gradient ecosystems and are increasingly recognized as important factors affecting the balance between accumulation and mineralization of subsoil organic matter. Using zero tension and tension lysimeters at three soil depths (20, 40, 60 cm) in paired forest and maize/soybean land uses, we compared dissolved organic C (DOC), dissolved organic N (DON) and DIN concentrations as well as DOM properties including hydrophilic-C (HPI-C), UV absorption (SUVA₂₅₄), humification index and C/N ratio. Soil moisture data collected at lysimeter locations suggest zero tension lysimeters sampled relatively rapid hydrologic flowpaths that included downward saturated flow through the soil matrix and/or rapid macropore flow that is not in equilibrium with bulk soil solution whereas tension lysimeters sampled relatively immobile soil matrix solution during unsaturated conditions. The effect of land use on DOC and DON concentrations was largely limited to the most shallow (20 cm) sampling depth where DOC concentrations were greater in the forest (only zero tension lysimeters) and DON concentrations were greater in the cropland (both lysimeter types). In contrast to DOC and DON concentrations, the effect of land use on DOM properties persisted to the deepest sampling depth (60 cm), suggesting that DOM in the cropland was more decomposed regardless of lysimeter type. DOC concentrations and DOM properties differed between lysimeter types only in the forest at 20 cm where soil solutions collected with zero tension lysimeters had greater DOC concentrations, greater SUVA₂₅₄, greater humification index and lower HPI-C. Our data highlight the importance of considering DOM quality in addition to DOC quantity, and indicate long-term cultivation reduced the delivery of relatively less decomposed DOM to all soil depths.     

Effects of reclamation of natural wetlands to a rice paddy on dissolved carbon dynamics in the Sanjiang Plain,Northeastern China

Natural wetlands play an important role in the global carbon cycle, and loss of dissolved carbon through water has been indicated as one of the most important carbon sources for riverine ecosystems. During the last century, a large natural wetland area was reported to be converted to other land use types such as rice paddy land around the world. In this study, we explored the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in two natural freshwater wetlands and a rice paddy field, which was reclaimed from the natural wetlands in the Sanjiang Plain, Northeastern China, during the growing season (May–October) of 2009. The DOC and DIC concentrations in the two ecosystems were significantly different (P < 0.05). The mean DOC concentrations during the growing season in the surface water of the Deyeuxia angustifolia and Carex lasiocarpa wetlands were 49.88 ± 5.44 and 27.97 ± 1.69 mg/L, respectively, while it was only 8.63 ± 2.54 mg/L in the rice paddy field. Specific ultra-violet light absorption at 254 nm (SUVA₂₅₄) of DOC increased by an average of 19.54% in the surface water from the natural wetlands to rice paddy, suggesting that DOC mobilized in the natural wetlands was more aromatic than that in the rice paddy field. The mean DIC concentration in surface water of the rice paddy was 5.25 and 5.04 times higher than that in the natural D. angustifolia and C. lasiocarpa wetlands, respectively. The average ratio of DIC to dissolved total carbon (DTC) for the water sampled from the artificial drainage ditch in the rice paddy field was 61.82%, while it was 14.75% from the nearby channel of natural wetlands. The significant differences in dissolved carbon concentration in surface water and channels originating from different land use types suggested that reclamation of natural wetlands to rice paddy field would reduce DOC runoff and increase the DIC concentration to adjacent watersheds. Our study results for the changed pattern in dissolved carbon after the natural wetland was transformed to paddy field could have important implications for studying the impacts of the large-scale land use change to carbon cycle and management.     

Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

     

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long-term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy-nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by-products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.     

Sources of dissolved organic matter (DOM) in a Rocky Mountain stream using chemical fractionation and stable isotopes

Dissolved organic matter (DOM) is an important vehicle for the movement of nutrients from terrestrial to aquatic systems. To investigate how the source and composition of aquatic DOM change in both space and time, we used chemical, spectroscopic, and isotopic analyses to characterize DOM in a headwater catchment in the Colorado Front Range. Streamwater samples for DOM analyses were collected from 2 sites, a lightly vegetated alpine site and a forested, subalpine site, in the North Boulder Creek catchment during the snowmelt runoff season (May–September). Concentrations of dissolved organic carbon (DOC) peaked on the ascending limb of the snowmelt hydrograph at both the alpine (2.6 mg C l⁻¹) and the subalpine sites (7.0 mg C l⁻¹) and decreased sharply on the descending limb of the hydrograph. Fractionation of DOM into operationally defined humic and non-humic components showed that the fulvic acid content of DOC decreased through the season at both sites and that spectroscopic (fluorescence and ultraviolet) properties of the humic DOM fraction shifted in a manner consistent with an increase in the proportion of humic DOM derived from instream sources as compared to terrestrial catchment sources. Humic and non-humic fractions of DOM isolated near peak flow in June and during low flows in September showed a seasonal enrichment in ¹⁵N and ¹³C as well as a seasonal decrease in the ratio of aromatic to aliphatic carbon, both of which were correlated with a decrease in the C:N ratio of the DOM fractions. These results suggest that seasonal shifts in the isotopic and chemical characteristics of DOM are a result of changes in catchment sources of DOM. In particular, it appears that DOM production in alpine lakes is an important contributor to the streamwater DOM load during late season low flows, especially in the alpine reach of the catchment. Our results further suggest that stable isotopes of C and N are useful tools, particularly when combined with ancillary data such as elemental analyses and catchment discharge, for evaluating sources and transformations of DOM at the catchment scale.     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Multi-scale habitat selection affects offspring survival in a precocial species

In theory, habitat preferences should be adaptive. Accordingly, fitness is often assumed to be greater in preferred habitats; however, this assumption is rarely tested and, when it is, the results are often equivocal. Habitat preferences may not directly convey fitness advantages if animals are constrained by tradeoffs with other selective pressures like predation or food availability. We address unresolved questions about the survival consequences of habitat choices made during brood-rearing in a precocial species with exclusive maternal care (mallard Anas platyrhynchos, n = 582 radio-marked females on 27 sites over 8 years). We directly linked duckling survival with habitat selection patterns at two spatial scales using logistic regression and model selection techniques. At the landscape scale (55–80 km²), females that demonstrated stronger selection of areas with more cover type 4 wetlands and greater total cover type 3 wetland area (wetlands with large expanses of open water surrounded by either a narrow or wide peripheral band of vegetation, respectively) had lower duckling survival rates than did females that demonstrated weaker selection of these habitats. At finer scales (0.32–7.16 km²), females selected brood-rearing areas with a greater proportion of wetland habitat with no consequences for duckling survival. However, females that avoided woody perennial habitats composed of trees and shrubs fledged more ducklings. The relationship between habitat selection and survival depended on both spatial scale and habitats considered. Females did not consistently select brood-rearing habitats that conferred the greatest benefits, an unexpected finding, although one that has also been reported in other recent studies of breeding birds.     

Spatial and temporal variability of dissolved organic matter quantity and composition in an oligotrophic subtropical coastal wetland

Dissolved organic matter (DOM) is an essential component of the carbon cycle and a critical driver in controlling variety of biogeochemical and ecological processes in wetlands. The quality of this DOM as it relates to composition and reactivity is directly related to its sources and may vary on temporal and spatial scales. However, large scale, long-term studies of DOM dynamics in wetlands are still scarce in the literature. Here we present a multi-year DOM characterization study for monthly surface water samples collected at 14 sampling stations along two transects within the greater Everglades, a subtropical, oligotrophic, coastal freshwater wetland-mangrove-estuarine ecosystem. In an attempt to assess quantitative and qualitative variations of DOM on both spatial and temporal scales, we determined dissolved organic carbon (DOC) values and DOM optical properties, respectively. DOM quality was assessed using, excitation emission matrix (EEM) fluorescence coupled with parallel factor analysis (PARAFAC). Variations of the PARAFAC components abundance and composition were clearly observed on spatial and seasonal scales. Dry versus wet season DOC concentrations were affected by dry-down and re-wetting processes in the freshwater marshes, while DOM compositional features were controlled by soil and higher plant versus periphyton sources respectively. Peat-soil based freshwater marsh sites could be clearly differentiated from marl-soil based sites based on EEM–PARAFAC data. Freshwater marsh DOM was enriched in higher plant and soil-derived humic-like compounds, compared to estuarine sites which were more controlled by algae- and microbial-derived inputs. DOM from fringe mangrove sites could be differentiated between tidally influenced sites and sites exposed to long inundation periods. As such coastal estuarine sites were significantly controlled by hydrology, while DOM dynamics in Florida Bay were seasonally driven by both primary productivity and hydrology. This study exemplifies the application of long term optical properties monitoring as an effective technique to investigate DOM dynamics in aquatic ecosystems. The work presented here also serves as a pre-restoration condition dataset for DOM in the context of the Comprehensive Everglades Restoration Plan (CERP).     

Depleted dissolved organic carbon and distinct bacterial communities in the water column of a rapid-flushing coral reef ecosystem

Coral reefs are highly productive ecosystems bathed in unproductive, low-nutrient oceanic waters, where microbially dominated food webs are supported largely by bacterioplankton recycling of dissolved compounds. Despite evidence that benthic reef organisms efficiently scavenge particulate organic matter and inorganic nutrients from advected oceanic waters, our understanding of the role of bacterioplankton and dissolved organic matter (DOM) in the interaction between reefs and the surrounding ocean remains limited. In this study, we present the results of a 4-year study conducted in a well-characterized coral reef ecosystem (Paopao Bay, Moorea, French Polynesia) where changes in bacterioplankton abundance and dissolved organic carbon (DOC) concentrations were quantified and bacterial community structure variation was examined along spatial gradients of the reef:ocean interface. Our results illustrate that the reef is consistently depleted in concentrations of both DOC and bacterioplankton relative to offshore waters (averaging 79 μmol l⁻¹ DOC and 5.5 × 10⁸ cells l⁻¹ offshore and 68 μmol l⁻¹ DOC and 3.1 × 10⁸ cells l⁻¹ over the reef, respectively) across a 4-year time period. In addition, using a suite of culture-independent measures of bacterial community structure, we found consistent differentiation of reef bacterioplankton communities from those offshore or in a nearby embayment across all taxonomic levels. Reef habitats were enriched in Gamma-, Delta-, and Betaproteobacteria, Bacteriodetes, Actinobacteria and Firmicutes. Specific bacterial phylotypes, including members of the SAR11, SAR116, Flavobacteria, and Synechococcus clades, exhibited clear gradients in relative abundance among nearshore habitats. Our observations indicate that this reef system removes oceanic DOC and exerts selective pressures on bacterioplankton community structure on timescales approximating reef water residence times, observations which are notable both because fringing reefs do not exhibit long residence times (unlike those characteristic of atoll lagoons) and because oceanic DOC is generally recalcitrant to degradation by ambient microbial assemblages. Our findings thus have interesting implications for the role of oceanic DOM and bacterioplankton in the ecology and metabolism of reef ecosystems.     

Habitat fragmentation rather than habitat amount or habitat split reduces the diversity and abundance of ground-dwelling anurans within forest remnants of the Brazilian Cerrado

Rampant deforestation has caused the loss and fragmentation of natural habitats, which has precipitated a global biodiversity crisis. Research on how land-use change contributes to a loss of biodiversity is urgently needed, especially in ecosystems that have undergone rapid anthropogenic changes. We sought to investigate the extent to which habitat loss, fragmentation, and habitat split (the separation of forest and aquatic habitats) negatively influenced taxonomic diversity, functional diversity, total abundance, and the individual abundances of five anuran species in the Brazilian Cerrado. We sampled anurans between December 2017 and March 2018 using pitfall traps at sites distributed along a gradient of habitat fragmentation/habitat split: unfragmented forest, forest fragments without habitat split, and forest fragments with habitat split. Forest cover was measured within a 1-km radius of each site. Sites within unfragmented forests had higher taxonomic and functional diversities than either fragment type. Taxonomic diversity was highly correlated with functional diversity, but we did not find a pattern to the loss of functional traits. Total anuran abundance and the abundances of Chiasmocleis albopunctata, Physalaemus cuvieri, and Rhinella diptycha were higher in unfragmented forests compared to forest fragments. No species was more abundant in fragments than in unfragmented forests. Our results indicate that the fragmentation of forests by agricultural land use is directly and indirectly responsible for the loss of taxonomic and functional diversity, as well as for reducing population sizes of ground-dwelling anurans. Although we did not find a distinct effect of habitat split on ground-dwelling anurans, our study underscores the importance of preserving continuous forest habitats for the maintenance of anuran diversity in the Cerrado.     

Migration stopover ecology of Cinnamon Teal in western North America

Identifying migration routes and fall stopover sites of Cinnamon Teal (Spatula cyanoptera septentrionalium) can provide a spatial guide to management and conservation efforts, and address vulnerabilities in wetland networks that support migratory waterbirds. Using high spatiotemporal resolution GPS‐GSM transmitters, we analyzed 61 fall migration tracks across western North America during our three‐year study (2017–2019). We marked Cinnamon Teal primarily during spring/summer in important breeding and molting regions across seven states (California, Oregon, Washington, Idaho, Utah, Colorado, and Nevada). We assessed fall migration routes and timing, detected 186 fall stopover sites, and identified specific North American ecoregions where sites were located. We classified underlying land cover for each stopover site and measured habitat selection for 12 land cover types within each ecoregion. Cinnamon Teal selected a variety of flooded habitats including natural, riparian, tidal, and managed wetlands; wet agriculture (including irrigation ditches, flooded fields, and stock ponds); wastewater sites; and golf and urban ponds. Wet agriculture was the most used habitat type (29.8% of stopover locations), and over 72% of stopover locations were on private land. Relatively scarce habitats such as wastewater ponds, tidal marsh, and golf and urban ponds were highly selected in specific ecoregions. In contrast, dry non‐habitat across all ecoregions, and dry agriculture in the Cold Deserts and Mediterranean California ecoregions, was consistently avoided. Resources used by Cinnamon Teal often reflected wetland availability across the west and emphasize their adaptability to dynamic resource conditions in arid landscapes. Our results provide much needed information on spatial and temporal resource use by Cinnamon Teal during migration and indicate important wetland habitats for migrating waterfowl in the western United States.     

Effect of catchment characteristics on aquatic carbon export from a boreal catchment and its importance in regional carbon cycling

Inland waters transport and emit into the atmosphere large amounts of carbon (C), which originates from terrestrial ecosystems. The effect of land cover and land‐use practises on C export from terrestrial ecosystems to inland waters is not fully understood, especially in heterogeneous landscapes under human influence. We sampled for dissolved C species in five tributaries with well‐determined subcatchments (total size 174.5 km²), as well as in various points of two of the subcatchments draining to a boreal lake in southern Finland over a full year. Our aim was to find out how land cover and land‐use affect C export from the catchments, as well as CH₄ and CO₂ concentrations of the streams, and if the origin of C in stream water can be determined from proxies for quality of dissolved organic matter (DOM). We further estimated the gas evasion from stream surfaces and the role of aquatic fluxes in regional C cycling. The export rate of C from the terrestrial system through an aquatic conduit was 19.3 g C m^?2(catchment) yr^?1, which corresponds to 19% of the estimated terrestrial net ecosystem exchange of the catchment. Most of the C load to the recipient lake consisted of dissolved organic carbon (DOC, 6.1 ± 1.0 g C m^?2 yr^?1); the share of dissolved inorganic carbon (DIC) was much smaller (1.0 ± 0.2 g C m^?2 yr^?1). CO₂ and CH₄ emissions from stream and ditch surfaces were 7.0 ± 2.4 g C m^?2 yr^?1 and 0.1 ± 0.04 g C m^?2 yr^?1, respectively, C emissions being thus equal with C load to the lake. The proportion of peatland in the catchment and the drainage density of peatland increased DOC in streams, whereas the proportion of agricultural land in the catchment decreased it. The opposite was true for DIC. Drained peatlands were an important CH₄ source for streams.     

扎龙湿地芦苇分株生态可塑性及其对土壤因子的响应

扎龙湿地的芦苇既可形成大面积的单优群落,也可形成不同群落斑块。采用大样本抽样调查与统计分析方法,对湿地内水生生境、湿生生境、旱生生境和盐碱生境芦苇种群分株高度和生物量进行比较。结果表明,6—10月份,4个生境芦苇种群分株高度及生物量均以水生生境最高,盐碱生境最低,水生生境株高为盐碱生境的1.5—2.3倍,分株生物量为2.0—5.1倍,生境间的差异性以及差异序位均相对稳定。4个生境株高生境间变异系数(19.45%—31.56%)均高于生境内变异系数(8.07%—17.61%),分株高度在生境间的可塑性更大;分株生物量中水生生境、湿生生境和盐碱生境3个生境间的变异系数(33.43%—55.61%)均低于生境内变异系数(44.85%—79.82%),分株生物量在生境内的可塑性更大。不同生境条件下芦苇种群分株,在生长和生产上均存在较大的生态可塑性,表现出明显的环境效应,其中土壤含水量是该地区芦苇分株生态可塑性变异的主要驱动因子(R0.80),为正向驱动。     

Dissolved and particulate organic carbon in two wetlands in southwestern New South Wales,Australia

Concentrations of dissolved (DOC) and particulate organic carbon (POC) in two billabong wetlands (Murrumbidgil Swamp and Lake Merrimajeel) in inland temperate Australia were positively correlated with leaf fall from river red gums Eucalyptus camaldulensis (DOC and POC in Murrumbidgil Swamp), phytoplankton productivity (DOC in Murrumbidgil Swamp, DOC and POC in Lake Merrimajeel), and with biomass of aquatic macrophytes (DOC in the two wetlands). DOC correlated with water level at both sites. Mean concentrations of DOC (Murrumbidgil Swamp, 36.0 gm^-3; Lake Merrinmajeel, 27.5 g m^-3) and POC (15.0 g m^-3; 15.5 g m^-3) were high compared with other water bodies worldwide, although average for wetlands. The relatively high concentrations of DOC and POC in the study wetlands appear to be the result of their high rates of plant production, and input of leaves from river red gums.     

Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems

While large herbivores can have strong impacts on terrestrial ecosystems, much less is known of their role in aquatic systems. We reviewed the literature to determine: 1) which large herbivores (> 10 kg) have a (semi‐)aquatic lifestyle and are important consumers of submerged vascular plants, 2) their impact on submerged plant abundance and species composition, and 3) their ecosystem functions. We grouped herbivores according to diet, habitat selection and movement ecology: 1) Fully aquatic species, either resident or migratory (manatees, dugongs, turtles), 2) Semi‐aquatic species that live both in water and on land, either resident or migratory (swans), 3) Resident semi‐aquatic species that live in water and forage mainly on land (hippopotamuses, beavers, capybara), 4) Resident terrestrial species with relatively large home ranges that frequent aquatic habitats (cervids, water buffalo, lowland tapir). Fully aquatic species and swans have the strongest impact on submerged plant abundance and species composition. They may maintain grazing lawns. Because they sometimes target belowground parts, their activity can result in local collapse of plant beds. Semi‐aquatic species and turtles serve as important aquatic–terrestrial linkages, by transporting nutrients across ecosystem boundaries. Hippopotamuses and beavers are important geomorphological engineers, capable of altering the land and hydrology at landscape scales. Migratory species and terrestrial species with large home ranges are potentially important dispersal vectors of plant propagules and nutrients. Clearly, large aquatic herbivores have strong impacts on associated species and can be critical ecosystem engineers of aquatic systems, with the ability to modify direct and indirect functional pathways in ecosystems. While global populations of large aquatic herbivores are declining, some show remarkable local recoveries with dramatic consequences for the systems they inhabit. A better understanding of these functional roles will help set priorities for the effective management of large aquatic herbivores along with the plant habitats they rely on.