Landscape-Scale Variability of Organic Carbon Burial by SW Greenland Lakes

Ecosystems - Lakes are a key feature of arctic landscapes and can be an important component of regional organic carbon (OC) budgets, but C burial rates are not well estimated. 210Pb-dated sediment...     

Local and Regional Determinants of an Uncommon Functional Group in Freshwater Lakes and Ponds

A combination of local and regional factors and stochastic forces is expected to determine the occurrence of species and the structure of communities. However, in most cases, our understanding is incomplete, with large amounts of unexplained variation. Using functional groups rather than individual species may help explain the relationship between community composition and conditions. In this study, I used survey data from freshwater lakes and ponds to understand factors that determine the presence of the floating plant functional group in the northeast United States. Of the 176 water bodies surveyed, 104 (59.1%) did not contain any floating plant species. The occurrence of this functional group was largely determined by local abiotic conditions, which were spatially autocorrelated across the region. A model predicting the presence of the floating plant functional group performed similarly to the best species-specific models. Using a permutation test, I also found that the observed prevalence of floating plants is no different than expected by random assembly from a species pool of its size. These results suggest that the size of the species pool interacts with local conditions in determining the presence of a functional group. Nevertheless, a large amount of unexplained variation remains, attributable to either stochastic species occurrence or incomplete predictive models. The simple permutation approach in this study can be extended to test alternative models of community assembly.     

Effects of Eutrophication on Young-of-the-year Freshwater Fish Communities in Coastal Areas of the Baltic

Abundance, growth, habitat and food choice of young-of-the-year (Y-O-Y) freshwater fishes were investigated in a gradient of eutrophication in a Baltic inlet. Secchi-disc depth decreased from 3 to 0.5m along the gradient with an accompanying reduction in submerged vegetation coverage. There were positive trends in Y-O-Y abundance with increasing turbidity for cyprinids, pikeperch, ruffe and smelt, and a negative trend for perch, leading to a shift from a perch dominated community in the reference area to a cyprinid dominated in the eutrophic area. The species composition of adult fishes resembled the Y-O-Y species community, indicating that community structure was determined already during early juvenile interval. Perch and cyprinid Y-O-Y preferred areas with submerged macrophytes while pikeperch and smelt were found more frequently in open waters. Perch Y-O-Y growth was reduced in eutrophic areas compared to references, especially in the size range where they change from foraging on zooplankton to benthic organisms. The deterioration of visual conditions caused by high turbidity is suggested to be negative for feeding of perch especially in deeper water. There was no evident growth reduction for pikeperch, which may be explained by a visual physiology well adapted to turbid conditions. The combined effects of turbidity and competition for food resources on growth and survival in eutrophic waters are discussed.     

The Nature and Longevity of Agricultural Impacts on Soil Carbon and Nutrients: A Review

Since the domestication of plant and animal species around 10,000 years ago, cultivation and animal husbandry have been major components of global change. Agricultural activities such as tillage, fertilization, and biomass alteration lead to fundamental changes in the pools and fluxes of carbon (C), nitrogen (N), and phosphorus (P) that originally existed in native ecosystems. Land is often taken out of agricultural production for economic, social, or biological reasons, and the ability to predict the biogeochemical trajectory of this land is important to our understanding of ecosystem development and our projections of food security for the future. Tillage generally decreases soil organic matter (SOM) due to erosion and disruption of the physical, biochemical, and chemical mechanisms of SOM stabilization, but SOM can generally reaccumulate after the cessation of cultivation. The use of organic amendments causes increases in SOM on agricultural fields that can last for centuries to millennia after the termination of applications, although the locations that provide the organic amendments are concurrently depleted. The legacy of agriculture is therefore highly variable on decadal to millennial time scales and depends on the specific management practices that are followed during the agricultural period. State factors such as climate and parent material (particularly clay content and mineralogy) modify ecosystem processes such that they may be useful predictors of rates of postagricultural biogeochemical change. In addition to accurate biogeochemical budgets of postagricultural systems, ecosystem models that more explicitly incorporate mechanisms of SOM loss and formation with agricultural practices will be helpful. Developing this predictive capacity will aid in ecological restoration efforts and improve the management of modern agroecosystems as demands on agriculture become more pressing.     

Oxygen Deficits,Clarity, and Eutrophication in some Madison Lakes

Oxygen deficits, rates of oxygen depletion, and Secchi disk measurements were the major indices examined in this study to determine trends in trophic conditions in Lakes Mendota, Monona, and Waubesa near Madison, Wisconsin. On the basis of available data, there is little that can be said reliably with respect to documenting eutrophication, particularly in Lake Mendota in the past several decades, that could not be attributed to normal variability. Better evidence for eutrophication comes from cores of some of the lakes. A comparison of oxygen deficits without simultaneous information on mean hypolimnetic temperatures and the lateness of ice out may provide a misleading interpretation of trophic variations over time.     

Vertical Distribution of Potentially Pathogenic Free-Living Amoebae in Freshwater Lakes1

The vertical distribution of thermotolerant (37°C and 45°C) free-living amoebae (FLA) in warm monomictic lakes was determined in relation to the onset of thermal stratification and associated physical and chemical changes. The position of abiotic or biotic paniculate layers in the water column was located by using a submersible horizontal beam transmissometer that measures attenuance, or the absorption and scattering of light by participates in the water column. During mixis, the vertical distribution of amoebae was sporadic with significant numbers of FLA only occurring in clay layers caused by runoff after heavy rains. With the onset of thermal stratification in the lakes, phytoplankton layers began to form. Few amoebae were isolated from layers containing flagellated phytoplankton; however, significant (P < 0.005) numbers of FLA were isolated from two paniculate layers dominated by the filamentous blue-green algae Aphanizomenon and Lyngbya, respectively. By late June, a persistent detrital or decomposition layer formed in the lower metalimnion, as well as a hypolimnetic iron layer where the Fe²⁺ state was predominant. In this midsummer period, 13 Naegieria fowleri were isolated, with three from the detrital layer and seven from the iron layer. The presence of attenuation zones was found to be the best indicator of the vertical distribution of FLA in the water column, and such layers represent an important, previously undescribed habitat for potentially pathogenic FLA.     

Impacts of Agricultural Management and Climate Change on Future Soil Organic Carbon Dynamics in North China Plain

Dynamics of cropland soil organic carbon (SOC) in response to different management practices and environmental conditions across North China Plain (NCP) were studied using a modeling approach. We identified the key variables driving SOC changes at a high spatial resolution (10 km×10 km) and long time scale (90 years). The model used future climatic data from the FGOALS model based on four future greenhouse gas (GHG) concentration scenarios. Agricultural practices included different rates of nitrogen (N) fertilization, manure application, and stubble retention. We found that SOC change was significantly influenced by the management practices of stubble retention (linearly positive), manure application (linearly positive) and nitrogen fertilization (nonlinearly positive) – and the edaphic variable of initial SOC content (linearly negative). Temperature had weakly positive effects, while precipitation had negligible impacts on SOC dynamics under current irrigation management. The effects of increased N fertilization on SOC changes were most significant between the rates of 0 and 300 kg ha⁻¹ yr⁻¹. With a moderate rate of manure application (i.e., 2000 kg ha⁻¹ yr⁻¹), stubble retention (i.e., 50%), and an optimal rate of nitrogen fertilization (i.e., 300 kg ha⁻¹ yr⁻¹), more than 60% of the study area showed an increase in SOC, and the average SOC density across NCP was relatively steady during the study period. If the rates of manure application and stubble retention doubled (i.e., manure application rate of 4000 kg ha⁻¹ yr⁻¹ and stubble retention rate of 100%), soils across more than 90% of the study area would act as a net C sink, and the average SOC density kept increasing from 40 Mg ha⁻¹ during 2010s to the current worldwide average of ∼55 Mg ha⁻¹ during 2060s. The results can help target agricultural management practices for effectively mitigating climate change through soil C sequestration.     

Impacts of Alien Invasive Species on Freshwater Fauna at Risk in Canada

Freshwater aquatic organisms in North America are disproportionately imperilled when compared to their terrestrial counterparts due to widespread habitat alteration, pollution, overexploitation and the introduction of alien species. In this review, we examine the threat factors contributing to the endangerment of freshwater fishes and molluscs in Canada and further examine the nature of alien invasive species introductions affecting aquatic species at risk. Habitat loss and degradation is the predominant threat factor for Canadian freshwater fishes and molluscs that are listed as Extinct, Extirpated, Endangered and Threatened. Alien invasive species are the second most prevalent threat for fishes, affecting 26 of 41 listed species. Alien invasive species are a threat in most parts of Canada where listed fishes are found. Most (65%) of the alien invasive species affecting listed fishes are the result of intentional introductions related to sport fishing, and the majority of these introductions are unauthorized. Fifteen fishes and two plant species are cited as alien invasive species that impact listed fishes with brown bullhead (Ameiurus nebulosus) and pumpkinseed (Lepomis gibbosus) being the most prevalent. Alien species are a threat to 6 of 11 listed mollusc species. All six species are threatened by the alien zebra mussel (Dreissena polymorpha) in the Great Lakes basin. An erratum to this article is available at .     

Effects of Carbon Substrates on Nitrite Accumulation in Freshwater Sediments

The contribution of the biochemical pathways nitrification, denitrification, and dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA) to the accumulation of NO₂⁻ in freshwaters is governed by the species compositions of the bacterial populations resident in the sediments, available carbon (C) and nitrogen (N) substrates, and environmental conditions. Recent studies of major rivers in Northern Ireland have shown that high NO₂⁻ concentrations found in summer, under warm, slow-flowing conditions, arise from anaerobic NO₃⁻ reduction. Locally, agricultural pollutants entering rivers are important C and N sources, providing ideal substrates for the aquatic bacteria involved in cycling of N. In this study a range of organic C compounds commonly found in agricultural pollutants were provided as energy sources in 48-h incubation experiments to investigate if the chemical compositions of the pollutants affected which NO₃⁻ reduction pathway was followed and influenced subsequent NO₂⁻ accumulation. Carbon stored within the sediments was sufficient to support DNRA and denitrifier populations, and the resulting NO₂⁻ peak (80 μg of N liter⁻¹ [approximate]) observed at 24 h was indicative of the simultaneous activities of both bacterial groups. The value of glycine as an energy source for denitrification or DNRA appeared to be limited, but glycine was an important source of additional N. Glucose was an efficient substrate for both the denitrification and DNRA pathways, with a NO₂⁻ peak of 160 μg of N liter⁻¹ noted at 24 h. Addition of formate and acetate stimulated continuous NO₂⁻ production throughout the 48-h period, caused by partial inhibition of the denitrification pathway. The formate treatment resulted in a high NO₂⁻ accumulation (1,300 μg of N liter⁻¹ [approximate]), and acetate treatment resulted in a low NO₂⁻ concentration (<100 μg of N liter⁻¹).     

Effects of Landscape Elements on the Distribution of the Rare Bumblebee Species Bombus muscorum in an Agricultural Landscape

The regional distribution pattern of Bombus muscorum was studied in an agricultural landscape of central Germany, one of two remaining areas with the occurrence of this nationally endangered species in the Land Hesse. To determine the landscape characteristics that facilitate the occurrence of B. muscorum, grid-based observation records were analysed in a GIS environment at a regional scale. A significantly negative effect of the number of trees on the occurrence of B. muscorum and a significantly positive one of the proportion of arable land, strongly support the species’ preference for open landscapes. Yet, apart from open landscapes additional landscape features were shown to be important. A significantly positive effect of ditches in the final model revealed the importance of this landscape element for the occurrence of B. muscorum. This finding was additionally supported by recordings of nest-searching queens, nests, and flower visits along dithes. The positive effects of clover and fallow land indicate the species’ need for suitable food resources throughout the season. Because B. muscorum exhibits small foraging ranges, it is essential that landscape elements that provide nesting sites, foraging habitats and undisturbed hibernation structures are next to each other. The low numbers of individuals of B. muscorum recorded indicate that the supply of these habitat elements may have reached a critical threshold in the study region.     

Seasonal Viral Loop Dynamics in Two Large Ultraoligotrophic Antarctic Freshwater Lakes

The effect of viruses on the microbial loop, with particular emphasis on bacteria, was investigated over an annual cycle in 2003–2004 in Lake Druzhby and Crooked Lake, two large ultraoligotrophic freshwater lakes in the Vestfold Hills, Eastern Antarctica. Viral abundance ranged from 0.16 to 1.56 × 10⁹ particles L^-1;1 and bacterial abundances ranged from 0.10 to 0.24 × 10⁹ cells L^-1;1, with the lowest bacterial abundances noted in the winter months. Virus-to-bacteria ratios (VBR) were consistently low in both lakes throughout the season, ranging from 1.2 to 8.4. lysogenic bacteria, determined by induction with mitomycin C, were detected on three sampling occasions out of 10 in both lakes. In Lake Druzhby and Crooked Lake, lysogenic bacteria made up between 18% and 73% of the total bacteria population during the lysogenic events. Bacterial production ranged from 8.2 to 304.9 × 10⁶ cells L^-1;1 day^-1;1 and lytic viral production ranged from 47.5 to 718.4 × 10⁶ viruslike particles L^-1;1 day^-1;1. When only considering primary production, heterotrophic nanoflagellate (HNF) grazing and viral lysis as the major contributors to the DOC pool (i.e., autochthonous sources), we estimated a high contribution from viruses during the winter months when >60% of the carbon supplied to the DOC pool originated from viral lysis. In contrast, during the summer <20% originated from viral lysis. Our study shows that viral process in ultraoligotrophic Antarctic lakes may be of quantitative significance with respect to carbon flow especially during the dark winter period.     

Six-year Stable Annual Uptake of Carbon Dioxide in Intensively Managed Humid Temperate Grassland

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO₂) exchange of grassland ecosystems. We used 6 years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO₂ exchange (NEE), gross ecosystem production (GEP), and ecosystem respiration (ER) of an intensively managed grassland in the humid temperate climate of southern Ireland. In all the years of the study period, considerable uptake of atmospheric CO₂ occurred in this grassland with a narrow range in the annual NEE from −245 to −284 g C m⁻² y⁻¹, with the exception of 2008 in which the NEE reached −352 g C m⁻² y⁻¹. None of the measured environmental variables (air temperature (Ta), soil moisture, photosynthetically active radiation, vapor pressure deficit (VPD), precipitation (PPT), and so on) correlated with NEE on a seasonal or annual scale because of the equal responses from the component fluxes GEP and ER to variances in these variables. Pronounced reduction of summer PPT in two out of the six studied years correlated with decreases in both GEP and ER, but not with NEE. Thus, the stable annual NEE was primarily achieved through a strong coupling of ER and GEP on seasonal and annual scales. Limited inter-annual variations in Ta (±0.5°C) and generally sufficient soil moisture availability may have further favored a stable annual NEE. Monthly ecosystem carbon use efficiency (CUE; as the ratio of NEE:GEP) during the main growing season (April 1–September 30) was negatively correlated with temperature and VPD, but positively correlated with soil moisture, whereas the annual CUE correlated negatively with annual NEE. Thus, although drier and warmer summers may mildly reduce the uptake potential, the annual uptake of atmospheric CO₂, in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.     

Protozoan Colonization Rates and Trophic Status of Some Freshwater Wetland Lakes

Primary productivity, chlorophyll a, phosphorus and nitrogen nutrients, and other chemical and physical parameters were measured in 13 wetland lakes in northern lower Michigan. These lakes included several examples located in each of the four major wetland types—bogs, fens, marshes, and swamps. Of the four types, the brown-colored waters of the acid bog lakes generally had the highest levels of primary productivity, chlorophyll a, phosphorus, and nitrogen. Primary productivity correlated positively with water color, total-N, and NH₃-N (α≤ 0.05). By these measures, waters of the bogs were the most eutrophic of the four types of wetland lakes. These findings would seem to contradict the generally-held concept that “dystrophic” bog lakes are extremely oligotrophic. Protozoan colonization of artificial substrate islands was monitored at each wetland site. The correlation between protozoan colonization rates (G values in the MacArthur-Wilson noninteractive model) and primary productivity, measured by 24-h light and dark bottle incubations, was significant at the 95% confidence level (r= 0.850, P= 0.001) and with water color at the 90% confidence level (r= 0.599, P= 0.084). It was concluded that protozoan colonization rate was an excellent indicator of the trophic status of wetland lakes.     

Responses of Active Ammonia Oxidizers to Eutrophication and Oxygen Statuses in Taihu Freshwater Sediments

Abstract

Here, we employed DNA-based stable isotope probing (SIP) and molecular biology methods to investigate active ammonia oxidizer communities in suboxic sediments (0 to –2?cm) at the micromolar oxygen level and layers (–2 to –5?cm) at nanomolar oxygen concentrations from meso-eutrophic and light-eutrophic locations in Taihu Lake. The results revealed that ammonia-oxidizing archaea (AOA) were less active in the anoxic layer of meso-eutrophic sites, while ammonia-oxidizing bacteria (AOB) were less active in suboxic sediments of light-eutrophic sites after 8?weeks of incubation. The active AOA in the meso- and light-eutrophic sediments belonged to the Nitrosopumilus, Nitrosotalea, and Nitrososphaera clusters and the Nitrosopumilus and Nitrososphaera clusters, respectively, with Nitrosopumilus cluster as the predominant AOA, which took up a higher ratio in the light-eutrophic and suboxic layers than their counterparts. The advantageous active AOB were numerically predominated by the Nitrosomonas cluster in the suboxic layers, and the Nitrosospira cluster in the anoxic layers, respectively, both of which were distributed in diverse frequencies in different eutrophication statuses. The role and community composition diversities of active ammonia oxidizers in freshwater sediments were attributed to the different eutrophication (including nitrogen and organic carbon content) and oxygen statuses.     

Railway Embankments as New Habitat for Pollinators in an Agricultural Landscape

Pollinating insect populations, essential for maintaining wild plant diversity and agricultural productivity, rely on (semi)natural habitats. An increasing human population is encroaching upon and deteriorating pollinator habitats. Thus the population persistence of pollinating insects and their associated ecosystem services may depend upon on man-made novel habitats; however, their importance for ecosystem services is barely understood. We tested if man-made infrastructure (railway embankments) in an agricultural landscape establishes novel habitats that support large populations of pollinators (bees, butterflies, hoverflies) when compared to typical habitats for these insects, i.e., semi-natural grasslands. We also identified key environmental factors affecting the species richness and abundance of pollinators on embankments. Species richness and abundance of bees and butterflies were higher for railway embankments than for grasslands. The occurrence of bare (non-vegetated) ground on embankments positively affected bee species richness and abundance, but negatively affected butterfly populations. Species richness and abundance of butterflies positively depended on species richness of native plants on embankments, whereas bee species richness was positively affected by species richness of non-native flowering plants. The density of shrubs on embankments negatively affected the number of bee species and their abundance. Bee and hoverfly species richness were positively related to wood cover in a landscape surrounding embankments. This is the first study showing that railway embankments constitute valuable habitat for the conservation of pollinators in farmland. Specific conservation strategies involving embankments should focus on preventing habitat deterioration due to encroachment of dense shrubs and maintaining grassland vegetation with patches of bare ground.     

Effect of Habitat Patch Characteristics on Abundance and Diversity of Insects in an Agricultural Landscape

The objective of this study was to test for general effects of patch size, patch isolation, disturbance frequency, and patch life span, on density and diversity of organisms. We sampled predominantly herbivorous insects in 31 alfalfa fields that varied in size, isolation, frequency of disturbance by cutting, and age (number of years planted in alfalfa). Effects on insect relative density and diversity were examined at three taxonomic levels: all insects, eight separate orders, and six legume-specialist weevil species. We found that (a) more isolated alfalfa fields had higher overall insect richness, (b) fields with higher disturbance frequency had lower overall insect richness, and (c) fields of intermediate age had highest insect richness. In some cases these patterns were reflected at lower taxonomic levels, but in many cases they were not. These results are important because they indicate that, although we cannot simultaneously tailor a landscape for each of thousands of species, we may be able to produce desired effects at a more general level. Received 8 August 1997; accepted 2 January 1998.     

Paleolimnologic Evidence for Recent Eutrophication in the Valley of the Great Lakes (Mongolia)

Climate warming and major land-use changes have profoundly affected the Mongolian landscape in the past several decades. Previous studies have recognized the impacts of a warmer, more arid climate and Mongolia’s 1991 transition from a command to a market economy on terrestrial ecosystems, including impaired sustainability of subsistence herding and threats to wild animals. In this study, we examined the combined effects of changing climate and herding practices on lake eutrophication in Western Mongolia. We sampled 65 lakes for modern nutrients and found the majority of lakes were eutrophic to hyper-eutrophic. Sediment cores were taken from five of the lakes to compare current lake status to paleolimnologial measures of lake eutrophication over the past 100–2000 years, including changes in diatom assemblages, diatom-inferred total phosphorus, biogenic silica, organic matter, and sediment accumulation rates. Variance partitioning analysis showed that recent shifts in diatom assemblages were related to changes in both climate and herding practices. The results presented here demonstrate a need for further study and long-term monitoring of water quality in Mongolia to understand the complicated interactions of climate and land use on aquatic resources and to preserve water quality in this remote and ecologically important region.