Cahoon S.M.P.,Pennsylvania State University |
Sullivan P.F.,University of Alaska Anchorage |
Shaver G.R.,The Ecosystems Center |
Welker J.M.,University of Alaska Anchorage |
Post E.,Pennsylvania State University
Ecology Letters | Year: 2012
Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO2 flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one-half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO2 exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (> 10 °C) were net sources of CO2, whereas woody sites with cold soils (< 10 °C) were strong sinks. Our results indicate that transition to a shrub-dominated Arctic will increase the rate of C cycling, and may lead to net C loss if soil temperatures rise. © 2012 Blackwell Publishing Ltd/CNRS.
Deegan L.A.,The Ecosystems Center |
Peterson B.J.,The Ecosystems Center
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2015
Increased growth rates for adult and young-of-the-year Arctic grayling (Thymallus arcticus) were measured after the addition of fertilizer to an oligotrophic Alaskan tundra river. The strongest response to the fertilization was seen in young-of-the-year grayling; the adult response was more variable. Whole-river phosphorus fertilization of the Kuparuk River, Alaska, during 1985-90 resulted in a 1.4- to 1.9-fold increase in the size of age 0+ fish and a 1.5- to 2.4-fold increase in the weight gain of adult grayling in some years. Neutral lipid storage in adult grayling was increased 1.3- to 3.4-fold in the fertilized zone compared with that in fish from the control zone. There was no detectable difference between the zones in gonad mass, percent lipid in eggs, or egg size. These results suggest that autochthonous production is an important energy source for fish in tundra streams and that nutrient limitation of stream ecosystems affects the highest trophic levels. These findings have importance for understanding the function of river ecosystems, for assessing human impacts on rivers, and for managing fisheries. On a mesuré laugmentation du taux de croissance des ombres arctiques (Thymallus arcticus) adultes et jeunes de lannée après ajout dengrais dans une riviére oligotrophe de la toundra, en Alaska. C'est chez les jeunes de lannée qu'on a observé la réaction la plus forte; la reaction des adultes était plus variable. L'enrichissement en phosphore, à léchelle de tout le cours deau, de la riviére Kuparuk (Alaska), entre 1985 et 1990, a provoqué une augmentation de l'ordre de 1,4 à 1,9 de la taille des poissons d'âge 0 +, et une augmentation de 1,5 à 2,4 du gain de poids chez les adultes certaines années. Le stockage des lipides neutres chez les ombres arctiques adultes a augmenté d'un ordre de grandeur de 1,3 à 3,4 dans la zone enrichie par rapport aux poissons de la zone témoin. On n'a pas détecté de différences entre les zones en ce qui concerne la masse gonadique, le pourcentage des lipides dans les oeufs ou la taille des oeufs. Ces résultats semblent indiquer que la production autochtone est une source importante dénergie pour les poissons dans les cours d'eau de la toundra, et que la limitation par les nutriants dans les écosystèmes lotiques a un effet sur les niveaux trophiques supérieurs. Ces découvertes ont une importance pour la comprehension de la fonction des écosystèmes lotiques, pour lévalua- tion des effets anthropiques sur les cours d'eau et pour la gestion des pêches. © 2015, National Research Council of Canada. All rights reserved.
Fox L.,The Ecosystems Center |
Valiela I.,The Ecosystems Center |
Kinney E.L.,The Ecosystems Center
Estuaries and Coasts | Year: 2012
Nitrogen inputs restructure ecosystems and can interact with other agents of ecological change and potentially intensify them. To examine the effects of nitrogen combined with those of elevation and competition, in 2005 we mapped vegetation and elevation within experimental plots that have been fertilized since 1970 in Great Sippewissett salt marsh, Cape Cod, MA, USA and compared the resulting effects on marsh vegetation. Decadal-scale chronic nutrient enrichment forced changes in cover and spatial distribution of different species. With increasing enrichment, there was a shift in species cover primarily involving loss of Spartina alterniflora and an increase in Distichlis spicata. Percent cover of near monocultures increased with nitrogen fertilization, owing mainly to the proliferation of D. spicata. The experimental fertilization prompted a shift from the short form of S. alterniflora to taller forms, hence increasing above-ground biomass, where this species managed to remain. Chronic enrichment increased upper and lower limits of the elevation range within which certain species occurred. The shift to increased cover of D. spicata was also associated with faster accretion of the marsh surface where this species was dominant, but not where S. alterniflora was dominant. Interactions among nutrient supply, elevation, and competition altered the direction of competitive success among different species of marsh plants, and forced changes in the spatial distribution and composition of the salt marsh plant communities. The results imply that there will be parallel changes in New England salt marshes owing to the widespread eutrophication of coastal waters and the increasing sea level rise. Knowing the mechanisms structuring marsh vegetative cover, and their role in modification of salt marsh accretion, may provide background with which to manage maintenance of affected coastal wetlands. © 2012 Coastal and Estuarine Research Federation.
Moran X.A.G.,Spanish Institute of Oceanography |
Moran X.A.G.,The Ecosystems Center |
Lopez-Urrutia T.,Spanish Institute of Oceanography |
Calvo-Diaz A.,Spanish Institute of Oceanography |
LI W.K.W.,Bedford Institute of Oceanography
Global Change Biology | Year: 2010
The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric size-scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the -0.6 to 22 °C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems. © 2009 Blackwell Publishing Ltd.
Rocha A.V.,The Ecosystems Center |
Shaver G.R.,The Ecosystems Center
Global Change Biology | Year: 2011
Fires produce land cover changes that have consequences for surface energy balance and temperature. Three eddy covariance towers were setup along a burn severity gradient (i.e. Severely, Moderately, and Unburned tundra) to determine the effect of fire and burn severity on arctic tundra surface energy exchange and temperature for three growing seasons (2008-2010) following the 2007 Anaktuvuk River fire. The three sites were well matched before the fire, experienced similar weather, and had similar energy budget closure, indicating that the measured energy exchange differences between sites were largely attributable to burn severity. Increased burn severity resulted in decreased vegetation and moss cover, organic layer depth, and the rate of postfire vegetation recovery. Albedo and surface greenness steadily recovered with Moderately matching Unburned tundra by the third growing season. Decreased albedo increased net radiation and partly fueled increased latent and ground heat fluxes, soil temperatures, and thaw depth. Decreases in moss cover and the organic layer also influenced the ground thermal regime and increased latent heat fluxes. These changes either offset or decreased the surface warming effect from decreased albedo, resulting in a small surface warming in Severely and a small surface cooling in Moderately relative to Unburned tundra. These results indicate that fires have a significant impact on surface energy balance and highlight the importance of moss and permafrost thaw in regulating arctic surface energy exchange and temperature. © 2011 Blackwell Publishing Ltd.
Neumann R.B.,University of Washington |
Cardon Z.G.,The Ecosystems Center
New Phytologist | Year: 2012
Hydraulic redistribution (HR) - the movement of water from moist to dry soil through plant roots - occurs worldwide within a range of different ecosystems and plant species. The proposed ecological and hydrologic impacts of HR include increasing dry-season transpiration and photosynthetic rates, prolonging the life span of fine roots and maintaining root-soil contact in dry soils, and moving rainwater down into deeper soil layers where it does not evaporate. In this review, we compile estimates of the magnitude of HR from ecosystems around the world, using representative empirical and modeling studies from which we could extract amounts of water redistributed by plant root systems. The reported average magnitude of HR varies by nearly two orders of magnitude across ecosystems, from 0.04 to 1.3mm H 2O d -1 in the empirical literature, and from 0.1 to 3.23mm H 2O d -1 in the modeling literature. Using these synthesized data, along with other published studies, we examine this variation in the magnitude of upward and downward HR, considering effects of plant, soil and ecosystem characteristics, as well as effects of methodological details (in both empirical and modeling studies) on estimates of HR. We take both ecological and hydrologic perspectives. © 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.
Valiela I.,The Ecosystems Center |
Bartholomew M.,The Ecosystems Center
Estuaries and Coasts | Year: 2015
Adjoined watershed–estuary–coastal ecosystems are coupled by biogeochemical and hydrodynamic processes, as Scott Nixon repeatedly argued in his many contributions. Case histories from Waquoit Bay and the Pacific Coast of Panama, supplemented by information from other sites, make evident that the couplings that enable connectivity among spatially separate landscape units, while highly subject to detailed local contingencies, take place in every coastal zone, can be powerfully affected by human activities on land, and by global-scale forcings, as Scott Nixon often reminded us. While the factors that determine the details of land–sea coupling differ significantly from one coastal zone to the next, estuarine systems manage, to different degrees, to furnish ecological services not only as filters or transformers of land-derived inputs but also as exporters of energy-rich subsidies to coastal food webs. © 2014, Coastal and Estuarine Research Federation.
Pollarda P.C.,Griffith University |
Ducklow H.,The Ecosystems Center
Limnology and Oceanography | Year: 2011
We studied trophic dynamics in a warm eutrophic subtropical river (Bremer River, Australia) to determine potential sources of dissolved organic carbon (DOC) and the fate of heterotrophic bacterial production. Sustained high rates of bacterial production suggested that the exogenous DOC was accessible (labile). Bacterial specific growth rates (0.2 h-1 to 1.8 h-1) were some of the highest measured for natural aquatic ecosystems, which is consistent with high respiration rates. Bacteria consumed 10 times more organic carbon than that supplied by the daily algal production, a result that implies that terrestrial sources of organic carbon were driving the high rates of bacterial production. Viruses (1011 L-1) were 10 times more abundant than bacteria; the viral to bacterial ratio ranged from 3.5 to 12 in the wet summer and 11 to 35 in the dry spring weather typical of eutrophic environments. Through a combination of high bacterial respiration and phage lysis, a continuous supply of terrestrial DOC was lost from the aquatic ecosystem in a CO2-vented bacterial-viral loop. Bacterial processing of DOC in subtropical rivers may be contributing disproportionately large amounts of CO2 to the global carbon cycle compared to temperate freshwater ecosystems. © 2011, by the American Society of Limnology and Oceanography, Inc.
Neill C.,The Ecosystems Center
Philosophical transactions of the Royal Society of London. Series B, Biological sciences | Year: 2013
The expansion and intensification of soya bean agriculture in southeastern Amazonia can alter watershed hydrology and biogeochemistry by changing the land cover, water balance and nutrient inputs. Several new insights on the responses of watershed hydrology and biogeochemistry to deforestation in Mato Grosso have emerged from recent intensive field campaigns in this region. Because of reduced evapotranspiration, total water export increases threefold to fourfold in soya bean watersheds compared with forest. However, the deep and highly permeable soils on the broad plateaus on which much of the soya bean cultivation has expanded buffer small soya bean watersheds against increased stormflows. Concentrations of nitrate and phosphate do not differ between forest or soya bean watersheds because fixation of phosphorus fertilizer by iron and aluminium oxides and anion exchange of nitrate in deep soils restrict nutrient movement. Despite resistance to biogeochemical change, streams in soya bean watersheds have higher temperatures caused by impoundments and reduction of bordering riparian forest. In larger rivers, increased water flow, current velocities and sediment flux following deforestation can reshape stream morphology, suggesting that cumulative impacts of deforestation in small watersheds will occur at larger scales.
Hobbie J.E.,The Ecosystems Center |
Hobbie E.A.,University of New Hampshire
Frontiers in Microbiology | Year: 2013
Understanding microbial transformations in soils is important for predicting future carbon sequestration and nutrient cycling. This review questions some methods of assessing one key microbial process, the uptake of labile organic compounds. First, soil microbes have a starving-survival life style of dormancy, arrested activity, and low activity. Yet they are very abundant and remain poised to completely take up all substrates that become available. As a result, dilution assays with the addition of labeled substrates cannot be used. When labeled substrates are transformed into 14CO2, the first part of the biphasic release follows metabolic rules and is not affected by the environment. As a consequence, when identical amounts of isotopically substrates are added to soils from different climate zones, the same percentage of the substrate is respired and the same half-life of the respired 14CO2 from the labeled substrate is estimated. Second, when soils are sampled by a variety of methods from taking 10 cm diameter cores to millimeter-scale dialysis chambers, amino acids (and other organic compounds) appear to be released by the severing of fine roots and mycorrhizal networks as well as from pressing or centrifuging treatments. As a result of disturbance as well as of natural root release, concentrations of individual amino acids of ~10 μM are measured. This contrasts with concentrations of a few nanomolar found in aquatic systems and raises questions about possible differences in the bacterial strategy between aquatic and soil ecosystems. The small size of the hyphae (2-10 μm diameter) and of the fine roots (0.2-2 mm diameter), make it very difficult to sample any volume of soil without introducing artifacts. Third, when micromolar amounts of labeled amino acids are added to soil, some of the isotope enters plant roots. This may be an artifact of the high micromolar concentrations applied. © 2013 Hobbie and Hobbie.