Showing papers in "Limnology and Oceanography in 2009"

Lakes and reservoirs as regulators of carbon cycling and climate

Abstract: We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.

Lakes as sentinels of climate change

Abstract: While there is a general sense that lakes can act as sentinels of climate change, their efficacy has not been thoroughly analyzed. We identified the key response variables within a lake that act as indicators of the effects of climate change on both the lake and the catchment. These variables reflect a wide range of physical, chemical, and biological responses to climate. However, the efficacy of the different indicators is affected by regional response to climate change, characteristics of the catchment, and lake mixing regimes. Thus, particular indicators or combinations of indicators are more effective for different lake types and geographic regions. The extraction of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication or acidification, and the equivalent reverse phenomena, in addition to other land-use influences. In many cases, however, confounding factors can be addressed through analytical tools such as detrending or filtering. Lakes are effective sentinels for climate change because they are sensitive to climate, respond rapidly to change, and integrate information about changes in the catchment.

Lakes and reservoirs as sentinels, integrators, and regulators of climate change

Abstract: Climate change is generating complex responses in both natural and human ecosystems that vary in their geographic distribution, magnitude, and timing across the global landscape. One of the major issues that scientists and policy makers now confront is how to assess such massive changes over multiple scales of space and time. Lakes and reservoirs comprise a geographically distributed network of the lowest points in the surrounding landscape that make them important sentinels of climate change. Their physical, chemical, and biological responses to climate provide a variety of information-rich signals. Their sediments archive and integrate these signals, enabling paleolimnologists to document changes over years to millennia. Lakes are also hot spots of carbon cycling in the landscape and as such are important regulators of climate change, processing terrestrial and atmospheric as well as aquatic carbon. We provide an overview of this concept of lakes and reservoirs as sentinels, integrators, and regulators of climate change, as well as of the need for scaling and modeling these responses in the context of global climate change. We conclude by providing a brief look to the future and the creation of globally networked sensors in lakes and reservoirs around the world.

Microbial oceanography of the dark ocean's pelagic realm

Abstract: The pelagic realm of the dark ocean represents a key site for remineralization of organic matter and long-term carbon storage and burial in the biosphere. It contains the largest pool of microbes in aquatic systems, harboring nearly 75% and 50% of the prokaryotic biomass and production, respectively, of the global ocean. Genomic approaches continue to uncover the enormous and dynamic genetic variability at phylogenetic and functional levels. Deep-sea prokaryotes have comparable or even higher cell-specific extracellular enzymatic activity than do microbes in surface waters, with a high fraction of freely released exoenzymes, probably indicative of a life mode reliant on surface attachment to particles or colloids. Additionally, evidence increases that chemoautotrophy might represent a significant CO2 sink and source of primary production in the dark ocean. Recent advances challenge the paradigm of stable microbial food web structure and function and slow organic-matter cycling. However, knowledge of deep-ocean food webs is still rudimentary. Dynamics of particle transformation and fate of the exported material in deep waters are still largely unknown. Discrepancies exist between estimates of carbon fluxes and remineralization rates. Recent assessments, however, suggest that integrated respiration in the dark ocean’s water column is comparable to that in the epipelagic zone, and that the dark ocean is a site of paramount importance for material cycling in the biosphere. The advent of new molecular tools and in situ sampling methodologies will improve knowledge of the dark ocean’s microbial ecosystem and resolve current discrepancies between carbon sources and metabolic requirements of deep-sea microbes. The dark ocean is the largest habitat in the biosphere, comprising 1.3 3 10 18 m 3 . It is characterized by the absence of sufficient light to support photosynthesis, although the dim light below the epipelagic zone reaching to about 1000m depth (the so-called ‘‘twilight zone’’) is sufficient to guide organism locomotion and orientation. This realm distinctly differs from the epipelagic ocean by its high pressure, low temperature, and high inorganic nutrient concentrations. It is subdivided into the mesopelagic zone (in this review considered as the depth range from 200- to 1000-m depth), with a water-mass residence time of decades and confined between the seasonal and the permanent thermocline, and the bathypelagic (1000–4000-m depth) and abyssal zones (.4000 m) with water-mass residence times of centuries. Most of the organic carbon resulting from photosynthesis in the epipelagic zone and transported into the dark ocean is respired in the mesopelagic zone by prokaryotes (Aro´stegui et al. 2005b) and hence may be returned back to the atmosphere as carbon dioxide in months to years. However, some of the organic carbon escapes microbial degradation in the mesopelagic and reaches the bathypelagic zone, where it may be respired and sequestered as CO2 for centuries until the overturning circulation returns this carbon to the upper ocean, allowing exchange with the atmosphere.

Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean

Abstract: Pelagic food webs drive a flux of .10 310 12 kg C yr 21 that exits surface waters, mostly via sinking particles through the ocean’s ‘‘biological pump.’’ Much of this particle flux is remineralized in the poorly studied waters of the twilight zone, i.e., the layer underlying the euphotic zone and extending to 1000 m. We present a reanalysis of selected upper-ocean studies of particulate organic carbon (POC) flux and relate these observations to a simple one-dimensional biological model to shed light on twilight zone processes. The ecosystem model first predicts particle flux from the base of the euphotic zone, and then its attenuation below based on transformations by heterotrophic bacteria and zooplankton, and active downward transport of surface-derived particles by zooplankton. Observations and simulations both suggest that future sampling strategies for the twilight zone should take regional variability of the euphotic zone depth (Ez) into account. In addition, conventional curvefitting of particle flux data (i.e., power law or exponential) skews our interpretation of twilight zone processes. To overcome these artifacts, we introduce two new terms: the Ez-ratio (POC flux at Ez relative to net primary production [NPP]) and T100 (the ratio of POC flux 100 m below Ez to POC flux at Ez). A comparison of NPP, Ezratios, and T100 provides a new set of metrics to classify the ocean into different regimes, representing high and low surface export and subsurface flux attenuation. The ocean’s ‘‘twilight zone’’ is defined as the layer below the sunlit euphotic zone extending to depths of about 1000 m. The use of this term dates back to at least Russell (1931) and colleagues, who were interested in controls on the vertical distribution of marine macroplankton. The concept of sinking particles raining through this layer to the deep sea is also not new, as Agassiz (1888) and others explored deep-sea organisms and their nourishment from above. However, the advent of modern sediment trapping to intercept and capture the flux of settling ocean particles began much later, in the 1960s–1970s (Berger 1971; Honjo 1976; Soutar et al. 1977). Sediment traps allowed assessment of the relationship among surface algal productivity, particle export, and attenuation through the twilight zone (Berger et al. 1989). In addition to sinking particles, mixing

Cyanobacteria dominance: Quantifying the effects of climate change

Abstract: An increase in cyanobacteria bloom formation within lakes has been forecasted as a result of global warming. We investigated the particular physical and chemical thresholds for cyanobacteria performance in a lake model system, the polymictic eutrophic Muggelsee, which has been affected by significant warming trends and substantial reductions in external nutrient load. To identify key physical and nutrient thresholds favoring cyanobacterial performance, we applied classification tree analysis to water temperature, Schmidt stability, oxygen, pH, nutrients (including phosphorus, nitrogen, and their relative ratios), and zooplankton abundance during periods of summer thermal stratification. Although total phosphorus (TP) concentration was the principal force driving cyanobacteria contribution to total algal mass, climate-induced changes in the thermal regime, rather than direct temperature effects, positively influenced cyanobacteria dominance. Stratification periods exceeding 3 weeks and exhibiting a Schmidt stability of .44 g cm cm22 favored cyanobacteria proliferation within a critical TP concentration range (70-215 m gL 21). The dominating genera Aphanizomenon, Anabaena, and Microcystis achieved the highest biomass in cases in which total nitrogen concentrations exceeded 1.29 mg L21, stratified conditions exceeded a duration of 3 weeks, and TP concentrations exceeded 215 m gL 21, respectively. Given the observed broad range of TP thresholds within which climate warming enhances the probability of cyanobacteria dominance, the incidence of cyanobacteria blooms will certainly increase in many lakes under future climate scenarios.

Organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source

Abstract: We compared the burial efficiency of organic carbon (buried OC: deposited OC) in a diverse set of 27 different sediments from 11 lakes, focusing on the potential effects of organic matter source, oxygen exposure, and protective sorption of OC onto mineral surfaces. Average OC burial efficiency was high (mean 48%), and it was particularly high in sediments receiving high input of allochthonous organic matter (mean 67%). Further, OC burial efficiency was strongly negatively related to the oxygen exposure time, again particularly so in sediments receiving high allochthonous loads. On the other hand, OC burial efficiency was not related to the mineral surface area, which was used as a proxy of the sorption capacity of the mineral phase for OC. The high OC burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes, as well as to a strong dependence of OC burial efficiency on oxygen exposure time. This study demonstrates that the carbon sink in lake sediments alters the OC export from the continents to the sea and that the fate of OC in lake sediments (burial vs. mineralization to carbon dioxide and methane) is highly sensitive to environmental conditions.

Evidence for geochemical control of iron by humic substances in seawater

Abstract: A new method, based on the direct detection of iron-humic substance (HS) species by cathodic stripping voltammetry (CSV), is used to determine the iron binding capacity and complex stability of fulvic acid (FA), humic acid (HA), and the natural HS in the seawater. The FA binds 16.7 6 2.0 nmol iron (Fe) (mg FA)21, whereas the HA and the marine HS bind 32 6 2.2 nmol Fe (mg FA)21. The complex stabilities are (log K9Fe9HS values) 10.6 for FA and 11.1 for HA and coastal HS. Measurements of coastal waters (Irish Sea) show that the HS occur in a widespread fashion, the HS concentration decreasing with increasing salinity and occurring at levels of 400 (at salinity 30) to 70 (at salinity 34) m gL 21. A sample from the deep Pacific was found to contain 36 m gH S L21, amounting to 4% of the dissolved organic matter. Comparative measurement of the total iron complexing capacity by CSV with competitive ligand exchange showed that the natural HS can account for the entire ligand concentration in the shallow coastal and deep ocean waters tested. Measurements of the iron solubility showed that FA added to seawater and the HS in coastal waters maintain iron in solution at a level just below the iron binding capacity. The preliminary data for the open ocean indicate that the same may be true for HS in deep ocean waters. The data are consistent with a mechanism by which iron is transported from land to sea, associated with land-derived HS. Humic substances (HS) are ubiquitous hydrophobic components of the natural organic matter present in soil and aquatic environments that consist predominantly of polyphenols and benzoic/carboxylic acids (Buffle 1990). HS are operationally divided into humic acids (HAs) and fulvic acids (FAs) on the basis of their solubility, as HAs precipitate at pH 1, while FAs are soluble. Their presence in seawater and remote areas of the oceans is in the range of tens to hundreds of m gH S L 21 (Obernosterer and Herndl

The effects of elevated carbon dioxide concentrations on the metamorphosis, size, and survival of larval hard clams (Mercenaria mercenaria), bay scallops (Argopecten irradians), and Eastern oysters (Crassostrea virginica)

Abstract: We present experiments that examined the metamorphosis, growth, and survivorship of larvae from three species of commercially and ecologically valuable shellfish (Mercenaria mercenaria, Argopecten irradians, and Crassostrea virginica) at the levels of CO2 projected to occur during the 21st century and beyond. Under CO2 concentrations estimated to occur later this century (,66 Pa, 650 ppm), M. mercenaria and A. irradians larvae exhibited dramatic declines (.50%) in survivorship as well as significantly delayed metamorphosis and significantly smaller sizes. Although C. virginica larvae also experienced lowered growth and delayed metamorphosis at ,66 Pa CO2, their survival was only diminished at ,152 Pa CO2. The extreme sensitivity of larval stages of shellfish to enhanced levels of CO2 indicates that current and future increases in pelagic CO2 concentrations may deplete or alter the composition of shellfish populations in coastal ecosystems. The combustion of fossil fuels and the resultant increase in atmospheric CO2 during the past century has had a multitude of effects on this planet, including acidification of the world’s oceans. The oceans have absorbed nearly half of the anthropogenically produced CO2 during the past century (Sabine et al. 2004), altering its inorganic carbon chemistry and pH. Model simulations indicate that combustion of the world’s fossil fuel supply in the coming centuries could result in a fivefold increase in atmospheric CO2 levels to nearly 203 Pa (2000 ppm) and a decrease in surface ocean pH by 0.77 units (Caldeira and Wickett 2003). This decline in the pH of surface waters will concurrently reduce carbonate ion (CO 2{ 3 ) concentrations

Lakes as sentinels and integrators for the effects of climate change on watersheds, airsheds, and landscapes

Abstract: Lakes provide unique sentinels and integrators of events in their catchments and airsheds and in the total landscapes in which they are embedded. A variety of physical, chemical, and biological properties of lakes are amenable to simple, precise, and inexpensive long-term monitoring. Changes to watersheds caused by climate warming can in turn affect the properties of lakes to which they drain. Examples include changes to nutrient inputs, the balance between base cations and strong acid anions, carbon cycles, and mercury, in some cases associated with insect outbreaks and forest fires caused by warmer weather. Paleolimnology also allows integration and interpretation of changes in lakes and catchments for millennia. Such studies indicate that much drier conditions occurred in the past in central and western Canada, causing the closing of lake basins, increased salinity, eutrophication, and even the disappearance of some lakes, as forested catchments were invaded by grasslands. Such historical perspectives indicate that large areas of western Canada may be adversely affected by climate warming.

Downwelling and deep-water bottom currents as food supply mechanisms to the cold-water coral Lophelia pertusa (Scleractinia) at the Mingulay Reef complex

Abstract: In 2006 and 2007, multiple deployments of current meters and optical sensors on landers and moorings were made in the first detailed in situ study of the particle supply to the coral community in the Mingulay Reef complex in the Sea of Hebrides at 140-m water depth. Two distinct and predictable supply mechanisms were resolved. One mechanism consisted of the rapid downwelling of surface water caused by hydraulic control of tidal flow that transports particles from the surface to the corals in less than an hour. The rapid downwelling was recorded on the reef top as a pulse of warm, fluorescent, and relatively clear water at the onset of the flood and ebb tides. The pulse was strongest after flood tide and lasted for up to 3 h. The second mechanism consisted of advection onto the reef of deep bottom water with a high suspended matter load. This advection occurred during peak tides and was combined with topographical current acceleration on the reef top, enhancing delivery of particles to the corals.

Effects of phytoplankton community on production, size, and export of large aggregates: A world-ocean analysis

Abstract: We recorded vertical profiles of size distributions of large particles (<100 µm) to a 1000-m depth in the Atlantic, Indian, and Pacific Oceans and in the Mediterranean Sea with the Underwater Video Profiler. Of the 410 profiles used in our analysis, 193 also included temperature, salinity, and high-performance liquid chromatography (HPLC)-resolved pigments, which were used to characterize the size structure of the phytoplankton community. Classification analysis identified six clusters of vertical profiles of size distributions of particles. Each cluster was characterized by the size distribution of its particles in the mesopelagic layer and the change of the particle-size distribution with depth. Clusters with large particles in the mesopelagic layer corresponded to surface waters dominated by microphytoplankton, and those with small particles corresponded to surface waters dominated by picophytoplankton. We estimated the mass flux at 400 m using a relationship between particle size and mass flux. Principal-component regression analysis showed that 68% of the variance of the mass flux at 400 m was explained by the size structure of the phytoplankton community and integrated chlorophyll a in the euphotic zone. We found that coefficient k in the Martin power relationship, which describes the decrease in the vertical mass flux with depth, varies between 0.2 and 1.0 in the world ocean, and we provided an empirical relationship to derive k from the size structure of phytoplankton biomass in the euphotic zone. Biogeochemists and modelers could use that relationship to obtain a realistic description of the downward particle flux instead of using a constant k value as often done.

Terrestrial organic matter and light penetration: Effects on bacterial and primary production in lakes

Abstract: We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.

The origin of methylmercury in open Mediterranean waters

Abstract: We present high-resolution vertical profiles of methylated mercury (MeHgT 5 monomethylmercury + dimethylmercury) concentrations in the water column of the open and marginal areas of the Mediterranean Sea. MeHgT concentrations ranged from ,0.015 pmol L21 to 0.430 pmol L21 (n 5 130), with maximal concentrations occurring within the maxima of oxygen consumption. Within the most biologically active zone (0–600 m), the MeHgT vertical profiles exhibited a nutrient-like pattern. The MeHgT vs. phosphate relationships were highly significant (p , 0.001), whichever the station. We argue that most of the MeHgT in the water column originates from in situ methylation of inorganic HgII associated with the regeneration of the organic matter. The demethylation of MeHgT as water ages seems responsible for the MeHgT distribution in deep waters. The influences of the methylmercury inputs from the atmosphere, rivers, and coastal or abyssal sediments appear to be locally restricted.

Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification

Abstract: We measured denitrification rates using a field 15N–NO { tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban–urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWden) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO { removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH z concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO { concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO { concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis– Menten equation. Although Uden increased with increasing NO { concentration, the efficiency of NO { removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO { load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO { concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO { concentration.

Quantification of zooplankton trophic position in the North Pacific Subtropical Gyre using stable nitrogen isotopes

Abstract: We quantify the trophic positions of subtropical open-ocean zooplankton species using amino acid–specific (AA) stable nitrogen isotopic compositions. We model animal trophic position by computing trophic 15N enrichment of glutamic acid relative to phenylalanine, and find that trophic position for primary copepod consumers (Oithona spp., Neocalanus robustior) and secondary copepod consumers (Pleuromamma xiphias and Euchaeta rimana) varied little over a 5–10-yr period in the North Pacific Subtropical Gyre (NPSG; mean 6 SD: 2.1 6 0.1 and 2.9 6 0.1, respectively). Comparison of AA 15N enrichment patterns in different copepod species suggests that trophic 15 N enrichment is most consistent in glutamic acid, aspartic acid, and alanine, ‘‘trophic’’ AAs that are intimately involved in the citric acid cycle and energy production. We further test equations involving these trophic AAs and ‘‘source’’ AAs (which appear to retain the nitrogen isotopic composition of the food-web base), and find that such compound-specific models give results that are identical to those calculated using whole-animal (bulk) stable isotopic compositions. However the benefits of our AA-based approach (i.e., the relatively few samples needed for precise TP estimation, elimination of the need for concurrent prey isotopic analyses, and the ability to utilize formalin-preserved specimens from archived collections), make this a powerful technique for the quantitative assessment of trophic position within the pelagic food web. We further discuss how stable isotopic analyses provide a new perspective on the structure of open-ocean food webs and can be used to trace large seasonal fluctuations in nitrogen source in the NPSG.

Death by dissolution: Sediment saturation state as a mortality factor for juvenile bivalves

Abstract: We show that death by dissolution is an important size-dependent mortality factor for juvenile bivalves. Utilizing a new experimental design, we were able to replicate saturation states in sediments after values frequently encountered by Mercenaria mercenaria in coastal deposits (Varagonite 5 0.4 and 0.6). When 0.2-mm M. mercenaria were reared in sediments at Varagonite 5 0.4 and 0.6, significant daily losses of living individuals occurred (14.0% and 14.4% d21, respectively), relative to supersaturated-control sediments (3.9% d21). For 0.4mm M. mercenaria, significant mortality occurred under the most undersaturated conditions (Varagonite 5 0.4, mortality 5 9.6% d21), although mortality at Varagonite 5 0.6 was not significant (mortality 5 2.7% d21; controlsaturated mortality 5 0.2% d21). For the largest size-class investigated, 0.6 mm, we show significant mortality for clams under the most undersaturated sediments (Varagonite 5 0.4, 2.8% d21). To test if buffered sediments would increase survivorship of juvenile bivalves during periods of recruitment, we manually manipulated sediment saturation state by adding crushed Mya arenaria shell to a mud flat in West Bath, Maine, U.S.A. Although we increased the average sediment saturation state within retrieved cores from V 5 0.25 6 0.01 to only 0.53 6 0.06, numbers of live M. arenaria in buffered sediment increased almost three-fold in 2 weeks. Buffering muds against the metabolic acids that cause lowered saturation states may represent a potentially important management strategy to decrease dissolution mortality. Shallow-water deposits can be significantly undersaturated with respect to both calcite and aragonite and dissolution may occur in surface sediments where juvenile

Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake

Abstract: We measured uptake length of 15NO { in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO { uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban–urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO { concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO { uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO { concentrations, showing a loss in removal efficiency in streams with high NO { concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO { removal. The fraction of catchment area as agriculture and suburban– urban land use weakly predicted NO { uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO { uptake lengths via directly increasing both gross primary production and NO { concentration. Gross primary production shortened SWtot, while increasing NO { lengthened SWtot resulting in no net effect of land use on NO { removal.

Simulations of climate effects on water temperature, dissolved oxygen, and ice and snow covers in lakes of the contiguous U.S. under past and future climate scenarios.

Abstract: Daily water temperature, dissolved oxygen (DO) profiles, and ice and snow covers (where applicable) were simulated for 27 types of small lakes (up to 10 km2 surface area) at 209 locations in the contiguous United States under past climate (observed from 1961 to 1979) and for projected doubled atmospheric carbon dioxide (23CO2) climate conditions. A verified, process-oriented, dynamic, and one-dimensional year-round lake water quality model (MINLAKE96) was applied to simulate water temperature and DO profiles continuously in daily time steps over a 19-yr simulation period. This regional lake model has no geographic constraints on the model’s physical and chemical processes, but the climate forcing is a function of geographic location. Model calibration parameters and initial conditions are correlated to lake geometry, trophic state, and location. The 23CO2 climate scenario is projected to increase lake surface temperatures by up to 5.2uC when the climate scenario projects an increase of mean annual air temperature up to 6.7uC. The 23CO2 climate scenario is projected to increase the duration of seasonal summer stratification by up to 67 d, to shorten ice cover by up to 90 d, and to reduce the maximum ice thickness by up to 0.44 m. Under a 23CO2 climate scenario, lake anoxia during the period of ice cover is projected to shorten so that fish winterkill can be avoided, but the periods of hypolimnetic summer anoxia are projected to lengthen. These projected changes will have many significant effects on ecological conditions and aquatic habitats in lakes in the contiguous United States. An increase of atmospheric CO2 and other greenhouse gases causes climate warming, which alters water temperature and dissolved oxygen (DO) in lake waters (Blumberg and Di Toro 1990). These water quality changes have a profound effect on fish habitat (Regier et al. 1990; Magnuson et al. 1990). Water temperatures and DO concentrations in Minnesota lakes under several projected climate scenarios were previously simulated for the open water season (Stefan et al. 1996). The following is a summary and analysis of simulations of long-term average water temperature, DO conditions, and ice and snow cover characteristics in small lakes (up to 10 km2 surface area) over the contiguous United States (U.S.) under past and

Paleolimnological evidence of the effects on lakes of energy and mass transfer from climate and humans

Abstract: The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.

The cold-water coral community as hotspot of carbon cycling on continental margins: A food-web analysis from Rockall Bank (northeast Atlantic)

Abstract: We present a quantitative food-web analysis of the cold-water coral community, i.e., the assembly of living corals, dead coral branches and sediment beneath, associated with the reef-building Lophelia pertusa on the giant carbonate mounds at ~800-m depth at Rockall Bank. Carbon flows, 140 flows among 20 biotic and abiotic compartments, were reconstructed using linear inverse modeling by merging data on biomass, on-board respiration, d15N values, and literature constraints on assimilation and growth efficiencies. The carbon flux to the coral community was 75.1 mmol C m-2 d-1 and was partitioned among (phyto)detritus (81%) and zooplankton (19%). Carbon ingestion by the living coral was only 9% of the carbon ingestion by the whole community and was portioned among (phyto)detritus (72%) and zooplankton (28%). Carbon cycling in the community was dominated by suspension- and filter-feeding macrofauna associated with dead coral branches. Sediment traps mounted on a bottom lander trapped 0.77 mmol C m-2 d-1 (annual average), which is almost two orders of magnitude lower than total carbon ingestion (75.1) and respiration (57.3 mmol C m-2 d-1) by the coral community. This discrepancy is explained in two ways: the coral community intercepts organic matter that would otherwise not settle on the seafloor, and through their action as ecosystem engineers, the increased turbulence generated by the coral framework and organic-matter depletion in the boundary layer augment the influx to the coral community. A comparison of macrofaunal biomass and respiration data with soft sediments reveals that coral communities are hot spots of biomass and carbon cycling along continental margins.

Why are diverse relationships observed between phytoplankton biomass and transport time

Abstract: Transport time scales such as flushing time and residence time are often used to explain variability in phytoplankton biomass. In many cases, empirical data are consistent with a positive phytoplankton-transport time relationship (i.e., phytoplankton biomass increases as transport time increases). However, negative relationships, varying relationships, or no significant relationship may also be observed. We present a simple conceptual model, in both mathematical and graphical form, to help explain why phytoplankton may have a range of relationships with transport time, and we apply it to several real systems. The phytoplankton growth-loss balance determines whether phytoplankton biomass increases with, decreases with, or is insensitive to transport time. If algal growth is faster than loss (e.g., grazing, sedimentation), then phytoplankton biomass increases with increasing transport time. If loss is faster than growth, phytoplankton biomass decreases with increasing transport time. If growth and loss are approximately balanced, then phytoplankton biomass is relatively insensitive to transport time. In analyses of several systems, portions of an individual system, or time periods, apparent insensitivity of phytoplankton biomass to changes in transport time could arise due to the superposition of cases with different phytoplankton-transport time relationships. Thus, in order to understand or predict responses of phytoplankton biomass to changes in transport time, the relative rates of algal growth and loss must be known.

Salt marshes and eutrophication: An unsustainable outcome

Abstract: Most plant production by emergent coastal marshes occurs belowground. This belowground production adds to the accumulation of organic matter sustaining salt marshes as sea level rises, thus preventing excessive flooding, eventual plant death, and habitat loss. The ubiquitous nutrient enrichment of coastal salt marshes stimulating aboveground plant growth may result in higher rates of inorganic matter accumulation that compensates for marsh flooding caused by sea level rise. Results from several short-term experiments, however, demonstrate that root and rhizome biomass and carbon accumulation is reduced with nutrient enrichment, suggesting that eutrophication of coastal waters may not be a compensatory counterbalance to the effects of global sea level rise on salt marshes. We show that the net effects of 36 yr of nutrient enrichment in replicated field experiments do not lead to higher organic or inorganic accumulation. Enrichment reduces organic matter belowground and may result in a significant loss in marsh elevation equivalent to about half the average global sea level rise rates. Sustaining and restoring coastal emergent marshes is more likely if they receive less, not more, nutrient loading.

Simulation of multiannual thermal profiles in deep Lake Geneva: a comparison of one-dimensional lake models.

Abstract: In this study, we report on the ability of four one-dimensional lake models to simulate the water temperature profiles of Lake Geneva, the largest water body in Western Europe, over a 10-yr period from 1996 to 2005, using lake models driven by a common atmospheric forcing. These lake models have already demonstrated their capability of reproducing the temperature distribution in smaller lakes and include one eddy-diffusive lake model, the Hostetler model; a Lagrangian model, the one-dimensional Dynamic Reservoir Simulation Model "DYRESM" a к - e turbulence model, "SIMSTRAT"; and one based on the concept of self-similarity (assumed shape) of the temperature-depth curve, the Freshwater Lake model "FLake." Only DYRESM and SIMSTRAT reproduce the variability of the water temperature profiles and seasonal thermocline satisfactorily. In layers in which thermocline variability is greatest, the temperature root mean square error is ≪2°C and 3°C (at the time of highest stratification) for these models, respectively. It is possible to apply certain one-dimensional lake models that simulate the behavior of temperature to investigate the potential future warming of the water column in Lake Geneva. Importantly, the metalimnion boundary is successfully modeled, which represents an encouraging step toward demonstrating the feasibility of coupling biogeochemical modules, such as, for example, a phytoplanktonic model, to assess the possible biological responses within lakes to climate change.

Photophysiology in two major Southern Ocean phytoplankton taxa: Photoprotection in Phaeocystis antarctica and Fragilariopsis cylindrus

Abstract: Phytoplankton communities in the Ross Sea, Antarctica, are characterized by the haptophyte Phaeocystis antarctica Karsten, which dominates deep mixed layers, and diatoms, including Fragilariopsis cylindrus Grunow, that thrive in shallower mixed layers. To investigate whether differences in photoprotective strategies explain these distributions, photosynthetic parameters, pigments, and fluorescence properties were measured in cultures grown under several irradiance regimes and during acclimation to increased irradiance. In P. antarctica, cellular concentrations of all pigments declined with increasing growth irradiance under continuous light, but xanthophyll cycle pigment concentrations increased with increasing irradiance under dynamic conditions without changes in chlorophyll. In contrast, F. cylindrus exhibited declines in chlorophyll cell−1 with increasing irradiance under both continuous and dynamic conditions, but xanthophyll cycle cell21 pigments increased under continuous irradiance and declined under dynamic irradiance. P. antarctica did not exhibit non-photochemical quenching (NPQ) unless exposed to irradiance in excess of the mean growth irradiance. F. cylindrus exhibited NPQ in response to lower irradiances but displayed less photoinhibitory quenching than P. antarctica after exposure to very high irradiance. Inhibitor experiments suggest that both taxa rely upon xanthophyll cycle photoprotection to maintain photosynthetic performance but only P. antarctica relies heavily upon protein synthesis, presumably for D1 protein repair. F. cylindrus can thrive in shallow mixed layers because its high capacity for heat dissipation minimizes photoinhibition. P. antarctica utilizes xanthophyll cycle photoprotection to a lesser degree, but is able to dominate deeper mixed layers by effectively repairing the photodamage incurred when it is mixed to the surface.

Production of nitrogen gas via anammox and denitrification in intact sediment cores along a continental shelf to slope transect in the North Atlantic

Abstract: We measured the production of N2 gas from anammox and denitrification simultaneously in intact sediment cores at six sites along a transect of the continental shelf (50 m) and deeper slope (2000 m) in the North Atlantic. Maximum rates of total N2 production were measured on the shelf and were largely due to denitrification, with anammox contributing, on average, 33% of this production. On the continental slope, the production of N2 gas decreased but the proportion due to anammox reached a maximum of 65%. This change in both amount and dominant pathway of N2 production could be explained largely by the concentration of organic carbon at each site. With increasing carbon the total production of N2 increased rapidly while the response of anammox was not significant. On the continental slope, total N2 production fell below 2 mmol N m22 h21 and anammox was strongly related (r 5 0.95) to denitrification but the relative magnitude of anammox to denitrification (1.65 : 1) suggested that anammox could not be fuelled by NO { from denitrification alone. On the shelf, however, where total N2 production was predominantly greater than 2 mmol N m22 h21, no relationship between anammox and denitrification was found and anammox remained constant at 1.4 mmol N m22 h21. Despite the constancy and greater availability of NO { and lower temperatures on the continental slope, the significance of anammox to the

Nonlinear response of dissolved organic carbon concentrations in boreal lakes to increasing temperatures

Abstract: Recent increases in concentrations of dissolved organic carbon (DOC) in lakes and rivers over large regionshave been related to both changes in the climate and in atmospheric deposition chemistry. ...

Ocean acidification exacerbates the effect of UV radiation on the calcifying phytoplankter Emiliania huxleyi

Abstract: National Basic Research Program of China [2009CB421207]; National Natural Science Foundation of China [40676063, 40876058]; Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET, Argentina) ; Shantou University and by the European Project on Ocean Acidification (EPOCA), through the European Community's 7th Framework Programme [211384]

Effect of Saharan dust inputs on bacterial activity and community composition in Mediterranean lakes and reservoirs

Abstract: We assessed the effects of Saharan dust inputs of particulate matter (PM), total phosphorus (TP), total nitrogen, and water soluble organic carbon (WSOC) on bacterial abundance (BA) in two alpine lakes and two reservoirs in the Mediterranean region. We also experimentally assessed the effects of dust inputs on bacterial activity and community composition and explored the presence of airborne bacteria. We found synchronous BA dynamics at least in one of the study years for each corresponding pair of ecosystems, suggesting an external control. The link between BA dynamics and inputs of PM, WSOC, or TP occurred only in those ecosystems with severe P-limitation and low dissolved organic carbon. The response was most intense in the most P-limited ecosystem. Dust addition had a significant positive effect on bacterial growth and abundance, but not on richness, diversity, or composition of the indigenous bacterial assemblages. We also obtained experimental evidence that some airborne bacteria could develop in oligotrophic waters by observing the growth of gamma-proteobacteria, a group poorly represented in natural aquatic environments.

The optical characterization of chromophoric dissolved organic matter using wavelength distribution of absorption spectral slopes.

Abstract: We use a new approach to characterize the absorption characteristics of chromophoric dissolved organic matter (CDOM) in standard solutions and in several tropical lake ecosystems (Lake Victoria, Lake Tanganyika, Laguna Ibera ´ ). Compared with the common methods where a single slope of CDOM absorption spectrum is determined over a broad spectral region, we estimate the variations in spectral slope as a function of wavelength from 200 to 700 nm. Hence, our results show a distribution of spectral slope, referred to as the spectral slope curve, Sl, which was used to identify similarities between lakes and standard solutions as well as to show the wavelength intervals in which photodegradation modifies spectral slope. In contrast to the plethora of wavelength intervals and ratios presented in the CDOM literature, a comparison of spectral slope curves between ecosystems provides a potentially important tool to examine the characteristics of CDOM in a consistent manner.