bluuebunny/crossref_metadata_2025 huggingface dataset:
How to surprise a copepod: Strike kinematics reduce hydrodynamic disturbance and increase stealth of suction‐feeding fish
Authors: Holzman Roi , Wainwright Peter C.
Published: January 2011

To capture prey with suction, fish must get sufficiently close to their prey to allow the suction flow to overwhelm the prey and draw it into the mouth. Both swimming towards the prey and suction flow create a hydrodynamic disturbance, which can elicit an escape response by the prey. Using particle image velocimetry, we measured flow speeds and derived fluid deformation rates at the location of the prey as bluegill sunfish fed. In front of the mouth, flows had a composite time‐dependent nature. First, the bow wave pushed water away from the fish, but when the mouth opened and suction commenced, flow reversed and water deformation rates increased rapidly. Our inferences indicate that, at the prey, the approaching bluegill is detected primarily based on its suction‐induced disturbance, rather than its bow wave‐induced disturbance. A comparison of suction‐induced disturbance with the signal produced by active suspension feeders indicated that fish are able to produce a more subtle disturbance than expected based on their flow speeds and mouth size alone. Jaw protrusion and the rapid opening of the mouth during the strike both help to minimize the signal available to the prey. We propose that the temporally quick strikes and high jaw protrusion that are seen in many zooplanktivorous teleosts represent adaptations that minimize the time available to prey for executing an escape response.
A model of degradation and production of three pools of dissolved organic matter in an alpine lake
Authors: Miller Matthew P. , McKnight Diane M. , Chapra Steven C. , Williams Mark W.
Published: January 2011

We investigated the transport and production of dissolved organic matter (DOM) in an alpine lake in the Colorado Front Range during snowmelt and the summer ice‐free season by employing a new approach for distinguishing between three pools of DOM based on fractionation and spectroscopic characterization. Reactive transport modeling results confirm that terrestrially derived sources of humic DOM are dominant during snowmelt and that microbially derived humic and nonhumic DOM are produced in the lake in the summer, when rates of primary productivity are highest. DOM characterization and modeling results support the interpretation that losses of terrestrially derived humic DOM are dependent on photochemistry and indicate that the decay of nonhumic and microbially derived humic DOM is more influenced by heterotrophic degradation. Results suggest that production of nonhumic DOM can be directly related to chlorophyll a concentrations and that microbially derived humic DOM can be produced through condensation reactions. Furthermore, fluorescence and parallel factor analysis (PARAFAC) of DOM suggest that the rate of decay of microbially derived humic DOM decreases as the redox state of quinone‐like moieties of the DOM becomes more oxidized. This study quantifies the influence of DOM source on the chemical and biological reactivity of DOM in lakes. The methods presented in this study provide a framework for testing hypotheses related to the effects of a changing climate on lake ecosystem structure and function.
Multidecadal climate oscillations detected in a transparency record from a subtropical Florida lake
Authors: Gaiser Evelyn E. , Deyrup Nancy D. , Bachmann Roger W. , Battoe Larry E. , Swain Hilary M.
Published: January 2011

Synchronous interannual variability in water transparency observed in neighboring lakes has been linked to regional precipitation and resultant runoff of dissolved organic material, but many climate forcings oscillate over time scales longer than most limnological records can detect. A strong relationship (R2 5 0.86) between transparency and the previous two years' rainfall and lake stage in a 25‐yr record from a Florida lake enabled us to hindcast transparency from a longer 75‐yr record of rainfall and lake stage. Predictions revealed a ~30‐yr cycle in transparency linked to the Atlantic Multidecadal Oscillation (AMO). Transparency was greatest (4–8 m) in the cool phase of the AMO (~1962–1993) associated with below‐average rainfall in south Florida and lowest (0.1‐ 3.0 m) during two warm phases (~1932–1961, 1994–present) associated with above‐average, but more variable, annual rainfall. Models that predict effects of large‐scale hydrologic restoration projects on solute export from South Florida's expansive wetlands need to account for recent entry into a warm AMO phase, where teleconnections between the AMO phases and runoff are opposite of those shown for the U.S. interior.
Seasonal variability and controls on chromophoric dissolved organic matter in a large river‐dominated coastal margin
Authors: D'Sa Eurico J. , DiMarco Steven F.
Published: January 2011

The spring‐to‐summer seasonal transition in the spatial and vertical distribution of chromophoric dissolved organic matter (CDOM) optical properties was examined during surveys conducted in March, May, July, and August of 2005 in the waters influenced by the Mississippi‐Atchafalaya River system of the northwestern Gulf of Mexico. Strong seasonal river influences and a well‐correlated inverse relationship between CDOM absorption at 412 nm and salinity during spring of 2005 suggests conservative mixing in the water column between the riverine freshwater sources and the oceanic end members. Trends in the CDOM absorption and the spectral absorption slope (S) and deviations from the end‐member conservative mixing line in surface and bottom waters during the summer suggest apparent sources and sinks of CDOM. Elevated CDOM in lower‐salinity surface waters at many stations appears to be linked to high concentrations of dissolved oxygen (DO; >10 mg L−1) and elevated chlorophyll concentrations. Effects of photo‐oxidation in surface waters were observed as increasing S and enhanced CDOM loss in the ~27‐32 salinity range and were attributed to seasonal stratification and the increased solar radiation during the summer. In the bottom waters, elevated CDOM levels in the ~32‐36 salinity range were associated with low DO and relatively higher bacterial abundance; this suggests a potential CDOM source due to microbial remineralization of organic matter in the hypoxic zone in bottom waters. Seasonal dynamics associated with CDOM loss in surface waters and CDOM gains in bottom waters may be another pathway influencing hypoxia in coastal waters.
Organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source
Authors: Sobek Sebastian , Durisch-Kaiser Edith , Zurbrügg Roland , Wongfun Nuttakan , Wessels Martin , Pasche Natacha , Wehrli Bernhard
Published: January 2011

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.
Turbulent mixing induced by nonlinear internal waves in Mono Lake, California
Authors: MacIntyre Sally , Clark Jordan F. , Jellison Robert , Fram Jonathan P.
Published: January 2011

Describing the spatial and temporal occurrence of turbulent mixing and the mechanisms of turbulence production are fundamental problems in physical limnology and oceanography. Here we present results from Mono Lake, California, that illustrate pronounced steepening of the pycnocline upon cessation of strong wind forcing and the enhanced turbulence resulting from this nonlinearity. Three events occurred with wind speed of >10 m s−1. Upwind, the upper pycnocline upwelled, but on relaxation of the wind, it downwelled 6–10 m and compressed. The similarity of wave amplitude during compression to mixed layer depth and Lake numbers <4 were indicative of significant nonlinearity. Energy in the internal wave field increased by two orders of magnitude, but ~50% of the energy was lost within 24 h. The largest energy losses occurred at low frequencies rather than via an energy flux from low to moderate and high frequencies. Eddy diffusivities, computed over 6‐d intervals using the heat budget method, averaged ~10−6 m2 s−1 in the energized pycnocline but were five to 10 times higher during the initial upwelling and downwelling periods. Microstructure casts, taken 1‐2 d after the first event, revealed that the percentage of the pycnocline that was turbulent varied with bottom slope, being 80% turbulent where bottom slopes were up to 10%, 60% where bottom slopes were 3%, and less than 7% where bottom slopes were ~0.1%. The threshold for turbulent transport was exceeded only 10% of the time and only where bottom slopes were highest.
Lakes and reservoirs as sentinels, integrators, and regulators of climate change
Authors: Williamson Craig E. , Saros Jasmine E. , Vincent Warwick F. , Smol John P.
Published: January 2011

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.
Lakes as sentinels of climate change
Authors: Adrian Rita , O'Reilly Catherine M. , Zagarese Horacio , Baines Stephen B. , Hessen Dag O. , Keller Wendel , Livingstone David M. , Sommaruga Ruben , Straile Dietmar , Van Donk Ellen , Weyhenmeyer Gesa A. , Winder Monika
Published: January 2011

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 regulators of carbon cycling and climate
Authors: Tranvik Lars J. , Downing John A. , Cotner James B. , Loiselle Steven A. , Striegl Robert G. , Ballatore Thomas J. , Dillon Peter , Finlay Kerri , Fortino Kenneth , Knoll Lesley B. , Kortelainen Pirkko L. , Kutser Tiit , Larsen Soren. , Laurion Isabelle , Leech Dina M. , McCallister S. Leigh , McKnight Diane M. , Melack John M. , Overholt Erin , Porter Jason A. , Prairie Yves , Renwick William H. , Roland Fabio , Sherman Bradford S. , Schindler David W. , Sobek Sebastian , Tremblay Alain , Vanni Michael J. , Verschoor Antonie M. , von Wachenfeldt Eddie , Weyhenmeyer Gesa A.
Published: January 2011

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.
Modeling lakes and reservoirs in the climate system
Authors: MacKay M. D. , Neale P. J. , Arp C. D. , De Senerpont Domis L. N. , Fang X. , Gal G. , Jöhnk K. D. , Kirillin G. , Lenters J. D. , Litchman E. , MacIntyre S. , Marsh P. , Melack J. , Mooij W. M. , Peeters F. , Quesada A. , Schladow S. G. , Schmid M. , Spence C. , Stokesr S. L.
Published: January 2011

Modeling studies examining the effect of lakes on regional and global climate, as well as studies on the influence of climate variability and change on aquatic ecosystems, are surveyed. Fully coupled atmosphere‐land surface‐lake climate models that could be used for both of these types of study simultaneously do not presently exist, though there are many applications that would benefit from such models. It is argued here that current understanding of physical and biogeochemical processes in freshwater systems is sufficient to begin to construct such models, and a path forward is proposed. The largest impediment to fully representing lakes in the climate system lies in the handling of lakes that are too small to be explicitly resolved by the climate model, and that make up the majority of the lake‐covered area at the resolutions currently used by global and regional climate models. Ongoing development within the hydrological sciences community and continual improvements in model resolution should help ameliorate this issue.
Paleolimnological evidence of the effects on lakes of energy and mass transfer from climate and humans
Authors: Leavitt Peter R. , Fritz S. C. , Anderson N. J. , Baker P. A. , Blenckner T. , Bunting L. , Catalan J. , Conley D. J. , Hobbs W. O. , Jeppesen E. , Korhola A. , McGowan S. , RÜhland K. , Rusak J A. , Simpson G. L. , Solovieva N. , Werne J.
Published: January 2011

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.
Lakes as sentinels and integrators for the effects of climate change on watersheds, airsheds, and landscapes
Authors: Schindler D. W.
Published: January 2011

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.
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
Authors: Fang Xing , Stefan Heinz G.
Published: January 2011

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 (2×CO2) 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 2×CO2 climate scenario is projected to increase lake surface temperatures by up to 5.2°C when the climate scenario projects an increase of mean annual air temperature up to 6.7°C. The 2×CO2 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 2×CO2 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.
High Arctic lakes as sentinel ecosystems: Cascading regime shifts in climate, ice cover, and mixing
Authors: Mueller Derek R. , Van Hove Patrick. , Antoniades Dermot , Jeffries Martin O. , Vincent Warwick F.
Published: January 2011

Climate and cryospheric observations have shown that the high Arctic has experienced several decades of rapid environmental change, with warming rates well above the global average. In this study, we address the hypothesis that this climatic warming affects deep, ice‐covered lakes in the region by causing abrupt, threshold‐dependent shifts rather than slow, continuous responses. Synthetic aperture radar (SAR) data show that lakes (one freshwater and four permanently stratified) on Ellesmere Island at the far northern coastline of Canada have experienced significant reductions in summer ice cover over the last decade. The stratified lakes were characterized by strong biogeochemical gradients, yet temperature and salinity profiles of their upper water columns (5‐20 m) indicated recent mixing, consistent with loss of their perennial ice and exposure to wind. Although subject to six decades of warming at a rate of 0.5°C decade−1, these lakes were largely unaffected until a regime shift in air temperature in the 1980s and 1990s, when warming crossed a critical threshold forcing the loss of ice cover. This transition from perennial to annual ice cover caused another regime shift whereby previously stable upper water columns were subjected to mixing. Far northern lakes are responding discontinuously to climate‐driven change via a cascade of regime shifts and have an indicator value beyond the regional scale.
Alpine lake optical properties as sentinels of dust deposition and global change
Authors: Mladenov N. , Lípez-Ramos J. , McKnight D. M. , Rechea I.
Published: January 2011

We characterized dissolved organic matter in La Caldera, an alpine lake in Sierra Nevada (Spain), and water‐soluble organic compounds (WSOC) in dry and wet deposition originating from Saharan and marine air masses using ultraviolet‐visible absorbance and three‐dimensional fluorescence spectroscopy. Molar absorption coefficients at 250 and 280 nm in the lake were highly correlated with those in organic aerosol deposition, originating mainly from Saharan dust, and suggest that absorption in clear alpine lakes in unvegetated catchments may represent a unique sentinel of desertification and global change. Using parallel factor analysis modeling to resolve dominant fluorescent components, we identified a semiquinone‐like fluorophore (C9) in the WSOC of deposition that was traced into the lake. At least three fluorescent components, including C9, contributed significantly to absorption of WSOC from atmospheric deposition. Saharan dust supplied chromophoric, aromatic, and fluorescent organic matter to this alpine lake. In contrast, marine organic aerosols had lower absorption coefficients, lower fluorescence intensity, and more microbial fluorescence properties.
Climate‐related variations in mixing dynamics in an Alaskan arctic lake
Authors: MacIntyre Sally , Fram Jonathan P. , Kushner Paul J. , Bettez Neil D. , O'Brien W. J. , Hobbie J. E. , Kling George W.
Published: January 2011

Mean epilimnetic temperatures from mid‐June through mid‐August in a small, arctic kettle lake had no trend from 1975 to 2008 and varied annually up to ±3°C relative to the mean. Analysis of data from temperature arrays deployed on the lake from 1998 to 2007 showed that as mean summer temperatures shifted from 2.5°C below the mean, to the mean, and to 3°C above the mean, deepening of the mixed layer during cold fronts decreased, average metalimnetic thickness increased from 2 to 5 m, maximum values of water‐column stability increased fourfold, minimum values of Lake numbers (LN) increased from ≤1 to 10, the metalimnetic coefficient of eddy diffusivity (Kz) decreased from 10−5 m2 s−1 to 10−7 m2 s−1, and time scales for mixing across the metalimnion increased from days to months. Mean surface temperatures and mixing regimes were significantly correlated with mean air temperatures, but not with mean insolation, or mean wind speeds during summer. They also depended upon the frequency and persistence of events with higher winds, heating, or cooling. In summers with cold surface temperatures, the surface energy fluxes that induce mixing by heat loss were low but with frequent wind events heat was mixed downwards, leading to lower stability. The warmest surface temperatures resulted when atmospheric conditions led to persistent positive buoyancy flux in early summer and winds were elevated primarily on diel cycles as opposed to longer ones. Summers with cooler water temperatures and enhanced vertical mixing are linked to frontal activity and low atmospheric pressure near the northern Alaskan coast.
The physics of the warming of Lake Tanganyika by climate change
Authors: Verburga Piet , Hecky Robert E.
Published: January 2011

Climate warming over the 20th century has increased the density stratification and stability of Lake Tanganyika, a deep rift valley lake. Here we examine the physical processes involved in and affected by the warming of the lake, and we discuss effects on lake productivity. The rate of net heat absorption by Lake Tanganyika has been 0.4 W m−2 since 1913, twice the rate in the global ocean, indicating stronger climate forcing in the East African region. Lakes warm through increased incoming long‐wave radiation. While lakes in general will increase heat outputs in a warming climate, heat outputs will increase more slowly in deeper lakes than in shallower lakes. Temperatures have increased by 0.2°C at 1000 m in depth, in part because of reduced cool marginal inflows, while water surface temperatures have increased by about 1.3°C. This differential heating over depth has increased the density gradient through the water column, reducing the potential for vertical mixing and thereby limiting nutrient fluxes to the phototrophic zone. An increase in transparency, indicating a reduction in productivity as a result of the reduced vertical mixing, occurred both in Lake Tanganyika and in Lake Malawi, a similar deep tropical lake in which warming has also been documented.
Influence of climate change on algal community structure and primary productivity of Lake Malawi (East Africa) from the Last Glacial Maximum to present
Authors: Castañeda Isla S. , Werne Josef P. , Johnsona Thomas C.
Published: January 2011

Biomarkers of aquatic algae and compound‐specific carbon isotopes are used to examine past variations in algal community composition and primary productivity in Lake Malawi. We note major changes at the Pleistocene‐Holocene boundary. From the Last Glacial Maximum until ~11.8 calendar years before present (kyr B.P.), Lake Malawi was characterized by either low rates of primary productivity or algal productivity that was dominated by a group other than diatoms. At the onset of the Holocene, conditions similar to those of modern Lake Malawi were initiated, with diatoms and nitrogen‐fixing cyanobacteria becoming more important contributors to primary productivity. The transition at ~11.8 kyr B.P. is likely related to a shift in the dominant wind direction over Lake Malawi, resulting from a southward shift in the mean latitudinal position of the Intertropical Convergence Zone during the last glacial. Throughout the 23‐kyr B.P. record, the effects of wind‐induced upwelling are important and may be the main control on the carbon isotopic composition of algal lipids through delivery of 13C‐depleted CO2, derived from organic matter decomposition in deeper waters, to the photic zone. Relationships are also suggested between thermal stratification and primary productivity, with cool conditions at ~12 and 8 kyr B.P. resulting in weaker thermal stratification and increased primary productivity. The opposite situation occurs at 4.9 kyr B.P., when significantly warmer temperatures and decreased algal productivity are observed. Thus, in lentic systems such as Lake Malawi the major influence of climate on algal ecology appears to be through feedbacks in physical circulation mechanisms.
Elevated CO2 increases sensitivity to ultraviolet radiation in lacustrine phytoplankton assemblages
Authors: Sobrino Cristina , Neale P. J. , Phillips-Kress J. D. , Moeller R. E. , Porter J. A.
Published: January 2011

This study tests the effects of elevated CO2 and ultraviolet radiation (UVR) on phytoplankton photosynthesis through in situ incubations in Lake Giles, Pennsylvania. In a first experiment, CO2 was supplied from a tank to simulate atmospheric CO2 concentrations predicted in scenarios of future global change. In a second experiment, elevated CO2 conditions were obtained by the mineralization of added colored dissolved organic matter (CDOM) of terrestrial origin (400 µmol L−1 final concentration). The results demonstrated that for natural assemblages from Lake Giles, atmospheric CO2 concentrations similar to those predicted for the end of the century can increase primary productivity up to 23% in the absence of photoinhibition. However, elevated CO2 concentrations also increased sensitivity of phytoplankton to UVR, making cells more susceptible and increasing photoinhibition. Increased sensitivity was observed in samples incubated with the artificial CO2 supply as well as with the CDOM addition, the latter resulting in CO2 partial pressures close to three times present atmospheric levels. Photosynthetic rate modeled for elevated CO2 and midday solar exposure just below the lake surface was 17% of potential production compared with 21% under usual CO2 levels in the lake, resulting in similar rates between phytoplankton assemblages grown under high and low CO2 levels. Understanding the effect on primary productivity of the interaction between factors that may be affected by global change is essential to predict future changes in ecosystems and climate.
Cyanobacteria dominance: Quantifying the effects of climate change
Authors: Wagner Carola , Adrian Rita
Published: January 2011

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 Müggelsee, 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 cm−2 favored cyanobacteria proliferation within a critical TP concentration range (70‐215 µg L−1). The dominating genera Aphanizomenon, Anabaena, and Microcystis achieved the highest biomass in cases in which total nitrogen concentrations exceeded 1.29 mg L−1, stratified conditions exceeded a duration of 3 weeks, and TP concentrations exceeded 215 mg L−1, 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.
Climate control of the spring clear‐water phase through the transfer of energy and mass to lakes
Authors: Dröscher Iris , Patoine Alain , Finlay Kerri , Leavitt Peter R.
Published: January 2011

We analyzed decade‐long time series of water transparency, algal abundance, zooplankton density, heat income, hydrology, and climate from six polymictic lakes of the northern Great Plains to determine how energy and mass transfers interact to regulate lake structure during the spring clear‐water phase (CWP). Timing (date of occurrence) and intensity (Secchi transparency) of CWP were highly variable among lakes (mean lake‐pair synchrony, S = 0.072, p = 0.53); however, CWP occurred in all lakes when water overlying the sediments reached 16.1°C ± 3.7°C and Daphnia cleared the water column of diatoms. At a decadal resolution, timing and intensity of CWP were correlated strongly and positively (r2 > 0.90, p < 0.05) with the net heat income (θ) and lake volume, but not with other independent lake, catchment, or climate features. Instead, at an annual resolution, the strength of correlations between CWP characteristics and heat income (rCWP‐θ) was itself correlated inversely with the winter index of the North Atlantic Oscillation (r < −0.55, p < 0.05) and with precipitation during February‐April (r < −;0.675, p < 0.05). We conclude that the transfer of energy interacts with lake volume to regulate differences in timing and intensity of CWP among lakes, whereas the transfer of mass overrides energetic regulation to introduce temporal variation in CWP characteristics among years.
Extreme weather events alter planktonic communities in boreal lakes
Authors: Graham Mark D. , Vinebrooke Rolf D.
Published: January 2011

Climate warming has been shown to increase the frequency of extreme weather effects on small lakes by increasing the variability of terrigenic inputs and surface water temperatures. We hypothesized that the effect of thermal variability on boreal plankton depends on dissolved terrigenic matter (i.e., temperature‐terrigenic interaction). A two‐factor mesocosm (1500‐L capacity) experiment consisting of three terrigenic treatment levels (control, [−] runoff, [+] runoff) and three temperature treatment levels (control, warm, and cold) was conducted in triplicate for a total of 27 mesocosms deployed in Lake 302S of the Experimental Lakes Area in Canada. The warming treatment amplified the positive effect of terrigenic amendment on total phytoplankton biomass by stimulating large (>35‐µm Greatest Axial Linear Dimension; GALD) taxa during the 50‐d experiment. In comparison, removal of terrigenic matter increased the abundance of smaller (<35‐µm GALD) phytoplankton along with copepods and cladocerans under cold and warm conditions, respectively. We also attempted to corroborate our experimental findings by comparing planktonic communities collected from reference Lake 239 during climatically contrasting summers between 1970 and 2001. Although planktonic communities in Lake 239 also differed significantly between years characterized by cold, wet vs. warm, dry ice‐free conditions, their responses ran opposite to those detected during the experiment, highlighting the potential overriding importance of other scale‐dependent factors (e.g., fish predation, vertical migration) mediating the effects of climate on lake communities.
Disrupted seasonal clockwork in the population dynamics of a freshwater copepod by climate warming
Authors: Winder Monika , Schindler Daniel E. , Essington Timothy E. , Litt Arni H.
Published: January 2011

Life history responses are expected to accompany climate warming, yet little is known how long‐term effects of climate and environmental change affect the seasonal dynamics of planktonic organisms. We used an historical data set from Lake Washington (U.S.A.) to quantify population responses of a calanoid copepod (Leptodiaptomus ashlandi) to long‐term changes in temperature and resource availability and explore potential mechanisms for the responses. Increasing water temperatures (annual mean increase of 1.5°C in the upper 10‐m water volume) and longer stratification periods (about 4 weeks) were observed between 1962 and 2005, coincident with a pronounced decline in Leptodiaptomus densities. However, production was maintained because of an increase in the production to biomass ratio and a life cycle shift in Leptodiaptomus from an annual to a 6‐month cycle. Cross‐wavelet analyses demonstrated that the annual thermal forcing of copepod recruitment observed during the first two decades of the study weakened substantially, leading to more stochastic population dynamics during the past two decades. This shift from one to two generations per year was most likely produced by a longer and warmer growing period combined with changing fluctuations in resource (phytoplankton) availability. Climate change can lead to higher‐frequency voltinism in ectothermic organisms and to temporal reorganization of their population dynamics.
Response of the invertebrate predator Bythotrephes to a climate‐linked increase in the duration of a refuge from fish predation
Authors: Manca Marina , DeMott William R.
Published: January 2011

Climate warming can affect ecological food chains directly by increasing the rates of physiological processes and indirectly by changing habitat use and altering predator‐prey interactions. In Lake Maggiore, Italy, a greater than 10‐fold increase in the mean annual population density of Bythotrephes longimanus (Cladocera Onychopoda) was recorded between 1987 and 1993, when high values of the North Atlantic Oscillation winter index indicate warmer winter and spring temperatures across Europe. Bythotrephes remained abundant and further increased during the following 10 yr, as water temperature continued to increase. We analyzed changes in water temperature, timing of thermal stratification, and hypolimnion depth to test whether changes in the duration and thickness of a warm, low‐light, deep‐water refuge from fish predation can account for increases in the invertebrate predator's abundance and seasonal duration. Using a 21‐yr data set, we found that the sharpest increase in Bythotrephes abundance coincided with a shift from late August to May in its first appearance in the water column. The appearance of the planktonic population was, in turn, linked with earlier thermal stratification and earlier establishment of the predation refuge. The duration and thickness of the refuge increased as a result of earlier warming and deeper mixing, factors affected by climate rather than decreased water clarity. Daphnia hyalina galeata, the dominant grazer and a prey of Bythotrephes, decreased sharply as Bythotrephes increased. This study provides an example of how climate warming can indirectly affect the population density and phenology of a key invertebrate predator and, hence, the functioning of the pelagic food web.
Nonlinear response of dissolved organic carbon concentrations in boreal lakes to increasing temperatures
Authors: Weyhenmeyer Gesa A. , Karlsson Jan
Published: January 2011

Recent increases in concentrations of dissolved organic carbon (DOC) in lakes and rivers over large regions have been related to both changes in the climate and in atmospheric deposition chemistry. Using a data set of 1041 boreal lakes along a 13° latitudinal gradient, sampled in 1995, 2000, and 2005, and an additional data set of 90 lakes along a 1000‐m altitudinal gradient at 68°N, we show that DOC concentrations increase in a nonlinear way along a latitudinal and altitudinal temperature gradient. The nonlinear relation of DOC to increasing temperatures was consistent over space and time. Out of 14 meteorological, catchment, morphometric, and atmospheric deposition variables tested, the variable best explaining this kind of nonlinear pattern was the number of days when air temperatures exceeded 0°C, i.e., the duration of the main growing and runoff season (DT>0). Using DT>0 as an input variable, we were able to predict the nonlinear temperature response of DOC concentrations, both spatially (R2 = 0.90, p < 0.0001) and temporally (R2 = 0.90, p < 0.0001). DT>0 has an advantage over other variables because it includes the time factor, which is decisive for the duration that biogeochemical processes can take place. We suggest that DOC concentrations in lakes are influenced by climate change and that present temperature increases over Sweden result in an accelerated DOC increase toward warmer geographical regions.
Nitrogen deposition, catchment productivity, and climate as determinants of lake stoichiometry
Authors: Hessen Dag O. , Andersen Tom , Larsen Søren , Skjelkvåle Brit Lisa , de Wit Heleen A.
Published: January 2011

Nearly 1000 Norwegian lakes in catchments with low human activity were surveyed. By covering a wide range of nitrogen (N) deposition (0.1‐2 g m−2 yr−1) along a latitudinal and climatic gradient, we clearly demonstrate how nitrogen (N) deposition, climate, and a few key catchment properties, notably the terrestrial vegetation density and the fraction of bogs, together serve as major predictors of concentrations and ratios of carbon, nitrogen (N), phosphorus (P), and silicate (Si) in downstream lakes. Inorganic N in lakes was positively correlated with N deposition, while organic N was closely associated with allochthonous dissolved organic carbon. The ratio of NO3 to total N as well as NO3 to total P and NO3 to SiO2 were highly variable, and most of this variability was explained by N deposition; terrestrial vegetation density, as inferred from the Normalized Difference Vegetation Index; temperature; runoff; and the fraction of bogs in the catchment. Climate‐induced changes in element concentrations and elemental ratios could profoundly affect the lake metabolism and community composition. By linking these data with downscaled climate change predictions we may also predict future shifts in element export and element ratios in various lakes with reasonable accuracy.
Deciphering the effect of climate change and separating the influence of confounding factors in sediment core records using additive models
Authors: Simpsona Gavin L. , Anderson N. J.
Published: January 2011

We describe a new approach to modeling sediment core records, one that uses additive models (AMs) incorporating a serial correlation structure to model residual autocorrelation. Species assemblages, for example, are reduced to ordination axis scores that capture major changes in the data through time. Each set of axis scores is then modeled using an AM, where covariates represent forcing variables (e.g., tree‐ring‐inferred temperature or proxies for atmospheric deposition) and/or trend and, where necessary, periodic components. The effect of forcing variables on species composition through time can be determined via the contribution each covariate makes to the fitted model, which can be used to separate effects due to competing forcing variables. We illustrate the approach using data from Kassjön, northern Sweden, and Loch Coire Fionnaraich, northwest Scotland.
Climate and CO2 saturation in an alpine lake throughout the Holocene
Authors: Catalan Jordi , Pla Sergi , García Joan , Camarero Lluís
Published: January 2011

This study shows that diatom sediment records can be used to investigate the long‐term inorganic carbon dynamics in oligotrophic and poorly acid‐buffered lakes. Using a training set of 115 high‐mountain lakes in the Pyrenees, we found that both alkalinity and potential hydrogen (pH) independently explained some of the variability in diatom assemblages. Transfer functions for both variables were developed and applied to a Holocene record from Lake Redon and CO2 changes calculated. CO2 saturation broadly followed alkalinity, which in turn was related to summer and autumn air‐temperature fluctuations. In general, warmer climate during the ice‐free period led to higher supersaturation, due to increased alkalinity, which facilitated retention of CO2 from respiration, and decreased primary production (assessed by diatom fluxes). Only during the early Holocene, there were periods of extreme undersaturation, corresponding to cold periods of low alkalinity (<20 microequivalents per liter [µeq L−1]), and suggesting carbon limitation of primary production. The winter and spring climate, which determines the ice cover duration, appears to be relevant for CO2 saturation only during periods when the organic‐matter content of the sediments was low (<22%). Longer periods of ice cover led to lower lake CO2 saturation, suggesting that the ice cover influence on internal nutrient loading may regulate lake productivity fluctuations under low allocthonous nutrient and organic‐matter inputs. Alkalinity ~20 µeq L−1 and sediment organic matter ~22% appear as critical thresholds in the way lake CO2 levels respond to climate fluctuations.
Regulation of spatial and temporal variability of carbon flux in six hard‐water lakes of the northern Great Plains
Authors: Finlay K. , Leavitt P. R. , Wissel B. , Prairie Y. T.
Published: January 2011

Six hard‐water lakes were sampled May‐August for 14 yr in a 52,000 km2 catchment to identify the mechanisms that regulate the spatial and temporal variability of net atmospheric exchange of CO2 of lakes on the Northern Great Plains. Annual mean daily fluxes ranged from ‐100 to >200 mmol C m−2 d−1, while pCO2 values varied between 0.3 and 5500 Pa. We observed periods of net CO22 uptake (1995, 2000) and release (1998, 2006) resulting in synchronous variations in net CO2 flux among lakes. Furthermore, pCO2, pH, and chemical enhancement of CO2 influx all varied coherently among sites. Interannual variation in net CO2 flux and pCO2 was correlated strongly with pH, correlated weakly with other physical and chemical conditions, and was uncorrelated to algal biomass, productivity, or ecosystem respiration. In contrast, spatial variability of water‐column pCO2 was correlated negatively to concentrations of soluble reactive phosphorus, total dissolved nitrogen, pH, and gross primary productivity, suggesting an important role of lake metabolism at large spatial scales. Finally, comparison with an additional 20 saline lakes demonstrated that changes in mean annual pH, pCO2, and CO2 flux during 2002‐2007 were coherent in diverse lakes within a region of >100,000 km2 and suggest that climatic control of pH and pCO2 had an unexpectedly great effect on net CO2 flux through productive hard‐water lake ecosystems.