bluuebunny/crossref_metadata_2025 huggingface dataset:
Initiation of cyanobacterial blooms in a frontal region at the entrance to the Gulf of Finland, Baltic Sea
Authors: Kononen Kaisa , Kuparinen Jorma , Mäkelä Kalervo , Laanemets Jaan , Pavelson Juss , Nõmmann Sulev
Published: May 2010

A 2‐week multidisciplinary study of the physical, chemical, and biological mechanisms controlling the initiation of the late summer blooms of the diazotrophic cyanobacteria, Aphanizomenon flos‐aquae Ralfs and Nodularia spumigena Mertens, in the Baltic Sea was carried out in a frontal region at the entrance to the Gulf of Finland in July 1993. The front is formed by inflowing saltier waters of the northern Baltic proper and outflowing fresher waters from the gulf, and its position and shape are largely controlled by wind conditions. In general, the waters of the northern Baltic proper are less stratified than the outflowing lesssaline waters. At the time of the study, the two major water masses differed in terms of phytoplankton community structure, both at species level and at the level of functional groups. Wind‐induced vertical mixing was instrumental in bringing nutrient pulses to the upper mixed layer in the less‐stratified, high‐saline water mass. Nutrient pulses were followed by enhancement of primary productivity and assimilation number (primary productivity/Chl a) in cyanobacterial (> 20 µm) and flagellate (<20 µm) size fractions. It is proposed that mesoscale blooms of A. flos‐aquae benefit from the nutrient‐pulsing events. Calm weather and solar heating, as reflected by rising temperatures in the upper mixed layer and overriding of water masses in the frontal region, resulted in substantial shallowing of the upper mixed layer, which initiated the bloom of N. spumigena.
Spatial structure and seasonality of decapod crustacean populations on the northwestern Mediterranean slope
Authors: Maynou Francesc , Conan Gérard Y. , Cartes Joan E. , Company Joan B. , Sardà Francisco
Published: May 2010

Two trawl surveys designed for geostatistical analysis were made in spring and fall 1991 on Nephrops norvegicus fishing grounds off Barcelona, Spain. High‐resolution geostatistical techniques‐a new tool for analyzing species assemblages‐were used to map and assess species abundances. Of the 43 decapod crustacean species collected, the spatial structures of the nine most abundant were analyzed and mapped by kriging. The maps of abundance drawn by kriging showed that the populations within the depth range of a species were not spread uniformly but were segregated in areas of high density. The locations of high concentration patches were generally mutually exclusive between pairs of species. The depth‐related zonation of the megafaunal assemblage studied was complemented by a strong alongshore spatial heterogeneity. The spatial structure revealed by the geostatistical analysis, namely local dominance by one or a few species and a spatial pattern of high‐density patches, made it difficult to predict the relative species composition at a given place, thus challenging the concept of a community of ecologically interacting decapod crustaceans over the continental slope. Local factors, such as canyon‐mediated hydrographic processes and sediment resuspension, helped account for the horizontal heterogeneity in the spatial distribution of decapod crustaceans.
Mortality estimation for planktonic copepods: Pseudocalanus newmani in a temperate fjord
Authors: Ohman Mark D. , Wood Simon N.
Published: May 2010

The population surface method for estimating mortality was modified to incorporate diapause of copepodid stage 5 and missing naupliar stages and applied to a 2‐yr time series of the planktonic copepod Pseudocalanus newmani in a temperate fjord. The fitted model captures nearly all of the observed dynamics of the population and yields age‐, stage‐, and time‐specific measures of mortality along with confidence limits. Average mortality rates for eggs and adult females were quite similar, as would be expected for species that carry eggs until hatching. Instantaneous mortality rates of adult males were on average 11–12 times higher than those for adult females. Average mortality rates were high for copepodid 1 (C1), declined in stages C2 through C4, and increased again at C5. This bimodal pattern of mortality with developmental stage probably results from changing susceptibility to different guilds of predators with ontogeny. Mortality rates for C5 and adult females were not significantly different between the overwintering period and the active growth season. Mortality rates of adult female P. newmani varied in a nonlinear manner with the abundance of predators as a result of predator avoidance behavior, underscoring the importance of zooplankton behavior in interpreting population dynamics.
Feeding by a heterotrophic dinoflagellate (Noctiluca scintillans) in marine snow
Authors: Shanks Alan , Walters Keith
Published: May 2010

Marine snow samples were collected on two dates (20 and 27 August 1992) in the San Juan Islands, Washington. On both sampling dates the heterotrophic dinoflagellate Noctiluca scintillans was concentrated in aggregates. On 20 August “small” and “large” aggregates were sampled separately. The data indicate that on this date, all of the N. scintillans were in aggregates and that most were in the large aggregates. About 60% of the N. scintillans contained food, which, given their association with aggregates, likely resulted from feeding in marine snow. On 27 August, 51% of the N. scintillans were in aggregates and 80% of these individuals contained food. Only 20% of the individuals in the water surrounding aggregates contained food. The data suggest that on the two sampling dates, aggregates were the preferred habitat of N. scintillans and that N. scintillans associated with marine snow had greater success feeding than did individuals in the surrounding water. During periods when water‐column concentrations of N. scintillans prey are reduced, marine snow, with its typically high concentrations of phytoplankton, may act as a refuge habitat.
Seiche‐induced mixing: Its impact on lake productivity
Authors: Ostrovsky I. , Yacobi Y. Z. , Walline P. , Kalikhman I.
Published: May 2010

Analysis of temperature and oxygen data at an inshore station in Lake Kinneret (Israel) indicates that cross‐isopycnal mixing occurs at the littoral zone as a consequence of seiche activity. We suggest that nearbottom mixing at the lake periphery is highly important for nutrient supply from the hypolimnion to the epilimnion. Temperature and temperature gradient distribution patterns along a cross‐lake transect indicate horizontal transport via the metalimnion of the mixed water from the littoral toward deeper parts of the lake. Inshore localization of mixing and dilution of the mixed water during its transport cause the hypolimnetic water supply into the epilimnion to be higher nearshore than in the middle of the lake. Addition (5‐10%) of hypolimnetic water to epilimnetic water in laboratory experiments results in enhanced primary production and increased algal biomass. The stimulating effect of hypolimnetic water on primary producers (algae) and the deduced pattern of nutrient‐rich hypolimnetic water supply are consistent with observed spatial distributions of algae and fish (mainly planktivorous), which are more abundant in the lake periphery. We suggest that the import of hypolimnetic waters into the epilimnion enhances biological productivity of the lake.
Ecosystem‐specific patterns in the relationship between zooplankton and POM or microplankton del13C
Authors: del Giorgio Paul A. , France Robert L.
Published: May 2010

Measurements of plankton δ13C from 28 temperate lakes show that zooplankton are depleted in 13C relative to smaller planktonic size fractions. A broad literature survey indicates that this is a general pattern in lakes but not in marine and estuarine plankton communities, where zooplankton are generally enriched in 13C relative to particulate organic matter (POM) or microplankton. Marine plankton thus conform to the assumptions that pelagic food webs are essentially driven by phytoplankton and that POM or small planktonic size classes largely reflect algal carbon, which is transferred to zooplankton with a slight enrichment in 13C. The plankton of lakes and possibly of estuaries, however, do not conform to this expectation. We show that there is a continuum in the δ13C differences between POM and zooplankton from open ocean to coastal areas and estuaries and that a similar pattern occurs from unproductive to highly productive lakes. These differences probably reflect both the degree in POM dilution by nonalgal sources of organic carbon and depthrelated changes in the isotopic signature of phytoplankton in lakes and suggest systematic differences in organic carbon flow pathways among aquatic ecosystems.
Antagonisms between cadmium and zinc toxicity and manganese limitation in a coastal diatom
Authors: Sunda William G. , Huntsman Susan A.
Published: May 2010

Long‐term growth and metal (Mn and Cd) uptake experiments and short‐term uptake kinetic experiments with the coastal diatom Thalassiosira pseudonana revealed antagonistic interactions between toxic metals (Zn and Cd) and the micronutrient manganese. Cd and Zn inhibited algal growth rate only at low Mn ion concentrations, an effect that could be totally accounted for by an inhibition of cellular Mn uptake by the toxic metals and the resultant generation of Mn‐deficient cells. Mn and Zn inhibited cellular Cd uptake, indicating reciprocal effects among the metals with respect to uptake. Saturation kinetics modeling of the uptake data was consistent with all three metals competing with each other for binding to the Mn uptake system and with both Cd and Mn being transported into the cell by that system (uptake of Zn was not measured). Mathematical modeling of Mn and Cd uptake data revealed evidence for a Cd efflux system that was induced only at high cellular Cd concentrations and prevented cellular Cd from reaching toxic levels.
Controls on iron(III) hydroxide solubility in seawater: The influence of pH and natural organic chelators
Authors: Kuma Kenshi , Nishioka Jun , Matsunaga Katsuhiko
Published: May 2010

Iron solubilities of Fe(III) hydroxide in coastal and oceanic waters and in the ultraviolet (UV)‐irradiated seawaters over a pH range of 5.7–8.2 at 20°C were determined by a simple filtration (0.025 µm) involving γ‐activity measurement of 59Fe. At pH ranges of 5.7–7.2 (coastal water) and of 5.7–7.6 (oceanic water), only the Fe(OH)2+ species is significant. The calculated solubility products, log*KSO, for coastal and oceanic waters were 4.8–5.0 and 4.4–4.6, respectively. The solubilities within the pH range of 7.8–8.2 are relatively independent of pH and aging time. Solubility in the oceanic water was about one order of magnitude lower than that in the coastal water, and UV irradiation reduced solubility to ≤0.1 nM. The vertical profiles of ambient Fe(III) solubility (pH 8.0–8.2) in oceanic waters have the following features in common: solubility in the surface mixed layer is high and variable (0.3–0.6 nM), generally corresponding with the depth of high chlorophyll a concentrations; solubility minima (0.15–0.2 nM) occur at a depth of 50–200 m. These results suggest that natural organic Fe(III) chelators exist in significant concentrations and control the dissolved iron concentration in seawaters.
Chernobyl 137Cs used to determine sediment accretion rates at selected northern European coastal wetlands
Authors: Callaway J. C. , DeLaune R. D. , Patrick W. H.
Published: May 2010

Sediment cores were collected from five coastal wetlands along the North Sea (England and Netherlands) and Baltic Sea (Poland). 137Cs dating was used to assess sediment accretion rates, including rates based on the 137Cs peak from the 1986 accident at Chernobyl. Peaks from the Chernobyl fallout were found in cores from the Oder and Vistula Rivers in Poland, from the Eastern Scheldt in the Netherlands, and in one of the two cores from Stiffkey Marsh, U.K. No evidence of Chernobyl fallout was found in cores from Dengie Marsh, U.K. The Chernobyl 137Cs peak serves as an excellent marker for short‐term accretion rates because of its high activity. Vertical accretion rates (cm yr−1) based on 1963 and 1986 peaks were similar at most sites; differences may be due to large inputs of sediment from storms or recent accumulation of organic matter. Large differences in sediment characteristics and accretion rates were found between samples from Poland and western Europe. Vertical accretion rates over the period 1963–1986 ranged from 0.26 to 0.85 cm yr−1 and from 0.30 to 1.90 cm yr−1 over the 1986–1991 period. Vertical accretion rates for the period 1963–1991 were greater than the rates of relative sea‐level rise for all cores, so it does not seem that any of these sites are in imminent danger of excessive flooding. The Chernobyl 137Cs peak will be especially useful for studies of short‐term (i.e. very recent) sedimentation in the near future and for comparisons of sediment processes over different time scales.
Survival of marine larvae under the countervailing selective pressures of photodamage and predation
Authors: Morgan Steven G. , Christy John H.
Published: May 2010

Unlike most zooplankters, larvae of decapod crustaceans and fish often are pigmented and may hatch and ascend to the sea surface during the day. Chromatophores possibly protect these larvae from ultraviolet radiation (UVR), but may increase their visibility to planktivorous fish. We exposed larvae of four species of tropical crabs to sunlight and to planktivorous fish in the field to address this paradox. Most lightly pigmented larvae of three species died from exposure to sunlight withir 1 d, and 94–97.5% of them died after 2 d. The more darkly pigmented Pachygrapsus transversus larvae survived significantly better; only 57% of them died after 2 d of exposure to sunlight. These darkly pigmented larvae survived encounters with fish as well as or better than larvae of two other species that did not have large melanophores. Larval chromatophore systems may block UVR without greatly increasing the visibility of larvae to fish. Larvae that migrate from adult habitats to nursery grounds in surface waters throughout the day likely have chromatophore systems that are effective against both of these countervailing selective pressures; however, increasing UVR may stress larvae. The timing of larval release relative to the diel cycle was not related to the susceptibility of larvae to photodamage.
Inducible aggression and intraspecific competition for space in a marine bryozoan, Membranipora membranacea
Authors: Padilla Dianna K. , Harvell C. Drew , Marks Jessica , Helmuth Brian
Published: May 2010

We experimentally investigated the causes and consequences of stolon production in the marine bryozoan Membranipora membranacea. Inducible stolons appear to mediate contests for space among conspecifics by slowing the growth of adjacent competitors. However, because stolons are produced in some but not all competitive interactions, we examined whether environmental factors such as food and hydrodynamic regime regulate their production. Furthermore, because stolons are ephemeral, persisting for only 24 h, we removed stolons to determine whether stolon production affected the outcome of competitive contests. Removing stolons from large colonies reduced their ability to surround a small neighbor, but increased their growth rate. Control colonies, producing their full complement of stolons, grew more slowly than colonies with stolons removed, suggesting that there is a tradeoff between stolon production and growth. The rate of stolon production was higher in colonies grown under lower food conditions. The likelihood that waterborne chemical or hydrodynamic cues induce stolon formation is diminished by our finding that the frequency of stolon production was unaffected by the relative positions of two interacting colonies in unidirectional flow.
Hydrogen peroxide formation: The interaction of ultraviolet radiation and dissolved organic carbon in lake waters along a 43–75°N gradient
Authors: Scully N. M. , McQueen D. J. , Lean D. R. S.
Published: May 2010

Hydrogen peroxide (H2O2) formation rates (nM h−1), photoproductive capacity (nM W−1 h−1 m−2), and H2O2 formation efficiency (ϕH2O2) were measured on water samples exposed to a standard light source with a spectral composition similar to natural sunlight. Samples were from lakes and wetlands with varying levels of dissolved organic C (DOC), P, Fe, Ca, and pH. The relationship between H2O2 formation rates and DOC was a power function (H2O2 = 49.65 DOC1.71; r2 = 0.94), whereas the relationships between formation rates and DOC fluorescence (DOCFL) (H2O2 = 118.32 + 33.06 DOCFL; r2 = 0.98) and absorption coefficients at 310 (Ka310) were linear (H2O2 = 185.0 + 55.50 Ka310; r2 = 0.91). ϕH2O2 was independent of DOC (r2 = 0.12). Apparent quantum yields decreased with increasing wavelengths (300–400 nm). However, when apparent quantum yields were corrected with solar irradiance data, values were greatest in the UV‐A (320–400 nm) region. The patterns observed were consistent for samples from temperate to arctic regions.
Nitrous oxide emissions to the atmosphere from an artificially oxygenated lake
Authors: Mengis Martin , Gächter René , Wehrli Bernhard
Published: May 2010

Nitrous oxide (N2O) production at the sediment surface of eutrophic Lake Baldegg was quantified with three independent methods: pore‐water samplers, benthic chambers, and mass balances of the aerated‐oxygenated hypolimnion. N2O production at the sediment surface was the most important source in this lake and led to an accumulation in the hypolimnion during summer stratification. Highest rates of N2O emission to the atmosphere (24 µmol m−2 d−1) were observed after the onset of winter overturn, when hypolimnetic water enriched in N2O came in contact with the atmosphere. During summer stratification N2O emissions to the atmosphere decreased to ∼4 µmol m−2 d−1. The winter fluxes are close to the highest reported N2O emissions from marine systems.
Concentrations and uptake of neutral monosaccharides along 14°W in the equatorial Pacific: Contribution of glucose to heterotrophic bacterial activity and the DOM flux
Authors: Rich James H. , Ducklow Hugh W. , Kirchman David L.
Published: May 2010

We examined concentrations and uptake of dissolved neutral monosaccharides (DNMS) in order to determine the contribution of DNMS to heterotrophic bacterial production and to the flux of dissolved organic matler (DOM) in the equatorial Pacific. DNMS concentrations were greater during El Niño‐affected months of February–April 1992 than during August–October 1992; in contrast, glucose turnover was the opposite— turnover was faster in August–October than in February–April. The variation in sugar concentrations and turnover probably resulted from El Niño‐induced changes in primary production; as El Niño waned primary production increased, which appeared to stimulate bacterial activity, especially glucose turnover, that in turn forced down DNMS concentrations. In all months, however, DNMS concentrations were low, especially compared with total dissolved organic carbon concentrations (<1%). Glucose was the dominant neutral monosaccharide and alone supported 15–47% of bacterial production. Other monosaccharides apparently did not support much bacterial growth; concentrations of other sugars were low, as probably was turnover. Respiration of glucose (30–60% of uptake) and mannose (60–90%) was relatively high, suggesting that DNMS supported a large fraction of bacterial respiration as well as biomass production. These results point to the importance of DNMS and glucose in particular in supporting bacterial growth and in contributing to the flux of labile DOM.
Experimental evidence for aging food patches as a factor contributing to high deep‐sea macrofaunal diversity
Authors: Snelgrov Paul V. R. , Grassle J. Frederick , Petrecca Rosemarie F.
Published: May 2010

To determine whether deep‐sea macrofaunal succession is observed in response to small‐scale patches, we deployed colonization trays at 900‐m depth for 23 d and 28 months. Treatments included sediment that was unenriched or enriched with Sargassum natans. Short‐term Sargassum trays contained significantly higher and short‐term unenriched trays significantly lower total densities than the ambient sediment. Both Sargassum treatments attracted high densities of a low‐diversity fauna, but they differed in species composition. Short‐term, unenriched trays attracted low densities of a high‐diversity fauna, and long‐term unenriched trays attracted high densities of a moderately diverse fauna. All treatments showed low faunal affinity with the ambient community. In a 19‐d experiment with S. natans incubated in situ or in sterilized seawater, different species dominated. Thus, differences between 23‐d and 28‐month Sargassum trays may have been related to changes in the food source; a succession of different species colonize aging patches, indicating that patch age may be an important source of microheterogeneity for deep‐sea macrofauna.
Photo‐oxidative production of dissolved inorganic carbon in lakes of different humic content
Authors: Granéli Wilhelm , Lindell Måns , Tranvik Lars
Published: May 2010

Photo‐oxidation of dissolved organic carbon (DOC) to inorganic carbon (DIC) in five oligotrophic south Swedish lakes of different humic content (3.9–19 mg DOC liter−1, 0–140 mg Pt liter−1, Secchi depth 7.6–1.5 m) was investigated. Sterile‐filtered (0.2 µm) water was incubated in UV‐transparent quartz tubes and in tubes covered with aluminum foil. Samples were incubated at different depths (0, 0.20, 0.65, and 2.00 m) from sunrise to sunset (0400 to 2200 hours) in July. Inorganic C was measured before and after incubation. Total plankton respiration in unfiltered lake‐water (production of DIC) was also measured. At the surface, photo‐oxidative DIC production was 86–410 mg C m−3 d−1, while plankton community respiration was 101–274 mg C m−3 d−1. Photo‐oxidation integrated over depth was 44–171 mg C m−2 d−1, while respiration was 201–547 mg C m−2 d−1 to a depth of 2 m and 398–860 mg C m−2 d−1 over the depth of the epilimnion. Depth‐integrated photo‐oxidation was independent of water color or DOC concentration in the lakes. Photooxidation was detected deeper than the penetration of UV‐B radiation, indicating that longer wavelengths (UV‐A and possibly PAR) are also active. DIC production was linearly related to loss of fluorescence (excitation 355 nm, emission 455 nm) in light‐incubated samples. Our study shows that photo‐oxidation of DOC may be an important process causing the regularly observed supersaturation of DIC in lakes.
Density‐driven exchange between the basins of Lake Lucerne (Switzerland) traced with the 3H‐3He method
Authors: Aeschbach‐Hertig W. , Kipfer R. , Hofer M. , Imboden D. M. , Baur H.
Published: May 2010

Lake Lucerne is divided by sills into four major and two minor basins. During winter, differential vertical mixing due to non‐uniform wind exposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradients. These gradients induce density‐driven currents across the sills that contribute significantly to the deep water exchange in the basins and gradually reduce the density gradients during summer.

Over a period of 2 yr (1990–1992), the spatial and temporal evolution of water density and water age (measured by the 3H‐3He method) was investigated. Variation in water age between the different basins and the occasional occurrence of age inversions (older water overlying younger one) can be explained in terms of the observed density distribution and the exchange flows the density gradients cause. Water age provides an integral measure of the magnitude of deep‐water renewal, which in some basins can be roughly separated into contributions from density‐driven currents and wind‐induced vertical mixing.

By correlating water age with dissolved helium and oxygen concentrations, radiogenic helium fluxes and average oxygen depletion rates were determined for the main basins of the lake. The helium flux was between 1 and 2 × 1010 atoms m−2 s−1, indicating dynamic equilibrium with helium production in the underlying crust. Oxygen depletion rates per unit sediment area were roughly equal in all basins (between 220 and 290 g m−2 yr−1), whereas volumetric depletion rates varied from 1.8 g m−3 yr−1 in the deepest basin to 10 g m−3 yr−1 in the shallowest.
Approximation of the size distribution of marine particles by a sum of log‐normal functions
Authors: Jonasz Miroslaw , Fournier Georges
Published: May 2010

A simple algorithm is presented that decomposes the size distribution of marine particles into a sum of log‐normal components of the 0th order. The algorithm was applied to 412 particle‐size distributions in a particle‐diameter range of ∼0.5–200 µm measured by different researchers in various water bodies. A size distribution from this population may have from 1 to 6 log‐normal components. The variability of the number of components reflects the variability in the shape of the size distribution and variations in the size range in which the data are available. The full‐width‐at‐half‐maximum of a component is approximately proportional to the peak diameter of the component. The maximum value of a component is approximately proportional to the inverse of the square of the peak diameter.

Two standard components of the marine particle‐size distribution were identified. The peak diameter (Dpeak), width parameter (σ), and maximum value (FDmax), of the first component are, respectively, 0.66 µm, 0.673, and 1.34 × 104 µm−1 cm−3. These parameters for the second component are 10.5 µm, 0.366, and 3.3 µm−1 cm−3.
Avoidance by Daphnia magna of fish and macrophytes: Chemical cues and predator‐mediated use of macrophyte habitat
Authors: Lauridsen Torben L. , Lodge David M.
Published: May 2010

Recent biomanipulation studies suggest that macrophytes are an important refuge from fish predation for large pelagic zooplankton. We conducted two laboratory experiments that tested the behavioral responses of Daphnia magna to a macrophyte (Myriophyllum exalbescens L.) and a sunfish (Lepomis cyanellus Rafinesque) and whether responses were chemically (for fish) or structurally (for macrophytes) mediated. In the first experiment, we measured Daphnia response to four treatments in separate 38‐liter tanks. In controls without macrophytes and fish, ∼15% of the daphnids were found in the central zone (∼50% of the tank area); the others were found around the tank perimeter (especially in the corners). With macrophytes present, 80% of the daphnids were found in the central zone (unvegetated in all treatments). When fish or fish odor alone were present 35% and 45%, respectively, of the Daphnia occupied the central zone. Thus, chemically mediated avoidance of Lepomis caused Daphnia to increase its occupation of macrophytes. In the second experiment, we tested whether the repellent effect of Myriophyllum resulted from structural characteristics of the macrophyte; the results suggest that both chemical and structural cues contributed to Daphnia avoidance of the macrophyte. Overall, our results are consistent with the suggestion that large pelagic zooplankton may use macrophytes as a refuge in shallow lakes where vertical migration is restricted.
Measuring the sensitivity of southern Wisconsin lake ice to climate variations and lake depth using a numerical model
Authors: Vavrus Stephen J. , Wynne Randolph H. , Foley Jonathan A.
Published: May 2010

The sensitivity of lake ice phenology to climatic variations is tested using a numerical lake‐ice model (LIMNOS). The model simulates the evolution of ice and snow cover by time‐integrating equations of vertical heat conduction through ice and snow. The required input variables are mean lake depth, air temperature and moisture, wind speed, solar radiation, snowfall, and cloudiness.

The model simulates the ice‐on and ice‐off dates of three southern Wisconsin lakes to within 1 week of their historical averages, despite large differences in mean depth. Using hourly meteorological data from 1961–1990 as inputs, LIMNOS simulates the annual ice‐on and ice‐off dates of Lake Mendota with a median absolute error of only 2 d and 4 d, respectively.

The atmospheric variables are altered to determine the sensitivity of Lake Mendota ice phenology to climate change. The simulated ice‐off date shows stronger sensitivity than the ice‐on date to air temperature changes, and the sensitivity of both dates is greater for climatic warmings than coolings. Increased snowfall causes a monotonic delay in the breakup date, whereas decreased snowfall nonlinearly hastens ice decay.
Climate forcing and thermal feedback of residual lake‐ice covers in the high Arctic
Authors: Doran Peter T. , McKay Christopher P. , Adams W. Peter , English Michael C. , Wharton Robert A. , Meyer Michael A.
Published: May 2010

A discontinuous ice coverage and lake temperature record of 36‐yr duration has been compiled for Colour Lake (79°25′N, 90°45′W) on Axel Heiberg Island, Northwest Territories. About once every 6 yr, the ice cover remains through the entire summer, creating a residual ice cover the following winter. Residual ice covers are more frequent in the last 10 yr (1986–1995) than in the 20 yr from 1959 to 1978, indicating a reduction in climate factors controlling ice decay. These factors are identified as the number of thawing degree‐days, suggesting a tendency toward cooler summers over the last decade. We report year‐round meteorology observations that indicate a mean annual temperature of about ‒15.2°C, with ∼500 thawing degree‐days in summer. Following a residual ice year, spring lake‐water temperatures are significantly greater than they are following nonresidual years. The increased temperatures can be attributed to the stabilizing effect of the residual ice pan the previous fall. Without an ice pan, convection cools the entire water column before the surface freezes. When the lake has an ice pan at the start of freezing, the surface freezes over quickly, effectively trapping water heated during summer. The warmer spring water temperatures act as negative feedback, decreasing the potential for a second consecutive residual ice year.
Paleolimnological records of climatic change in North America
Authors: Fritz Sherilyn C.
Published: May 2010

Lacustrine fossil records provide long time series of data on limnological and climatic conditions; these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate‐ecosystem linkages. Shoreline features can indicate past lake‐level fluctuations that may reflect changes in moisture balance, but often these records are discontinuous and are evidence of only extreme conditions. The organisms, geochemistry, and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake‐climate interactions in the past. The most clearly interpretable paleolimnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate. In all cases, hydrologic setting mediates a lake’s response to climate and must be considered in interpreting sedimentary sequences.
Ecosystem processes at the watershed scale: Sensitivity to potential climate change
Authors: Band Lawrence E. , Mackay D. Scott , Creed Irena F. , Semkin Ray , Jeffries Dean
Published: May 2010

A distributed data and simulation system for forested watersheds was used to investigate the potential changes in watershed hydrological and ecological processes under hypothesized climate change scenarios. RHESSys (Regional HydroEcological Simulation System) incorporates a spatial representation of nested catchment and lake systems in a GIS, along with a set of process submodels to compute local flux and storage of energy, water, carbon, and nutrients. A hierarchy of potential climate change shifts in weather, forest canopy physiological processes, and forest cover were used to operate RHESSys for comparison with control simulations for present‐day conditions. Use of projected temperature and precipitation changes alone led to qualitatively different forecasts of watershed climate change impact when compared to simulations that also incorporated adjustment of canopy physiology to elevated concentrations of atmospheric CO2. In addition, ecosystem processes may be more resilient to climate change due to the existence of a series of offsetting effects. Annual net effects on specific processes such as watershed outflow and forest productivity may qualitatively vary from year to year rather than showing consistent increases or decreases relative to current conditions. The model results illustrate the significance of incorporating a reasonable description of terrestrial ecosystem processes within the contributing watershed when assessing the impact of climate change.
Hydrochemical modeling of Emerald Lake watershed, Sierra Nevada, California: Sensitivity of stream chemistry to changes in fluxes and model parameters
Authors: Wolford Ross A. , Bales Roger C.
Published: May 2010

We applied a detailed hydrochemical model of watershed processes to assess the response of the Emerald Lake watershed in the southern Sierra Nevada to changes in inputs of water, chemicals, and energy and to changes in model parameters. Observations during the 1986 and 1987 water‐years were used as base conditions. In 1986 snowfall in the Sierra Nevada was well above average, although chemical fluxes were lower than in 1987, which was a low‐accumulation year. The chemical composition of stream water during the early part of snowmelt was sensitive to chemical loading in the snowpack and to snowmelt rate, both of which are subject to year‐to‐year natural variations and to anthropogenic perturbations. Stream‐water chemical composition was also sensitive to uncertainties in the model, including rate of chemical release from the snowpack, nitrogen reactions, and flow routing in the catchment. The largest effects were limited to a period lasting up to a few days, coinciding with the ionic pulse in spring snowmelt. Some significant effects were also apparent during summer rains. Stream waters in late summer and fall were also affected by a faster snowmelt rate, primarily associated with soil drying and lower late‐season streamflows. Overall, the analysis suggests that the Emerald Lake basin may be particularly sensitive to acidic loadings in precipitation and to faster snowmelt and thus earlier runoff associated with climate warming.
Effects of glacial meltwater inflows and moat freezing on mixing in an ice‐covered antarctic lake as interpreted from stable isotope and tritium distributions
Authors: Miller Laurence G. , Aiken George R.
Published: May 2010

Perennially ice‐covered lakes in the McMurdo Dry Valleys have risen several meters over the past two decades due to climatic warming and increased glacial meltwater inflow. To elucidate the hydrologic responses to changing climate and the effects on lake mixing processes we measured the stable isotope (δ18O and δD) and tritium concentrations of water and ice samples collected in the Lake Fryxell watershed from 1987 through 1990. Stable isotope enrichment resulted from evaporation in stream and moat samples and from sublimation in surface lake‐ice samples. Tritium enrichment resulted from exchange with the postnuclear atmosphere in stream and moat samples. Rapid injection of tritiated water into the upper water column of the lake and incorporation of this water into the ice cover resulted in uniformly elevated tritium contents (>3.0 TU) in these reservoirs. Tritium was also present in deep water, suggesting that a component of bottom water was recently at the surface. During summer, melted lake ice and stream water forms the moat. Water excluded from ice formation during fall moat freezing (enriched in solutes and tritium, and depleted in 18O and 2H relative to water below 15‐m depth) may sink as density currents to the bottom of the lake. Seasonal lake circulation, in response to climate‐driven surface inflow, is therefore responsible for the distribution of both water isotopes and dissolved solutes in Lake Fryxell.
Ultraviolet radiation in North American lakes: Attenuation estimates from DOC measurements and implications for plankton communities
Authors: Williamson Craig E. , Stemberger Richard S. , Morris Donald P. , Frost Thomas M. , Paulsen Steven G.
Published: May 2010

Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV‐B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV‐B and UV‐A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin.

In some regions of North America 25% of the lakes have 1% attenuation depths for UV‐B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV‐B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV‐A radiation are ∼2.5 times as deep as those for UV‐B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.

The strong dependence of 1% attenuation depth on DOC below the 1–2 mg liter‒1 DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.
Consequences of fall warming for zooplankton over wintering success
Authors: Chen C. Y. , Folt C. L.
Published: May 2010

Increased temperatures in fall as predicted by recent climate models will have important consequences for the overwintering strategies and seasonal phenologies of organisms in temperate lakes. We compare fall thermal responses of species representatives of the copepod Epischura lacustris and the cladoceran Daphnia catawba. Daily temperatures and weekly population densities were measured in a lake where these species co‐occur, and demographic variables for each species (survival, reproduction, egg hatching, and production of resting stages) were assessed over a temperature range of 15 to 30°C in the laboratory. This study provides three types of evidence that the success of these species could be altered by fall warming events (temperatures > 15°C). First, resting eggs of E. lacustris could be stimulated to hatch from the “seed bank” in fall. If so, they would be unable to reach maturity before winter, and the cohort could be lost. Second, sexually reproducing D. catawba populations in fall could be induced to switch from sexual to asexual reproduction. This switch could delay production and reduce the number of ephippial resting eggs. Third, on the basis of thermal tolerances, E. lacustris is representative of a species whose geographic range is likely to be altered by warming at any time. Over the long term, the disruption of overwintering strategies by warming in fall could be a mechanism that alters the geographic ranges of species.
Potential habitat loss and population fragmentation for cold water fish in the North Platte River drainage of the Rocky Mountains: Response to climate warming
Authors: Rahel Frank J. , Keleher Christopher J. , Anderson Jeffry L.
Published: May 2010

We used three approaches to examine potential habitat loss in relation to climate warming for cold water species of fish in the North Platte River drainage in Wyoming. The projected loss of habitat varied among approaches, but all methods indicated a noticeable loss of habitat for even minor increases in temperature. An approach based on the use of summer air temperatures to define the thermal limits of cold water species estimated a loss of 9–76% of the present geographic range for temperature increases of 1–5°C. A second approach, also based on air temperature limits, projected a loss of 7–64% of the stream distance currently having thermally suitable habitat for cold water fish for temperature increases of 1–5°C. A third approach, based on the use of summer water temperatures to define the thermal limits of cold water species, projected a loss of 16–69% of the stream distance currently having thermally suitable habitat for temperature increases of 1–5°C. In addition to habitat loss, population fragmentation would occur as remaining enclaves of cold water fish are forced to retreat to increasingly isolated headwater stream reaches.
Grazing‐activated production of dimethyl sulfide (DMS) by two clones of Emiliania huxleyi
Authors: Wolfe Gordon V. , Steinke Michael
Published: May 2010

Emiliania huxleyi clones CCMP 370 and CCMP 373 produced similar amounts of dimethylsulfoniopropionate (DMSP) during axenic exponential growth, averaging 109 mM internal DMSP. Both clones had detectable DMSP lyase activity, as measured by production of dimethyl sulfide (DMS) during in vitro assays of crude cell preparations, but activities and conditions differed considerably between clones. Clone 373 had high activity; clone 370 had low activity and required chloride. For both strains, enzyme activity per cell was constant during exponential growth, but little DMS was produced by healthy cells. Rather, DMS production was activated when cells were subjected to physical or chemical stresses that caused cell lysis. We propose that DMSP lyase and DMSP are segregated within these cells and reaction only under conditions that result in cell stress or damage. Such activation occurs during microzooplankton grazing. When these clones were grazed by the dinoflagellate Oxyrrhis marina, DMS was produced; ungrazed cells, as well as those exposed to grazer exudates and associated bacteria, generated no DMS. Grazing of clone 373 produced much more DMS than grazing of clone 370, consistent with their relative in vitro DMSP lyase activities. DMS was only generated when cells were actually being grazed, indicating that ingested cells were responsible for the DMS formation. We suggest that even low levels of grazing can greatly accelerate DMS production.
Cycling of high‐molecular‐weight dissolved organic matter in the Middle Atlantic Bight as revealed by carbon isotopic (13C and 14C) signatures
Authors: Guo Laodong , Santschi Peter H. , Cifuentes Luis A. , Trumbore Susan E. , Southon John
Published: May 2010

Carbon isotopes (13C and 14C) and elemental composition (C and N) in two fractions of colloidal organic matter (COM) were measured to study the origin and cycling of dissolved organic matter (DOM) in the Middle Atlantic Bight (MAB). COM1 (1 kDa–0.2 µm) was 59% of the bulk DOM in surface Chesapeake Bay waters and decreased to 30–35% in waters of the MAB. COM10 (10 kDa–0.2 µm), which was the high‐molecular‐weight (HMW) component of COM1, comprised 3–12% of the bulk DOM, with highest concentrations in Chesapeake Bay waters and the lowest in deep waters in the MAB. Δ14C values of COM1 decreased from nearshore (‒21 to +12‰) to offshore and from surface (‒166 to ‒85‰) to bottom waters (‒400 to ‒304‰). Although Δ14C values of surface‐water HMW COM10 were generally high (‒2 to ‒7‰), values for bottom‐water COM10 were much lower (‒129 to ‒709‰). The high Δ14C values in the surface water suggest a particulate origin of pelagic COM, consistent with the contemporary Δ14C values of particulate organic matter (POM). The very low Δ14C values of bottom‐water COM10 imply that in addition to the pelagic origin, sedimentary organic C may serve as an important source for the benthic colloids in the bottom nepheloid layer. The general flow direction of organic carbon is from POM to HMW and to LMW DOM. Three colloidal end‐members were identified in the MAB as well as in the Gulf of Mexico: estuarine colloids with high Δ14C values, high C : N ratios, and lower δ13C values; offshore surface water colloids with intermediate Δ14C values, lower C : N ratios, and higher δ13C values; and offshore deep‐water colloids with low Δ14C values, intermediate C : N ratios, and variable δ13C values.
The seasonal cycle of phytoplankton, macronutrients, and the microbial community in a nearshore antarctic marine ecosystem
Authors: Clarke Andrew , Leakey Raymond J. G.
Published: May 2010

Seawater chlorophyll a concentration (size‐fractionated at 20, 2, and 0.2 µm) and temperature have been measured weekly from December 1988 to August 1994 at a nearshore shallow‐water station at Signy Island in the maritime Antarctic. Macronutrients (N, P, Si) were measured monthly, and the microbial community enumerated during a 15‐month period. The duration of winter sea‐ice, summer seawater temperature, and peak chlorophyll biomass all exhibited strong interannual variability. The summer peak of microplankton (>20 µm) chlorophyll was dominated by large diatoms and colonial forms, and there were late summer‐early winter blooms in two of the six seasons examined. Peak concentrations of nanoplankton (20–2 µm) chlorophyll, predominantly from flagellates, were much lower but the bloom lasted longer and winter biomass was higher than in the microplankton. Seasonal macronutrient patterns suggested a preferential utilization of ammonium at the start of spring followed by a major utilization of nitrate by the summer diatom bloom. The microbial community was complex and showed a clear seasonality in all components, although the timing of the summer bloom differed between taxa. Cell numbers were low in winter, increasing in summer with flagellate taxa especially important in late summer. This study emphasizes the complex nature of the nearshore marine ecosystem and highlights the need for year‐round oceanographic studies in the highly seasonal polar marine environment.
Exuvium analysis: A nondestructive method of analyzing copepod growth and development
Authors: Twombly Saran , Burns Carolyn W.
Published: May 2010

Quantitative measurements of individual growth and developmental rates are necessary to study copepod life‐history ecology, population dynamics, and secondary production, yet these measurements are difficult to obtain, particularly for naupliar instars. Copepods molt several times during their life cycle, and the resulting exuvium provides an accurate record of an individual’s size at molting. Newly hatched nauplii of two freshwater calanoid copepods (Boeckella triarticulata and Boeckella hamata) were reared in the laboratory under identical conditions and examined daily for the presence of exuviae, which were used to quantify individual growth and development. Both size and age at successive molts were variable among individuals of each species. Some individuals consistently developed more slowly or more rapidly than others, and individual differences in growth rates also were highly significant. Exuvium analysis provides accurate, quantitative data on individual patterns of growth and development throughout the life cycle.