bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow‐induced uptake of particulate matter in permeable sediments
Authors: Huettel M. , Ziebis W. , Forster S.
Published: May 2010

We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z < 4 cm) of permeable sediments with topography‐related interfacial water flows. The transport is driven by pressure gradients (▵ P ≤ 3 Pa) generated when bottom flows (u ≤ 10 cm s−1) are deflected by small surface structures (z < 3 cm) of hydrodynamical or biological origin. Acrylic pigment grains of 1‐ and 10‐µm diameter traced the intrusion of particulate matter into sandy sediment (k > 2 × 10−11 m2) incubated in a laboratory flume. Increased pressure up‐ and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the core, carrying suspended particles (1 µm) to 2.2‐cm sediment depth within 10 h. Simultaneously, decreased pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z ≤ 10 cm) to the surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation and layers of increased particle concentration. Ripple topography (0.8–2.8 cm high) enhanced interfacial particle (1 µm) flux by a factor 2.3 when compared to a level control core. The pathways of the particle and solute tracers below a sediment ripple are explained with a source‐sink model that describes the pore flow velocity field. Our results suggest that bedform‐induced interfacial flows are important for the uptake of particulate organic matter into permeable shelf sediments.
Atmospheric CO2 fluxes in a highly polluted estuary (the Scheldt)
Authors: Frankignoulle Michel , Bourge Isabelle , Wollast Roland
Published: May 2010

Dissolved CO2 concentration and exchange with the atmosphere were investigated simultaneously in the Scheldt estuary. CO2 partial pressures as high as 5,700 µatm, corresponding to oversaturation with respect to the atmosphere of 1,600%, were observed in the upper estuary. The corresponding atmospheric CO2 fluxes reached values of up to 1.2 mol m−2 d−1. The estimated flux for the entire estuary amounts to 600 t of C d−1 for a river discharge of <1×106 m3 d−1.
The biogeochemical cycling of Zn, Cu, Fe, Mn, and dissolved organic C in a seasonally anoxic lake
Authors: Hamilton‐Taylor J. , Davison W. , Morfett K.
Published: May 2010

Detailed time‐depth distributions of dissolved Zn, Cu, Fe, Mn, and dissolved organic C (DOC), together with pH, were obtained for the water column and near‐surface sediment pore waters of a seasonally anoxic lake (Esthwaite Water, U.K.) over a 1‐yr period. Dissolved Fe and Mn foll owed well‐known patterns linked to redox recycling. The variation in DOC resulted mostly from its coupling with Fe, DOC being coprecipitated with Fe oxide and released into solution on the reductive remobilization of the oxide. This process produced a > 7‐fold seasonal variation in DOC concentrations in the pore waters, with a maximum of ∼7 mg liter−1. Dissolved Zn and Cu exhibited coherent and similar distribution patterns unrelated in any simple way to the distributions of dissolved Fe and Mn, pH, and DOC. The most ubiquitous feature of the distribution of dissolved Zn, and to a lesser degree that of dissolved Cu, was a concentration maximum at the sediment‐water interface, which disappeared completely only when anoxia was maximal in mid‐ and late summer. The interfacial maxima were attributed to rapid release from freshly deposited particles allied to sulfide precipitation in the sediments. The timing of events, associated with an increase in interfacial dissolved Cu and Zn concentrations before and during a period of ice cover, indicated that the Cu was derived from Fe and possibly Mn oxides and Zn from planktonic algae. The cumulative downward diffusive fluxes of Zn and Cu from the interfacial maxima were unimportant 1% of total) compared to total metal sedimentation rates. Chemical equilibrium calculations indicated that Zn‐ and Cu‐sulfide complexes dominated in the pore waters under reducing conditions, with humic binding being insignificant, and that precipitation as pure sulfides or coprecipitation with FeS were likely major removal processes for Zn and Cu under anoxic conditions.
Diagenetic behavior of methylmercury in organic‐rich coastal sediments
Authors: Gagnon Christian , Pelletier Emilien , Mucci Alfonso , Fitzgerald William F.
Published: May 2010

Conditions regulating the formation and diagenetic behavior of methylmercury, CH3Hg(II), in marine sediments are poorly characterized, although understanding these conditons is essential to understanding the modes of transfer of this contaminant to the water column and biota. We report the first high‐resolution profiles of CH3Hg(II) concentrations in pore waters extracted under rigorous oxygen‐free and trace metal clean conditions from organic‐rich contaminated marine sediments. Methylmercury was present at concentrations reaching 10 ng liter−1 as Hg (50 pM) in pore waters extracted from the anoxic sediment layers but was below our detection limit in pore waters recovered from the surficial oxic layers. Dissolved CH3Hg(II) in the anoxic layers accounts for up to 30% of the total dissolved Hg but <1% of the CH3Hg(II) adsorbed onto particles. Our data suggest that the surficial oxic sediment layer serves as a geochemical barrier to the diffusion of dissolved CH3Hg(II) accumulated in anoxic sediments to the overlying waters. Burrowing organisms, however, likely assimilate CH3Hg(II) directly from the underlying anoxic sediments and become active vectors of methylmercury to benthic predators, such as nordic shrimp and cod.
The effects of spatial and temporal variations in acetate and sulfate on methane cycling in two Michigan peatlands
Authors: Shannon Robert D. , White Jeffrey R.
Published: May 2010

In this 3‐yr study, we investigated temporal and spatial changes in pools of pore‐water methane, sulfate, and acetate (a methanogenic substrate), along with corresponding methane fluxes. Acetate accumulated to high concentrations (>500 µM) in the shallow pore waters in winter and spring, indicating that acetogenic bacteria were metabolically active at 2–10°C. High sulfate concentrations in the upper pore waters of one site delayed onset of acetate accumulation, suggesting that acetate consumption by sulfate‐reducing bacteria or competition between H2‐consuming sulfate‐reducers and autotrophic acetogens caused the delay. Sites dominated by grasses and sedges had larger accumulations of acetate; pore‐water pools and fluxes of methane increased rapidly following diminution of the acetate pools at these sites. At a site dominated by Chamaedaphne calyculata, an ericaceous shrub, methane fluxes were lower by 2−3 orders of magnitude; little accumulation of acetate or methane was observed, except in the third year of the study when water levels were consistently high. The seasonal accumulation of acetate and subsequent methane production and flux seem to be dependent on long‐term anaerobic conditions in the upper 20 cm of surface peat.
Mesocosm tracer studies. 1. Zooplankton as sources and sinks in the pelagic phosphorus cycle of a mesotrophic lake
Authors: Lyche Anne , Andersen Tom , Christoffersen Kirsten , Hessen Dag O. , Berger Hansen Paul H. , Klysner Annette
Published: May 2010

The impact of various zooplankton groups on the cycling and retention of phosphorus in a mesotrophic lake in late summer was estimated in a 10‐d mesocosm experiment with addition of 33P. Specific assimilation rates for various dissolved and particulate fractions and individual zooplankton species were estimated by application of tracer models. By means of mathematical inverse methods, these estimates were used to construct a balanced P budget that included the different zooplankton groups.

Altogether, the consumers (including the mixotrophic Dinobryon) were responsible for >60% of the estimated P sedimentation (∼1% d−1). The microconsumers had the largest specific P assimilation (0.28 d−1) and regenerated 45% of assimilated P; Daphnia had an intermediate P assimilation (0.17 d−1), but regenerated <20%, whereas Cyclops assimilated P quite slowly (0.07 d−1) and regenerated 230% of its assimilation of algal P <20 µm, due to its feeding on microconsumers.

The microconsumers, and the daphnids in particular, thus act mainly as sinks for P, whereas the cyclopoids act mainly as sources of P by assimilating and regenerating P otherwise trapped in the microconsumers. The mechanisms responsible for these different roles seem related to growth rate and P content of the consumers relative to that of their diet.
Molecular characterization of suspended and sedimentary organic matter in an Arctic delta
Authors: Peulvé S. , Sicre M.‐A. , Saliot A. , De Leeuw J. W. , Baas M.
Published: May 2010

Curie‐point pyrolysis‐gas chromatography (CuPy‐GC) and Curie‐point pyrolysis‐gas chromatography‐mass spectrometry (CuPy‐GC‐MS) were used to characterize the macromolecular organic content of suspended particles in surface waters, at intermediate water depths, and in the surface sediments of the Lena delta and the adjacent Laptev Sea, eastern Siberia. The chromatograms of the pyrolysates of the Lena River particles revealed several compound classes of biogenic significance: polysaccharides, phenolic substances, and lipid constituents of higher plants. The pyrolysates of the surface‐suspended particles of the Laptev Sea suggested mixed inputs. Some fatty acids, phytadienes, and n‐alkylnitriles were assigned to planktonic organisms. Pyrolysis products from terrigenous material encountered in the river were also present. At intermediate water depths, major differences with respect to the surface particles were evident. In all cases we observed a substantial decrease of the precursors of phytadienes and the fatty acids that undergo biotransformation processes. A peculiar pattern of three series of C14, C16, and C18 alkanes and alkenes [normal alkane, the alk‐1‐ene, the (E)‐alk‐2‐ene, and the (Z)‐alk‐2‐ene] was the prominent feature of these pyrolysates. Further investigations of the particle solvent extract and the corresponding residue suggested that these alkenes, as well as the fatty acids, derived from wax esters. The composition of the sedimentary material in the Laptev Sea indicated preservation of the refractory terrigenous constituents. However, the presence of some algal constituents suggested that a fraction of marine‐derived macromolecules is resistant to degradation.
he swarming behavior of the copepod Dioithona oculata: In situ and laboratory studies
Authors: Buskey Edward J. , Peterson Jay O. , Ambler Julie W.
Published: May 2010

The behavior of the swarm‐forming copepod Dioithona oculata was studied both in situ and in the laboratory using a video‐computer system for motion analysis. In nature, swarms form in light shafts between the prop roots of red mangroves. Swarms maintain their position within these light shafts despite currents of up to 2 cm s−1. In the laboratory, swimming speeds and turning rates of swarming copepods in still water were lower than those observed in the field. Copepods studied in a flowthrough chamber in the laboratory had swimming behaviors similar to those observed in nature; the stimulation from water movement caused increases in both swimming speed and rate of change of direction. Increased current speeds also caused the swarms to become more tightly packed within the center of a vertical light shaft. Nonswarming copepods were unable to maintain their position in a current in darkness. In laboratory experiments, the presence of actively feeding planktivorous fish caused swarms to temporarily disperse due to escape responses of the copepods. However, planktivorous fish were rarely observed feeding on swarms in nature, perhaps due to the presence of predatory fish hiding among the prop roots.
Seasonal dynamics of phytoplankton and Calanus finmarchicus in the North Sea as revealed by a coupled one‐dimensional model
Authors: Carlotti François , Radach Günther
Published: May 2010

A population dynamics model for Calanus finmarchicus was coupled with a one‐dimensional physical and biological upper layer model for phosphate and phytoplankton to simulate the development of the successive stages of Calanus and study the role of these stages in the dynamics of the northern North Sea ecosystem. The copepod model links trophic processes and population dynamics, and simulates individual growth within stages and the changes in biomass between stages.

Simulations of annual cycles contain two or three generations of Calanus and indicate the importance of growth of late stages to total population biomass. The spring peak of zooplankton lags that of phytoplankton by a month due to growth of the first cohort. When compared with observations, the simulation shows a broad phytoplankton bloom and a low biomass of Calanus. A higher initial overwintering stock changes the dynamics of Calanus, but not the annual biomass. The timing of the spring ascent of overwintering individuals influences subsequent dynamics.

Simulations of spring dynamics compared with data obtained during the Fladen Ground Experiment in 1976 show that grazing by Calanus cannot be the only major cause limiting the phytoplankton bloom because development of the first stages of Calanus is slow, and the last copepodite stages arrive after the bloom. Calanus never attains realistic biomasses feeding on phytoplankton as a single food source. These simulations led us to add a compartment of pelagic detritus, which provides another food source to enable Calanus population growth.
Pelagic responses to changes in dissolved organic carbon following division of a seepage lake
Authors: Christensen David L. , Carpenter Stephen R. , Cottingham Kathryn L. , Knight Susan E. , LeBouton Joseph P. , Schindler Daniel E. , Voichick Nicholas , Cole Jonathan J. , Pace Michael L.
Published: May 2010

Within 2 yr of dividing a multibasin lake into discrete lakes for experimentation, dissolved organic carbon (DOC) concentrations and water color (absorption coefficient g at 440 nm, g440) increased in the east basin and decreased slightly in the west basin. These changes were not explainable by watershed vegetation or groundwater chemical composition. However, g440 increased from 0.7 to 4.2 m−1 for water moving through the sediment‐water interface in the east basin. In the west basin, g440 of groundwater (0.6 m−1), in‐seeping water (0.7 m−1), and lake water (0.7 m−1) were all similar. Patterns of DOC distribution matched time trends in the surface waters. In the east basin, DOC concentration doubled and g440 increased 3‐fold from 1990 to 1993. Trends in the west basin were more complex, but in general, there was a decrease in DOC and g440 over the same period. Changes in the light regime affected phytoplankton vertical distribution, but total areal chlorophyll and epilimnetic chlorophyll concentrations were not altered. The depth of oxygenation was changed by altered mixing characteristics and phytoplankton distribution. Changes in light‐attenuating DOC affected pelagic responses to nutrient inputs.
Grazing on filamentous cyanobacteria by Daphnia pulicaria
Authors: Epp Gregory T.
Published: May 2010

I conducted short‐term grazer‐gradient experiments to assess direct mortality effects of Daphnia pulicaria on phytoplankton species in natural lake phytoplankton assemblages that contained 20–80% filamentous cyanobacteria by biovolume. Filamentous cyanobacteria often interfere with feeding by large cladoceran zooplankton such as D. pulicaria and can limit their distribution, yet some D. pulicaria populations coexist with filamentous cyanobacteria. In my trials, D. pulicaria from Oneida Lake and Cazenovia Lake, New York, consumed three species of filamentous cyanobacteria (Aphanizomenon flos‐aquae, Anabaenallos‐aquae, Anabaena wisconsinense) but avoided three other species of Anabaena. Despite high filament concentrations, Daphnia consumed diatoms, flagellates, and chlorococcales and reduced overall phytoplankton abundance. Coexistence of Daphnia and filamentous cyanobacteria in these lakes apparently is facilitated by undeterred consumption of some species and avoidance of others, without substantial interference.
Flexible digestion strategies and trace metal assimilation in marine bivalves
Authors: Decho Alan W. , Luoma Samuel N.
Published: May 2010

Pulse‐chase experiments show that two marine bivalves take optimal advantage of different types of particulate food by varying food retention time in a flexible two‐phase digestive system. For example, carbon is efficiently assimilated from bacteria by subjecting nearly all the ingested bacteria to prolonged digestion. Prolonging digestion also enhances assimilation of metals, many of which are toxic in minute quantities if they are biologically available. Detritus‐feeding aquatic organisms have always lived in environments naturally rich in particle‐reactive metals. We suggest that avoiding excess assimilation of metals could be a factor in the evolution of digestion strategies. We tested that suggestion by studying digestion of particles containing different Cr concentrations. We show that bivalves are capable of modifying the digestive processing of food to reduce exposure to high, biologically available, Cr concentrations. The evolution of a mechanism in some species to avoid high concentrations of metals in food could influence how effects of modern metal pollution are manifested in marine ecosystems.
In vivo substitution of zinc by cobalt in carbonic anhydrase of a marine diatom
Authors: Yee Donald , Morel François M. M.
Published: May 2010

We examined the mechanism by which Co additions partially alleviate the growth limitation of marine diatoms in cultures with low Zn concentrations. Cultures of the coastal diatom Thalassiosira weissflogii were grown under low‐Zn conditions and provided with low and high levels of Co. Enzyme assays and autoradiographs of electrophoresis gels of cellular proteins revealed that a large fraction of the Co is found in the main isoform of the enzyme carbonic anhydrase (CA), where most of the Zn is located. The affinity of CA for Co is lower than for Zn, and the Co‐substituted enzyme is less active than the native Zn form. Comparisons of growth under different partial pressures of CO2 show that Co can ameliorate the growth of carbon‐limited cultures. We conclude that Co alleviates Zn limitation via direct substitution in CA, which is important for carbon acquisition.
Pelagic‐benthic coupling: Profundal benthic community response to spring diatom deposition in mesotrophic Lake Erken
Authors: Goedkoop Willem , Johnson Richard K.
Published: May 2010

Phytodetrital sedimentation, bacterial activity in sediments, and benthic macro‐ and meiofauna abundance were measured to calculate a carbon budget for pelagic‐benthic coupling in Lake Erken. Bacterial activity in surficial sediment was limited by temperature in spring, although a significant correlation with the input of spring‐bloom phytodetritus was noted. Population dynamics of benthic meiofauna during a period of increased food availability in spring were highly taxon‐specific and closely correlated to feeding behavior. Abundances of ostracods and nematodes rapidly increased when phytodetritus became available in hypolimnetic water and surficial sediment, respectively. Considerable time lags (months) were found between phytodetritus deposition and population development of harpacticoid copepods and chydorids.

Carbon budget calculations based on sedimentation data (traps) and surficial sediment C content showed that during spring between 1.1 and 7.2% of deposited phytodetritus is assimilated by benthic meiofauna, whereas the dominant macroinvertebrates, sedentary chironomids, assimilated between 2.4 and 6.0%. On average, between 1.9 and 12.4% of the deposited phytodetritus was mineralized by sediment bacteria. A significant negative correlation was found between abundances of benthic meiofauna and bacteria in the sediment. Chlorophyll a analysis of surficial sediment was a necessary, complementary tool to detect sedimentation that was not detected by the traditional, cylindric sediment traps.
Naupliar and copepodite growth and survival of two freshwater calanoids at various food levels: Demographic contrasts, similarities, and food needs
Authors: Hart Rob C.
Published: May 2010

Various demographic responses of Tropodiaptomus spectabilis and Metadiaptomus meridianus (Copepoda: Calanoida) were measured at 17°C at seven food levels of Cryptomonas (0.05–2.5 mg C liter−1). In both species, naupliar, copepodid, and total postembryonic durations declined monotonically, while survivorship, fecundity, and adult size increased asymptotically with rising food supply. M. meridianus matured faster than T. spectabilis up to ∼0.5 mg C liter−1, and its development responses were saturated at lower food levels (∼0.35 mg C liter−1). High naupliar mortality led to lower overall survival in M. meridianus (maximally 40%) than in T. spectabilis (90%). Survival asymptotes varied between ∼0.35 and 0.5 mg C liter−1. Clutch sizes were 5‐fold larger in M. meridianus than in T. spectabilis. Although respective regression estimates of food threshold levels for maturation (11 and 5), egg production (18 and 60), and population increase (46 and 79 µg C liter−1) in T. spectabilis and M. meridianus did not differ, the derived estimates of interspecific equivalence for egg production and population increase (82 and 165 µg C liter−1) suggest that T. spectabilis may be the better competitor below these food levels.
Recruitment in a temperate demersal fish: Does larval supply matter?
Authors: Levin Phillip S.
Published: May 2010

The importance of larval supply to spatial patterns of larval settlement and recruitment was determined for a common Gulf of Maine fish (the cunner Tautogolabrus adspersus). The number of presettlement pelagic fish differed among the four sites sampled. Inshore sites had significantly greater densities than offshore sites, and this difference was consistent over time. The average size of presettlement fish also differed; fish at inshore sites averaged ∼2 mm less in standard length than those at offshore sites. Settlement was quantified by using artificial substrata suspended above the benthos, and a nonsignificant trend for greater settlement at offshore vs. inshore sites was revealed. Recruitment surveys after the settlement season showed that densities of recruits were significantly greater offshore than inshore. Correlation analyses showed either significant inverse or nonsignificant relationships between densities of presettlement fish and settlement depending on the size of presettlement fish. The relationship between recruitment and densities of presettlement fish was not significant with each of the size classes of presettlement fish analyzed, but there was a strong positive relationship between settlement and recruitment.
Sediment microhabitat effects on carbon stable isotopic signatures of microcosm‐cultured benthic foraminifera
Authors: Chandler G. Thomas , Williams Douglas F. , Spero Howard J. , Xiaodong Gao
Published: May 2010

Environmentally controlled sedimentary microcosms were used to experimentally determine temperature, salinity, and depth‐horizon effects on δ13CDIC profiles of dissolved inorganic carbon (DIC) in sediment pore water (PW) and near‐bottom water (BW). Simultaneously, benthic foraminiferal populations of Ammonia beccarii were cultured and assayed for calcite δ13C to establish how accurately foraminiferal shells recorded BW‐PW δ13CDIC. One treatment population was restricted to the 0–1‐cm sediment depth; in another treatment, the population could freely “roam” up and down in the sediment microcosm. Pore‐water DIC between the uppermost 0–0.5‐cm sediment layer and the overlying BW (i.e. 3 cm above the sediment‐water interface) showed steep gradients of core‐top Δδ13CPW averaging −3.6±1.6‰ at 25°C and −2.6±0.5‰ at 20°C. Correspondingly, shell calcite δ13C for A. beccarii collected from sediment microhabitats recorded a strong PW influence; δ13C shell values averaged 1.1±0.8‰ lighter than BW δ13CDIC but 1.6±0.6% enriched relative to average 0–5‐mm PW δ13CDIC. Foraminifera restricted to the uppermost 1‐cm sediment depth throughout their lives exhibited shell calcite Δδ13C values not significantly different from foraminifera allowed to free‐roam through the sediment column.
Spatial and temporal distribution of dissolved oxygen in Crater Lake,Oregon
Authors: McManus James , Collier Robert , Dymond Jack , Wheat C. Geoff , Larson Gary L.
Published: May 2010

Small temporal and spatial variations in the distribution of dissolved oxygen in Crater Lake, Oregon, are used to estimate the mean age of the lake’s deep water, the flux of labile organic carbon to the deep lake, and the influence of hydrothermal activity on the concentration of dissolved oxygen within the lake. An increase in the concentration of dissolved oxygen in the deep water during winter 1988–1989 indicates that 30–45% of deep water was replaced with well‐oxygenated surface water. This deep‐water renewal corresponds to a mean deep water residence time of 2–4 yr. The deep‐water oxygen consumption rate is 21–26 µmol m−3 d−1, which occurs primarily through the oxidation of organic matter and, to a lesser extent, the oxidation of reduced inorganic species that are introduced to the lake via subsurface hydrothermal springs.
Synoptic study of variations in the fluorescence based maximum quantum efficiency of photosynthesis across the North Atlantic Ocean
Authors: Olaizola Miguel , Geider Richard J. , Harrison William G. , Graziano Lisa M. , Ferrari Giovanni M. , Schlittenhardt Peter M.
Published: May 2010

Using ship‐based phytoplankton fluorescence techniques on a 9‐d transect of the North Atlantic Ocean, we have produced the first synoptic view of variations in the maximum quantum yield of photosynthetic energy conversion (ϕm) on an ocean‐wide basis. Based on our ϕm measurements, the North Atlantic can be divided into two basins. The southeastern basin (from the African coast to the midocean ridge) is characterized by relatively little horizontal variation in ϕm over vast stretches of the ocean. In the northwestern basin (from the midocean ridge to the Canadian coast), we measured lower values of ϕm than in the eastern basin, particularly in the upper water column. Lowest surface values were found in an area of strong horizontal flow. Our results suggest that nutrient supply limits phytoplankton productivity over vast stretches of the North Atlantic in this period (early summer). However, surface values of ϕm across the North Atlantic appear to be independent of nutrient concentration and distance to nitrate supply. We propose that horizontal advection may play an important role in determining the local conditions to which phytoplankton productivity parameters respond. Our results also suggest that an apparent relationship between remotely sensed variables (ocean color, sea surface height) and phytoplankton photosynthetic parameters exists that may provide the basis for the development of new algorithms used to estimate phytoplankton productivity from satellites.
Declines in oxygen concentrations along the Norwegian Skagerrak coast, 1927‐1993: A signal of ecosystem changes due to eutrophication?
Authors: Johannesse Tore , Dahl Einar
Published: May 2010

Every year since 1927 oxygen concentration, temperature, and salinity have been measured at 31 stations along the Norwegian Skagerrak coast during the latter half of September. At all analyzed depths (10 m, 30 m, and bottom water) there have been significant decreases in oxygen saturation all along the coast. In some inner coastal areas, this has led to oxygen deficiency. At intermediate depths (10 and 30 m), there is no trend in oxygen saturation until the middle of the 1960s, after which an almost linear decrease is observed until the 1990s. An explanation for the decreased oxygen saturation in the intermediate layer is increased heterotrophic activity relative to primary productivity. In the bottom water, on the other hand, oxygen saturation did not change until the beginning of the 1970s, when it decreased rapidly to a significantly lower level within a few years and then stabilized at this low level after the middle of the 1970s. The higher oxygen consumption in the bottom water may be due to increased sedimentation of phytoplankton and phytodetritus as a result of greater phytoplankton biomass and, in particular, to less grazing by herbivores. No corresponding changes in meteorological or hydrographical variables were found; we therefore conclude that the decreasing oxygen concentrations are most likely caused by increased nutrient load of the coastal waters. The present evidence suggests that the decrease in bottom‐water oxygen is due to structural changes in the pelagic community.
Accounting for nutrient processing time in mathematical models of phytoplankton growth
Authors: Davidson K. , Cunningham A.
Published: May 2010

In mathematical models, the decoupling of the kinetics of nutrient assimilation and growth under transient conditions is traditionally handled by using either time lags (which are dynamically unsatisfactory) or multiple intracellular nutrient compartments (which are difficult to paramaterize). We use the estimated time required for nutrient processing to calculate the quantity of processed nutrient held within the cells. We then determine growth rate by the mean cell content of processed nutrient. The resulting model successfully simulates the transient growth of nitrogen‐limited cultures of Chlamydomonas reinhardii and Isochrysis galbana. The model can be fully parameterized from relatively simple data sets and generates overshoots and time delays without exhibiting the dynamical instability characteristic of previously published time‐lag formulations.
Flow cytometric determination of bacterial abundance in lake plankton with the green nucleic acid stain SYTO 13
Authors: Giorgio Paul A. del , Bird David F. , Prairie Yves T. , Planas Dolors
Published: May 2010

We have developed a protocol for the cytometric enumeration of bacterioplankton in lakes with the nucleic acid stain SYTO 13; our protocol can be used with any standard bench‐top flow cytometer equipped with an argon laser with excitation at 488 nm. The protocol performs well in the range of bacterial densities from 105 to 106 ml−1. Estimates of bacterial abundance using the cytometric technique ranged from 72 to 141% of the epifluorescence estimates with DAPI staining. The average C.V. of replicate cytometric counts was 2.4%, and the average C.V. for cytometric counts in replicate water samples was 6.7%. The precision of cytometric counts was 10‐fold higher than corresponding epifluorescence bacterial counts, and the sample processing time is decreased by 10–15‐fold relative to the epifluorescence technique. This cytometric protocol can be applied to samples from a wide variety of lakes and offers an accurate, precise, and fast alternative to standard epifluorescence bacterial counts.
Decrease in Daphnia egg viability at elevated pH
Authors: Vijverberg Jacobus , Kalf Dennis F. , Boersma Maarten
Published: May 2010

The effect of high pH on the reproduction of two Daphnia galeata clones was experimentally investigated in the laboratory. We observed that the mortality of juveniles and adults did not increase with increasing pH in the range pH 9.0–10.5, which agrees with what is generally reported in the literature for cladoceran zooplankton and suggests that the threshold value for mortality is between pH 10.5 and 11.5. However, both egg mortality and the proportion of stillborn neonates increased at pH 10.0 and above, and the two clones differed in their sensitivities to pH. Consequently, pH already affects population growth rate markedly from pH 10.0 onward. Because pH values ≥ 10.0 are common during spring and summer in many eutrophic and hypertrophic lakes due to intense photosynthetic activity, we expect that high pH has a much larger effect on the population structure of Daphnia and the community composition of microcrustacean zooplankton in such water bodies than was assumed previously.
Feeding‐deterrent properties of common oceanic holoplankton from Bermudian waters
Authors: McClintock James B. , Swenson Daniel P. , Steinberg Deborah K. , Michaels Anthony A.
Published: May 2010

The feeding‐deterrent properties of a suite of common holoplankton (nine species representing five phyla) collected from oceanic waters near Bermuda were investigated. The common planktivorous fish Abudefduf saxatilus (sergeant major) was used as a model predator. With the exception of the salp Pegea bicaudata, all plankton had significant ichthyodeterrent properties compared to squid tissue controls. These plankton generally lack structural defenses and are passive or sluggish swimmers; our observations suggest that they may derive protection by harboring defensive chemistry. Chemical defenses would have important implications for the regulation of material and energy flux in oceanic food webs, and such defenses may help explain the abundance of coexisting species, all competing for similar resources, in a seemingly homogeneous habitat (i.e. the paradox of the plankton).
Evidence of recent warming and El Niño‐related variations in ice breakup of Wisconsin lakes
Authors: Anderson Wendy L. , Robertson Dale M. , Magnuson John J.
Published: May 2010

Ice breakup dates from 1968 to 1988 were examined for 20 Wisconsin lakes to determine whether consistent interannual and long‐term changes exist. Each ice record had a trend toward earlier breakup dates, as demonstrated by a negative slope with time, indicating a recent warming trend. The average change in breakup dates was 0.82 d earlier per year for the lakes in southern Wisconsin, which was more extreme than that for the northern Wisconsin lakes (0.45 d yr‒1). Interannual variation in breakup dates was related to the warm phase of El Niño/Southern Oscillation (ENSO) episodes. El Niño events occurred five times during this period (1965, 1972, 1976, 1982, and 1986). Average breakup dates were significantly earlier than average (5–14 d) during the mature phase of El Niño. This variability was affected by the location of the lake: El Niño‐related variation was more evident for the southern lakes than the northern lakes. This difference was caused by the average date of breakup for the southern lakes being in late March directly following the period when air temperatures were strongly related to El Niño events, whereas the average dates of breakup of the northern lakes was in mid‐ to late April following a period when air temperatures were not significantly related to El Niño events. Overall, the interannual and long‐term patterns across Wisconsin were relatively consistent, indicating that recent warming and El Niño‐related variation are regional climatic responses.
The hydrologic regime of perched lakes in the Mackenzie Delta: Potential responses to climate change
Authors: Marsh Philip , Lesack Lance F. W.
Published: May 2010

To illustrate potential impacts of climate change on perched or high‐closure lakes in the Mackenzie Delta, we developed and tested a simulation model. Comparison of model output with measured lake level demonstrates that the model simulates the major features of the annual hydrologic regime of high‐closure delta lakes. To demonstrate potential changes in lake regime, we used a 12‐yr empirical data set of temperature, solar radiation, precipitation, and main‐channel water level for the model runs, with the temperature and precipitation modified using results from a 2 × CO2 GCM (general circulation model) run. With these data, modeled water levels for a high‐closure lake decline more rapidly between episodes of flooding, ice covers are thinner, and the ice‐covered season is much shorter than at present. If precipitation were to decrease slightly instead of increasing slightly as suggested by the GCM, the decrease in lake level between flooding episodes is much faster, and a typical high‐closure lake would disappear within 10 yr.
Century scale paleoclimatic reconstruction from Moon Lake, a closed‐basin lake in the northern Great Plains
Authors: Laird Kathleen R. , Fritz Sherilyn C. , Grimm Eric C. , Mueller Pietra G.
Published: May 2010

Estimates of past lake‐water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains. A good correspondence between diatom‐inferred salinity and historical records of mean annual precipitation minus evapotranspiration (P ‒ ET) strongly suggests that the sedimentary record from Moon Lake can be used to reconstruct past climatic conditions. Century‐scale analysis of the Holocene diatom record indicates four major hydrological periods: an early Holocene transition from an open freshwater system to a closed saline system by 7300 b.p., which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate; a mid‐Holocene period of high salinity from 7300 to 4700 b.p., indicating low effective moisture (P ‒ ET); a transitional period of high salinity from 4700 to 2200 b.p., characterized by poor diatom preservation; and a late Holocene period of variable lower salinity during the past 2,200 yr, indicating fluctuations in effective moisture.
Climate change scenarios for Great Lakes Basin ecosystem studies
Authors: Mortsch Linda D. , Quinn Frank H.
Published: May 2010

Significant change in global climate could occur due to human‐induced changes in the chemistry of the atmosphere. We provide a basis for the continuing assessment of potential impacts of climate change on aquatic ecosystems. A series of climate change scenarios have been developed for the Great Lakes Basin using general circulation models (GCMs), climate spatial transpositions, and historical climate analogs. The direct impacts of climate change on the Great Lakes ecosystem would occur through higher air and water temperatures. Indirect climate change impacts include both positive and negative changes in precipitation, decreases riverine runoff, less snowfall and snowpack accumulation, higher evapotranspiration, and a reduction in lake levels and connecting channel flows. These climate and hydrologic changes affect the quantity and quality of wetland and aquatic habitats, alter the frequency and timing of lake turnover, and change dissolved oxygen, and alter fish community composition and dynamics. We provide an integration of Great Lakes climate scenarios. We also illustrate, for the first time, the spatial variability of the climate change scenarios on a tributary river‐basin scale.
Effects of lake size, water clarity, and climatic variability on mixing depths in Canadian Shield lakes
Authors: Fee E. J. , Hecky R. E. , Kasian S. E. M. , Cruikshank D. R.
Published: May 2010

The depth of the summer mixed (Ed) of 21 Canadian Shield lakes in northwestern Ontario was examined in relation to lake size (surface area, Ao) and water clarity (extinction coefficient, Kd) for periods ranging from 2 to 23 yr (n = 1,408). The lakes range in Ao from 4 to 4.9 × 105 ha, and midsummer (mid‐June through mid‐August mean) Kd varied from <0.2 to >2.5 m‒1 with greatest variation in small lakes that were experimentally eutrophicated or acidified or whose terrestrial basins were burned. Over the full spectrum of lake sizes, Ao was the primary determinant of Ed; transparency significantly modified this relationship but only in small lakes (Ao <500 ha). In noneutrophic shield lakes, transparency is controlled by the concentration of dissolved organic C (DOC). Lower DOC concentrations (a likely consequence of 2 × CO2 climate change) would cause transparency to increase, resulting in 1–2‐m‐deeper epilimnia in small lakes; there would be no similar effect in large lakes.
Variations in discharge and dissolved organic carbon and nitrogen export from terrestrial basins with changes in climate: A neural network approach
Authors: Clair Thomas A. , Ehrman James M.
Published: May 2010

We use a neural network approach to examine relationships between climate and geography on discharge and dissolved organic C (DOC) and N (DON) from 15 river basins in Canada’s Atlantic region over a 10‐yr period from 1983 to 1992. Model predictions emphasize the importance of the evapotranspiration‐precipitation link in establishing basin discharge because even large increases in precipitation can lead to decreasing (Q) when accompanied by higher temperatures. Varying temperature and precipitation inputs to the DOC model revealed that both increases in mean monthly temperature and precipitation will only slightly increase exports from river basins, matching the pattern for Q. Surprisingly, lower temperatures and precipitation will also increase the exports. Increases in temperature with decreases in precipitation will lead to large decreases in export. The DON model predictions were not as accurate as DOC but showed similar tendencies.
Sensitivity of vertical mixing in a large saline lake to variations in runoff
Authors: Romero José R , Melack John M.
Published: May 2010

To examine the influences of changes in lake level and salinity on the seasonal mixing regime, we applied a one‐dimensional vertical mixing model to Mono Lake, California. Assuming climate change leads to less precipitation in the Mono basin, we simulated the frequency and duration of meromixis for three hydrological scenarios in which precipitation and runoff were 100, 87.5, and 75% of those recorded for the 50‐yr period from 1940 to 1990 with the assumption of no diversion of streamflow. Simulations indicate that Mono Lake would be susceptible to meromixis over a large range of lake levels for all three scenarios during very large runoff years. Climatic reconstructions and hydrological data also indicate a recent trend of higher interannual variability in precipitation and runoff in the Sierra Nevada. To assess the effect of higher interannual runoff variability on the frequency of meromixis in Mono Lake, we ran the 87.5 and 75% 50‐yr freshwater inputs with no reductions in the eight largest runoff years. A higher incidence of meromixis was predicted when high runoff years were interspersed with drought periods that kept the lake at levels susceptible to long‐term chemical stratification.
Potential methane emission from north‐temperate lakes following ice melt
Authors: Michmerhuizen Catherine M. , Striegl Robert G. , McDonald Michael E.
Published: May 2010

Methane, a radiatively active “greenhouse” gas, is emitted from lakes to the atmosphere throughout the open‐water season. However, annual lake CH4 emissions calculated solely from open‐water measurements that exclude the time of spring ice melt may substantially underestimate the lake CH4 source strength. We estimated potential spring CH4 emission at the time of ice melt for 19 lakes in northern Minnesota and Wisconsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littoral zone sediment type. Regression analyses indicated that lake area explained 38% of the variance in potential CH4 emission for relatively undisturbed lakes; as lake area increases potential CH4 emission per unit area decreases. Inclusion of a second term accounting for the presence or absence of soft organic‐rich littoral‐zone sediments explained 83% of the variance in potential spring CH4 emission. Total estimated spring CH4 emission for 1993 for all Minnesota lakes north of 45° with areas ≥4 ha was 1.5 × 108 mol CH4 assuming a 1 : 1 ratio of soft littoral sediment to hard littoral sediment lakes. Emission estimates ranged from 5.3 × 107 mol assuming no lakes have soft organic‐rich littoral sediments to 4.5 × 108 mol assuming all lakes have soft organic‐rich littoral sediments. This spring CH4 pulse may make up as much as 40% of the CH4 annually emitted to the atmosphere by small lakes.
The effects of climatic warming on the properties of boreal lakes and streams at the Experimental Lakes Area, northwestern Ontario
Authors: Schindler David W. , Bayley Suzanne E. , Parker Brian R. , Beaty Ken G. , Cruikshank Dana R. , Fee Everett J. , Schindler Eva U. , Stainton Michael P.
Published: May 2010

A period of prolonged warmer, drier‐than‐normal weather in northwestern Ontario during the 1970s and 1980s resulted in severe forest fires that caused dramatic changes to lake and stream catchments. The changed interactions of weather with catchments and hydrological processes caused unexpected changes in physical, chemical, and biological processes in lakes and streams. Permanent first‐order streams became ephemeral. Flows at spring melt were lower, and chemical exports from catchments were reduced. Although catchments burned by forest fire had slightly higher flows and chemical exports than unburned basins in the years following fires, chemical exports generally declined due to lower streamflow. Decreased exports of silica indicated lower rates of weathering. Base cation exports also decreased, as did the ratio of base cations to strong acid anions in streams.

Changes in lakes included warmer temperatures, clearer waters, deeper thermoclines and euphotic zones, higher alkalinities, and higher concentrations of base cations and nitrogen, but lower concentrations of dissolved organic C, silica, and P. The increase in alkalinity was caused by increases in the ratio of base cations to strong acid anions, resulting from the interaction of increased water retention, microbial sulfate reduction, and exchanges of cations between water and sediments. Declines in chlorophyll and increases in phytoplankton biomass were observed, but there was no detectable effect on areal phytoplankton production. Summer subthermocline habitats for cold stenotherms were reduced slightly in extent as the results of thermocline deepening and lower hypolimnetic oxygen. There is considerable potential for interaction between climatic change and other human perturbations affecting boreal lakes, including acidification, increased incident UV radiation, eutrophication, and overharvesting.
Effects of natural phenolic material (tannin) on phytoplankton growth
Authors: Herrera‐Silveira Jorge A. , Ramírez‐Ramírez Javier
Published: May 2010

Natural phenolic materials were found to influence the growth of two phytoplankton species in Celestún Lagoon, a tropical coastal lagoon on the northern Yucatan Peninsula (Gulf of Mexico). The input of natural phenolic materials from mangrove forests imparts a reddish color to the water column of the lagoon. Natural water from the lagoon and tannic acid were used to assess the effects of natural phenolic material on the growth of two microalgae, Dunaliella salina and Skeletonema costatum. Both species were inhibited by tannic acid concentrations equivalent to those observed in the lagoon during winter (18 mg liter‒1). The pattern was similar in both species, but cultures of S. costatum achieved higher cell numbers (>400 × 104 cells ml‒1). However, growth of both species at lower concentrations, equivalent to 3 mg liter‒1 of tannic acid in the natural water, was similar to growth in the control assay, suggesting a positive effect of other organic substances on phytoplankton growth. The climatic conditions during the nortes season (November–February) result in high concentrations of natural phenolic materials in the inner zone of the lagoon, which can play an important role in controlling the seasonal changes in phytoplankton and could control the response of the lagoon ecosystem to climate change.