bluuebunny/crossref_metadata_2025 huggingface dataset:
Light dependence of quantum yields for PSII charge separation and oxygen evolution in eucaryotic algae
Authors: Flameling Inez A. , Kromkamp Jacco
Published: May 2010

Quantum yields of photosystcm II (PSII) charge separation (ΦP and oxygen production () were determined by simultaneous measurements of oxygen production and variable fluorescence in four different aquatic microalgae representing three different taxonomic groups: the freshwater alga Scenedesmus protuberans (Chlorophyceae) and the marine algae Phaeocystis globosa (Prymnesiophyceae), Emiliania huxleyi (Prymnesiophyceae), and Phaeodactylum tricornutum (Bacillariophyceae). In S. protuberans, P. tricornutum, and E. huxleyi, light‐dependent variability was observed in the ratio of to ΦP i.e. in the number of oxygen molecules produced per electron generated by PSII. The ratio :ΦP was highly variable at low light intensities (E < 0.5 Ek), and at higher light intensities (E > 0.5 EK) :ΦP showed a nonlinear decrease with increasing light intensity. In contrast, in P. globosa a trend in :ΦP could not be distinguished, and this species showed a decrease in :ΦP during the day, indicating a dependency of :ΦP on light history. Additionally, considerable interspecific quantitative differences in :ΦP were observed. Two possible interpretations to explain the variability in :ΦP are discussed. Assuming that ΦP is a reliable measure of the quantum yield for charge separation at PSII, one interpretation is that net oxygen production is influenced by processes that consume oxygen or affect linear electron transport (e.g. cyclic electron transport around PSII, pseudocyclic electron transport in the Mehler reaction, Rubisco oxygenase activity, and lightdependent mitochondrial respiration). A second interpretation, however, suggests that at saturating light, changes in photosynthesis turnover time occur, such that ΦP does not predict the steady‐state O2 yield.
Nanoscale patchiness of bacteria in lake water studied with the spatial information preservation method
Authors: Krembs Christopher , Juhll Andrew R. , Long Richard A. , Azam Farooq
Published: May 2010

Direct evidence that free‐living bacteria can form small patches in the water column has been unavailable, despite long‐standing hypotheses, because traditional sampling methods destroy the aggregations. The recently developed spatial information preservation (SIP) method sufficiently preserves the two‐dimensional, relative spatial distribution of aquatic microorganisms to resolve patches on the scale of tens to hundreds of micrometers (nanoscale patchiness). We collected samples from lake water using the SIP method. Nanoscale patches were not found in several unmanipulated lake samples. However, we were able to stimulate patch formation by filtering out organisms larger than bacteria and adding freeze‐thawed diatoms. Patchiness developed within 3 h and then dispersed by 16 h after the addition of the algae. Patches did not form in an unfiltered sample that was rotated. The potential for free‐living aquatic bacteria to occur in patches has important implications for aquatic microbiology.
Simultaneous measurement of O2, Mn, Fe, I−, and S(—II) in marine pore waterswith a solid‐state voltammetric microelectrode
Authors: Luther George W. , Brendel Paul J. , Lewis Brent L. , Sundby Bjørn , Lefrançois Lucie , Silverberg Norman , Nuzzio Donald B.
Published: May 2010

A solid‐state Au/Hg voltammetric microelectrode was used to measure, simultaneously and with millimeter spatial resolution, the vertical distributions of O2, Mn2+, Fe2+, HS−, and I− in the pore water of sediments from the Canadian continental shelf and slope. The electrode was used shipboard to analyze undisturbed sediment cores and in a sediment mesocosm to determine the three‐dimensional distribution of redox species in the sediment surrounding an actively irrigated worm burrow. In cores from Emerald Basin and Cabot Strait, O2 disappeared and Mn2+, Fe2+, and HS− appeared in the vertical sequence predicted on the basis of thermodynamics. In a core from the Scotian slope, O2 disappeared at ~14‐mm depth but Mn2+ was not detected over the 50‐mm depth examined with the probe. In all three cores, I− was detected in the pore water below the oxygen‐penetration depth. In none of the cores did the distributions of O2 and Mn2+ overlap. The thickness of the layer within which neither O2 nor Mn2+ could be detected ranged from 8 to 40 mm; we suggest that nitrate, not oxygen, is used to oxidize Mn2+. The upwarddirected Mn2+ gradients of 3.5 and 10 mol mm4 in the Emerald Basin and Cabot Strait sediments, respectively, drive manganese fluxes that are more than one order of magnitude lower than the O2 fluxes, which are estimated at 4‐S mmol mm−2 d−1. The three‐dimensional distributions of solutes in the pore water in the sediment surrounding an irrigated worm burrow demonstrate that both the O2‐penetration depth and the depth where Mn2+ is first detected are deeper in the vicinity of the worm burrow than away from it. This is consistent with the notion that irrigation brings oxygenated water into the worm tube, allowing oxygen to diffuse across the burrow wall into the sediment and react with Mn2+ and other reduced porewater constituents. Three‐dimensional heterogeneity in porewater composition cannot be detected by conventional one‐dimensional coring and slicing techniques. Because these latter techniques average the porewater composition in a large volume of sediment, they may indicate that the distributions of individual species overlap when in fact they do not.
Denitrification in estuarine sediments determined by membrane inlet mass spectrometry
Authors: Kunu Todd M. , Sullivan Matthew B. , Cornwell Jeffrey C. , Groxzkowski Kevin M.
Published: May 2010

Steady‐state and transient‐state denitritication rates were measured in sediment cores from a brackish river of the Chesapeake Bay using high‐precision, membrane‐inlet mass spectrometry. Denitrification was independent of salinity over the range of 1–13 ppt and was directly dependent on nitrate concentration over the range of 0–200 µM in the over‐lying water. Denitrification was observed when the water‐column nitrate concentration was <l µM, indicating that nitrification in the sediments was occurring. There was no detectable lag in the response of denitrification to an abrupt increase in nitrate in the overlying water column; moreover, the enhanced rate under nitrate enrichment was either stable or changed slowly over periods of days. Thus, the microbial flora remained poised to utilize increased nitrate supplies, suggesting that the denitrifiers were facultative. An analysis based on diffusion theory supports a view that denitrification was controlled by properties that affected the physical transport of nitrate from the water to the sites of denitrification. Our results indicate that denitrification in this river can respond rapidly and directly to episodic events that cause changes in watercolumn nitrate concentration.
Factors potentially preventing trophic cascades: Food quality, invertebrate predation, and their interaction
Authors: MacKay Neil A. , Elser Jumes J.
Published: May 2010

Recent laboratory work on food quality constraints on zooplankton growth and reproduction, as well as several examples of weak effects of food‐web manipulations on lower trophic levels in lakes with phosphorus‐deficient phytoplankton, suggests that food quality effects may have currently unappreciated effects on zooplankton success and food‐web interactions under field conditions. We experimentally manipulated two factors that we anticipated might play a role in suppressing Daphnia in P‐limited lakes—the quality of phytoplankton food and the presence of the invertebrate predator Chaoborus punctipennis. We used a two‐factor design, manipulating food source and presence of Chaoborus, and measured growth rate, survivorship, and fecundity of Daphnia rosea neonates incubated at fixed food levels in flow‐through growth chambers. D. rosea grew significantly faster and was significantly more fecund when fed seston from a high‐food quality lake (Lake 979) relative to a treatment fed seston from a low‐food quality lake (Lake 110). Chaoborus reduced survivorship of D. rosea but the food source–predator interaction term was not significant, indicating that invertebrate predation and phytoplankton food quality did not influence Daphnia populations synergistically in this experiment. A second experiment was conducted to determine if variation in Daphnia growth rate and fecundity when fed food of varying quality was caused by a change in feeding rate. Daphnia feeding rate increased with improved food quality, suggesting that Daphnia responds to increases in food quality, at least in part, by increasing feeding rate. We conclude that food quality can strongly affect Daphnia feeding, growth, and reproduction, thereby constraining food‐web dynamics in nutrient‐deficient lakes.
Nutrient recycling by Daphnia reduces N2 fixation by cyanobacteria
Authors: MacKay Neil A. , Elser James J.
Published: May 2010

A field experiment in an experimentally eutrophied lake (Lake 227) tested the hypothesis that differential recycling of nitrogen and phosphorus by Daphnia (a zooplankter with low body N:P ratio) affects the physiological status of cyanobacteria, including rates of N2 fixation (stoichiometric recycling hypothesis). After a 5‐d incubation in 2.4‐liter bottles, Daphnia treatments had lower standing stocks of algae (based on particulate carbon or Chl a) and higher concentrations of dissolved nutrients than did Epischura (a high N:P ratio consumer) or control treatments lacking macrozooplankton. The N:P ratio of dissolved nutrients was higher in the Daphnia treatments than in control or Epischura treatments, consistent with the stoichiometric recycling hypothesis, and was associated with greater algal P deficiency. Measurements of absolute and C‐specific N2 fixation rates following the incubation showed that Daphnia treatments experienced a 50% reduction in N2 fixation relative to Epischura and control treatments. This reduction is consistent with the higher N:P ratio of the dissolved pool in the Daphnia treatment, as N4‐N inhibits N2 fixation. Thus, by differentially recycling N4‐N relative to P, Daphnia reduce the advantage (N2 fixation) cyanobacteria have~over other phytoplankton. We suggest that this mechanism may be an important factor in the ability of Daphnia to reduce the incidence of cyanobactetial blooms in lakes with low N:P loading ratios.
Contrasts between marine and freshwater bacterial community composition: Analyses of communities in Lake George and six other Adirondack lakes
Authors: Methé Barbara A. , Hiorns William D. , Zehr Jonathan P.
Published: May 2010

The bacterial communities of seven freshwater lakes in the Adirondack Mountains of New York state were examined using culture‐independent methods. β‐Proteobacteria 16S rRNA sequences were recovered from all seven lakes and their presence was confirmed by direct DNA hybridization The results are consistent with phylogenetic and in situ hybridization‐based studies in other freshwater environments, but are significantly different than the results of marine oceanic studies, where β‐Proteobacteria are noticeably absent. This relationship between evolutionary history and environmental distribution is striking, since these phylogenetic clades have not been correlated with consistent physiological features or biochemical capabilities, and there is no a priori reason to expect differences in phylogenetic composition between the environments. In contrast, freshwater relatives to marine phylogenetic clusters, in particular the SAR 11 cluster of the α‐Proteobacteria, were identified. The data imply an underlying physiological distinction between the β‐ and other Proteobacteria groups and potentially an important difference between the composition of bacterial communities in marine and freshwater environments.
Light attenuation and photosynthesis of aquatic plant communities
Authors: Krause‐Jensen Dorte , Sand‐Jensen Kaj
Published: May 2010

We compiled 414 studies from the literature to test if general relationships exist between chlorophyll concentration, light attenuation, and gross photosynthesis across phytoplankton communities, macrophyte stands, and attached microalgal mats. We also evaluated the upper limit to photosynthesis in the various communities. Along with increasing chlorophyll concentration, the photic zone diminishes from >100 m in sparse phytoplankton communities to centimeters‐meters in macrophyte stands to <l mm in dense microalgal mats. This compression of the photic zone is paralleled by a marked increase in volumetric photosynthesis (mmol O2 m−3 h−1). In contrast, integral photosynthesis (mmol O2 m−2 h−1) remains relatively constant, depending on both the photic zone and volumetric photosynthesis. A similar upper limit (˜60 mmol mmol O2 m−2 h−1) is attained in dense macrophyte and phytoplankton communities when most of the irradiance is captured by plants. Integral photosynthesis, however, is often far below this upper limit because background light attenuation by particles and dissolved organic matter restrict photosynthesis in many aquatic plant communities. Moreover, limitation of CO2 and O2 exchange may constrain integral photosynthesis in dense plant communities, which are often subject to severe CO2 depletion and O2 supersaturation.
Inhibition of marine photosynthesis by ultraviolet radiation: Variable sensitivity of phytoplankton in the Weddell‐Scotia Confluence during the austral spring
Authors: Neale Patrick J. , Cullen John J. , Davis Richard F.
Published: May 2010

To assess the potential impacts of ozone depletion on photosynthesis in the Southern Ocean, we need to know more about effects of ultraviolet radiation (UV) on phytoplankton in Antarctic waters, where, in addition to variable stratospheric ozone, temporal and regional differences in vertical mixing might influence photosynthesis and photoacclimation of phytoplankton assemblages. Toward this end, we quantified the responses to UV of Antarctic phytoplankton in the Weddell‐Scotia Confluence during the austral spring of 1993. Experimental results on spectral sensitivity of photosynthesis were fit statistically to a model that incorporated uninhibited photosynthesis as a function of photosynthetically available radiation (PAR), wavelength‐dependence of inhibition, and the kinetics of photosynthesis during exposure to UV. In contrast to previous results on UV‐induced photoinhibition in a diatom culture at 20°C natural phytoplankton from open waters of the Antarctic showed no ability to counter UV‐induced inhibition of photosynthesis during exposures of 0.5–4 h: the rate of photosynthesis declined exponentially as a function of cumulative exposure, and inhibition was not reversed during incubations for up to 3.5 h under benign conditions. The results suggest that nonlinear exposure‐response relationships are necessary for modeling UV dependent photosynthesis in the surface mixed layer of the springtime Weddell‐Scotia Confluence. Consequently, we modified our laboratory‐based model of photosynthesis and photoinhibition to describe photoinhibition as a nonlinear function of biologically weighted cumulative exposure to damaging irradiance. The model described ˜90% of the spectrally dependent experimental variation in photosynthetic rate, and yielded six biological weighting functions (BWFs) for phytoplankton in the Weddell‐Scotia Confluence. Assemblages from different stations showed substantial variability in sensitivity to UV. Tolerance of UV was generally highest in assemblages from shallower mixed layers, which presumably had experienced higher irradiance, including UV, prior to sampling. The BWFs of assemblages that seemed acclimated to low irradiance showed the highest sensitivity to UV yet seen for Southern Ocean phytoplankton. The pattern of UV sensitivity was consistent with acclimation, but also with selection against less tolerant species.
Consumption of picoplankton‐size particles by marine ciliates: Effects of physiological state of the ciliate and particle quality
Authors: Christaki Urania , Dolan John R. , Pelegri Silvia , Rassoulzadegan Fereidoun
Published: May 2010

Ingestion of picoplankton‐size particles was studied in two marine ciliates, a typical grazer of pica‐ and nanoplankton, the oligotrich Strombidium sulcatum, and a bacteriovorous scuticociliate, Uronema sp. In laboratory experiments, both logarithmic‐ (food‐unlimited) and stationary‐phase (food‐limited) populations were presented with particles of different sizes and surface properties: plain microspheres, protein‐adsorbing carboxylate microspheres, fluorescently labelled heterotrophic bacteria, and cyanobacteria. In both log‐ and stationary‐phase populations, Strombidium clearance rates varied linearly as function of prey size based on the ingestion of plain microspheres. In contrast, Uronema clearance rates were invariant with particle size for log‐phase populations but increased with particle size in stationary‐phase cells. Discrimination among prey with different surface properties was also dissimilar in the two ciliates. For Strombidium, log‐phase populations cleared 1‐µm‐diameter plain microspheres at higher rates than those with carboxylate surfaces. The pattern was repeated by stationary‐phase cells for both 0.5 and 1‐µm‐diameter microspheres. In Uronema, there was no significant difference in clearance rates with surface property in log‐phase populations, similar to the findings with regard to particle size. Only in stationary‐phase populations of Uronema was some selection evident, as 0.97‐µm carboxylate microspheres were cleared at rates higher than were 0.95 µm plain microspheres. Overall, selection was higher in stationary‐phase ciliates, and Strombidium seemed to feed more size‐selectively among picoplankton particles than did Uronema.
Giant aggregates: Importance as microbial centers and agents of material flux in the mesopelagic zone
Authors: Silver Mary W. , Coale Susan L. , Pilskaln Cynthia H. , Steinberg Deborah R.
Published: May 2010

By using a remotely operated vehicle (ROV), we studied large mucous aggregates produced by the larvacean Bathochordaeus in Monterey Bay, California. These fragile structures or “houses,” cannot be sampled by typical water or net collecting devices, and likely represent a class of cosmopolitan, widely spaced aggregates in subsurface waters. ROV samples allowed us to determine the contribution of the aggregate‐associated communities to populations of microorganisms at depths of 100–500 m. Because the giant houses only average ˜1 per 100 m3, they harbor <1% of the microorganisms in the water, although they are locally enriched. However, once abandoned, houses may play a significant role in transporting materials and organisms to depth. Flux calculations indicate they may seasonally account for 11–100% of some classes of sinking materials at 500 m. These calculations are based on house‐flux contributions predicted from house age (estimated), house abundance (ROV measured), and particle flux (measured by a nearby sediment trap). Based on our ROV observations of spatial and temporal characteristics of the giant aggregates, we calculate the probability that sediment traps collect the correct number of sinking aggregates. Our results suggest that rare particles, such as large aggregates and some sarcodines and pteropods, may be undersampled by commonly used, small sediment traps. Furthermore, the predictably infrequent capture of such comparatively rare particles can lead to erroneous interpretations of temporal or spatial patterns in the flux of material through the upper ocean.
Mean mixed depth of sediments: The wherefore and the why
Authors: Boudreau Bernard P.
Published: May 2010

The activities and consequently the bioturbational effects of deposit‐feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide, environmentally invariant mean of 9.8 cm with a standard deviation of 4.5 cm. Currently available theories of infaunal behavior cannot predict quantitatively this observation. A new simple model that accounts for the feedback between resource (food) abundance, its reactivity, and the intensity of bioturbation leads to a quantitative estimate of 9.7 cm. This model constitutes a fundamental advance in our understanding bioturbation.
Predicting diet composition from body length in the zooplankton predator Leptodora kindti
Authors: Branstrator Donn K.
Published: May 2010

Leptodora kindti and many other species of zooplankton predator range widely throughout temperate‐zone lakes, yet these species are rarely included in size‐based studies of food webs. This oversight derives mainly from a lack of predictive functions that quantify their effects on prey communities. With laboratory experiments I measured the handling time and phosphorus ingestion of L. kindti fed three cladoceran species radiolabeled with 33P as a tracer of dry weight (DW). In general, laboratory results show that predator body length exerts strong influence on maximum prey length, on prey handling time (and indirectly predation rate), and on prey profitability (expressed as µg DW ingested time−1) in systematic ways that may be useful in predicting predatory effects of L. kindti on natural prey communities. The prey‐capturing structure of L. kindti, called the trap basket, constrains the maximum length of prey that is consumable, and handling times on prey increase exponentially as prey length approaches the maximum dimensions of the trap basket. The empirical relationship is given by the following equation: Handling time = 1/[0.067(Trap basket length/Prey length — 1)0.536]. I use handling time and body length data to evaluate the predictive nature of two previously published regression models that relate L. kindti trap basket length to body length. This note offers a length‐based approach to analysis of diet composition in L. kindti that will hopefully encourage increased consideration of this species in future studies of food webs.
Spectrofluorometric quantification of neutral and polar lipids suggests a food‐related recruitment bottleneck for juveniles of a deposit‐feeding polychaete population
Authors: Hentschel Brian T
Published: May 2010

A growing body of evidence suggests that juveniles of species that deposit feed as adults are more susceptible to food limitation than are conspecific adults. To quantify ontogenetic variations in the nutritional condition of the depositfeeding polychaete Pseudopolydora kempi japonica, I modified existing spectrofluorometric assays of neutral and polar lipids. The hydrophobic fluorochrome Nile red fluoresces yellow‐orange when bound to neutral lipids and red‐orange when bound to polar lipids. The ratio of neutral lipids (energy storage) to polar lipids (cell membranes) provides an index of nutritional condition that is scaled to body size. For individual P. kempi japonica ranging in length from 2 to 12 mm, I quantified the florescence from each lipid class and analyzed sizedependent variations in the neutral/polar lipid ratio. P. kempi japonica showed a size‐dependent increase in the neutral/polar lipid ratio up to a body length of −6–8 mm; little size‐dependent change in the ratio occurred beyond this size. Ontogenetic changes in both particle‐size selection and the assimilated diet of P. kempi japonica display size‐dependent patterns that are similar to the lipid data. Together, the three lines of evidence suggest that P. kempi japonica juveniles experience a food‐related recruitment bottleneck until they grow to −6–8 mm. Once juveniles grow beyond this size, they appear to forage like adults and achieve adult levels of neutral lipid. Because deposit‐feeding benthos ingest a low‐quality and nutritionally dilute diet that will be difficult for small juveniles to digest, ontogenetic niche shifts and food‐related recruitment bottlenecks during the juvenile stage may be common in the population ecology of species that deposit feed as adults.
Effect of Raman scattering on the average cosine and diffuse attenuation coefficient of irradiance in the ocean
Authors: Berwald Juli , Stramski Dariusz , Mobley Curtis D. , Kiefer Dale A.
Published: May 2010

We have investigated the effect of Raman scattering on the average cosine of underwater irradiance, , the diffuse attenuation coefficient of irradiance, K, and the Raman scattering source coefficient, P*, throughout the water column and into the asymptotic field. The Raman scattering source coefficient is the fractional gain in the scalar irradiance from local Raman scattering at a given depth, and it can be found from the Gershun equation with a Raman source term. In particular, by using calculations from a radiative transfer model (Hydrolight 3.0), we compare , K, and P* from simulations that include and that omit Raman scattering. These simulations are performed at wavelengths between 355 and 665 nm in a vertically homogeneous ocean with inherent optical properties (IOPs) determined from case 1 bio‐optical models for three different chlorophyll concentrations (0.05, 0.5, and 5 mg m−3). We also investigated several two‐layered oceans that assumed different inherent optical properties in the surface and deep layers. Our calculations showed that when Raman scattering is present, there may exist strong vertical gradients in the average cosine and the diffuse attenuation coefficient, especially at long wavelengths for low chlorophyll concentrations. In addition, a quasi‐asymptotic field may be a feature of the vertical profiles of the average cosine and the diffuse attenuation coefficient. In the asymptotic field, there is no effect of Raman scattering at shorter wavelengths, there is a strong effect of Raman scattering at longer wavelengths and between these two spectral regions, and there is a transition that increases from 500 to 590 nm as the concentration of chlorophyll increases from 0.05 to 5 mg m−2. Over a broad range of IOPs, the asymptotic parameters , K∞*, and P*∞, are related by a quadratic relationship. Neither the magnitude or angular distribution of surface light nor changes in the absorption and scattering coefficients in the surface layer of a two‐layered ocean have auy effect on the values of , K∞*, and P*∞; the values of these parameters depend only upon the IOPs in the deepest layer.
Vertical distribution and temporal variation of marine planktonic archaea in the Gerlache Strait, Antarctica, during early spring
Authors: Massana Ramon , Taylor Lance T. , Murray Alison E. , Wu Ke Y. , Jeffrey Wade H. , DeLong Edward F.
Published: May 2010

A station located in the Gerlache Strait (Antarctic Peninsula) was sampled during early spring to determine the
vertical distribution of marine planktonic archaea and to further describe the dynamic environment where they live.
Chlorophyll concentration indicated that sampling occurred during the early stages of austral spring algal development. As expected, prokaryote abundance was higher at surface than at depth, and prokaryotic activity estimated
by leucine incorporation was low at the surface and extremely low at 500 m. The relative abundance of planktonic
archaeal, eucaryal, and bacterial ribosomal RNA was determined by quantitative rRNA hybridization, and the
performance of two different universal probes used to normalize group‐specific probe hybridization response was
compared. Archaeal rRNA was detected in all samples analyzed, and was more abundant at depth (up to 25% of
total rRNA) than at the surface. In both years, the archaeal signal decreased during the sampling period, particularly
at surface. Most of the archaeal signal was attributable to group I archaea, affiliated with the kingdom Crenarchaeota.
Planktonic euryarchaeotes (group II archaea) showed a larger contribution to the archaeal assemblage in surface
waters than at depth. In total, our results verify that planktonic archaea are dynamic and abundant components in
marine picoplankton assemblages of the Antarctic Peninsula.
Evidence of deep circulation in two perennially ice‐covered Antarctic lakes
Authors: Tyler Scott W. , Cook Peter G. , Butt Anya Z. , Thomas James M. , Doran Peter T. , Lyons W. Berry
Published: May 2010

The perennial ice covers found on many of the lakes in the McMurdo Dry Valley region of the Antarctic have been postulated to severely limit mixing and convective turnover of these unique lakes. In this work, we utilize chlorofluorocarbon (CFC) concentration profiles from Lakes Hoare and Fryxell in the McMurdo Dry Valley to determine the extent of deep vertical mixing occurring over the last 50 years. Near the ice‐water interface, CFC concentrations in both lakes were well above saturation, in accordance with atmospheric gas supersaturations resulting from freezing under the perennial ice covers. Evidence of mixing throughout the water column at Lake Hoare was confirmed by the presence of CFCs throughout the water column and suggests vertical mixing times of 20–30 years. In Lake Fryxell, CFC‐11, CFC‐12, and CFC‐113 were found in the upper water column; however, degradation of CFC‐ 11 and CFC‐ 12 in the anoxic bottom waters appears to be occurring with CFC‐ 113 only present in these bottom waters. The presence of CFC‐113 in the bottom waters, in conjunction with previous work detecting tritium in these waters, strongly argues for the presence of convective mixing in Lake Fryxell. The evidence for deep mixing in these lakes may be an important, yet overlooked, phenomenon in the limnology of perennially icecovered lakes.
Organic enrichment of submarine‐canyon and continental‐shelf benthic communities bymacroalgal drift imported from nearshore kelp forests
Authors: harrold Christopher , Light Karen , Lisin Susan
Published: May 2010

Remotely operated vehicle (ROV) surveys were conducted over a 3‐year period to evaluate the importance of macroalgal drift exported from nearshore forests of giant kelp (Macrocystis pyrifera) to adjacent submarine‐canyon and continental‐shelf benthic habitats. Abundant macroalgal drift was found in the Carmel Submarine Canyon (153– 454‐m depth) but was rare at Pt. Joe, a continental shelf habitat 9 km away (87–357‐m depth). Most of the drift parcels consisted of brown algae, but 10 other categories were found, including green and red algae and the surfgrass Phyllospadix spp. M. pyrifera drift accounted for as much as 20% of the density of drift parcels in the Carmel Submarine Canyon, and up to 50% of the drift parcels at Pt. Joe. From previously determined turnover rates of drift M. pyrifera and the standing stock estimates from the present study, we estimated that the rate of carbon flux associated with drift M. pyrifera to the benthos of the Cannel Submarine Canyon was 45.2 mg Cm−2 d−1. Based on published estimates of carbon flux from vertically sinking particulate organic carbon at 400‐m depth in the nearby Monterey Submarine Canyon, and our estimates of kelp‐derived carbon flux, drift M. pyrifera can account for 20–83% of the total particulate organic carbon reaching the sea floor in the Carmel Submarine Canyon. We conclude that drift macrophytes provide significant enrichment of organic carbon to the benthos above that provided by vertically sinking particulate organic material. The extent to which this alters the structure of benthic communities compared to areas without organic enrichment by drift macrophytes remains unknown and awaits further study.
A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature
Authors: Geider Richard J. , Maclntyre Hugh L. , Kana Todd M.
Published: May 2010

A new regulatory model can describe acclimation of phytoplankton growth rate (µ), chlorophyll a:carbon ratio and nitrogen: carbon ratio to irradiance, temperature and nutrient availability. The model uses three indices of phytoplankton biomass‐phytoplankton carbon (C), phytoplankton nitrogen (N), and chlorophyll a (Chl). The model links the light‐saturated rate of photosynthesis to N:C, requires that Chl a synthesis be coupled to nitrogen assimilation, and includes several regulatory features. These include feedback inhibition of the nitrogen assimilation rate by increases in the N:C ratio, as well as regulation of Chl a synthesis by the balance between light absorption and photosynthetic carbon fixation. The model treats respiration as the sum of the maintenance metabolic requirement and the cost of biosynthesis. In addition, the model can account for accumulation and mobilization of energy reserves (i.e. variability of N:C) and photoacclimation (i.e. variability of Chl:N and Chl:C) in response to variations in irradiance and nutrient availability. The assumptions of the model are shown to be in agreement with experimental observations and the model output compares favorably with data for cultures in balanced and unbalanced growth.
Calibration of an optical plankton counter for use in fresh water
Authors: Sprules W. G. , Jin E. H. , Herman A. W. , Stockwell J. D.
Published: May 2010

A towed optical plankton counter (OPC‐1T) developed for automated counting and sizing of marine zooplankton was calibrated for use in fresh water. Zooplankton in lakes are generally smaller in size, more variable in shape, and more abundant than in oceans. Our objectives were to study the conditions and parameters for utilizing the OPC‐1T in the relatively high zooplankton concentrations typical of fresh water while maintaining its full capability for measuring large and small organisms. By passing low concentrations of monospecific and mixed zooplankton assemblages through a laboratory OPC‐lL, we first verified that the OPC can measure freshwater zooplankton size very accurately. We then compared simultaneously collected plankton net and OPC‐1T field samples from Lakes Erie and Ontario in order to quantify the known measurement bias of the OPC caused by coincident effects in the high zooplankton concentrations (up to 212 liter−1) typical of these lakes. The correlation between plankton net and OPC‐1T biomass estimates was greatest when each organism detected by the OPC was treated as an ellipsoid with major axis (a) equal to the recorded OPC diameter and minor axis equal to a/1.33. The effect of increasing zooplankton abundance, and hence coincident counts, on zooplankton size measurements of the OPC‐1T was also quantified. Results suggest that with proper choice of beam length and calibration algorithms, lake zooplankton biomass, numerical concentrations, and size distributions can be accurately measured at concentrations up to 100 organisms liter−1.
Relaxation of phosphorus limitation due to typhoon‐induced mixing in two morphologically distinct basins of Lake Biwa, Japan
Authors: Robarts Richard D. , Waiser Marley J. , Hadas Ora , Zohary Tamar , Maclntyre Sally
Published: May 2010

We measured changes in hydrodynamics and phosphorus cycling due to a typhoon in two morphologically distinct basins of Lake Biwa, Japan. The typhoon, with wind speeds up to 20 m s−1, led to physical, chemical, and physiological responses that differed dramatically in the shallow (mean depth of 3.5 m), eutrophic south basin compared to the deep (mean depth of 45.5 m), oligo‐mesotrophic north basin. Prior to the typhoon, PO4‐P concentrations were <l µg liter−1 in both basins. A suite of physiological tests (protein:carbohydrate ratio, sestonic C:N:P ratios, nutrient‐enrichment bioassays, alkaline phosphatase activity, 32P uptake) all indicated that the plankton was P‐deficient. Bacteria dominated (avg of 65%) the uptake of 32PO43−. As a result of the typhoon, P concentrations increased to −2.5 pg liter−1 and suspended solids increased from 4.5 to ~17 mg liter−1 in the south basin. The day after the typhoon, all physiological tests indicated that the south basin plankton was P‐sufficient and that phytoplankton dominated (65%) 32P‐uptake. A week after the typhoon, P concentrations had dropped to near pre‐typhoon levels and the physiological tests indicated that the plankton was again P‐deficient. Only on the day of the typhoon were wavg‐induced shear stresses great enough to cause considerable entrainment of sediment and associated pore waters in the South Basin. Because interstitial porewater P concentrations were ≥2 orders of magnitude greater than the overlying water concentration, we concluded that the increase in P was caused by entrainment of nutrient‐rich pore water. In the north basin the typhoon‐induced upwelling and mixing in the thermocline did not penetrate to depths where P concentrations were elevated. Consequently, plankton were as P deficient immediately after the typhoon as they were prior to it.
An iron limitation mosaic in the California upwelling regime
Authors: Hutchins D. A. , DiTullio G. R. , Zhang Y. , Bruland K. W.
Published: May 2010

Although Fe limitation is well documented in open ocean high ‐nutrient, low ‐chlorophyll (HNLC) areas, little is known about the potential for Fe limitation in coastal environments. We present a series of four Fe addition experiments that demonstrate varying degrees of Fe limitation in the central California coastal upwelling area. Fe concentrations vary widely here (<0.1 to >8.0 nM) because inputs from rivers and resuspended shelf sediments are unevenly distributed. The biological response to Fe availability is also extremely variable. Fe‐replete areas experience extensive blooms of large diatoms and almost complete depletion of nutrients. In slightly Fe‐stressed areas, Fe limits the growth of large diatoms but does not control nutrient biogeochemistry or growth of other planktonic organisms. In severely Fe‐limited waters, Fe exerts a fundamental control on nitrate and silicic acid drawdown, particulate organic carbon production, and the growth of phytoplankton, zooplankton, and bacteria. We propose a four ‐level classification scheme for Fe limitation in coastal California waters. Each level is characterized by a set of specific biological and biogeochemical responses to Fe. Parameters that show a characteristic response to Fe addition and thus define a region's Fe limitation status include particulate Si:N and Si:C production ratios, NO3− and H2SiO3 drawdown, C fixation, large diatom and picoplankton growth, bacterial production, and zooplankton grazing and biomass. Fe limitation of coastal upwelling regions needs to be recognized as an important biogeochemical process that could profoundly affect global new production and carbon cycling. The physical, chemical, and biological complexity of coastal upwelling regimes requires that Fe limitation effects be addressed with a more sophisticated approach than has generally been used to describe oceanic HNLC regimes.
Interactions among Cu2+, Zn2+, and Mn2+ in controlling cellular Mn, Zn, and growth rate in the coastal alga Chlamydomonas
Authors: Sunda William G. , Huntsman Susan A.
Published: May 2010

Culture experiments with the green alga Chlamydomonas sp. in metal ion‐buffered media revealed antagonistic interactions between toxic metals (Cu and Zn) and nutrient metals (Zn and Mn) in regulating cellular Mn and Zn uptake and specific growth rate. High levels of ionic Cu2+ and Zn2+ inhibited cellular Mn uptake, and high Cu2+ inhibited Zn uptake rates. These effects were associated with growth rate inhibition at combined conditions of high Zn2+ and low Mn2+ and at high Cu2+ and low Zn2+. Zinc inhibited Mn uptake by competitively blocking Mn binding to a high‐affinity Mn uptake system which was under negative feedback regulation. Kinetic modeling suggested that Zn was taken up by this system, and as a result, cellular Zn concentrations increased as external Mn2+ was decreased. Zinc uptake behavior was complex, and in addition to the Mn system, uptake appeared to involve a high‐affinity Zn system induced at low Zn2+ and a separate constitutive low‐affinity system. High Cu2+ levels inhibited zinc uptake by the high‐affinity transport system. Such complex competitive interactions among metals provide important controls on cellular metal accumulation, toxicity, and nutrition.
Direct and interactive effects of allochthonous dissolved organic matter, inorganic nutrients, and ultraviolet radiation on an alpine littoral food web
Authors: Vinebrooke Rolf D. , Leavitt Peter R.
Published: May 2010

Allochthonous dissolved organic matter (DOM) may regulate littoral food webs by both modifying nutrient availability and attenuating potentially damaging ultraviolet radiation (UVR). These hypotheses were tested in a three‐factor (DOM, inorganic nutrients, and UVR) experiment using 24 littoral enclosures in an alpine lake located in Banff National Park, Canada. DOM was extracted from treeline soils and added (3 mg DOM liter−1) to +DOM enclosures over 1 month. These amendments were intended to simulate increases in allochthonous inputs that might occur as climate warming promotes the development of treeline soils and vegetation.

DOM amendments significantly increased epilithon biomass (as total chlorophyll, carotenoids) but did not affect the abundance of either epipelon or phytoplankton. In addition, natural UV irradiance significantly enhanced the positive effect of DOM on epilithon and directly increased the abundance of epipelon. Threefold amendments of ambient nitrogen (N) and phosphorus (P) (20 N:1 P, by weight) significantly increased epilithon abundance. The positive effects of DOM, NP, and UVR on phytobenthos were primarily attributable to the increased abundance of diatoms, whereas NP affected phytoplankton by increasing chrysophytes and dinoflagellates. DOM amendments did not significantly affect the final abundances of heterotrophic bacteria and protists but did result in significantly higher densities of omnivorous copepod nauplii. These results show that allochthonous DOM is an important resource for littoral food webs in oligotrophic lakes; however, its effects are mediated by UVR and differ between benthic and planktonic habitats. Our findings suggest that littoral food‐web structure in clear, shallow lakes and ponds is altered by fluctuations in allochthonous inputs and UVR exposure arising from droughts and long‐term climatic change
Diel patterns in optical properties of the chlorophyte Nannochloris sp.: Relating individual‐cell to bulk measurements
Authors: DuRand Michele D. , Olson Robert J.
Published: May 2010

To investigate how cell growth and division affect the optical properties of phytoplankton, cultures of the chlorophyte Nannochloris were sampled over a diel cycle to measure cell size and concentration, light‐beam attenuation and absorption, flow cytometric forward light scattering and chlorophyll fluorescence, and carbon content. Refractive index was calculated using the anomalous diffraction approximation. At six different light levels, ranging from 60 to 1,500 µmol photons m−2 s−l, cell division was tightly phased to the light: dark cycle, occurring soon after dark. There were pronounced diel patterns, with minima near dawn and maxima near dusk, in cell size, cell‐specific beam attenuation and absorption, flow cytometric forward light scatter, and carbon content. The diel variations in attenuation cross section were primarily caused by changes in cell size due to growth and division, although there was some influence from changes in refractive index. Because the major constituents of many phytoplankton communities are small eukaryotic cells similar to Nannochloris in cell size and division pattern, the results presented here have important consequences for the interpretation of diel variations in optical properties observed in the ocean.
Relationships between plankton community structure and plankton size distribution in lakes of northern Germany
Authors: Tittel Jörg , Zippel Barbara , Geller Walter , Seeger Juliane
Published: May 2010

To test existing hypotheses about the plankton size distribution (PSD) in lakes, we intensively sampled the plankton of a eutrophic, stratified lake (Arendsee) and constructed size spectra for particles ranging in size from bacteria to herbivorous cladocerans. The Arendsee Lake annual mean spectrum extended over nearly 10 decades of body size and 2.1 decades of biomass per size class (minimum, autotrophic picoplankton; maximum, colonial cyanobacteria). The spectrum was continuous (no size class without associated biomass). These data indicate that continuous PSDs are not restricted to the plankton in large pelagic zones (e.g., central oceanic systems or large freshwater lakes). We conclude that differences in the biomass of the various size classes are a consequence of eutrophic conditions in Arendsee Lake, not of littoral or benthic influences. In addition, we evaluated the impact of daphnids on the shape of the PSD with a data set comprising single samples from each of 28 lakes in northern Germany plus seasonal data from Arendsee Lake. In lakes or situations with a large Daphnia biomass (≥0.5 mg fresh weight liter−1), the slopes of the straight lines fitted to the normalized PSDs were less negative. In the 28 northern lakes, but not in Arendsee Lake, there was greater variation about the fitted line when Daphnia biomass was high, indicating a less homogenous PSD. Discontinuities in the spectra were found only in samples from the 28 northern lakes and for higher Daphnia biomass. Our interpretation is that the feeding mode of large filter‐feeders results in a higher transfer efficiency from small to larger individuals (less negative slopes), which produces changes to a nonhomogenous PSD.
Genetic variation and reproductive isolation among phenotypically divergent amphipod populations
Authors: McPeek Mark A. , Wellborn Gay A.
Published: May 2010

We examined the degree of reproductive isolation and patterns of genetic structure and diversification within and among seven populations of the freshwater amphipod Hyalella azteca. Hyalella occurs across ecologically dissimilar habitats in southeastern Michigan and exhibits substantial, and apparently adaptive, genetically based phenotypic variation among populations. Habitats with predatory centrarchid fish contain a small‐bodied Hyalella ecotype, whereas habitats lacking these predators contain a large‐bodied ecotype. Hierarchical F statistics showed that allele frequency variation at six electrophoretic loci is greater among populations within an ecotype than between ecotypes, a result also reflected in a cluster analysis based on genetic distances among the seven populations. Despite the lack of clear genetic differentiation between ecotypes, gene flow between ecotypes appeared restricted or absent. Physically adjacent large‐ and small‐ecotype populations in the same drainage differed appreciably in allele frequencies. Within‐population genetic structuring also differed between ecotypes. Whereas genotype frequencies never differed from Hardy‐Weinberg expectations in large‐ecotype populations, all small‐ecotype populations exhibited a significant deficiency of heterozygous genotypes at one or more loci. Interbreeding trials demonstrated that individuals from different populations of the same ecotype readily interbreed, but no interbreeding was observed in crosses involving individuals of dissimilar ecotypes. Interbreeding results, considered together with between‐ecotype differences in allele and genotype structure, suggest that Hyalella ecotypes in southeast Michigan form at least two separate species. Furthermore, the lack of distinct allelic differences and the relative levels of allele frequency differentiation within and between ecotypes suggest recent local speciation in this taxon.