bluuebunny/crossref_metadata_2025 huggingface dataset:
Importance of lacustrine physical factors for the distribution of anthropogenic 60Co in Lake Biel
Authors: Albrecht Achim , Goudsmit Gerrit , Zeh Markus
Published: May 2010

The fate of particle reactive substances in a surface aquatic system is controlled by both geochemical and physical parameters. Scavenging processes in Lake Biel (Switzerland) have been studied using the anthropogenic tracer 60Co. This isotope originates mostly in dissolved form from the Muhleberg nuclear power plant and is transported 18 km down by the River Aare to the lake. Radionuclide data from sediments, combined with inventory calculations, indicate that between 30% and 55% of the discharged 60Co is transferred to sediments. Scavenging efficiency is higher during the winter period than during summer, in spite of higher particle fluxes during the summer. An intrusion model, based on buoyancy calculations, predicts that Aare water, including its radionuclide load, remains in the epilimnion during the warm period; thus, mixing is restricted to only a fraction of the lake volume and the residence time is reduced to 1 week. Buoyancy in the summer is controlled mostly by temperature. During winter, when the lake and Aare waters are close to 4°C, the higher dissolved load of Aare water increases its density and forces it to intrude into deeper sections of the lake, increasing the residence time and thus the time available for adsorption and particle settling. The predictive capability of the model was tested for the summer period using a fluorescent dye (uranin) to track Aare water from the inflow to the outflow of the lake. Higher fluxes of both particles and associated radionuclides in deep sediment traps confirm the results of the intrusion model for the winter period. Comparison between modeled and measured 60Co activities in lake water at the end of the winter validates the credibility of the intrusion model further. This combined geochemical and physical approach may be applied to transport modeling of radionuclides and of any particle reactive substance, inorganic or organic.
Photophysiological acclimation of Phaeocystis antarctica Karsten under light limitation
Authors: Moisan Tiffany A. , Mitchell B. Greg
Published: May 2010

Primary production models and pigment algorithms for remote optical systems including satellites, moorings, or drifters depend on an improved understanding of the relationship between spectral light absorption, pigments, and photosynthesis for species of phytoplankton that are widespread and numerically abundant. Cultures of colonial Phaeocystis antarctica, a prymnesiophyte that can dominate the phytoplankton community in the Southern Ocean, were grown under blue light at seven different levels ranging from 14 to 542 µmol quanta m−2 s−1 at 4°C under nutrient‐replete conditions. Chlorophyll‐specific absorption (a*ph) at 436 nm increased linearly from 0.03 to 0.11 m2 mg chlorophyll a−1 with increasing light intensities. This variability is attributed to pigment packaging effects and pigmentation. The 2.5‐fold range in the values of a*ph(676 nm) demonstrates significant pigment packaging effects due to the intracellular pigment content, single cell diameter within the colony, and thylakoid stacking. Quantum yield for growth (Φμ) varied 30‐fold, ranging from 0.003 to 0.09 mol carbon fixed (mol quanta absorbed)−1. Under low light conditions, the relatively high (Φμ) and high a*ph(λ) may enable Phaeocystis to accumulate a seed population to initiate blooms at the beginning of spring when light levels are low, mixed layers are deep, and sea ice is still significant. These aspects of its photophysiology may contribute to the ecological success of Phaeocystis in polar regions.
Picoplankton and nanoplankton and their trophic coupling in surface waters of the Sargasso Sea south of Bermuda
Authors: Caron David A. , Peele Emily R. , Lim Ee Lin , Dennett Mark R.
Published: May 2010

Studies were conducted during August and March–April in the Sargasso Sea south of Bermuda to examine rates of bacterial growth and picoplankton consumption by microbial consumers. Bacterial growth rates were estimated from [3H]thymidine (TdR) incorporation rates, while grazing rates were determined using fluorescently labeled prey (FLP). In addition, net bacterial growth rates were calculated from changes in bacterial abundance during 24‐h incubations. The latter measurements were compared to net growth rates obtained by subtracting picoplankton grazing mortality rates from bacterial TdR growth rate estimates (TdR minus FLP). Overall, bacterial growth rates determined by TdR uptake were similar to FLP grazing rates during the March–April cruise, indicating an approximate balance between production and removal processes. Bacterial growth rates during August, however, were approximately twice the rates of grazer removal. Net bacterial growth rates determined from TdR growth rates minus FLP grazing rates were similar to net growth rates estimated from changes in abundance for samples collected near the surface during both cruises. However, rates of change of bacterial abundances during incubations were generally greater than rates predicted from TdR minus FLP for samples collected in the deep euphotic zone during both cruises. These discrepancies might be explained by several factors, including the inclusion of prochlorophytes in the bacterial counts and/or the stimulation of bacterial growth during containment. The TdR conversion factor also was an important consideration when comparing net bacterial growth rates estimated from changes in bacterial abundance to net growth rates determined from TdR minus FLP. Small nanoplanktonic protists (<5 µm) were responsible for most of the picoplanktonic grazing activity. Doubling times of 0.9–18.3 d for the heterotrophic nanoplankton were estimated based on the removal rates of picoplankton. The complexity of the microbial food web of this oligotrophic ecosystem is such that relatively little carbon may be recovered from nonliving organic material and passed on to metazoa.
Origins and scale dependence of temporal variability in the transparency of Lake Tahoe, California–Nevada
Authors: Jassby Alan D. , Goldman Charles R. , Reuter John E. , Richards Robert C.
Published: May 2010

Secchi depth has been measured in Lake Tahoe an average of every 12 d since July 1967. Because of the unusual clarity of the lake, Secchi depth measurement is responsive to small changes in light‐attenuating particles, and the record exhibits strong variability at the seasonal, interannual, and decadal scales. Using recently developed methods of applied time‐series analysis, the mechanisms of change were delineated at each scale. The seasonal pattern is a bimodal one, with two minima at approximately June and December. The June minimum is due mostly to cumulative discharge of suspended sediments following melting of the snowpack. The December minimum is probably a result of mixed‐layer deepening as the thermocline passes through layers of phytoplankton and other light–attenuating particles that reach a maximum below the summer mixed layer. The interannual scale exhibits two modes of variability, one during the weakly stratified autumn–winter period and the other during the more stratified spring–summer period. The first mode is a result of variable depth of mixing in this unusually deep lake, while the second results from year‐to‐year changes in spring runoff. A decadal trend also exists (–0.25 m yr−1), resulting from accumulation of materials in the water column. It is not yet understood, however, how much of this change is due to phytoplankton or recent phytoplankton‐derived materials and how much is due to other materials such as mineral suspensoids. Based on the available measurements and physical considerations, both categories may play a significant role.
Bacterioplankton dynamics in northern San Francisco Bay: Role of particle association and seasonal freshwater flow
Authors: Murrell M. C. , Hollibaugh J. T. , Silver M. W. , Wong P. S.
Published: May 2010

Bacterioplankton abundance and metabolic characteristics were observed in northern San Francisco Bay, California, during spring and summer 1996 at three sites: Central Bay, Suisun Bay, and the Sacramento River. These sites spanned a salinity gradient from marine to freshwater, and sampling occurred during a period of seasonally declining river flow. The microbial measures included radio‐labeled amino acid uptake (L‐leucine, L‐proline, L‐serine), ectoenzyme activity (aminopeptidase and β‐D‐glucosidase), and bacterial abundance using 1‐µm filters to separate free from particle‐associated bacteria. A seasonal decline in all bacterial metabolic measures was observed at all stations, suggesting that a system‐wide variable may be important in controlling bacterial activity. One such variable is freshwater flow into the Bay (as a proxy for organic matter flux), which positively covaried with all metabolic measures. A sharp decline in particle‐associated bacteria was also observed in Suisun Bay and the Sacramento River between July and August. This decline may have been due to combined effects of declining nutritive value of the aging particles and increasing grazing pressure by benthic filter feeders. Aminopeptidase activity was positively related with increasing salinity, and β‐D‐glucosidase was negatively correlated with increasing salinity, indicating a gradient in the relative quality of organic matter from carbohydrate‐rich riverine to protein‐rich oceanic material. Overall, Suisun Bay had the highest mean proportion of particle‐associated bacteria (49%), followed by Sacramento River (36%) and Central Bay (11%). Particles were the sites of enhanced ectoenzyme activity but not amino acid incorporation. Bacteria may be actively dissolving the particulate organic matter, but their growth rates on particles are not significantly enhanced.
The contribution of macrophyte‐derived organic matter to microbial biomass in salt‐marsh sediments: Stable carbon isotope analysis of microbial biomarkers
Authors: Boschker H.T.S. , de Brouwer J. F. C. , Cappenberg T. E.
Published: May 2010

Stable carbon isotope ratios of bacterial biomarkers were determined to infer sources of organic carbon used by bacteria in the sediments of three salt marshes. Biomarkers studied were polar lipid‐derived fatty acids (PLFA), mainly bacteria‐specific, methyl‐branched i15:0 and a15:0. Experiments showed that isotopic fractionation between substrate and biomarkers was relatively constant (–4 to –6‰, on average) compared to the wide range in 13C/12C ratios of carbon sources found in the studied marshes. At the Spartina site of the Waarde Marsh (The Netherlands), biomarker 13C/12C ratios were depleted by approximately 6‰ more than expected for bacteria growing on Spartina litter and were similar to an unvegetated control sediment. This pattern suggested that local macrophyte production was of little importance and that other material (probably of algal origin) was the dominant carbon source for bacterial growth. Spartina contributed about half of the carbon in bacterial PLFA at the Kattendijke Marsh (The Netherlands) and dominated at the Great Marshes (U.S.). The variation in bacterial carbon sources in these marshes was probably related to estimated inputs of nonmacrophyte organic matter to the sediment. At the Waarde Marsh, a clear plant species effect was found as coupling between plant and bacteria was more important in Scirpus maritimus than in Spartina anglica. The contribution of local plant production to bacterial biomass in salt‐marsh sediments is highly variable between marshes and depends on the input of nonmacrophyte material by sedimentation in comparison to local plant input, which in turn may differ among plant species.
Stable isotope response to lake eutrophication: Calibration of a high‐resolution lacustrine sequence from Baldeggersee, Switzerland
Authors: Teranes Jane L. , McKenzie Judith A. , Lotter Andre F. , Sturm Michael
Published: May 2010

Stable isotope analyses of discrete seasonal layers from a 108‐yr annually laminated freeze‐core from Baldeg‐gersee, a small, eutrophic lake in central Switzerland, provide information on the climatological and environmental factors, including lake eutrophication, that control oxygen and carbon isotopic composition of epilimnic biologically induced calcite precipitate.

During the last 100 yr, Baldeggersee has undergone major increases in productivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake's watershed. Calibration of the isotopic signal in Baldeggersee to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. δ18O values from the spring/summer “light” sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured δ18O values were up to 21.5‰ more negative than calculated equilibrium δ18O values. Evidence for 13C depletion in the calcite, relative to equilibrium values, is more difficult to ascertain because of an overall dominance of isotopic enrichment in the dissolved inorganic pool as productivity in Baldeggersee increases. A positive association exists between the degree of oxygen‐18 depletion and the calcite crystal size. Thus, large amorphous calcite grains can be used as a proxy for recognizing apparent isotopic nonequilibrium in sediment sequences from highly productive lacustrine environments from all geologic time scales.

In contrast to the light layers, the oxygen isotopic composition of the calcite in the late summer/fall “dark” sediment layers is unaffected by the apparent isotope nonequilibrium. Oxygen and carbon isotope values from the dark laminae in the Baldeggersee sediment therefore provide environmental and climatological proxies that can be calibrated with known environmental and regional climate data for the last century.
Predicted impacts of elevated temperature on the magnitude of the winter‐spring phytoplankton bloom in temperate coastal waters: A mesocosm study
Authors: Keller Aimee A. , Oviatt Candace A. , Walker Henry A. , Hawk Jamie D.
Published: May 2010

An experiment was conducted with six 13‐m3 land‐based mesocosms (5 m deep) in December 1996/February 1997 to address the impact of increased temperature on the trophic structure of nutrient‐rich coastal systems. All mesocosms were exposed to a high nutrient loading rate (2.31 mmol N m−3 d−1:0.18 mmol P m−3 d−1:0.165 mmol Si m−3 d−1). Three treatment mesocosms were maintained at a temperature elevated ∼1C relative to the long‐term (1977–1989) average, ambient temperature in the parent system, Narragansett Bay, Rhode Island, and elevated ∼3C from three control mesocosms. Warmer temperatures were hypothesized to result in lower phytoplankton biomass during the winter‐spring bloom period as a result of increased grazing related to greater metabolic activity of both zooplankton and the benthos. Mean phytoplankton biomass and abundance were lower in the mesocosms with warmer temperatures. Well‐developed phytoplankton blooms occurred in two of the three cool systems. The presence of high numbers of filter‐feeding mussels (Mytilus edulis) prevented a bloom from occurring in the third cool system. Unlike most benthic organisms, mussels continue to filter at high rates even at very low temperatures. Analyses of variance (ANOVAs), after adjusting for mussel biomass, revealed significant (P <0.05) or near significant (P < 0.10) differences in phytoplankton (abundance and biomass), zooplankton abundance, and sedimentation rates between warm and cool treatments. Experimental and literature data were combined to develop carbon budgets for the six systems. Budgets for the warm systems indicated that carbon produced by phytoplankton was lost primarily by grazing of zooplankton, mussels, or both (29–55%) and to a lesser degree, by sedimentation (29–43%). In the cool systems without mussels, losses via sedimentation (73–82%) predominated, with an average ninefold increase in the amount of material supplied to the benthos relative to warm systems.
Zooplankton effects on vertical particulate flux: Testable models and experimental results
Authors: Sarnelle Orlando
Published: May 2010

The effects of herbivorous zooplankton on the sedimentation of particles out of the euphotic zone are examined with mathematical models, a large‐scale field experiment, and descriptive data from a eutrophic lake. The theory is rooted in the population dynamics of phytoplankton and zooplankton and so explicitly accounts for the potential effect of zooplankton grazing on primary production and the connection between the rate at which phytoplankton cells sink and sustainable zooplankton biomass. The models predict positive, negative, or unimodal zooplankton effects, depending on the values of four parameters: rate of direct phytoplankton sinking, fraction of zooplankton fecal material exiting the euphotic zone, zooplankton assimilation efficiency, and system productivity. Models were parameterized with data from a eutrophic lake to make a priori predictions about the shape and direction of zooplankton effects. Predictions were tested against the results of an independent experiment in which a gradient of Daphnia biomass was established in large enclosures. Daphnia negatively affected sedimentation rates of carbon, nitrogen, and phosphorus in the enclosure experiment, which confirmed model predictions. Daphnia had a strong negative effect on phytoplankton biomass, and phytoplankton biomass was positively correlated with sedimentation in the enclosures. Experimental results were congruent with relationships between Daphnia biomass and sedimentation rate in the lake. Successful application of the theory suggests that these models may be of utility for assessing the direction of zooplankton effects on vertical flux in other systems. More generally, the models help to identify parameters that should be measured in studies of zooplankton effects on downward particle flux.
Acoustic characterization of Mysis relicta
Authors: Gal Gideon , Rudstam Lars G. , Greene Charles H.
Published: May 2010

It is possible to increase the sampling resolution and the available information on zooplankton populations by applying acoustic sampling techniques. Knowledge of an organism's acoustic target strength (TS) is critical for translating acoustic data into meaningful biological information, such as numerical abundance. We have taken several approaches to evaluate the acoustic TS of Mysis relicta, a key benthic‐pelagic invertebrate in the Laurentian Great Lakes. This included in situ TS measurements with a dual‐beam 420‐kHz system and comparison among net‐based, optical plankton counter (OPC)‐based, and acoustic‐based estimates of numerical abundance of mysids. In addition, based on the in situ TS data, we modified a model previously applied primarily to marine zooplankton taxa. The resulting predicted TS was compared to other TS estimates. The estimated mean TSs from the in situ TS measurements and the OPC‐acoustic comparison were within a narrow range (–76 to –74.8 dB). Analysis of the net‐acoustic data comparison resulted in an estimated average TS of –73.1 dB. We suggest that the net‐acoustic discrepancy relative to other methods is due to net avoidance and low filtering efficiency. The model predicted similar mean TSs for the size distribution present when R (reflectivity coefficient) was changed to 0.0675. We hypothesized that the modification of the model parameter values was necessary because of the differences in the medium surrounding marine vs. freshwater zooplankton.
Environmental stress and local adaptation in Daphnia magna
Authors: De Meester Luc , Boersma Maarten , Spaak Piet
Published: May 2010

The effects of fish kairomones, crowding chemicals, and day length on the life–history traits of a set of 16 Daphnia magna clones, derived from four populations that differ in fish‐predation pressure, were studied. Significant among‐population differences were observed, the differences being in concordance with the hypothesis of local adaptation. The among‐population genetic differences were not mediated through a change in response to fish kairomones, but through an overall smaller body size, smaller eggs, and a higher number of eggs in clones derived from habitats in which fish are present. Using a model, we show that the observed changes in life‐history characteristics may lead to differences in fitness under different predation regimes, such that populations from habitats with fish have highest fitness under fish‐predation regimes and populations without a fish background have higher fitness values under invertebrate predation regimes.
Frequency distributions of phytoplankton single‐cell fluorescence and vertical mixing in the surface ocean
Authors: Dusenberry Jeffrey A. , Olson Robert J. , Chisholm Sallie W.
Published: May 2010

Because of photoacclimation, frequency distributions of phytoplankton single‐cell fluorescence reflect variation in light histories among individual cells. As a result, phytoplankton can be used as biological tracers for vertical mixing. A time series of Prochlorococcus single‐cell fluorescence distributions in the Sargasso Sea revealed changes that were qualitatively consistent with changes in water‐column physical dynamics. Applying a photoacclimation‐diffusion model to Prochlorococcus fluorescence from the time series yields a vertical diffusivity near 30 cm2 s−1 for a remnant mixed layer beneath a diurnal thermocline, which is consistent with estimates obtained by other means. This estimate results from an integrated view of how Prochlorococcus responds to changes in light intensity induced by vertical mixing and therefore, may be an appropriate measure of vertical diffusion rates for productivity models that include photoacclimation dynamics. This approach provides a novel way to examine vertical mixing on time scales relevant to phytoplankton physiological responses.
Ancient blue‐green blooms
Authors: McGowan Suzanne , Britton George , Haworth Elizabeth , Moss Brian
Published: May 2010

Recent decades have seen a large increase in surface scums (blooms) of cyanophytes (blue‐green algae and blue‐green bacteria) in inland waters. These are potentially toxic to mammals, including humans, and have caused considerable public concern in Europe, Australasia, and North America. They are often associated with eutrophication, and much has been invested in their control. Not all blooms, however, are necessarily the results of human interference with lakes. Scattered paleolimnological evidence indicates that some blooms may be associated with pristine conditions, though this message has largely been ignored. Evidence is given here of a long history of blooms in Whitemere, U.K., from extraction and identification of specific carotenoids from dated sediment cores. Whitemere is representative of a large group of lakes in the West Midlands of the U.K. and is likely to be representative also of those similar postglacial kettle hole lakes in North America and Eurasia, which are groundwater fed with long retention times and thermally stratified. Blue‐green blooms may thus be a normal feature of such lakes and not necessarily a pathology to be controlled.
The nutrient stoichiometry of benthic microalgal growth: Redfield proportions are optimal
Authors: Hillebrand Helmut , Sommer Ulrich
Published: May 2010

Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so‐called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C:N:P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C:N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C:N ratio ranged uniformly around 7.5. N:P ratios <13 indicated N limitation, while N:P ratios >22 indicated P limitation. Under P limitation, the C:P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C:N:P = 119:17:1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106:16:1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.
Influence of fish kairomones on the ovipositing behavior of Chaoborus imagines
Authors: Berendonk Thomas U.
Published: May 2010

The phantom midge, Chaoborus, is a dipteran, the larvae of which occur in many freshwater habitats and are known to be vulnerable to predation by fish. The hypothesis was tested that ovipositing imagines of Chaoborus use chemical cues to detect predatory fish and avoid depositing eggs into waterbodies containing fish kairomones. The experimental setup consisted of cages with two small water‐filled plastic containers, in which Chaoborus imagines could oviposit. One container was connected to a flow‐through system holding fish kairomones, while the other container (the control) was connected to a flow‐through system without fish kairomones. Three different Chaoborus species were tested: two pond‐dwelling species, Chaoborus crystallinus and C. obscuripes, which do not coexist with fish, and a lake‐dwelling species, C. flavicans, which does coexist with fish. The imagines of the two pond‐dwelling species showed an ovipositing preference for fish‐free water. No significant difference in ovipositing preference was found for the lake‐dwelling species (C. flavicans). The present experiment suggests that in addition to direct predatory effects, adult oviposition behavior may contribute to the lack of local coexistence between fish and pond‐dwelling Chaoborus.
Boundary mixing and nutrient fluxes in Mono Lake, California
Authors: MacIntyre Sally , Flynn Kevin M. , Jellison Robert , Romero José R.
Published: May 2010

Temperature‐gradient microstructure and nutrient profiling were undertaken at both an inshore and an offshore site on Mono Lake, California, to determine whether boundary mixing occurred and the effects on nutrient flux within the lake. Turbulence, as quantified by rates of dissipation of turbulent kinetic energy, was two to three orders of magnitude higher at the inshore site where the pycnocline intersected the bottom than at the same depths at an offshore station. The intense turbulence primarily occurred within 3.5 m of the sediment–water interface. In addition, temperature profiles were more incrementally stepped in the pycnocline inshore than offshore. The Turner angle indicated that double‐diffusive processes may have augmented turbulent transport in the upper 10 m, where temperatures were inversely stratified, but not in the main pycnocline. Within the pycnocline, ϵ exceeded the threshold value for buoyancy flux ( ϵthr = 15υN2) in 21% of the turbulent layers inshore but in only 1% of the layers offshore. The coefficient of vertical eddy diffusivity, Κz, was two to four orders of magnitude higher within 4 m of the bottom inshore than offshore at the same depths. Spatially averaged values of Κz, obtained from the heat‐flux method using data obtained from both conductivity–temperature–depth (CTD) profiles and moored thermistor chains, were two orders of magnitude less than those obtained nearshore with microstructure profiling. From the differences in Κz, we inferred that most heat flux occurred due to boundary mixing at the base of the pycnocline inshore with the heat redistributed laterally by advection. Boundary mixing was initiated after winds were strong enough for the Lake number to decrease to a value of 2; thermocline compression and steepening of internal waves at the base of the pycnocline occurred, followed by packets of high‐frequency internal waves critical for wave breaking. Calculated ammonium fluxes at the inshore site were sufficient to support daily rates of primary productivity in the deep chlorophyll maximum throughout the lake. These results indicate the vertical flux of nutrients across the nutricline in Mono Lake occurs over a limited area during intense mixing events initiated by high winds.
Gross and net nitrogen uptake and DON release in the euphotic zone of Monterey Bay, California
Authors: Bronk D. A. , Ward B. B.
Published: May 2010

15N tracer techniques were used to measure rates of NH4+ and NO3− uptake, dissolved organic nitrogen (DON) release resulting from both ammonium (NH4+) and nitrate (NO3−) uptake, and NH4+ regeneration at discrete depths throughout the euphotic zone in Monterey Bay, California, at a site, H3, outside the plume of direct upwelling influence. In March 1993, 94% of the inorganic nitrogen taken up was NH4+, and the primary fate of nitrogen uptake was particle production. In September 1993, NH4+ and NO3− uptake were more in balance, and the primary fate of nitrogen uptake was DON. We suggest that grazing was an important mechanism resulting in DON release in March and that a combination of grazing and a more physiologically stressed phytoplankton population produced the higher observed rates of DON release in September. During both cruises, the percentage of nitrogen released as DON increased with depth, suggesting that deeper in the water column, a smaller percentage of the nitrogen taken up is incorporated into sinking particles. Based on these data, we suggest that the DON pool acts as an intermediate between DIN assimilation and the net formation of particles for export and will thus affect carbon flow in Monterey Bay.
Responses of phytoplankton to varied resource availability in large lakes of the Greater Yellowstone Ecosystem
Authors: Interlandi Sebastian J. , Kilham Susan S. , Theriot Edward C.
Published: May 2010

We assessed phytoplankton dynamics in three lakes in the Greater Yellowstone Ecosystem to better understand the connections between changing environmental conditions and aquatic communities. This work primarily describes the connections between resource availability and phytoplankton seasonal succession in these lakes. We hypothesized that algal species efficient at utilizing a given resource (including N, P, Si, and light) would be correlated with low relative concentrations of those resources. The lakes generally exhibited moderate resource limitation, which is characteristic of lakes in subalpine and subarctic regions. Although in proximity, the lakes all exhibited different resource relationships: Lewis Lake was most P limited, Jackson Lake was most N limited, and Yellowstone Lake exhibited a moderate degree of N limitation along with periodic Si limitation. Mixing depths and light penetration were also variable among lakes. In 1996, spring diatom biomass was dominated by Stephanodiscus minutulus, Asterionella formosa, Aulacoseira subarctica, and Synedra sp. Relative abundances and dominance varied among the lakes. A. formosa and Synedra sp. abundances were positively correlated with total N:total P (TN:TP) levels in an analysis of data from all three lakes. A. subarctica was negatively correlated with TN:TP and all light:nutrient ratios. Species exhibiting late season maxima included Cyclotella bodanica, Fragilaria crotonensis, and Stephanodiscus niagarae. C. bodanica abundances corresponded to high‐light/low‐N situations, whereas S. niagarae maxima were found in high‐TN:TP/low‐light conditions. F. crotonensis abundances were most strongly positively correlated with total Si:TP and TN:TP. Environmental correlations were generally in good agreement with the measured physiological requirements of these species. Additionally, local population maxima of major species of diatoms never coincided.
Effect of CO2 concentration on C:N:P ratio in marine phytoplankton: A species comparison
Authors: Burkhardt Steffen , Zondervan Ingrid , Riebesell Ulf
Published: May 2010

The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,aq], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient‐replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO2,aq]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length‐dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO2,aq] led to large changes in C:N:P ratios. Significant CO2‐related changes in elemental ratios were observed at [CO2,aq] ≪ 10 μmol kg-1 and correlated with a CO2‐dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern for CO2‐related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2‐related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon‐rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient‐replete conditions are representative for natural phytoplankton populations, CO2‐related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle.
Homeostasis in the essential amino acid composition of the marine copepod Euterpina acutifrons
Authors: Guisande Cástor , Maneiro Isabel , Riveiro Isabel
Published: May 2010

The essential amino acid composition (EAA) of females and eggs of Euterpina acutifrons and their food was analyzed on copepods fed with different microalgae and collected from the field. There were not significant differences in the EAA of females, but the EAA of eggs varied according to food source. As predicted by homeostasis theory, a higher reproductive success was observed as the EAA of females and food became more similar. This chemical homeostasis explains the selective retention of amino acids observed in copepods, tells why conversion efficiencies of ingested nitrogen may vary according to the food source, and shows the importance of food quality on reproductive success in copepods.
Analytical solution of Boudreaús equation for a tracer subject to food‐feedback bioturbation
Authors: Swaney Dennis P.
Published: May 2010

Boudreau (1998) obtains a differential equation for a tracer at steady state, subject to mixing associated with bioturbation in a mixed layer of thickness L. As he suggests, the equation may be solved numerically, but an expression for the solution in terms of conventional functional forms would be convenient for the researcher. Analytical solutions corresponding to two different standard boundary conditions are derived below, and their behaviors are discussed briefly.
Production of chlorinated hydrocarbons and methyl iodide by the red microalga Porphyridium purpureum
Authors: Scarratt M. G. , Moore R. M.
Published: May 2010

Two experiments were performed using axenic batch cultures of the red microalga Porphyridium purpureum. The cultures were grown in sealed 5‐liter glass vessels under a high‐purity artificial atmosphere and analyzed for the production of several halocarbons, including chloroform (CHCl3), methylene chloride (CH2 Cl2), methyl iodide (CH3l), trichloroethylene (C2 HCl3), and tetrachloroethylene (C2Cl4). Two cultures and a control were used in each experiment. The first experiment followed the system for 17 d at low light intensity (20 μmol quanta m−2 s−1), while the second was comprised of 7 d at low light followed by 24 h at high light intensity (800 umol quanta m−2 s−1). In both experiments, only chloroform and methyl iodide were produced in measurable quantities. This is the first report of chloroform production in a microalgal species. The chlorophyll a (Chl a) normalized production rate of chloroform in the various cultures ranged between 1.3 × 10−7 and 7.8 × 10−7 mol (g Chl a)−1 d−1 (15–93 μg [g Chl a]−1 d−1). Chloroform production peaked during the logarithmic growth phase. Methyl iodide production ranged between 2.0 × 10−7 and 1.2 × 10−6 mol (g Chl a)−1 d−1 (28‐170 μg [g Chl a]−1 d−1). Exposure of the cultures to high irradiance (800 μmol quanta m−2 s−1) did not stimulate the production of any of the compounds. In contrast to an earlier published account involving this species of algae, the production of tri and tetrachloroethylene was not detected in any of the cultures under either low or high irradiance.
Bicarbonate addition promotes coral growth
Authors: Marubini Francesca , Thake Brenda
Published: May 2010

The addition of 2 mM bicarbonate to aquaria containing tropical ocean water and branches of Porites porites caused a doubling of the skeletal growth rate of the coral. Nitrate or ammonium addition (20 μM) to oligotrophic seawater caused a significant reduction in coral growth, but when seawater containing the extra bicarbonate was supplemented with combined nitrogen, no depression of the higher growth rate was evident. We infer that (1) the present dissolved inorganic carbon (DIC) content of the ocean limits coral growth, (2) this limitation is exacerbated by nitrate and ammonium, and (3) adding DIC increases coral calcification rates and confers protection against nutrient enrichment.
Does UV play a role in changes in predation and zooplankton community structure in acidified lakes?
Authors: Williamson Craig E. , Hargreaves Bruce R. , Orr Pamela S. , Lovera Patricia A.
Published: May 2010

Changes in both zooplankton community structure and zooplanktivorous predators often accompany the anthropogenic acidification of lakes. While changes in pH can account for many of the observed changes, combined observations from laboratory bioassay and field experiments as well as surveys suggest that these patterns cannot be explained by changes in pH alone. Dissolved organic carbon (DOC) also declines with lake acidification. Because DOC is the primary factor regulating variation in the depth of penetration of ultraviolet radiation (UVR) in lakes, there is also likely to be an increase in UVR levels during acidification. This suggests that changes in UVR may play some role in changes in predator and prey communities during acidification. As a first step toward testing this hypothesis, we examined the UVR tolerance of larvae of two widespread and abundant zooplanktivorous predators. We performed a series of in situ incubation experiments with the sunfish Lepomis and the midge Chaoborus in a low DOC (high UVR) lake and in a moderate DOC (low UVR) lake. Substantial UVR‐induced mortality of both predators was observed in the surface waters of the low DOC lake. The predators differed in their UVR tolerance levels: the sunfish survived for more than a day under high solar radiation conditions in the surface waters of a low DOC lake, while the midge perished in less than a day. These data and past literature are consistent with the hypothesis that UVR may play a role in changes in planktivorous predators and their prey during lake acidification and recovery.
Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm
Authors: Williamson Craig E. , Morris Donald P. , Pace Michael L. , Olson Olaf G.
Published: May 2010

The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient‐productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2‐axis model in separating lake types.
Influences of water and substrate quality for periphyton in a montane stream affected by acid mine drainage
Authors: Niyogi Dev K. , McKnight Diane M. , Lewis William M.
Published: May 2010

St. Kevin Gulch, a headwater stream of the Rocky Mountains of Colorado, receives acid mine drainage that maintains low pH, high concentrations of heavy metals, and high rates of metal hydroxide deposition. An acid‐tolerant alga, Ulothrix sp., is present below the source of mine drainage in St. Kevin Gulch, but its biomass is limited by the deposition rates of iron hydroxides, which are especially high near the source. An experimental diversion of the mine drainage increased the quality of water and improved the substrate condition through a reduction of deposition rates. During the first year of the experiment,Ulothrix ecame abundant in this reach. During the second year, pH increased to the point at which aluminum hydroxides precipitated from the stream water onto the streambed; this change inhibited the growth of all periphyton, includingUlothrixThe deposition rate of aluminum hydroxides, however, was less than that of iron hydroxides in stream reaches with high Ulothrix biomas uggesting that metal hydroxides vary by type in their effect on periphyton.
National Center for Ecological Analysis and Synthesis, Santa Barbara, California 93101 and Center for Limnology,University of Wisconsin, Madison, Wisconsin 53706
Authors: Cottingham Kathryn L.
Published: May 2010

Anthropogenic alterations of nutrient inputs and food‐web structure are two of the primary stressors affecting lake phytoplankton. This paper explores the independent and interactive effects of nutrients and food‐web structure (as indexed by zooplankton size) on phytoplankton size structure by fitting time‐series models to data from three lakes where both factors have been manipulated. I assessed phytoplankton size structure using three different approaches: small and large size classes, normalized size spectra, and average phytoplankton size. Increased phosphorus loading and increased zooplankton size had positive effects on large phytoplankton, slope of size spectra, and mean phytoplankton size, but negative effects on the relative abundance of small phytoplankton. The absolute abundance of small phytoplankton was increased by increased nutrients and decreased by increased zooplankton size. There was some evidence for nutrient 3 zooplankton interactions, particularly from interlake comparisons which revealed that phytoplankton size structure responded to enrichment in qualitatively similar but quantitatively different ways in lakes with different food webs. Specifically, small phytoplankton increased more strongly with enrichment in lakes with many planktivores and few large zooplankton, while large phytoplankton increased more in lakes with few planktivores and many large zooplankton. Thus, food‐web structure may influence the extent to which small vs. large phytoplankton respond to nutrient enrichment. Overall, size structure was an excellent descriptor of shifts in phytoplankton communities following manipulation of nutrient inputs and food‐web structure. Size spectra and size classes were highly effective approaches for summarizing size structure; average phytoplankton size was somewhat less effective.
Synergism and antagonism among multiple stressors
Authors: Folt C. L. , Chen C. Y. , Moore M. V. , Burnaford J.
Published: May 2010

This study was designed to test for synergism (increased stress) or antagonism (decreased stress) among multiple environmental stressors using additive, multiplicative, and simple comparative effects models. Model predictions were compared to empirical results of laboratory experiments measuring interactions among thermal stress, toxin exposure, and low food on reproduction and survival of two species of cladoceran zooplankton. Stress was defined operationally as a reduction in reproduction or survival relative to optimal conditions over a 7‐d period. These experiments are particularly applicable to episodic stresses such as those associated with short‐term heat waves. Toxin or low food in combination with 30C temperatures were generally more harmful than high temperature alone. However, most multiple stress effects were antagonistic, in that effects in combination were not as severe as predicted based on the sum or the product of their individual effects. In rare cases, interaction among stressors even diminished effects of the worst single stressor. Optimal conditions for reproduction and survival occurred at 25C, high food and 0 mg liter‐1 toxin (a surfactant, sodium dodecyl sulfate). Suppressive effects of stressors examined individually ranked: high temperature (30C) > SDS (10 mg liter‐1 ɥ low food (∼100 mg C liter‐1) > low temperature (20C). Daphnia pulex isolated from a pond which experiences high summer temperatures throughout was more tolerant of 30C conditions than Daphnia pulicaria isolated from a lake with a cold‐water refuge. Differences were observed in individuals exposed as either adults or as 24‐h neonates.
Using chlorophyll a fluorescence to detect the onset of anthracene photoinduced toxicity in Lemna gibba, and the mitigating effects of a commercial humic acid
Authors: Gensemer Robert W. , Dixon D. George , Greenberg Bruce M.
Published: May 2010

The influence of a commercial humic acid (Aldrich; AHA) on the development of anthracene photoinduced toxicity to the duckweed Lemna gibba was examined using both room‐ and low‐temperature (77°K) chlorophyll fluorescence assays. Plants were exposed to 2 mg liter‐1 anthracene both with and without 6.2 mg liter‐1 of AHA and grown under either visible light or simulated solar radiation that mimics the natural abundance of UV radiation. Exposure periods ranged from 1 to 48 h to examine temporal changes in chlorophyll degradation and chlorophyll a fluorescence induction in response to these light and HA treatments. The onset of anthracene photoinduced toxicity followed a definite sequence; chlorophyll a fluorescence induction parameters (Fv/Fm , and t½) responded earliest to anthracene exposure, with observable chlorophyll degradation requiring up to 24 to 48 h. Of these, t½ was the most sensitive, with significant inhibition apparent within 1 h of exposure. Throughout the entire 48‐h exposure, 6.2 mg liter‐1 AHA ameliorated the photoinduced toxicity of anthracene, in terms of both chlorophyll degradation and Fv/Fm inhibition. In contrast, AHA delayed the complete inhibition of t½ by only 1 to 24 h rather than permanently protecting the plants from anthracene damage to PS2. This suggests that AHA may slow, but not prevent, the entrance of either intact anthracene or its photooxidized byproducts under these exposure conditions.