bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic isotope effect and biochemical characterization of form IA RubisCO from the marine cyanobacterium Prochlorococcus marinus MIT9313
Authors: Scott K. M. , Henn-Sax M. , Harmer T. L. , Longo D. L. , Frame C. H. , Cavanaugh C. M.
Published: November 2010

In an effort to better understand the factors influencing carbon fixation by Prochlorococcus, and to elucidate the effects of these cyanobacteria on the ocean carbon cycle, the kinetic parameters and isotopic discrimination of form IA ribulose 1,5‐bisphosphate carboxylase/oxygenase (RubisCO) from Prochlorococcus marinus MIT 9313 were determined. The RubisCO genes (cbbL and cbbS) were cloned and expressed in Escherichia coli. Enzyme was purified via ammonium sulfate precipitation, and the optimum pH and temperature, as well as Michaelis‐Menton constants, were determined radiometrically. The degree to which this RubisCO discriminates against 13CO2 during fixation was determined by the high‐precision substrate depletion method. Purified enzyme had a pH optimum of 7.5, was most active between 20°C and 30°C, had a moderate Vmax (0.41 µmol min−1 mg protein−1), and had the highest ΚCO2 value (0.75 mmol L−1) for a form I RubisCO characterized to date. The  value,  = 1,000[(Κ12/Κ13) ‐ 1], for the enzyme was determined to be 24.0‰ (95% C.I. = 22.2‐25.6‰), within the range observed for other form I RubisCOs. This  value is a critical baseline for interpreting the δ13C values of marine environmental samples, particularly those collected from the open ocean, where P. marinus is responsible for a substantial fraction of carbon fixation.
Competition and facilitation between unicellular nitrogen‐fixing cyanobacteria and non—nitrogen‐fixing phytoplankton species
Authors: Agawin Nona S. R. , Rabouille Sophie , Veldhuis Marcel J. W. , Servatius Lidewij , Hol Suzanne , van Overzee Harriët M. J. , Huisman Jef
Published: November 2010

Recent discoveries show that small unicellular nitrogen‐fixing cyanobacteria are more widespread than previously thought and can make major contributions to the nitrogen budget of the oceans. We combined theory and experiments to investigate competition for nitrogen and light between these small unicellular diazotrophs and other phytoplankton species. We developed a competition model that incorporates several physiological processes, including the light dependence of nitrogen fixation, the switch between nitrate assimilation and nitrogen fixation, and the release of fixed nitrogen. Model predictions were tested in nitrogen‐limited and lightlimited chemostat experiments using the unicellular nitrogen‐fixing cyanobacterium Cyanothece sp. Miami BG 043511, the picocyanobacterium Synechococcus bacillaris CCMP 1333, and the small green alga Chlorella cf sp. CCMP 1227. Parameter values of the species were estimated by calibration of the model in monoculture experiments. The model predictions were subsequently tested in a series of competition experiments at different nitrate levels. The model predictions were generally in good agreement with observed population dynamics. As predicted, in experiments with high nitrate input concentrations, the species with lowest critical light intensity (S. bacillaris) competitively excluded the other species. At low nitrate input concentration, nitrogen release by Cyanothece enabled stable coexistence of Cyanothece and S. bacillaris. More specifically, model simulations predicted that fixed nitrogen release by Cyanothece enabled S. bacillaris to become four times more abundant in the species mixture than it would have been in monoculture. This intricate interplay between competition and facilitation is likely to be a major determinant of the relative abundances of unicellular nitrogen‐fixing cyanobacteria and non—nitrogen—fixing phytoplankton species in the oligotrophic ocean.
Nitrogen‐fixation strategies and Fe requirements in cyanobacteria
Authors: Berman-Frank Ilana , Quigg Antonietta , Finkel Zoe V. , Irwin Andrew J. , Haramaty Liti
Published: November 2010

Diazotrophic (nitrogen‐fixing) cyanobacteria are important contributors of new nitrogen to oligotrophic environments and greatly influence oceanic productivity. We investigated how iron availability influences the physiology of cyanobacterial diazotrophs with different strategies for segregating nitrogen fixation and photosynthesis. We examined growth, photosynthesis, nitrogen fixation, and Fe requirements of the filamentous nonheterocystous Trichodesmium, the filamentous heterocystous Anabaena, and the unicellular Cyanothece under a range of Fe concentrations. Under similar Fe concentrations the three species differed in N2‐fixation rates, photosynthetic activity, the relative abundance of the photosynthetic units PSI : PSII, elemental stoichiometry, and Fe use efficiency. Complex colonial forms such as Trichodesmium and Anabaena are more likely to be Fe limited in their natural environments and are more efficient at utilizing Fe than unicellular diazotrophs such as Cyanothece. The varied physiological responses to Fe availability of the three cyanobacteria reflect their nitrogen fixation strategies, cell size, unicellular or colonial organization, and may explain, at least in part, the ecological distribution of these photosynthetic bacteria.
Coupled changes in the cell morphology and elemental (C, N, and Si) composition of the pennate diatom Pseudo‐nitzschia due to iron deficiency
Authors: Marchetti Adrian , Harrison Paul J.
Published: November 2010

We investigated the changes in cell morphology and elemental composition with varying iron nutritional status in four oceanic and two coastal isolates of the marine pennate diatom Pseudo‐nitzschia. When iron‐deficient, most isolates exhibited slower specific growth rates (m), reduced maximum photochemical yields (WM), and lower chlorophyll a (Chl a) contents than iron‐replete cultures. Iron‐deficient cells exhibited reduced cell volumes, primarily as the result of decreased valve transapical widths, which increased the valve aspect ratios. The increase in aspect ratios varied among isolates and was not correlated with the degree of growth limitation due to irondeficiency. In all Pseudo‐nitzschia isolates, the mean carbon (Ccell), nitrogen (Ncell), and silicon (Sicell) cell quotas of iron‐deficient cells decreased when compared to iron‐replete cells. Similarly, in five out of the six isolates, the mean C and N quotas normalized per unit cell volume (Cvol and Nvol) of iron‐deficient cells also decreased, but by a lesser extent. In contrast, there were no clear differences in the changes in the mean Si quotas normalized per unit cell surface area (Sisa). The Sicell:Ncell and Sicell : Ccell ratios (quotas normalized per cell) of iron‐deficient cells increased compared to iron‐replete conditions due to greater reductions in C and N cell quotas compared to Si cell quotas. This increase in the Sicell:Ncell ratio was proportional to the changes in the valve aspect ratios. Our results show that iron‐deficient Pseudo‐nitzschia cells acclimate to low iron concentrations by increasing their valve aspect ratios, thus reducing their cell volumes and increasing their surface area‐to‐volume (SA : V) ratios. These changes in cell morphology increase the Si‐containing valve surface area relative to the volume of the internal components, which may then influence the cellular elemental composition, in particular, the Si :N and Si :C ratios.
Simulation and observations of annual density banding in skeletons of Montastraea (Cnidaria: Scleractinia) growing under thermal stress associated with ocean warming
Authors: Wírum Ferenc P. , Carricart-Ganivet Juan P. , Benson Larry , Golicher Duncan
Published: November 2010

We present a model of annual density banding in skeletons of Montastraea coral species growing under thermal stress associated with an ocean‐warming scenario. The model predicts that at sea‐surface temperatures (SSTs) <29°C, high‐density bands (HDBs) are formed during the warmest months of the year. As temperature rises and oscillates around the optimal calcification temperature, an annual doublet in the HDB (dHDB) occurs that consists of two narrow HDBs. The presence of such dHDBs in skeletons of Montastraea species is a clear indication of thermal stress. When all monthly SSTs exceed the optimal calcification temperature, HDBs form during the coldest, not the warmest, months of the year. In addition, a decline in mean‐annual calcification rate also occurs during this period of elevated SST. A comparison of our model results with annual density patterns observed in skeletons of M. faveolata and M. franksi, collected from several localities in the Mexican Caribbean, indicates that elevated SSTs are already resulting in the presence of dHDBs as a first sign of thermal stress, which occurs even without coral bleaching.
Bioturbation changes the patterns of benthic emergence in zooplankton
Authors: Viitasalo Satu , Katajisto Tarja , Viitasalo Markku
Published: November 2010

We investigated the effects of two functionally different deposit feeders, the amphipod Monoporeia affinis and the bivalve Macoma balthica, on the benthic emergence of copepods (Acartia spp., Eurytemora affinis, Temora longicornis), cladocerans (Bosmina longispina maritima, Daphnia spp.), and rotifers (Synchaeta spp., Keratella spp., Notholca spp.) in the northern Baltic Sea. We performed 3.5‐week laboratory experiments in June‐July and in September‐October using surface sediment containing zooplankton eggs and various combinations of amphipod and bivalve densities. Both zoobenthic species increased egg mortality rates in Synchaeta spp. and in calanoids. Each amphipod consumed a maximum of one Synchaeta spp. egg and 31 calanoid eggs per day, while the bivalves consumed six Synchaeta and 143 calanoid eggs per day. Monthly consumption throughout the community matches or even exceeds the field densities of eggs. In summer, M. affinis decreased the daily emergence rate of Synchaeta spp. by 31‐53% and of M. balthica by 57%. The effect on calanoids was less severe: the emergence rate of Acartia spp. and E. affinis was significantly suppressed only by M. balthica (the reduction was 48% and 28% in Acartia spp. and in E. affinis, respectively). T. longicornis was the only species that benefited from macrofauna: together the two zoobenthic species resulted in a 2.7‐fold increase in the emergence of this calanoid. Bioturbation significantly influences the benthic emergence of zooplankton, with effects varying among species, and may thus affect the dynamics of pelagic populations.
Biochemical food quality effects on a Daphnia hybrid complex
Authors: Brzeziński Tomasz , von Elert Eric
Published: November 2010

Correlative field studies have shown that the food quality of natural seston for Daphnia is highly correlated with the content of eicosapentaenoic acid (EPA), indicating that the growth of Daphnia in nature is at least seasonally limited by a low availability of this polyunsaturated fatty acid. Here we test the hypothesis that a putative limitation due to limited availability of EPA in natural seston should lead to genetic adaptation in physiological traits reflected in genotype‐dependent differences in the ability of sympatric clones of Daphnia to cope with these shortages. We further test if this adaptation to the low availability of EPA provides another explanation for the coexistence of hybrids with their parental taxa. Standardized growth experiments were performed with and without dietary EPA using clones of three taxa from the Daphnia longispina complex: Daphnia galeata, Daphnia hyalina, and their interspecific hybrids. Fitness was estimated from juvenile somatic growth rates, g, and from intrinsic population growth rates, r. There was marked interclonal variability in the susceptibilities of g and r to the absence of EPA from the food, with considerable intraspecific variability. At the taxon level, we noted differences in susceptibility to EPA limitation with regard to the intrinsic rate of population increase r, which demonstrates that the availability of EPA affects the relative fitness of hybrids, but not of the parental species. On average, hybrids seemed less susceptible to the absence of EPA than do parental species. Environmental heterogeneity driven by EPA‐mediated changes in the quality of food may in consequence contribute to the maintenance of genetic diversity in Daphnia and may be a reason for the temporal dominance of hybrids.
Population dynamics of a freshwater calanoid copepod: Complex responses to changes in trophic status and climate variability
Authors: Seebens Hanno , Straile Dietmar , Hoegg Rainer , Stich Hans-Bernd , Einsle Ulrich
Published: November 2010

The long‐term dynamics of the calanoid copepod Eudiaptomus gracilis were analyzed in Lake Constance, a deep lake that has been influenced strongly by changes in trophic status and climate variability during the last few decades. Although copepod abundances remained rather stable throughout the study period (1970‐1995), the seasonal pattern changed strongly, including a seasonal shift in the timing of peak abundance from spring to late summer with increasing oligotrophication. This shift in seasonality resulted from seasonally opposing long‐term trends in abundances (a reduction in abundances in spring and an increase in summer) possibly due to seasonally opposing long‐term trends in naupliar mortality (an increase in late winter mortality and a decrease in early summer mortality). The changes in seasonality were most pronounced within the early copepodid stages, whereas the long duration of the adult stage seemed to buffer the adult dynamics against seasonal environmental variability. The effects of climate variability on population dynamics were more subtle and consisted of faster development in spring in years with faster vernal warming. Furthermore, the accelerated development in spring resulted in earlier occurrence of the population minimum in summer and subsequently in an earlier abundance increase in late summer/autumn. Hence, abundances in summer/autumn were still related to spring climate variability, which shows that spring environmental variability can be transferred further into the season by the cohort dynamics of continuously reproducing populations.
Efficacy of open‐ocean ballast water exchange as a means of preventing invertebrate invasions between freshwater ports
Authors: Gray Derek K. , Johengen Thomas H. , Reid David F. , MacIsaac Hugh J.
Published: November 2010

Ballast water is a major vector of nonindigenous species invasion globally. Mandatory ballast water exchange (BWE) was implemented for vessels carrying ballast water into the Great Lakes in 1993. Despite the implementation of this policy, few data are available on its effectiveness, and invasions have continued to be reported in the Great Lakes. In this study, we conducted experiments to assess the efficacy of BWE on six operational transoceanic vessels traveling from the Great Lakes to European ports. Each vessel had paired ballast tanks, one of which was designated as a control that remained filled with Great Lakes water, while the other was exchanged with mid‐ocean water. Community composition was assessed immediately after tanks were filled and again prior to water discharge in European ports. BWE was verified by ship records and, in two cases, by in situ water quality sensors. BWE was highly effective (>99% loss) for reducing concentrations of freshwater zooplankton. Live sentinel amphipods and oligochaetes deployed in incubator chambers sustained nearly universal mortality in tanks that experienced BWE, but not in unexchanged tanks. Finally, BWE reduced in situ recruitment of zooplankton from diapausing eggs present in ballast sediments in additional incubator chambers deployed in these tanks. Collectively, these studies support the contention that BWE by transoceanic vessels traveling between freshwater ports results in ballast water that would exceed proposed International Maritime Organization (2004) ballast water performance standards if these standards were applied to freshwater species only. Thus, BWE provides strong protection to freshwater ecosystems against invasions by both pelagic and benthic freshwater species.
The effects of grazing by the snail, Lymnaea elodes, on benthic N2 fixation and primary production in oligotrophic, arctic lakes
Authors: Gettel Gretchen M. , Giblin Anne E. , Howarth Robert W.
Published: November 2010

This study assessed whether grazing by the snail, Lymnaea elodes, limits benthic dinitrogen (N2) fixation and primary production in nitrogen (N)‐limited oligotrophic lakes near Toolik Field Station on the North Slope of Alaska. We also tested whether snail excretion increased N and the ratio of N and phosphorus (P) supply ratio to benthic algae, which could indirectly affect production and the N2 fixation rate. We performed in situ, randomized block experiments in two lakes in 3 years in which snail density was manipulated and compared to open cage controls. Snails significantly decreased areal rates of N2 fixation in both lakes in all years (p < 0.05), but did not appear to cause a reduction in cyanobacterial abundance or filament size (p > 0.05). Snails did not significantly affect measures of benthic production, including gross primary production, respiration, net ecosystem production, and chlorophyll biomass (p > 0.05). Snail‐induced declines in N2 fixation probably did not result from snail excretion. The molar N: P excretion ratio of ammonium (NH4+) and phosphate (PO4+) was very low (4.8), indicating that snails likely exacerbated N limitation, a response that would tend to favor enhanced rather than reduced N2 fixation. Furthermore, the excretion rate of N‐NH4+ was several orders of magnitude lower than the N2 fixation rate (0.002‐0.02 mg N m−2 day−1 vs. 0.1‐0.4 mg N m−2 day−1, respectively) and met almost none (<<1%) of the N demand by primary producers. Although the mechanism by which Lymnaea elodes caused a decline in N2 fixation is unknown, the effect was small, and accounted for a reduction of N inputs of only 0.12 mg N m−2 summer−1 or by 0.85‐1.8% at ambient snail densities. Because N2 fixation is a new N input able to support new production, this effect may be important across long time scales or where densities of L. elodes are higher.
Variations in the optical properties of terrigenous mineral‐rich particulate matter suspended in seawater
Authors: Stramski Dariusz , Babin Marcel , Woźniak Slawomir B.
Published: November 2010

From laboratory measurements, we determined the spectral mass‐specific absorption, ap*(λ), and scattering, bp*(λ, coefficients for terrigenous mineral‐rich particulate assemblages suspended in seawater. The samples were derived mostly from surface soils in different locations and consisted of small particles (,10 mm). Both ap*(λ and bp*(λ showed large variability associated with variations in particle size distribution (PSD) and origin of samples. Variations in ap*(λ produced by changes in PSD are consistent with the package effect, in that samples with a higher percentage of small‐sized particles have higher ap*(λ. The variability among the samples is also associated with composition of particulate matter. For example, ap*(λ at blue wavelengths varied from ~0.05 m2 g−1 for organic‐dominated soil dust to ~0.1‐0.5 m2 g−1 for mineral‐dominated samples. The effects of particulate composition are reflected in a broad range of imaginary refractive index of particles, which in the blue can exceed 0.2‐0.3 for mineral‐dominated samples rich in iron oxides. The patterns of the variability in the scattering coefficient among the samples are quite intricate because of the effects of PSD and composition. In general, bp*(λ ranged from about 0.5 to 1.5 m2 g−1, and the spectral behavior varied from nearly flat spectra to the spectral dependency ~l2 λ with a slope λ as high as ~1.3 for the sample with the largest contribution of small particles.
Quantitative estimates of labile and semi‐labile dissolved organic carbon in the western Arctic Ocean: A molecular approach
Authors: Davis Jenny , Benner Ronald
Published: November 2010

A novel molecular approach based on carbon‐normalized yields of combined amino acids was developed to quantify concentrations of labile (L), semi‐labile (S), and refractory (R) dissolved organic carbon (DOC) in shelf and basin waters of the Western Arctic Ocean. Concentrations of L‐DOC were seasonally and spatially variable (0.1‐14.2 µmol L−1). In contrast, concentrations of S‐DOC were much less variable (20.2 ± 0.68 µmol L−1 SE). Average concentrations of L‐DOC in shelf waters increased from 0.7 µmol L−1 to 2.4 µmol L−1 between the spring and summer of 2002 and from 1.4 µmol L−1 to 3.9 µmol L−1 between the spring and summer of 2004. Primary productivity increased 2‐3‐fold between spring and summer, indicating a strong linkage between plankton and L‐DOC production. Patterns of L‐DOC abundance in surface waters are suggestive of multiple mechanisms of L‐DOC production, including direct release from phytoplankton and release during grazing. Concentrations of L‐DOC were not correlated with those of total DOC. Elevated concentrations of L‐DOC in halocline waters (40‐200‐m depth) of the Canada Basin indicated rapid transport of shelf‐produced DOC into the basin. Chemical and physical properties of basin waters with elevated L‐DOC concentrations indicated a sediment‐derived source of basin L‐DOC. The approach presented here for quantifying the labile and semilabile fractions of DOC is a potentially powerful tool for understanding processes controlling the distribution, production, and utilization of dissolved organic matter.
Light‐stimulated production of dissolved DMSO by a particle‐associated process in the Ross Sea, Antarctica
Authors: del Valle Daniela A. , Kieber David J. , John Bisgrove , Kiene Ronald P.
Published: November 2010

Dimethylsulfoxide (DMSO) is an abundant form of methylated sulfur in marine systems and it is known to be produced from dimethylsulfide (DMS). Using radiolabeled 35S‐DMS and gas chromatography techniques, we quantified the dissolved DMSO (DMSOd) produced from photo‐ and biological oxidation of dissolved DMS and compared the DMSOd production from these pathways with the net change in DMSOd concentrations in unfiltered seawater samples. The net change in DMSOd in light‐exposed treatments exceeded DMSOd production from photo‐ plus biological oxidation of dissolved DMS. This indicated that DMSOd was produced by one or more light‐driven processes likely associated with particulate material. Results from in situ incubation arrays showed that the relative importance of DMSOd production processes was dependent on irradiation depth, with the unidentified particle‐associated process and dissolved DMS photooxidation the main DMSOd sources close to the surface (0–10 m) and biological oxidation of dissolved DMS the main process at depths at which the light level was low (>10 m). Deckboard and in situ incubations revealed that DMSOd production from the particle‐associated process was stimulated by ultraviolet radiation. Higher particle‐associated production of DMSOd in samples more prone to suffer light‐induced stress supports the hypothesis that this process was related to phytoplanktonic biosynthesis and release of DMSO because of oxidative stress. Our results suggest that particle‐associated DMSOd production is an important source of DMSOd in surface waters of the Ross Sea and also help to explain why DMSOd is periodically the main organosulfur compound detected in the upper water column.
Distribution and redox speciation of dissolved iron on the European continental margin
Authors: Ussher Simon J. , Worsfold Paul J. , Achterberg Eric P. , Laës Agathe , Blain StÉphane , Laan Patrick , de Baar Hein J. W.
Published: November 2010

To investigate the biogeochemistry of iron in the waters of the European continental margin, we determined the dissolved iron distribution and redox speciation in filtered (<0.2 µm) open‐ocean and shelf waters. Depth profiles were sampled over the shelf slope southeast of the Chapelle Bank area (47.61°N, 4.24°W to 46.00°N, 8.01°W) and a horizontal surface‐water transect over the shelf and through the English Channel (la Manche) and the southern North Sea (46°N, 8°W to 52°N, 4°E). An abrupt trace‐metal front was found near the shelf slope, indicated by a horizontal gradient of dissolved iron (DFe) and aluminium (DAl), which correlated with changing salinities (r2 = 0.572 and 0.528, respectively, n = 92). Labile Fe(II) concentrations varied from <12 pmol L−1 in North Atlantic surface waters to >200 pmol L−1 in the near bottom waters of the shelf break. Labile Fe(II) accounted for ~5% of the dissolved iron species in surface shelf waters (mean 5.0 ± 2.7%), whereas higher Fe(II) fractions (i.e., >8%) were observed near the sea bottom on the shelf break and during a midday solar maximum in surface waters in the vicinity of the Scheldt river plume. Benthic processes (resuspension and diagenesis) constituted important sources of Fe(II) and DFe in this region, and photoreduction of Fe(III) species in shelf waters caused enhanced labile Fe(II) concentrations. These processes increased the lability of iron and its potential availability to marine organisms in the shelf ecosystem.
Iron limitation across chlorophyll gradients in the southern Drake Passage: Phytoplankton responses to iron addition and photosynthetic indicators of iron stress
Authors: Hopkinson Brian M. , Mitchell B. Greg , Reynolds Rick A. , Wang Haili , Selph Karen E. , Measures Christopher I. , Hewes Christopher D. , Holm-Hansen Osmund , Barbeau Katherine A.
Published: November 2010

Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron‐enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1–2 nmol L−1) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream highchlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (Fv:Fm, σPSII, and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.
Dinosterols or dinocysts to estimate dinoflagellate contributions to marine sedimentary organic matter?
Authors: Mouradian Maggy , Panetta Robert J. , de Vernal Anne , GÉlinas Yves
Published: November 2010

Dinosterol (4α,23,24‐trimethyl‐5α‐cholest‐22E‐en‐3β‐ol) is frequently used as an alternative to dinoflagellate cyst (dinocyst) counting in paleoceanography to assess dinoflagellate inputs to marine sediments. However, recent studies have shown poor correlation between these two proxies in continental‐margin sediments. We reevaluated the relationship and expanded it to include a suite of biogeochemical transformation products of the parent dinosterol (dinosterone, dinostanone, and dinostanol). These dinoflagellate‐specific 4α,23,24‐trimethyl steroidal species (𐎣dinosterol) are compared to dinocyst counts in sediments from the western Mexican margin (375–3,500 m). Samples were taken from subsurface (3‐6 cm) and down core (16‐27 cm) to reflect widely contrasting organic carbon content and redox conditions. A strong correlation was found between the sum of all dinoflagellate‐derived sterols, 𐎣dinosterols, and total dinocyst counts, highlighting the importance of including diagenetic alteration products of the parent molecule when exploiting organic biomarkers in paleoceanographic studies. In low‐energy environments and for well‐preserved samples, such as those studied in this work, both methods provide robust, internally consistent data, suggesting that when diagenetic transformation products of dinosterol are taken into account, gas chromatography and optical microscopy could be used interchangeably to estimate dinoflagellate inputs to marine sediments.
Assessing environmental control on dinoflagellate cyst distribution in surface sediments of the Benguela upwelling region (eastern South Atlantic)
Authors: Bockelmann Frank-D. , Zonneveld Karin A. F. , Schmidt Martin
Published: November 2010

Organic‐walled dinoflagellate cyst assemblages in surface sediments of the Benguela Current Upwelling System (eastern South Atlantic) show geographic patterns that cannot entirely originate from cyst production or transport. Aimed at answering how far these variations are due to taphonomic control, this study investigated a possible correlation with the changes in bottom‐water oxygen concentrations typifying this region. Based on 36 samples, multivariate statistics were used to analyze community variability with respect to bottom‐water oxygen concentration, temperature, salinity, nutrient content, chlorophyll a (Chl a) concentration, the organic carbon content of surface sediments, and a measure of water column stratification. Determined relationships to salinity, nutrient supply, nutrition, and environmental steadiness point out the requirements for dinoflagellate cyst production, while cross‐shelf transport processes could have introduced variability prior to burial of cysts in surface sediments. The offshore decrease in the relative abundance of protoperidinacean cyst types was consistent with their lower preservation potential under oxygenated conditions and coincided with a change in assemblage composition toward oxidation resistant species. On elimination of covariation, bottom‐water oxygenation was significantly related to this pattern and determined together with seasonal salinity, Chl a, and annual phosphate concentration, the parameter combination best explaining community variability. These results suggest that postdepositional degradation of peridinioid dinoflagellate cysts would partly explain the onshore‐offshore gradient in species distributions and could be responsible for more variability in assemblage compositions than is presently acknowledged.
Top‐down and bottom‐up controls on sediment organic matter composition in an experimental seagrass ecosystem
Authors: Spivak Amanda C. , Canuel Elizabeth A. , Duffy J. Emmett , Richardson J. Paul
Published: November 2010

We tested the singular and interactive effects of resource availability (light) and community composition (food chain length and herbivore species richness) on eelgrass (Zostera marina) ecosystem properties and functioning with an experimental mesocosm system. Food chain length was manipulated through the presence or absence of blue crab (Callinectes sapidus) predators, whereas grazer species richness varied across three levels (zero, two, or four crustacean species). We found important and interacting effects of bottom‐up and top‐down forcings on sediment organic matter (SOM) composition. Light increased eelgrass and algal biomass and sediment organic carbon and nitrogen content. Increasing grazer diversity generally decreased algal biomass and ecosystem production but interacted with food chain length (i.e., presence of predatory crabs) and light. Predators generally increased algal biomass and ecosystem production through a trophic cascade, which was stronger at high grazer diversity and under ambient light. SOM composition, determined with fatty acid (FA) biomarkers, was sensitive to all manipulated variables. Increasing grazer species richness often decreased the contributions of FAs derived from plant and algal sources, whereas increasing light had the opposite effect. Food chain length was generally a less important determinant of SOM composition than light, although predators did increase FAs representative of heterotrophic bacteria. Overall, resource availability and epibenthic community composition strongly influenced organic matter cycling, SOM composition, and the bacterial community in seagrass‐bed sediments.
Horizontal structure and excitation of primary motions in a strongly stratified lake
Authors: Shimizu Kenji , Imberger Jörg , Kumagai Michio
Published: November 2010

A modal analysis in the horizontal plane was extended to a layer‐stratified basin with irregular bathymetry, and the theory was applied to Lake Biwa to investigate the horizontal structure and excitation of the basin‐scale internal waves and gyres. The horizontal structure of the basin‐scale internal waves consisted of cyclonic and anticyclonic elliptic cells, each of which appeared to follow the dispersion relationship of Kelvin and PoincarÉ waves in elliptic basins. The internal waves were preferentially excited depending on the arrangement of the cells and the wind direction, but the spatial distribution of wind stress curl over the lake primarily determined the horizontal structure of the ensuing gyres. Decoupled evolutionary equations for the individual modes provided a good approximation for excitation of the internal waves and early stages of excitation of the gyres before nonlinear effects and damping become significant. The modal decomposition of hydrodynamic simulation results also showed that the primary action of the wind was to excite the internal waves; however, these internal waves were damped within a few days, and the dynamics during calm periods were dominated by the gyres, illustrating the importance of internal waves on mixing and gyres on long‐term horizontal transport.
The seasonal evolution of high vertical‐mode internal waves in a deep reservoir
Authors: Vidal Javier , Rueda Francisco J. , Casamitjana Xavier
Published: November 2010

The causal mechanism and seasonal evolution of the internal wave field in a deep, warm, monomictic reservoir are examined through the analysis of field observations and numerical techniques. The study period extends from the onset of thermal stratification in the spring until midsummer in 2005. During this time, wind forcing was periodic, with a period of 24 h (typical of land‐sea breezes), and the thermal structure in the lake was characterized by the presence of a shallow surface layer overlying a thick metalimnion, typical of small to medium sized reservoirs with deep outtakes. Basin‐scale internal seiches of high vertical mode (ranging from mode V3 to V5) were observed in the metalimnion. The structure of the dominant modes of oscillation changed as stratification evolved on seasonal timescales, but in all cases, their periods were close to that of the local wind forcing (i.e., 24 h), suggesting a resonant response. Nonresonant oscillatory modes of type V1 and V2 became dominant after large frontal events, which disrupted the diurnal periodicity of the wind forcing.
Role of across‐shelf currents in the dynamics of harmful dinoflagellate blooms in the northwestern Iberian upwelling
Authors: Crespo B. G. , Figueiras F. G. , Groom S.
Published: November 2010

Harmful dinoflagellate blooms are frequent in the Rías Baixas, which are made up of four open bays on the northwest part of the Iberian Peninsula. The relationship between surface currents resulting from wind forcing and the microplankton composition on the shelf offshore of the Rías Baixas was studied from May 2001 to April 2002. The aim was to assess the role of shelf currents in the dynamics of harmful dinoflagellate blooms in the Rías Baixas shelf system. In summer, upwelling‐favorable northerly winds forced a variable but persistent southwestward flow on the shelf, which reverted to an onshore flow following the change to downwellingfavorable southwesterly winds in autumn. During winter and spring, there was an intense alongshore flow to the north. Diatoms and dinoflagellates were common components of the microplankton community in summer. Although diatoms increased their abundance during summer upwelling events, a dinoflagellate assemblage was also present on the shelf. This dinoflagellate assemblage acquired higher importance during the intermediate periods of upwelling relaxation and during the autumn downwelling in particular, when satellite images revealed the accumulation of phytoplankton in a narrow band close to the coast. A dinoflagellate bloom of Prorocentrum minimum (10,564 cells mL−1) and Karenia cf. mikimotoi (3,022 cells mL−1) developed in the interior section of the Ría de Vigo coincident with the onshore flow imposed by the autumn downwelling. As these two species were components of the dinoflagellate assemblage present on the shelf in summer, it is inferred that downwelling caused their accumulation in the interior section of the Ría de Vigo.
Eutrophication‐induced phosphorus limitation in the Mississippi River plume: Evidence from fast repetition rate fluorometry
Authors: Sylvan J. B , Quigg A. , Tozzi S. , Ammerman J. W.
Published: November 2010

We assessed nutrient limitation in the Mississippi River plume and Louisiana continental shelf during the summer of 2002 (04‐08 July). We measured nutrient concentrations, alkaline phosphatase (AP) activities, chlorophyll a (Chl a) concentrations, and four fast repetition rate fluorescence (FRRF) parameters: the maximum quantum yield of photochemistry in photosystem II (PSII), Fv:Fm; the functional absorption cross section for PSII, σPSII; the time for photosynthetic electron transport on the acceptor side of PSII, τQa; and the connectivity factor, p, in 24‐h‐long nutrient addition bioassays near the Mississippi River delta. Low phosphorus (P) concentrations, elevated inorganic nitrogen‐to‐phosphorus ratios, high AP activities, and Chl a increases in response to P additions in the bioassays all indicated phosphorus limitation that was confirmed by the response of FRRF parameters. This is the first study to use FRRF to confirm results from basic oceanographic methods to demonstrate phosphorus limitation in a marine setting. Fv:Fm and p responded positively to phosphorus addition, while σPSII and τQa decreased in the same treatments. When nitrate alone was added, none of the measured parameters differed significantly from the control. We therefore suggest that FRRF can be used to rapidly detect phosphorus limitation in marine ecosystems.
Persistence of iron(II) in surface waters of the western subarctic Pacific
Authors: Roy Eric G. , Wells Mark L. , King D. Whitney
Published: November 2010

The distribution of dissolved iron(II) [Fe(II)] was studied in surface waters of the western subarctic Pacific during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study‐II (SEEDS II) iron enrichment experiment using highly sensitive flow injection‐based luminol chemiluminescence. Vertical profiles of Fe(II) and total dissolved iron were measured outside of the fertilized patch to investigate the chemical speciation of iron in this high‐nitrate low‐chlorophyll (HNLC) region. Ambient total dissolved iron concentrations ranged from 50 pmol L−1 to 150 pmol L−1 depending on depth and sampling times. Unexpectedly, Fe(II) accounted for up to half of total dissolved iron, with concentrations up to ~50 pmol L−1. Fe(II) concentrations decreased exponentially with depth and were undetectable at depths below 50 m. There was no evidence of increased Fe(II) concentrations associated with the subsurface chlorophyll maximum, indicating that photolysis, rather than biological reduction of Fe(III), was the primary source of Fe(II). Because Fe(II) concentrations in the fertilized patch remained elevated for more than a week after enrichment, Fe(II) oxidation rates at near‐ambient concentrations were measured. Indeed, the temperature‐dependent Fe(II) oxidation rates were significantly slower than predicted by Fe(II) oxidation models and rates measured in ligand‐free seawater. These findings suggest that Fe(II) binding ligands may exist in these HNLC waters, with conditional stability constants on the order of 108‐ 109 with respect to Fe2+. The accumulation of Fe(II) during daylight hours did not alleviate iron limitation of eukaryotic phytoplankton in these waters, contrary to expectations from recent iron uptake models.
Response of the methanogenic microbial community of a profundal lake sediment (Lake Kinneret, Israel) to algal deposition
Authors: Schwarz Julia I. K. , Eckert Werner , Conrad Ralf
Published: November 2010

An algal bloom of Peridinium gatunenese generally precedes the annual maximum of methane release from the profundal sediment of Lake Kinneret. Therefore, we investigated the response of the sediment methanogenic microbial community to simulated algal deposition. Addition of algal biomass on top of sediment cores resulted in increased CH4 production rates and concentrations of the fermentation products acetate and propionate in the upper 4‐cm layers with maximum values at 1‐cm depth. Addition of algae to sediment slurries also resulted in increased CH4 production rates and a transient increase of H2, propionate, and acetate concentrations within the first 10 d after addition. The composition of the active microbial community was determined by analysis of terminal restriction fragment polymorphism (T‐RFLP) targeting ribosomal RNA and cloning and sequencing of reverse‐transcribed 16S rRNA. Analysis of the sediment in the presence and absence of algae indicated that among the Bacteria, members of Deltaproteobacteria and Clostridiales responded by synthesis of ribosomes after 1 d of incubation and those of Bacteroidetes after 6 d. Among the Archaea, ribosomal RNA of the Methanosaetaceae (i.e., acetate‐utilizing methanogens) slightly increased after 6 d. Algal deposition apparently stimulated ribosomal synthesis in these sediment microorganisms, thus resulting in increased activity. We conclude that these microorganisms were involved in degradation of the algal biomass resulting in transient release of acetate and other fermentation products and increased production of CH4.
Mangrove tannins in aquatic ecosystems: Their fate and possible influence on dissolved organic carbon and nitrogen cycling
Authors: Maie Nagamitsu , Pisani Oliva , JaffÉ Rudolf
Published: November 2010

We describe the fate of mangrove leaf tannins in aquatic ecosystems and their possible influence on dissolved organic nitrogen (DON) cycling. Tannins were extracted and purified from senescent yellow leaves of the red mangrove (Rhizophora mangle) and used for a series of model experiments to investigate their physical and chemical reactivity in natural environments. Physical processes investigated included aggregation, adsorption to organic matter‐rich sediments, and co‐aggregation with DON in natural waters. Chemical reactions included structural change, which was determined by excitation‐emission matrix fluorescence spectra, and the release of proteins from tannin‐protein complexes under solar‐simulated light exposure. A large portion of tannins can be physically eliminated from aquatic environments by precipitation in saline water and also by binding to sediments. A portion of DON in natural water can coprecipitate with tannins, indicating that mangrove swamps can influence DON cycling in estuarine environments. The chemical reactivity of tannins in natural waters was also very high, with a half‐life of less than 1 d. Proteins were released gradually from tannin‐protein complexes incubated under light conditions but not under dark conditions, indicating a potentially buffering role of tannin‐ protein complexes on DON recycling in mangrove estuaries. Although tannins are not detected at a significant level in natural waters, they play an important ecological role by preserving nitrogen and buffering its cycling in estuarine ecosystems through the prevention of rapid DON export/loss from mangrove fringe areas and/or from rapid microbial mineralization.
Seasonal variability of the dynamics of dimethylated sulfur compounds in a coastal northwest Mediterranean site
Authors: Vila-Costa Maria , Kiene Ronald P. , Simí Rafel
Published: November 2010

We studied the seasonal variation of biotic and abiotic processes and the physico‐chemical forcing factors involved in the production and consumption of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) at a coastal sampling station in the northwestern Mediterranean. Monthly samplings of surface seawater for an 18‐month period revealed that algal‐associated DMSP and dimethylsulfoxide (DMSO) did not follow total phytoplankton biomass (measured as chlorophyll a [Chl a]). DMSP concentrations peaked 1 or 2 months later than the late winter Chl a bloom, following phytoplankton succession, whereas particulate DMSO was maximal in summer. Both Chl a‐normalized concentrations (DMSP : Chl a and DMSO: Chl a) exhibited a clear seasonality with maxima in summer, which was indicative of concurrent phytoplankton succession and physiological acclimation toward higher dimethylated sulfur‐producing taxa in summer. DMS concentrations also showed clear maxima in mid‐summer and minima in winter, which is anticorrelated with Chl a. Gross DMS production rates were higher in summer, coinciding with higher DMSP‐to‐DMS conversion yields and exceeded microbial DMS consumption in this season. Heterotrophic bacteria and DMSP‐assimilating phytoplankton only accounted for a portion (annual average 52%) of total DMSP transformations, suggesting that phytoplankton DMSP‐lyases, either in stressed cells or upon grazing by herbivores, must play a more important role in DMS production than is generally believed. Calculated photolysis and measured microbial consumption alternated in dominance as DMS sinks over the year, with ventilation to the atmosphere generally being a minor loss process. Under higher solar radiation (from March to September), calculated photolysis followed variations of colored dissolved organic matter, a known DMS photosensitizer.
Sinking particle properties from polyacrylamide gels during the KErguelen Ocean and Plateau compared Study (KEOPS): Zooplankton control of carbon export in an area of persistent natural iron inputs in the Southern Ocean
Authors: Ebersbach F. , Trull T. W.
Published: November 2010

The Kerguelen ocean and plateau compared study (KEOPS) examined the origin of elevated phytoplankton biomass in naturally iron‐fertilized waters over the Kerguelen plateau during midsummer (January‐February 2005). We report sinking particle characteristics determined from image analysis of thousands of individual particles collected in viscous polyacrylamide gels placed in free‐drifting sediment traps at two sites: a high phytoplankton biomass site over the central plateau (A3) and a moderate biomass site at its periphery (C5). The particles were divided into three types (1) oval fecal pellets, (2) cylindrical fecal pellets, and (3) aggregates. The aggregates were most abundant and mainly consisted of agglomerations of the cylindrical fecal pellets. Conversion of the pellet and aggregate volumes to carbon contents suggests export fluxes of 50‐60 mg C m−2 d−2 at 100‐m depth, in reasonable agreement with independent estimates from carbon and 234Th measurements. Our observation that the majority of the particle flux was processed through the heterotrophic foodweb contrasts with the results of artificial iron‐fertilization experiments and with models for export from productive diatomdominated waters that emphasize direct export of phytoplankton detritus. The KEOPS results may offer more appropriate scaling for the response of ecosystem structure and carbon export to persistent iron fertilization in the Southern Ocean.