bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical and biogeochemical limits to internal nutrient loading of meromictic Lake Kivu
Authors: Pasche Natacha , Dinkel Christian , Müller Beat , Schmid Martin , Wüest Alfred , Wehrli Bernhard
Published: January 2011

Lake Kivu is one of the large African Rift lakes situated between the Democratic Republic of the Congo and Rwanda. In its permanently stratified hypolimnion, unusually high methane concentrations have increased further in recent decades. Because methanogenesis is, in part, dependent on supply of organic material from the photic zone, it is necessary to quantify upward nutrient fluxes from the saline, nutrient‐rich deep waters. These upward fluxes are mainly driven by advection caused by subaquatic springs. Biogenic calcite precipitation drives surface‐water depletion and deep‐water enrichment of Ca2+, Sr2+, and Ba2+. Methane is mainly oxidized aerobically at the redox interface at 60 m, with a small contribution of anaerobic methane oxidation. A subaquatic spring that sustains the major chemocline at 250 m depth was depleted of N, P, and CH4, and concentrations of major ions were slightly lower than in the lake water of the same depth. Enrichment of the deep waters with nutrients and CH4 are driven by mineralization of settling organic material, whereas SiO2 is influenced by uptake and mineralization of diatoms and inputs through subaquatic springs. Dissolved inorganic phosphorus and Si fluxes supplied by internal loading through upwelling were found to be lower than the estimations for Lakes Malawi and Tanganyika. In contrast, N flux was within the lower range for Lake Malawi, whereas it was assumed to be totally lost by denitrification in Lake Tanganyika. In Lake Kivu, nutrient uptake by primary production is three times higher than nutrient upward fluxes.
Diffusive boundary layers do not limit the photosynthesis of the aquatic macrophyte, Vallisneria americana, at moderate flows and saturating light levels
Authors: Nishihara Gregory N. , Ackerman Josef D.
Published: January 2011

Hydrodynamic models of mass transport assume that diffusive processes next to the surface limit transport and that there are no biological and chemical processes that control the supply and demand of the scalar. The validity of these assumptions was examined by measuring the momentum boundary layer (via particle image velocimetry) and the concentration boundary layer (via O2 microsensors) over the leaves of Vallisneria americana. The O2 flux (Jobs) was highest at x = 2 cm downstream from the leading edge of the leaf and was 1.8 to 1.4 times higher than Jobs measured at the trailing edge of the leaf at 0.5 cm s−1 and 6.6 cm s−1 mean velocity (U), respectively. The maximum Jobs was 0.44 ± 0.07 (mean ± SE) vs. 0.50 ± 0.09 µmol m−2 s−1 at 0.5 vs. 6.6 cm s−1. Interestingly, the surface O2 potential (D[O2] = [O2]surface ‐ [O2]bulk) was also unimodal at the low velocity (D[O2]max = 36 ± 5 mmol m−3 at x = 3 cm) but was uniform at the higher velocity (D[O2] = 9 ± 0.7 mmol m‐3). An analysis of the time scale of nutrient diffusion (τD) vs. nutrient uptake (τup) through the measured diffusion boundary layer revealed that uptake was always the slower process (i.e., τD < τup; τD and τup increased with x and decreased with U). Under moderate water velocities and saturating irradiance, uptake rates rather than diffusive transport processes appear to control mass transfer rates regardless of the location on the leaf and the water velocity.
Complementary UV protective compounds in zooplankton
Authors: Hylander Samuel , Boeing Wiebke J. , Granéli Wilhelm , Karlsson Jan , Einem Jessica von , Gutseit Kelly , Hanssona Lars-Anders
Published: January 2011

Zooplankton accumulate several groups of photoprotective compounds to shield against damaging ultraviolet radiation (UV). One of these groups, the carotenoids, makes the animals more conspicuous to visually hunting predators, whereas others, such as the mycosporine‐like amino acids (MAAs) may not. The blend of photoprotective compounds is therefore important for the UV defense but also for the ability to escape predation through crypsis. Here we assess laboratory and field data from different latitudes to examine how UV, predation threat, and pigment availability (in food) affects the mixture of UV‐protective compounds in copepods. Overall, the blend of MAAs and carotenoids was partly explained by the availability of MAAs in the food, the UV‐threat, and the presence of predators. Copepods upregulated their MAA content when UV threat was increasing (i.e., if MAAs were abundant in food), and in field data this accumulation only occurred at high levels of predation threat. If MAAs were scarce, copepods instead compensated with higher carotenoid accumulation. However, when there was a high predation threat this carotenoid compensatory effect was disadvantageous, and low concentrations of both MAAs and carotenoids at high UV‐threat resulted in lower reproduction. In all, these results showed that carotenoids and MAAs are complementary substances, i.e., one is high when the other is low, and copepods are, hence, able to adjust their blend of different UV‐protective compounds to optimize their defenses to the threats of UV and predation. These defense systems may buffer against direct food‐web interactions and help the zooplankton to survive in environments with high UV threat.
Significant contribution of passively sinking copepods to downward export flux in Arctic waters
Authors: SampeiH Makoto , Sasaki iroshi , Hattori Hiroshi , Forest Alexandre , Fortiera Louis
Published: January 2011

Typically, all undecomposed metazoans found in formalin‐poisoned cups of sediment traps are considered to be active intruders (or “swimmers”) and are removed to prevent an overestimation of the downward particle flux. However, intact metazoans dead before entering the trap should be included in the estimation of downward flux of carbon. Arctic copepods collected in the field were killed either by formalin to mimic death by actively swimming into the formalin‐poisoned cups, or by crowding or high temperature and then preserved in formalin to simulate death before entering the trap. In the crowding and heat treatments, 64% of Calanus hyperboreus and C. glacialis and 44% of Pareuchaeta glacialis differed from copepods killed by formalin in the postmortem posture of the antennules or swimming legs. These frequencies were used to estimate the contribution of passively sinking copepods (PSCs) to the particulate organic carbon (POC) flux measured by a sediment trap moored at 70 m in the Beaufort Sea (Canadian sector of the Arctic Ocean). PSCs represented only a small fraction (<5%) of the copepods collected in the trap, thus justifying the removal of swimmers from samples to avoid a massive overestimation of fluxes. Nevertheless, PSCs contributed 36% of the overall POC flux of 6.8 g C m‐2 yr‐1, and discarding them along with the swimmers would have resulted in a significant underestimation of downward export. Given their rich carbon content (up to 50%), PSCs could be an important food resource for pelagic and benthic scavengers.
Viral control of bacterial growth efficiency in marine pelagic environments
Authors: Motegi Chiaki , Nagata Toshi , Miki Takeshi , Weinbauer Markus G. , Legendre Louis , Rassoulzadegand Fereidoun
Published: January 2011

We tested the hypothesis that viruses can control bacterial growth efficiency (BGE) in marine pelagic environments. In the Bay of Villefranche, Northwestern Mediterranean, three experiments were conducted on different months to determine bacterial and viral variables in seawater cultures. In December, phosphorus (P) addition enhanced bacterial growth 6‐9‐fold with concomitant increase in viral production (4‐15‐fold), but little enhancement of bacterial respiration (BR). In other months, P enrichment increased BR 2‐6‐fold and viral production 2‐5‐fold, but did not increase in bacterial abundance (Aug, Feb) or growth (Feb). BGE depended on the fraction of bacterial production destroyed by viruses (shunting efficiency, v; i.e., when v was low, nutrient enrichment enhanced BGE, whereas when v was high, nutrient enrichment mainly led to low BGE). Viral production and bacterial production and respiration in the Western North Pacific and other data from the literature showed that BGE was negatively correlated with shunting efficiency. Predictions from a carbon flow model were consistent with the above results showing that decreased BGE over a broad range of values (from 0.7 to 0.001) could be largely explained by viral‐induced conversion of bacterial biomass to dissolved organic carbon. Viruses exert the major influence on patterns in carbon fluxes mediated by bacteria in marine pelagic environments.
Geochemical ecosystem engineering by the mud shrimp Upogebia pugettensis (Crustacea: Thalassinidae) in Yaquina Bay, Oregon: Density‐dependent effects on organic matter remineralization and nutrient cycling
Authors: D'Andrea Anthony F. , DeWitt Theodore H.
Published: January 2011

We investigated the effect of the thalassinid mud shrimp Upogebia pugettensis on organic matter and nutrient cycling on Idaho Flat, an intertidal flat in the Yaquina River estuary, Oregon. Field studies were conducted to measure carbon and nitrogen remineralization rates and benthic fluxes relative to shrimp population density. Organic carbon remineralization rates estimated from incubations increased with burrow density. Similarly, benthic oxygen uptake increased linearly with burrow density. Shrimp bioirrigation enhanced carbon remineralization up to 2.9 times; its effects on nitrogen cycling were even more dramatic, enhancing ammonification rates up to 7 times, nitrification rates between 3 and 9 times, and estimated denitrification up to 4 times relative to portions of the tide flat lacking shrimp. Bioirrigation by shrimp also reduced pore‐water inventories of dissolved inorganic nitrogen (DIN), resulting in DIN fluxes that increased exponentially with burrow density up to 15‐fold relative to No‐shrimp plots. For both C and N fluxes, the strongest density‐dependent relationships occurred with burrow hole density rather than shrimp population density or biomass. Combined with population mapping of mud shrimp populations on Idaho Flat, we estimate that total areal C and DIN effluxes on the tide flat were enhanced 1.9 and 3.7 times, respectively, relative to areal fluxes expected if no shrimp were present. Mud shrimp thus perform as geochemical ecosystem engineers that strongly affect the flux and fate of C and N in Pacific estuaries.
Carbon isotope composition of fatty acids and sterols in the scleractinian coral Turbinaria reniformis: Effect of light and feeding
Authors: Treignier C. , Tolosa I. , Grover R. , Reynaud S. , sa C. Ferrier-Pagé
Published: January 2011

The effect of auto‐ and heterotrophy on the carbon isotope composition (δ13C) of fatty acids (FA) and sterols was studied in the scleractinian coral Turbinaria reniformis. Colonies were maintained in aquaria for 6 weeks under two light levels (100 and 300 mmol photons m−2 s−1) and/or two trophic conditions (starved and fed twice a week with zooplankton). In high‐light conditions, lipid biomarkers, both in the zooxanthellae and animal tissue, presented significantly higher δ13C values compared to those exposed to low light, supporting the widespread theory of enhanced 13C fixation with increasing irradiance and photosynthesis. This result and the similar lipid δ13C‐values of the host and the symbionts are a strong indication that most lipids are synthesized by the zooxanthellae. Feeding had a smaller effect than light on the lipid δ13C‐signatures and a more pronounced effect at low‐light level. Despite the depleted δ13C values of the zooplankton lipids, slight enrichment in 13C was observed in some FA from the host and zooxanthellae, whereas the main host sterol was depleted in 13C. These differences between FA and sterols give evidence for different biosynthesis pathways: FA might have been synthesized from the assimilation of carbohydrates or amino acids contained in the food, whereas sterols might derive from modification of dietary lipids.
Biogeneration of chromophoric dissolved organic matter by bacteria and krill in the Southern Ocean
Authors: Ortega-Retuerta Eva , Frazer Thomas K. , Duarte Carlos M. , Ruiz-Halpern Sergio , Tovar-Sánchez Antonio , Arrieta Jesús M. , Rechea Isabel
Published: January 2011

Chromophoric dissolved organic matter (CDOM), the optically active fraction of dissolved organic matter, is primarily generated by pelagic organisms in the open ocean. In this study, we experimentally determined the quantity and spectral quality of CDOM generated by bacterioplankton using two different substrates (with and without photoproducts) and by Antarctic krill Euphausia superba and evaluated their potential contributions to CDOM dynamics in the peninsular region of the Southern Ocean. CDOM was generated by bacteria in all experiments, and the presence of photoproducts influenced both the quantity and the spectral quality of the resultant CDOM. We confirmed a direct link between bacterial production and CDOM generation, which yielded in situ CDOM duplication times from 31 to 33 d. Antarctic krill as a direct source of CDOM was also confirmed experimentally. We estimated that CDOM generation by krill would lead to CDOM duplication times from 0.48 to 0.80 d within krill swarms. Our findings highlight the potential significance of bacteria and Antarctic krill swarms in the generation of CDOM and underscore the dynamic nature of CDOM in this area.
Effects of phytoplankton community on production, size, and export of large aggregates: A world‐ocean analysis
Authors: Guidi Lionel , Stemmann Lars , Jackson George A. , Ibanez Frédéric , Claustre Hervé , Legendre Louis , Picheral Marc , Gorskya Gabriel
Published: January 2011

We recorded vertical profiles of size distributions of large particles (<100 µm) to a 1000‐m depth in the Atlantic, Indian, and Pacific Oceans and in the Mediterranean Sea with the Underwater Video Profiler. Of the 410 profiles used in our analysis, 193 also included temperature, salinity, and high‐performance liquid chromatography (HPLC)‐resolved pigments, which were used to characterize the size structure of the phytoplankton community. Classification analysis identified six clusters of vertical profiles of size distributions of particles. Each cluster was characterized by the size distribution of its particles in the mesopelagic layer and the change of the particle‐size distribution with depth. Clusters with large particles in the mesopelagic layer corresponded to surface waters dominated by microphytoplankton, and those with small particles corresponded to surface waters dominated by picophytoplankton. We estimated the mass flux at 400 m using a relationship between particle size and mass flux. Principal‐component regression analysis showed that 68% of the variance of the mass flux at 400 m was explained by the size structure of the phytoplankton community and integrated chlorophyll a in the euphotic zone. We found that coefficient k in the Martin power relationship, which describes the decrease in the vertical mass flux with depth, varies between 0.2 and 1.0 in the world ocean, and we provided an empirical relationship to derive k from the size structure of phytoplankton biomass in the euphotic zone. Biogeochemists and modelers could use that relationship to obtain a realistic description of the downward particle flux instead of using a constant k value as often done.
Tidally regulated chemical fluxes across the sediment—water interface in Elkhorn Slough, California: Evidence from a coupled geochemical and hydrodynamic approach
Authors: Breier J. A. , Nidzieko N. , Monismith S. , Moore W. , Paytan A.
Published: January 2011

Measurements of four Ra isotopes, Ba, Mn, U, and dissolved nutrients were used in concert with hydrodynamic modeling to reveal a large, tidally regulated sediment‐surface‐water exchange within Elkhorn Slough, an estuary in Monterey Bay, California. Hydrologic and oceanographic data from in situ sensors and geophysical surveys of estuarine bottom sediments were integrated with the chemical measurements to develop Ra flux boundary conditions for a three‐dimensional hydrodynamic transport model of the estuary. Sediment and surface‐water exchange is the dominant source of Ra isotopes within this system. This process occurs without significant fresh terrestrial groundwater discharge. A number of known solute transport and pore‐water flushing mechanisms can explain the 224Ra input, including diffusion, evaporative transport, bioirrigation, and inundation and drainage of borrows and macropores. The 226Ra results suggest that some of these mechanisms, or others, are capable of mobilizing Ra from deeper sediments. The sediments supplying this flux may cover an area as large as the entire intertidal zone, primarily salt marsh, to a depth of 1–2 cm. In addition, the Ra delivery to the estuary appears to be temporally regulated by the tidal inundation of the salt marsh and subsequent drainage through tidal creek channels. These results illustrate the importance of the intertidal zone as a flowpath in the estuarine water cycle and the tidal phase lag that results when solute from this zone is transported into the estuary. This process is essential to understanding estuarine biogeochemical budgets of elements with significant sedimentary sources.
Metagenomic potential of microbial assemblages in the surface waters of the central Pacific Ocean tracks variability in oceanic habitat
Authors: Hewson Ian , Paerl Ryan W. , Tripp H. James , Zehr Jonathan P. , Karl David M.
Published: January 2011

Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local environmental conditions. To understand the relationship between microbial assemblage composition, functional capability, and habitat, a random genome shotgun sequencing (metagenomic) survey was conducted with surface‐water microbial assemblages (0.2–5‐µm size fraction) collected at seven locations along a meridional transect from the northern edge of the South Pacific subtropical gyre to the southern edge of the North Pacific subtropical gyre (16°S–13.5°N). A total of 1.1 million unique sequence reads were obtained, of which ~45% could be annotated to metabolic category. Microbial assemblages in equatorial divergence and countercurrent habitats were distinct phylogenetically from those in gyre waters. Ecotypes of dominant Cyanobacteria (Prochlorococcus and Synechococcus) had distinct distributions congruent with their physiological characteristics in cultivation. The metagenomic distribution of genes among metabolic pathways was very similar at all stations despite phylogenetic differences, but was unrelated to physicochemical habitat, suggesting that dominant microorganisms have a core suite of genes necessary for life in the open ocean. Among metabolic genes that varied across the transect, several patterns were observed. For example, phosphate (PO43‐) stress response genes were more common in gyre waters than at the equator. The variability in frequency of several metabolic pathways (e.g., chlorophyll biosynthesis, PO43‐ metabolism, and transcription initiation bacterial sigma factors) was related to physicochemical conditions, most of which were related to taxonomic differences among habitats. Microbial communities in the central Pacific Ocean are phylogenetically distinct to the oceanic provinces which they inhabit.
In situ diel cycles of photosynthesis and calcification in hermatypic corals
Authors: Schneider Kenneth , Levy Oren , Dubinsky Zvy , Erez Jonathan
Published: January 2011

Diel cycles of photosynthesis, respiration, and calcification were studied in situ on two hermatypic corals, Acropora eurystoma and Favia favus, using a submerged respirometer deployed at 5‐m depth in the Gulf of Eilat, Israel. A fast‐repetition‐rate fluorometer (FRRF) was placed in front of one coral in each incubation. Net photosynthesis (PN) and calcification were positively correlated with photosynthetically available radiation (PAR). A hysteresis phenomenon occurred in both photosynthesis and calcification relative to PAR: rates were higher in the afternoon than in the morning. This hysteresis is reversed (counterclockwise) compared to that observed in other photosynthetic organisms (plants and algae); however, the quantum yield (∆F′ : Fm′) and the rate of photosynthetic electron transport per PSII reaction center (Pf) showed a regular (clockwise) hysteresis. The photosynthetic compensation point (Ic) was significantly higher in the morning than in the afternoon, ~900 and ~300 µmol photons m−2 s−1, respectively. We suggest that PN, reversed hysteresis, and lower morning compensation points are caused by higher respiration in the morning than in the afternoon. This may be a result of higher predation rates by the corals during the night, when plankton is more abundant, followed by higher respiration during the digestion of the prey. Calcification counterclockwise hysteresis was observed during high‐PAR periods (>400 and 200 µmol photons m−2 s−1 for A. eurystoma and F. favus, respectively). We suggest that high morning respiration may induce a decrease in internal tissue pH, causing lower calcification rates.
Mixing and stratification in lakes of varying horizontal length scales: Scaling arguments and energy partitioning
Authors: Rueda Francisco , Schladow Geoffrey
Published: January 2011

This work analyzes the effects that changes in the surface area of stratified lakes have on the partitioning of energy among different pools, and, ultimately, on the strength of thermal stratification. Proposed restoration plans for the Salton Sea (USA), the surface area of which could be reduced by 80%, provided the original motivation of this work. Scaling arguments, based on a one‐dimensional model of lake stratification and three‐dimensional simulations, are consistent in predicting weaker mixing for smaller lakes. It is demonstrated that energy partitioning is affected by geometry for lakes that have a quarter internal wave period (Ti/4) that is shorter than the duration of wind forcing events, ∆tw. In these cases, it is demonstrated that mixing is, in general, stronger in larger basins due to greater mechanical energy fluxes across the free surface; larger shear production of turbulent kinetic energy Etk; and a larger fraction of Etk becoming background potential energy through diapycnal mixing. All these variables scale with the length of the basin. In lakes where Ti/4 > ∆tw, the rates of exchange among the different energy pools and the magnitudes of these pools are not sensitive to changes in lake size. Our results suggest that stratification in the reduced Salton Sea will be stronger than in the lake as it exists today, mainly in late summer. It is during that time when, due to high insolation values, stratification becomes strong and Ti/4 in the reduced sea will likely become small in comparison with the diurnal winds.
Predicting habitat suitability in temperate seagrass ecosystems
Authors: van der Heide T. , Peeters E.T.H.M. , Hermus D. C. R. , van Katwijk M. M. , Roelofs J. G. M. , Smolders A. J. P.
Published: January 2011

The worldwide observed dramatic decline of seagrasses has typically been attributed to multiple stressors such as eutrophication, disease, sedimentation, and toxicity events. Using principal component analysis and (multivariate) logistic regression, we investigated the importance of 30 commonly measured variables in explaining the presence and absence of the temperate seagrass species Zostera marina and Zostera noltii at 84 Western European locations. Although many interrelated variables influence seagrass presence in our dataset, presence or absence of both species could be reliably predicted by using only two easy‐to‐measure variables. A logistic regression model of Z. marina correctly predicted 77% of all observations by including water column light attenuation and sediment pore‐water reduction oxidation potential (RedOx). The Z. noltii model had an 86% accuracy based on only tidal location (intertidal or subtidal zone) and pore‐water RedOx. Applying the models to five evaluation sites demonstrated that both models can be usefully applied as tools for seagrass ecosystem restoration and conservation.
Molecular evidence for the occurrence of ctenophore Mertensia ovum in the northern Baltic Sea and implications for the status of the Mnemiopsis leidyi invasion
Authors: Gorokhova Elena , Lehtiniemi Maiju , Viitasalo-Frösen Satu , Haddock Steven H. D.
Published: January 2011

Nucleotide sequence analysis of 18S ribosomal RNA gene (rRNA), internal transcribed spacer, and 5.8S rRNA was used for taxonomic identification of ctenophores collected in the northern Baltic Sea, where invasive Mnemiopsis leidyi and native Pleurobrachia pileus have been reported to occur. Contrary to previous reports, sequence analysis of 53 randomly selected specimens from seven stations revealed that none of them were M. leidyi or P. pileus. The 18S rRNA and 5.8S rRNA sequences were 100% identical to those of Mertensia ovum, a ctenophore with a broad Arctic and circumboreal distribution, which has never been reported to occur in the Baltic Sea. Polymerase chain reaction screening with primers designed to amplify all three species, and using ctenophores collected by vertically stratified sampling, confirmed that all ctenophores collected in this survey were M. ovum. The ctenophore abundance was high, up to 4500 individuals m−2, positively correlating with salinity. Our findings emphasize the utility of applying molecular tools to biological surveys and the importance of rigorous species identification. They also indicate that M. leidyi, which is a threat to the southern Baltic ecosystem, does not occur in the northern part of the sea, and call for a pan‐Baltic survey to establish current distributions of ctenophores, both native and invasive.
Terrestrial organic matter and light penetration: Effects on bacterial and primary production in lakes
Authors: Ask Jenny , Karlsson Jan , Persson Lennart , Ask Per , Byström Pär , Jansson Mats
Published: January 2011

We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear‐water to brown‐water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear‐water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole‐lake (benthic + pelagic) energy mobilization was negatively correlated to the light‐extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light‐absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.
Flow paths and spatial heterogeneity of stream inflows in a small multibasin lake
Authors: Rueda Francisco J. , MacIntyre Sally
Published: January 2011

We describe the flow paths of negatively buoyant river inflows in a small lake with multiple basins separated by sills (Toolik Lake, Alaska) using field data and three‐dimensional simulations. Comparisons of field observations, analytical computations, and simulations show that in small basins in which the timescale for filling (τf) is less than the timescale of an event (τ0), overflow is the dominant mechanism of interbasin exchange. Exchange is mediated by internal wave displacements if upwelling reaches the height of the sills, zs. This criterion is met if the Lake number, the inverse of which is proportional to the degree of metalimnetic tilting, is less than zi /(zi − zs), where zi is the vertical displacement of the intrusion. Entrainment followed by horizontal dispersion is the dominant mechanism of interbasin transport and can account for ~65% of the exchange between large subbasins. Entrainment is enhanced when submerged sills force river water to flow close to the surface layer. Depths of intrusions depend upon discharge. They occurred in the lower metalimnion for the coldest events analyzed but were near the top of the metalimnion during low‐discharge events, such as during brief cold fronts. Persistence of intrusions depends upon the time interval between cold fronts; they ranged from a few days to three weeks. Timescales of horizontal mixing vary with meteorological forcing; they ranged from on the order of 1 d for wind events with speeds of up to 6–8 m s−1 to on the order of 10 d for light winds.
Top‐down and bottom‐up influences of jellyfish on primary productivity and planktonic assemblages
Authors: West Elizabeth J. , Pitt Kylie A. , Welsh David T. , Koop Klaus , Rissik David
Published: January 2011

A manipulative mesocosm experiment was done in a saline coastal lake in Australia to compare the top‐down and bottom‐up influences of jellyfish on primary production and planktonic assemblages. We hypothesized that non‐zooxanthellate jellyfish (Catostylus mosaicus) would exert both ‘top‐down’ (grazing) and ‘bottom‐up’ (nutrient excretion) effects, whereas zooxanthellate jellyfish (Phyllorhiza punctata) would only exert top‐down influences, as their dissolved excretory products are internally recycled to their zooxanthellae rather than released into the water column. Experimental treatments consisted of triplicate mesocosms that contained two C. mosaicus, two P. punctata, a combination of C. mosaicus and P. punctata, no jellyfish, and an open‐water control (sampling outside mesocosms). Both species of jellyfish preyed heavily on mesozooplankton, initiating a topdown trophic cascade that resulted in increased production of the heterotrophic dinoflagellate Protoperidinium sp. However, no increase in primary production or phytoplankton biomass was observed in the treatments containing P. punctata, indicating that top‐down processes did not extend to primary producers. Bottom‐up excretion of nutrients, however, caused phytoplankton biomass to more than double in the C. mosaicus treatment compared to all other treatments. Increased primary production was due largely to a 10‐fold increase in the diatom Chaetoceros sp. and was predominantly driven by C. mosaicus excreting phosphate (PO43‐), which was the limiting nutrient in the lake. Blooms of both zooxanthellate and non‐zooxanthellate jellyfish will deplete mesozooplankton and alter the composition of microzooplankton assemblages via top‐down processes. Excretion of nutrients by blooms of non‐zooxanthellate jellyfish, however, can also greatly increase phytoplankton production and could favor algal blooms.
The effects of elevated carbon dioxide concentrations on the metamorphosis, size, and survival of larval hard clams (Mercenaria mercenaria), bay scallops (Argopecten irradians), and Eastern oysters (Crassostrea virginica)
Authors: Talmage Stephanie C. , Gobler Christopher J.
Published: January 2011

We present experiments that examined the metamorphosis, growth, and survivorship of larvae from three species of commercially and ecologically valuable shellfish (Mercenaria mercenaria, Argopecten irradians, and Crassostrea virginica) at the levels of CO2 projected to occur during the 21st century and beyond. Under CO2 concentrations estimated to occur later this century (~66 Pa, 650 ppm), M. mercenaria and A. irradians larvae exhibited dramatic declines (>50%) in survivorship as well as significantly delayed metamorphosis and significantly smaller sizes. Although C. virginica larvae also experienced lowered growth and delayed metamorphosis at ~66 Pa CO2, their survival was only diminished at ~152 Pa CO2. The extreme sensitivity of larval stages of shellfish to enhanced levels of CO2 indicates that current and future increases in pelagic CO2 concentrations may deplete or alter the composition of shellfish populations in coastal ecosystems.
Climate variation, carbon flux, and bioturbation in the abyssal North Pacific
Authors: Vardaro Michael F. , Ruhl Henry A. , Smith Kenneth Jr. L.
Published: January 2011

We hypothesized that seasonal and interannual climate‐mediated changes in particulate organic carbon (POC) flux would affect bioturbation and ultimately the sequestration of organic carbon in the deep sea. An 18‐yr timeseries photographic record from 4100‐m depth in the northeast Pacific Ocean showed increased abundance of Echinocrepis rostrata, a common epibenthic echinoid and bioturbator, since the late 1990s. Abundance, size, and speed data were used to estimate bioturbation potential to track long‐term changes in the volume of sediment disturbed by E. rostrata. There was no secular increase in E. rostrata bioturbation over 18 yr despite increased population size, although periodic variations in bioturbation were significantly correlated with POC flux. Expected changes in POC flux and bioturbation rates due to climate variation could lead to altered rates of carbon sequestration in deep‐sea sediments, affecting the global carbon cycle.
Population genetics and hydrodynamic modeling of larval dispersal dissociate contemporary patterns of connectivity from historical expansion into European shelf seas in the polychaete Pectinaria koreni
Authors: Jolly M. T. , Guyard P. , Ellien C. , Gentil F. , Viard F. , Thiébaut E. , Jollivet D.
Published: January 2011

Using Pectinaria koreni as a biological model of larval dispersal, we coupled the analysis of differently evolving genetic markers (mitochondrial cytochrome oxidase I and four microsatellite loci) to hydrodynamic modeling of larval transport in the English Channel. To determine the influence of historical and contemporary processes on the genetic structure of our study populations, genetic relationships between English Channel, Irish Sea, and southern North Sea populations were assessed in relation to the long‐term pattern of marine currents and to postglacial colonization pathways. Although significant, low level of overall nuclear genetic differentiation was best explained by the recent origin of populations within the study area and the retention of ancestral polymorphism. Both genetic data sets suggest that two ancestral gene pools contributed to the origin of our study populations, and secondary contacts occurred in the English Channel and southern North Sea as a result of two migration routes around the British Isles. Although Irish Sea and Belgium populations appeared more recently connected, populations of the eastern English Channel were more isolated. English Channel patterns of connectivity indicated high dispersal and gene flow along the French coast, from Normandy to the southern North Sea. Despite significant genetic differentiation between both coasts, migration model selection favored cross‐channel gene flow and long‐distance migration following the coastlines. Our results highlight the influence of postglacial colonization on genetic patterns in the English Channel, and indicate that contemporary mesoscale connectivity inferred by hydrodynamic modeling cannot, alone, explain the present genetic structure of populations in this area.
Use of long‐term data and multivariate ordination techniques to identify environmental factors governing estuarine phytoplankton species dynamics
Authors: Rothenberger Meghan B. , Burkholder JoAnn M. , Wentworth Thomas R.
Published: January 2011

A continuous, 13‐yr record of environmental data and phytoplankton species and assemblage structure in the mesohaline Neuse River Estuary (biweekly, April‐October; monthly, November‐March) was used to evaluate phytoplankton assemblage responses to changing environmental conditions. Ordination techniques including nonmetric multidimensional scaling, indicator species analysis, and BIO‐ENV software were used to investigate potential environmental predictors of phytoplankton assemblage patterns under chronic eutrophication. Phytoplankton assemblages were strongly related to temperature and total nitrogen : total phosphorus ratios, with expected seasonal changes in species composition. Interannual changes in river discharge influenced whether phytoplankton assemblages were dominated by diatoms and phototrophic flagellates or by mixotrophic and heterotrophic dinoflagellates. Increasing ammonium concentrations also have been an important influence on phytoplankton assemblages. Raphidophytes (including the potentially toxic species Heterosigma akashiwo), haptophytes, chlorophytes, and the bloom‐forming dinoflagellate Heterocapsa rotundata have increased in more recent years (2000–2006), concomitant with increasing ammonium concentrations. Abundance of the potentially toxic dinoflagellates Prorocentrum minimum and the grouping Pfiesteria spp., “pfiesteria‐like” dinoflagellates, and Karlodinium veneficum remained stationary over time and rarely exceeded 103 cells mL−1. Abundance of P. minimum was positively related to dissolved organic nitrogen and suspended solids concentrations, whereas the highest abundance of the grouping Pfiesteria spp., “pfiesteria‐like” dinoflagellates, and K. veneficum occurred during summer and fall, related to high total phosphorus concentrations, temperature, and salinity. Overall, this study provides new species‐level insights to advance understanding about anthropogenic influences on phytoplankton assemblages. The data suggest an increasingly important role of ammonium in controlling phytoplankton assemblage structure, including increased abundance of some harmful species, in eutrophic estuaries.
Hydrologic control of nitrogen removal, storage, and export in a mountain stream
Authors: Hall Robert Jr. O. , Baker Michelle A. , Arp Christopher D. , Koch Benjamin J.
Published: January 2011

Nutrient cycling and export in streams and rivers should vary with flow regime, yet most studies of stream nutrient transformation do not include hydrologic variability. We used a stable isotope tracer of nitrogen (15N) to measure nitrate (NO3−) uptake, storage, and export in a mountain stream, Spring Creek, Idaho, U.S.A. We conducted two tracer tests of 2‐week duration during snowmelt and baseflow. Dissolved and particulate forms of 15N were monitored over three seasons to test the hypothesis that stream N cycling would be dominated by export during floods, and storage during low flow. Floods exported more N than during baseflow conditions; however, snowmelt floods had higher than expected demand for NO3− because of hyporheic exchange. residence times of benthic N during both tracer tests were longer than 100 d for ephemeral pools such as benthic algae and wood biofilms. Residence times were much longer in fine detritus, insects, and the particulate N from the hyporheic zone, showing that assimilation and hydrologic storage can be important mechanisms for retaining particulate N. Of the tracer N stored in the stream, the primary form of export was via seston during periods of high flows, produced by summer rainstorms or spring snowmelt the following year. Spring Creek is not necessarily a conduit for nutrients during high flow; hydrologic exchange between the stream and its valley represents an important storage mechanism.
The relative importance of heterotrophic bacteria to pelagic ecosystem dynamics varies with reservoir trophic state
Authors: Caston C. Brad , Nowlin Weston H. , Gaulke Alicia , Vanni Michael J.
Published: January 2011

We tested the hypothesis that nutrient sequestration and carbon flow through heterotrophic bacteria is relatively highest in oligotrophic systems and decreases with trophic state in 17 reservoirs in Texas and Ohio, U.S.A. The percentage of particulate carbon (C), nitrogen (N), and phosphorus (P) in bacteria (< 1 µm) was highest in oligotrophic reservoirs and decreased with chlorophyll a, our measure of trophic state. Patterns of nutrient sequestration in bacteria were very similar to Minnesota (U.S.A.) natural lakes. Bacterial production (BPr) and primary production (PPr) ranged from 13 µg C L−1 d−1 to 172 µg C L−1 d−1 and from 41 µg C L−1 d−1 to 1695 µg C L−1 d−1, respectively, and the BPr : PPr ratio decreased with trophic state. Inclusion of variables such as dissolved organic carbon (DOC), DOC: soluble reactive phosphorus, and DOC: dissolved inorganic nitrogen did not improve predictions of BPr : PPr over that based solely upon chlorophyll a. BPr : PPr was better predicted by DOC: particulate carbon (PC) and mixing depth (Zmix) than by chlorophyll a. The relationship between BPr : PPr and DOC: PC was primarily driven by the relationship between BPr : PPr and PC, which was a surrogate for trophic state. Unexpectedly, the strength of Zmix as a predictor BPr : PPr did not primarily reflect differences in the light environment. Our results support the hypothesis that the relative importance of heterotrophic bacteria is highest in oligotrophic systems and decreases with trophic state, but provide limited support for proposed mechanisms of this pattern. Patterns in microbial nutrient dynamics are similar in reservoirs and natural lakes.
Aerobic respiration and hypoxia in the Lower St. Lawrence Estuary: Stable isotope ratios of dissolved oxygen constrain oxygen sink partitioning
Authors: Lehmann Moritz F. , Barnett Bruce , Gélinas Y. , Gilbert Denis , Maranger Roxane J. , Mucci Alfonso , Sundby Bjorn , Thibodeau Benoit
Published: January 2011

We measured the concentration and the stable isotope ratios of dissolved oxygen in the water column in the Estuary and Gulf of St. Lawrence to determine the relative importance of pelagic and benthic dissolved oxygen respiration to the development of hypoxic deep waters. The progressive landward decrease of dissolved oxygen in the bottom waters along the axis of the Laurentian Channel (LC) is accompanied by an increase in the 18O:16O ratio, as would be expected from O‐isotope fractionation associated with bacterial oxygen respiration. The apparent O‐isotope effect, εO‐app, of 10.8% reveals that community O‐isotope fractionation is significantly smaller than if bacterial respiration occurred solely in the water column. Our observation can best be explained by a contribution of benthic O2 consumption occurring with a strongly reduced O‐isotope effect at the scale of sediment‐water exchange (εO‐sed ~ 7%). The value for εO‐sed was estimated from benthic O2 exchange simulations using a one‐dimensional diffusion‐reaction O‐isotope model. Adopting this εO‐sed value, and given the observed community O‐isotope fractionation, we calculate that approximately two thirds of the ecosystem respiration occurs within the sediment, in reasonable agreement with direct respiration measurements. Based on the difference between dissolved oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary and in the water that enters the LC at Cabot Strait, we estimate an average respiration rate of 5500 mmol O2 m−2 yr−1 for the 100‐m‐thick layer of bottom water along the LC, 3540 mmol O2 m−2 yr−1 of which is attributed to bacterial benthic respiration.
Seasonal dynamics of Posidonia oceanica in Magalluf Bay (Mallorca, Spain): Temperature effects on seagrass mortality
Authors: Díaz-Almela Elena , Marbà Núria , Martínez Regino , Santiago Rocío , Duarte Carlos M.
Published: January 2011

We recorded seasonal and interannual population dynamics of the endemic Mediterranean seagrass Posidonia oceanica in a shallow meadow, Magalluf Bay, Spain (39°30.25′N, 2°32.59′E), performing direct monthly‐bimonthly censuses from June 2001 to November 2004, in 12 permanent plots. Initially, there were 568 ± 54 (mean 6 SE) shoots m22. An average decline of ‐63 ± 10 SE shoots m−2 yr−1 was detected. Shoot mortality (median: 1.2% month−1; range: 0% to 12.1% month−1) increased in summer, enhanced by seawater temperature. There was also substantial interannual variability characterized by very high mortality in summer‐autumn 2001 and in autumn‐winter 2003–2004, coinciding with hot summers and meadow flowering in autumn (2001: 2.0% ± 1.4% of shoots; 2003: 53.8% ± 10.4% of shoots), but fruiting and seedling recruitment were only observed in 2004. Annual vegetative recruitment (0.034 ± 0.007 yr−1) was 53 times higher than annual sexual recruitment (0.0006 ± 0.0007 yr−1), and total recruitment (vegetative + sexual) was on average 4.5 times lower than mortality. Specific net population growth was always negative during the study. No seasonality was detected in shoot recruitment (median: 0.002 month−1; range: 0 to 0.021 month−1), which did not correlate with solar radiation or temperature. Shoots recruited during the study had a steeper exponential depletion rate (rr = ‐0.34 ± 0.05 SE yr−1) than the global population (rN0 = ‐0.16 ± 0.01 SE yr−1). Shoot probability to survive to the next age class (px) increased with shoot age. Our results suggest that the general decline of P. oceanica meadows could be enhanced by the decadal Mediterranean warming trend.
Microbial food web structure affects bottom‐up effects and elemental stoichiometry in periphyton assemblages
Authors: Fitter Anja , Hillebrand Helmut
Published: January 2011

Heterotrophic organisms constitute an important part of the overall biomass of periphyton assemblages, but the role of these components for the propagation of resource enrichment and top‐down control has rarely been addressed systematically. We report results from six laboratory experiments differing in their trophic structure by the inclusion of different functional groups of microbial benthic organisms (i.e., algae, bacteria, flagellates, ciliates, and rotifers). We manipulated the supply of organic carbon, phosphate, light, and the presence of the respective top consumer in full factorial designs. Increased supply of dissolved organic carbon (DOC) increased total periphyton biomass in almost all experiments, whereas increased phosphorus (P) supply increased total biomass only if algae were present. The effects of DOC and P on the ratio of heterotrophic to autotrophic abundance depended on trophic structure, where additional resources enhanced the autotrophic component when the basal heterotrophs were limited by low organic carbon (C) or by consumer pressure. Light had a comparably minor influence on total biomass and relative abundances. Purely heterotrophic biofilms had higher C: P ratios than autotrophic assemblages. Increased P supply decreased periphyton C: P throughout all experiments except for the experiment with the highest trophic complexity, which yielded low C: P ratios across all treatment levels. Our experimental microcosms with artificially constructed periphyton communities revealed the importance of the benthic microbial food‐web structure for both top‐down and bottom‐up propagating effects of consumers and resources, respectively.