bluuebunny/crossref_metadata_2025 huggingface dataset:
Synchronous variation of dissolved organic carbon and color in lakes
Authors: Pace Michael L. , Cole Jonathan J.
Published: June 2010

Temporal variation in dissolved organic carbon (DOC) and water color (light absorption at 440 nm) was measured in 20 lakes in northern Michigan that varied in DOC, pH, morphometry, and relative productivity as indicated by chlorophyll and total phosphorus (TP). Monthly observations during May—August over 6 yr revealed that DOC and color varied by 6‐ and 28‐fold among lakes and varied substantially through time within lakes. The pattern of temporal variation differed among years but was synchronous among lakes. Of the 190 possible correlations among lake time series, most were positive for DOC (158) and color (160), and 50% of the positive correlations were significant (P < 0.05). Other variables, such as total phosphorus, chlorophyll, and pH, were less synchronous and had fewer significant positive correlations (13—25%). Temporal dynamics in DOC and color were related to ice‐out date as well as spring and summer precipitation. Years of late ice‐out and high spring rain were associated with high DOC and color in spring. A summer drought in one year lead to declines in color (up to 40%) and DOC (up to 38%) in nearly all of the lakes. The common temporal dynamics of DOC and color were most likely the result of climatic conditions that affected loading of allochthonous carbon as well as losses due to photodegradation. The variations in DOC were sufficient to cause large changes in light penetration, standing stocks of carbon, and ecosystem metabolism.
Landscape effects of climate, agriculture, and urbanization on benthic invertebrate communities of Canadian prairie lakes
Authors: Quinlan Roberto , Leavitt Peter R. , Dixit Aruna S. , Hall Roland I. , Smol John P.
Published: June 2010

Paleoecological analyses of fossil chironomid assemblages from eight lakes of the Qu'Appelle Valley, Saskatchewan, Canada, were used to quantify the relative influence of climate, resource use, and urbanization on benthic invertebrate communities 1850–1995. Fossil analyses inferred that Qu'Appelle lakes are naturally productive but that invertebrate communities were altered by agriculture and urbanization. In western lakes, rates of community change (chord distance per 5 yr) were low and nonsignificant (P < 0.05) prior to European settlement, but increased twofold after ~1930–1940. In contrast, uniformly significant rates of community change were recorded in eastern downstream lakes only after the 1960s. In both cases, high rates of change corresponded to alterations in the balance between deep‐water (Chironomus) and littoral species (Cladotanytarsus mancus group, Tanytarsus s.lat. [s.l.]). Comparison of historical and fossil time series (~1920–1993) using variance partitioning analysis (VPA) explained up to 86.6% of past variations in chironomid community composition. Unexpectedly, climate (winter temperature) explained a significant (P , 0.05) and substantial (mean ± SD, n =5 8; 24.8 ± 21.9%) amount of community variance at all sites except Round Lake. In contrast, land‐use practices exhibited significant but less substantial (6.7 ± 6.1%, n = 8) impacts on zoobenthos of five lakes, whereas significant urban impacts (3.6 ± 7.5%, n = 8) were recorded only at two sites. Similarly, redundancy analysis showed that minimum winter or spring temperature significantly influenced the relative abundance of littoral taxa in seven lakes. Such strong effects of climate on benthic invertebrate communities contrast its weak effects on phytoplankton in these lakes and suggest that future environmental change may be expressed differentially among habitats.
The trophic role of marine pelagic ciliates and heterotrophic dinoflagellates in arctic and temperate coastal ecosystems: A cross‐latitude comparison
Authors: Levinsen Henrik , Nielsen Torkel Gissel
Published: June 2010

We compared seasonal studies of ciliates and heterotrophic dinoflagellates conducted in Disko Bay (West Greenland, ~69°N) and the Kattegat (Denmark, ~56°N). In both systems, ciliates and heterotrophic dinoflagellates were important components of the plankton. Their biomass was minute in the winter (October to April) in Disko Bay compared to the Kattegat, but from May to August/September, the biomass and composition of the ciliate and heterotrophic dinoflagellate assemblages were similar in the two systems. The seasonal biomass pattern was unimodal and bimodal for Disko Bay and the Kattegat, respectively. To evaluate top‐down versus bottom‐up control, experimentally derived maximum estimates of protozooplankton growth rates and copepod predation capacities from the study sites were applied to biomass data. This analysis showed that the effect of copepods was significant but that ciliates and heterotrophic dinoflagellates could effectively exploit prey during periods when top‐down pressure was relaxed. In Disko Bay, a high copepod biomass in spring is primarily caused by migration of an overwintering copepod population from deep waters into the photic zone prior to the spring bloom. We suggest that “regulation windows” for the protozooplankton are present even during the spring bloom when copepods occur at their peak levels because of food saturation. Bottom‐up regulation occurred during the winter and occasionally when copepod predation pressure relaxed, but it was difficult to separate food limitation from the effect of temperature. Multiple regression analysis supports the notion that ciliate and heterotrophic dinoflagellate biomass changed seasonally according to both top‐down and bottom‐up regulation, as well as to temperature control. Protozooplankton growth estimates were also used to calculate the fraction of primary production processed by the ciliates and heterotrophic dinoflagellates. When assuming complete algivory, 32–55% and 20–60% of the annual primary production was consumed by ciliates in Disko Bay and the Kattegat, respectively. Furthermore, because heterotrophic dinoflagellates were found to be as important grazers as ciliates in both systems, it was concluded that protozooplankton at high latitudes are also important in the cycling of primary production and should be considered if carbon and nutrient cycling in these systems is to be understood.
Compositional changes in free‐living bacterial communities along a salinity gradient in two temperate estuaries
Authors: Bouvier Thierry C. , del Giorgio Paul A.
Published: June 2010

There is now clear evidence for major differences in heterotrophic bacterial composition between freshwater and marine ecosystems. A fundamental question that remains unresolved is whether the compositional succession occurs by a gradual replacement of the major phylotypes, or whether there are drastic compositional shifts in discrete areas along the gradient from one system to another. The aim of this study is to examine the change in the phylogenetic composition of the free‐living bacterioplankton across the salinity gradient in both the Choptank and the Pocomoke Rivers, both subestuaries of the Chesapeake Bay, using fluorescence in situ hybridization (FISH). The proportion of free‐living cells that could be detected using FISH varied widely (3 to 80%), and the majority of cells detected belonged to bacteria, whereas archaea represented less than 3%. The distribution of the different members of bacteria exhibited a clear pattern along the salinity gradient, with the dominance of α‐proteobacteria in the lower, saltwater regions and of β‐proteobacteria in the upper freshwater regions. The cytophaga‐flavobacterium cluster prevailed in the turbidity maximum located in the middle estuary, and the γ‐proteobacteria showed sporadic peaks along the transect that may have been related to local events. The replacement of α‐proteobacteria along the salinity gradient was not gradual but rather occurred rapidly within the turbidity maximum region of the estuaries where the fresh and salt waters mix. The pattern of phylogenetic succession was linked to the development of the turbidity maximum, which is related to rainfall and the ensuing hydrological conditions. There is indication that the phylogenetic succession is accompanied by strong physiological changes in the bacterial assemblage, expressed as a decline in bacterial growth efficiency and community production. The transition appears not to be simply the result of conservative mixing of riverine and estuarine bacterial assemblages, but rather appears to involve cell inactivation and/or death.
The effects of nutrients and herbivory on competition between a hard coral (Poritescylindrica) and a brown alga (Lobophora variegata)
Authors: Jompa Jamaluddin , McCook Laurence J.
Published: June 2010

Coral reef degradation often involves a phase shift from coral‐ to macroalgal‐dominated reefs. Declining levels of herbivory or increasing supply of nutrients have both been suggested as a cause of increased algal abundance and consequent competitive overgrowth of corals. However, explicit demonstration of the processes involved and their relative strengths requires simultaneous tests of all three factors: competition, herbivory, and nutrient effects. We experimentally tested the factorial effects of nutrients and herbivory on the competitive interaction between a brown alga Lobophora variegata and a scleractinian coral Porites cylindrica. The results of the experiment show that coral tissue mortality was strongly enhanced by the presence of the competitor (L. variegata), and this effect was significantly greater when herbivores were excluded. In contrast, the coral growth (skeletal extension) of P. cylindrica was not significantly affected by any treatments. The addition of nutrients did not have a significant effect on corals overall, but had a small effect on algal growth and consequent coral tissue mortality when herbivores were excluded. The factorial combination of treatments in this experiment allows interpretation of the causal relationships between each factor, demonstrating that nutrient effects on algal growth only led to competitive effects on corals when herbivory was insufficient to consume excess algal growth and that both herbivore and nutrient effects on corals were dependent on the strength and outcome of the competitive interaction between corals and algae.
Observations of mixing near the sides of a deep lake in winter
Authors: Fer Ilker , Lemmin Ulrich , Thorpe S. A.
Published: June 2010

The dissipation rate of turbulent kinetic energy per unit mass, ɛ, vertical eddy diffusivity, Κz, and the rate of dissipation of temperature variance, ϰ, are estimated over the sloping sides of Lake Geneva both far from and near the bed, using temperature data from horizontal tracks of a submarine during periods of winter cooling. The estimated values are about one order of magnitude greater near the slope than those distant from it. The mean dissipation rate per unit mass averaged over the epilimnion varies between O(10−10) m2 s−3 in calm weather to O(10−8) m2 s−3 in winds of 8 m s−1, for surface buoyancy fluxes of 6 × 10−9 m2 s−3 and 1.1 × 10−8 m2 s−3, whereas near the slope ɛ has an average value of 3 × 10−8 m2 s−3. The relation between Κz and ɛN−2 (where N is the buoyancy frequency) is examined. Values of the mixing efficiency, Γ = ɛN−2Κz−1, of 0.15 ± 0.1 and 0.16 ± 0.1 21 z are obtained in the upper 10−m layer for calm and windy conditions, respectively. Near the slope, Γ is found to be 0.22 ± 0.2, slightly larger than in surface layers under windy conditions. Different mixing mechanisms in different regions, near the slope and in the surface waters of the epilimnion and relatively calm deeper layers, can be identified in the diagram of overturn Froude number versus overturn Reynolds number. Mixing near the slope in the epilimnion appears to be related to the gravitational winter cascading of cold water down the sloping sides of the lake.
14C excess in deep‐sea sediments porewater driven by diffusionߞSoutheast Mediterranean
Authors: Sivan Orit , Lazar Boaz
Published: June 2010

Core profiles in the deep SE Mediterranean Sea revealed a significant Δ14C excess in the total dissolved inorganic carbon (DIC) of porewater as compared to the Δ14C of the sediments, below the bioturbation zone down to core bottom (>2 m). The main observation was that the age difference between porewater and sediments, as calculated from the excess, was not constant but rather increasing with depth. The porewater apparent age was younger by 10% than the age of the sediment at the core top and gradually became younger by about 70% at the bottom of the core. Flux calculations and mass balances of the chemical and isotopic data suggest that this 14C excess stems from a downward diffusive flux of 95 ± 15 atoms of 14CDIC m−2 s−1 from the bioturbation zone and overlying bottom water into the sediment. The 14CDIC flux is opposite in direction to the DIC flux out of the sediment. It is suggested here that the 14CDIC flux may severely interfere with age determination of small submarine “brine lakes” formed in the deep Mediterranean. In addition, the 14C excess should show up in authigenic carbonate phases precipitating within the sediment and, hence, may serve as a tool to identify the extent and duration of authigenic carbonate precipitation.
Spatial‐temporal variability in surface layer deepening and lateral advection in an embayment of Lake Victoria, East Africa
Authors: MacIntyre Sally , Romero José R. , Kling George W.
Published: June 2010

Vertical and horizontal exchanges in Pilkington Bay, a shallow (9 m) embayment of Lake Victoria, were determined from a surface energy budget, time series measurements of temperature, and quasi synoptic transects of conductivity, temperature, and depth conducted over a 2‐d period. The surface energy budget is the first from a tropical lake over a diurnal timescale. Strong stratification developed during morning and early afternoon (>40 cycles h−1) but was eroded beginning in the afternoon by the combination of wind and heat loss. Surface heat losses contributed >70% of the energy for surface layer deepening 82% of the time from midafternoon until midmorning. Circulation times of the surface layer were <2 min as it deepened to 1.5 m in the afternoon and were <12 min at night even when mixing extended to the lake bottom. Spatial differences in the rates of heating and cooling and in the depth of wind mixing caused fronts to develop on spatial scales of kilometers within the bay. Convergence of these fronts led to downwelling of surface waters and upwelling of deep waters during the stratified period. Horizontal pressure gradients due to differential heating contributed to thermocline downwelling, lateral movement of deep, anoxic waters, and generation of high‐frequency internal waves, all of which contribute to vertical and horizontal transports. Although wind and heat loss at one location generally determine the depth of the surface layer and thermocline, the depths of these key features may be strongly influenced by rates of heating and cooling elsewhere in a basin.
Ultraviolet damage and counteracting mechanisms in the freshwater copepod Boeckella poppei from the Antarctic Peninsula
Authors: Rocco Vanina E. , Oppezzo Oscar , Pizarro Ramón , Sommaruga Ruben , Ferraro Marcela , Zagarese Horacio E.
Published: June 2010

The process of ozone depletion over the Antarctic continent has resulted in the increase of incident ultraviolet‐B (UVB) radiation, whose effects may be damaging to living organisms. To counteract the negative effects of ultraviolet radiation (UVR), aquatic organisms may display one or more strategies: (1) avoidance (i.e. deep distribution); (2) photoprotection through the use of “sunscreen” compounds, such as mycosporine‐like amino acids (MAAs); and (3) enzymatic repair of the damage. The effects of UVR were assessed on four populations of the copepod Boeckella poppei from Antarctic lakes using laboratory and field experiments. The results were related to measurements of DNA enzymatic repair activity and MAA concentration. This is the first study that combines these measurements in zooplankton. Boeckella poppei was highly tolerant to UVR (LD50 = 2.2–2.78 J cm−2). However, measurements of photorecovery (comparison of UVB mortality in the presence and absence of photoreactivating light) and dosage of photolyase activity indicated low rates of enzymatic repair, which may be the result of low temperatures typical of Antarctic lakes. Three different MAAs were identified, both in phytoplankton and copepods: porphyra‐334, mycosporine‐glycine, and shinorine. The population of B. poppei from Lake Boeckella had the lowest MAA concentration, as well as the lowest tolerance to artificial and natural UVR. These findings support the idea that UV tolerance in this species is related to the accumulation of MAAs. A comparison of the strategies used to cope with potentially damaging levels of UVR by different species of Boeckella indicates a high degree of plasticity in this genus, which has probably been key for its success to colonize a wide range of UV environments.
Light absorption by aquatic particles in the near‐infrared spectral region
Authors: Babin Marcel , Stramski Dariusz
Published: June 2010

In this study, we used a special measurement geometry with samples placed inside the integrating sphere to address whether significant absorption by aquatic particles exists in the near‐infrared (near‐IR) spectral region from about 700 to 850 nm. Our tests with inorganic dyes and MgCO3 particles showed that placing a small sample (1 cm cuvette) inside a relatively large integrating sphere (15 cm diameter) reduced the scattering error to a negligible level with no adverse effect on the absorption measurement. Our measurements of absorption by various particle suspensions suggest that absorption is generally negligible in the near‐IR regardless of the type of particles. We examined four species of phytoplankton, phytodetritus derived from phytoplankton cultures, three samples of natural assemblages of mineral particles that show distinct reddish or brownish color, and three samples of aquatic particles from coastal and inland waters that have varying proportions of organic and inorganic particles.
Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—I. Furcilia III larvae
Authors: Meyer B. , Atkinson A. , Stöbing D. , Oettl B. , Hagen W. , Bathmann U. V.
Published: June 2010

Physiological condition and feeding behavior of furcilia larvae were investigated in autumn (April 1999) in the southwestern Lazarev Sea prior to the critical overwintering period. Furcilia stage III (FIII) larvae were most abundant, so only these were used for all analyses (dry mass [DM], elemental and biochemical composition, gut content) and experiments (metabolic and ingestion rates, selective feeding behavior). Chlorophyll a (Chl a) concentrations in the mixed layer were ≪0.1 µg L-1. Respiration rates of freshly caught FIII larvae were between 0.4 and 1.2 µl O2 mg-1 DM h-1, similar to larvae fed for 7 d on high food concentrations (4 µg Chl a L-1). Excretion rates ranged between 0.01 and 0.02 µg NH4 mg-1 DM h-1. Their atomic O:N ratio of 72 indicated that lipids were the main metabolic substrate of FIII larvae in the field. The daily C ration ranged from 0.4% at the lowest food concentration of 3 µg C L-1 to 28% at the highest enriched food concentration of 216 µg C L-1, whereas clearance rates decreased with increasing food concentrations. In natural seawater, 115 ml mg-1 C h-1, and in natural seawater enriched with ice biota, 24 ml mg-1 C h-1, the clearance rates on specific phytoplankton taxa revealed no significant difference across a food size range of 12–220 µm. The study suggests that during periods of low food supply in the water column, larvae have to exploit ice biota to cover their metabolic demands.
Evidence for facilitated uptake of Hg(II) by Vibrio anguillarum and Escherichia coli under anaerobic and aerobic conditions
Authors: Golding George R. , Kelly Carol A. , Sparling Richard , Loewen Peter C. , Rudd John W. M. , Barkay Tamar
Published: June 2010

A mer‐lux bioreporter was used to study uptake of inorganic mercury, Hg(II), at trace concentrations by two facultatively anaerobic bacterial species, Vibrio anguillarum and Escherichia coli. Uptake of Hg(II) by these bacteria appeared to be facilitated, rather than by passive diffusion. Three lines of evidence support this conclusion. First, under anaerobic conditions Hg(II) uptake was greatly decreased compared with aerobic conditions, even though the chemical composition of the medium was identical except for the lack of oxygen (i.e., no reducing agents were used). Second, the uptake of Hg(II) under anaerobic conditions was not proportional to the abundance of lipophilic Hg species but was dependent on the total concentration of Hg in the samples. Third, at trace Hg(II) concentrations and under anaerobic conditions, Hg(II) uptake was enhanced by the addition of yeast extract and a variety of low molecular weight organic acids. In addition to demonstrating that Hg(II) uptake by these bacteria had the characteristics of facilitated transport, these lines of evidence also support the conclusion that processes under regulatory control of the cell affected Hg(II) uptake. If these findings apply to other bacteria as well, they mean that current models of Hg(II) uptake by microorganisms in aquatic systems, which are based solely on the role of lipophilic Hg species and passive diffusion, will need to be reconsidered.
Utilization of different nitrogen sources by the marine cyanobacteria Prochlorococcus and Synechococcus
Authors: Moore Lisa R. , Post Anton F. , Rocap Gabrielle , Chisholm Sallie W.
Published: June 2010

Prochlorococcus is the most abundant phytoplankter throughout the photic zone in stratified marine waters and experiences distinct gradients of light and nitrogen nutrition. Physiologically and genetically distinct Prochlorococcus ecotypes partition the water column: high‐B/A (low‐light adapted) ecotypes are generally restricted to the deep euphotic zone near or at the nitracline. Low‐B/A (high‐light adapted) ecotypes predominate in, but are not limited to, NO3-‐depleted surface waters, where they outnumber coexisting Synechococcus populations. The niche partitioning by different Prochlorococcus ecotypes begs the question of whether they also differ in their nitrogen (N) utilization physiology, especially with respect to NO3- utilization. To explore this possibility, we studied the capabilities of different Prochlorococcus and Synechococcus strains to grow on a variety of N sources. We found that all the isolates grew well on NH4+ and all were capable of urea utilization, occasionally at a lower growth rate. None of the Prochlorococcus isolates were able to grow with NO3-. Four high‐B/A Prochlorococcus isolates grew on NO3-, but all others did not. Whole genome analysis of the low‐B/A Prochlorococcus MED4 revealed that the genes required for NO uptake and reduction were absent. The genome of the high‐B/A Prochlorococcus MIT 9313 also lacked the NO3- utilization genes but has homologs of genes required for NO3- utilization consistent with its physiology and ecology. Thus, the utilization of different N sources in the marine environment is partitioned among closely related ecotypes, each with adaptations optimized for the environment where these sources are available.
Organic matter flux and reactivity on a South Carolina sandflat: The impacts of porewater advection and macrobiological structures
Authors: D'Andrea Anthony F. , Aller Robert C. , Lopez Glenn R.
Published: June 2010

Study of the flux and fate of reactive organic material (OM) within Debidue Flat, an intertidal sandflat in the North Inlet estuary, South Carolina, demonstrated that this coarse‐grained deposit is a dynamic, open system that experiences rapid OM decomposition and exchange of solutes in the top 30 cm of the sediment column. The fluxes of reactive OM through Debidue Flat were high during all seasons (27–170 mmol C m2 d-1) and were comparable to fluxes in muddy portions of the North Inlet estuary. Porewater decomposition products were N‐ and P‐rich, the modeled reactivity of organic carbon undergoing decomposition was high (first‐order rate constant, k = 0.02 d-1), and abundant extractable chlorophyll a was measured year‐round; all properties were consistent with marine algal derived substrates. Porewater solute profiles were controlled by advective flow that rapidly exchanged porewater with overlying waters to ~25 cm depth on timescales of hours. Thus, these sandflats act like an unsteady “trickling bed filter,” capturing or generating reactive organic particles, rapidly remineralizing OM, and recycling nutrients. Macrobiological structures within the flat altered the amounts and reaction rates of OM on various spatial and temporal scales. Relatively elevated OM decay rates were associated with the burrows of Callichirus major, a deepburrowing thalassinid shrimp. Large stingray feeding pits accumulated fine grained OM, locally clogging the “trickling bed filter” and inhibiting porewater advection. As illustrated by Debidue Flat, intertidal sands can be sites of high OM flux and turnover and play an important role in biogeochemical cycling in estuarine systems.
Sequential resuspension of protists by accelerating tidal flow: Implications for community structure in the benthic boundary layer
Authors: Shimeta Jeff , Amos Carl L. , Beaulieu Stace E. , Ashiru Oladipo M.
Published: June 2010

We measured resuspension thresholds of protists and bacteria at a subtidal coastal site with in situ flumes and by sampling the benthic boundary layer during tidal accelerations. Heterotrophic nanoflagellates, oligotrich ciliates, the diatom Navicula distans, and bacteria resuspended in weak flow (friction velocity u*crit = 0.25–0.80 cm s-1), likely associated with a surficial fluff layer of sediment. Hypotrich ciliates, scuticociliates, and the diatoms N. transitans and Pleurosigma sp. resuspended in moderate flow (u*crit = 0.82–1.3 cm s-1), followed by pigmented nanoflagellates and diatoms of two Nitzschia spp. in strong flow (u*crit >= 1.5 cm s-1). Hypotrichs and scuticociliates resuspended independent of sediment erosion thresholds, whereas most diatoms resuspended with bulk sediment. Differing thresholds may be due to cell size, specific gravity, behavior, or association with particles. As tidal currents accelerated to u* = 1.3 cm s-1, resuspension caused cell concentrations at 5 cm above bottom to increase by 2–16 times, varying among taxa. Community structure shifted accordingly, with total oligotrichs, hypotrichs, and scuticociliates changing from 75% to 96% of the ciliate community and the total diatom taxa listed above changing from 37% to 63% of the pennate cells. Sequential resuspension suggests that the species assemblage entering the water column during a resuspension event depends on the maximal bed shear stress, thus varying with the springneap cycle as well as atmospheric forcing and local hydrography. Flow‐induced fluctuations of community structure may influence microbial food‐web dynamics in the benthic boundary layer and sediment.
Meteorological controls of gas exchange at a small English lake
Authors: Frost Thomas , Upstill‐Goddard Robert C.
Published: June 2010

The relationships between gas transfer velocity, k600, wind speed, wind direction, rainfall, and relative humidity were examined using measurements of SF6 evasion from Coatenhill Reservoir, a small (0.017 km2), shallow (1.9 ± 0.1 m), man‐made lake in northeast England characterized by predominantly low to intermediate wind speeds ~1–10 m s-1. A graphical method was used to estimate the wind speed at a standard height of 10 m, U10, from wind speed measurements at 2 m and 3.8 m. Derived values of U10 normalized to remove thermal stability effects (U10‐n) were significantly correlated with k600 (R2 = 0.71, p < 0.001). A detailed analysis of surface roughness lengths estimated with this procedure showed the k600 versus U10‐n relationship to be sensitive to wind speed modification by the meteorological mast and sensors. With normalization to remove these effects, the correlation between k600 and U10‐n substantially improved (R2 = 0.86, p < 0.001). In contrast to previous laboratory findings, relative humidity was not significantly correlated with k600 and rainfall rate (Rn) was only weakly correlated with k600, possibly as a consequence of the effects of these variables being largely masked by the time scales of data averaging and SF6 sampling. Similarly, k600 was not correlated with wind direction (i.e., fetch). An empirical gas exchange model accounted for 88% of the total variance in k600 (p = 0.01) at Coatenhill, with U10‐n and Rn accounting for 86% and 2%, respectively. Future field investigations of the meteorological controls of k600 will require careful experimental design to allow for more detailed sampling than hitherto has been possible.
Food quality effects of unsaturated fatty acids on larvae of the zebra mussel Dreissena polymorpha
Authors: Wacker Alexander , Becher Paul , von Elert Eric
Published: June 2010

In standardized growth experiments, newly hatched larvae of the zebra mussel Dreissena polymorpha were fed diets representing different biochemical compositions. Algae that were rich in (n‐3) polyunsaturated fatty acids (PUFAs), except for long‐chained (>C18) PUFAs (Chlorella minutissima and Monoraphidium minutum) were of low food quality. Higher growth than on C. minutissima or M. minutum was supported by a culture of the cyanobacterium Aphanothece sp., which contained traces of a long‐chained (n‐3) PUFA, docosahexaenoic acid (DHA, 22 : 6n‐3). The alga Isochrysis aff. galbana, which contained high amounts of the longchained (n‐3) PUFAs DHA and eicosapentaenoic acid (EPA, 20 : 5n‐3), supported the highest growth. The alga Nannochloropsis limnetica, which differed from I. galbana by a deficiency in DHA, allowed slightly, but significantly lower, growth. Growth of larvae on N. limnetica was increased by enrichment of N. limnetica cells with a lipid extract of I. galbana, showing that larval growth on N. limnetica was limited by the deficiency of a compound that was present in I. galbana. Growth was also enhanced by feeding N. limnetica cells supplemented with DHA, but not by cells enriched with EPA, indicating that DHA was the limiting factor. We conclude that, on DHA‐deficient food, the larvae of D. polymorpha were not able to sufficiently convert C18‐PUFAs into long‐chained (n‐3) PUFAs and that the rates for elongation and desaturation of EPA into DHA limited growth.
Avoiding offshore transport of competent larvae during upwelling events: The case of the gastropod Concholepas concholepas in Central Chile
Authors: Poulin Elie , Palma Alvaro T. , Leiva Germán , Narvaez Diego , Pacheco Rodrigo , Navarrete Sergio A. , Castilla Juan C.
Published: June 2010

The coast of central Chile is characterized by the occurrence of coastal upwelling during the austral spring and summer seasons, which probably has important consequences for the cross‐shelf transport of larval stages of many species. Three cruises were conducted off the locality of El Quisco during upwelling‐favorable wind periods to determine the surface distribution of epineustonic competent larvae of the gastropod Concholepas concholepas during such events. Contrary to the predictions of a traditional model, where neustonic‐type larvae are transported offshore under such conditions, competent larvae of this species were exclusively found in the area between the shore and the upwelling front. Two additional cruises were conducted during calm periods to determine diel variation in the vertical distribution of C. concholepas competent larvae. The absence of competent larvae at the surface during early night hours suggests a reverse vertical migration. Thus, the retention of C. concholepas competent larvae in the upwelled waters could be the result of the interaction between their reverse diel vertical migration and the typical two‐layer upwelling dynamics.
Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study
Authors: Newell Roger I. E. , Cornwell Jeffrey C. , Owens Michael S.
Published: June 2010

Suspension‐feeding eastern oysters, Crassostrea virginica, were once abundant in Chesapeake Bay and may then have exerted top‐down control on phytoplankton and also reduced turbidities, thereby increasing light available to benthic plants. Alternatively, oysters may have simply recycled inorganic nutrients rapidly back to the water column, with no long‐lasting reduction in phytoplankton biomass resulting from oyster feeding activity. To help distinguish between these scenarios, we explored changes in nitrogen fluxes and denitrification in laboratory incubations of sediment cores held under oxic and anoxic conditions in response to loading by pelletized phytoplankton cells, an experimental analog for oyster feces and pseudofeces. When organics were regenerated under aerobic conditions, typical of those associated with oyster habitat, coupled nitrification‐denitrification was promoted, resulting in denitrification of ~20% of the total added nitrogen. In contrast, under anoxic conditions, typical of current summertime conditions in main‐stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. We postulate that denitrification of particulate nitrogen remaining in oyster feces and pseudofeces may enhance nitrogen removal from estuaries. In aerobic incubations with sufficient light (70 µmol m-2 s-1), a benthic microalgal/cyanobacterial community grew that not only absorbed the inorganic nitrogen released from the added organics but also fixed N2. This result suggests that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with bivalve feeding increases light penetration to a level that can sustain benthic microalgal production.
The systematics of Holarctic bosminids and a revision that reconciles molecular and morphological evolution
Authors: Taylor Derek J. , Ishikane Christine R. , Haney Robert A.
Published: June 2010

Bosminids are ubiquitous and abundant crustacean herbivores in freshwater ecosystems. They are among the best preserved zooplankters in limnological sediments, making them ideal paleolimnological indicators of ecosystem change. Moreover, their egg banks make possible the study of paleogenetics and resurrection ecology. A major limitation to these comparative disciplines is that the relatedness and identities of compared bosminid specimens or subfossils is often uncertain. We aimed to remedy this by developing the first robust molecular phylogeny of the group and inferring patterns of morphological evolution. We used sequence alignments of five nuclear rDNA genes (partial 18S rDNA, internal transcribed spacer 1 [ITS‐1], 5.8S, ITS‐2, and partial 28S rDNA) and one mitochondrial gene (partial 16S rDNA) from representative Holarctic species from all of the proposed subgenera and genera. No evidence of within‐individual sequence variation in the gene regions analyzed was found for the genus Bosmina and the genus Eubosmina. Our nuclear and mitochondrial genetic results revealed a robust phylogeny and were congruent with morphological changes. Competing morphological schemes of bosminid systematics were reconciled by reassigning several species to different subgenera, resurrecting the genus Eubosmina, erecting a new subgenus, and revising the character coding systems that created paraphyletic groups. The results provide an evolutionary framework with diagnostic genetic and morphologic characters for studies of bosminid paleolimnology and paleogenetics.
Dissolution of particle‐reactive radionuclides in deposit‐feeder digestive fluids
Authors: Shull David H. , Mayer Lawrence M.
Published: June 2010

Naturally occurring radionuclides such as 234Th, 7Be, and 210Pb are important tracers for quantifying sedimentmixing and sediment‐accumulation rates. Profiles of these radionuclides in marine sediments are strongly influenced by particle displacement due to deposit feeding. Observations of rapid dissolution and high concentrations of dissolved metals in deposit‐feeder digestive fluids suggest that particle‐bound radionuclides could also undergo dissolution during depositfeeder gut passage. We investigated this possibility in laboratory experiments examining radionuclide dissolution into the digestive fluids of the lugworm, Arenicola marina. Experiments with artificially labeled particles indicated that significant fractions of 234Th, 7Be, and 210Pb dissolved from labeled algal detritus and clay particles at low particle concentrations. 137Cs was also dissolved from clays. However, if unlabeled sediment particles were added to reach sediment : fluid ratios similar to those in A. marina midguts, little net dissolution occurred, which implies resorption of dissolved radionuclides by the added solid phases. Partition coefficients of these radionuclides in mixtures of digestive fluid and the various solid phases imply that relatively more 234Th resorbs to the residual organic phase following digestion, compared to 210Pb and 7Be, which partition more strongly to the inorganic sediment phases. Despite little net dissolution, the phase change from algal detritus to either mineral surfaces (for 210Pb) or undigested organic matter (for 234Th) implies that 234Th would serve as a better tracer for organic‐matter mixing in sediments compared to 210Pb, which would better trace bulk sediment mixing.
Do sediments from coastal sites accurately reflect time trends in water column phytoplankton? A test from Himmerfjärden Bay (Baltic Sea proper)
Authors: Bianchi Thomas S. , Engelhaupt Erika , McKee Brent A. , Miles Stephen , Elmgren Ragnar , Hajdu Susanna , Savage Candida , Baskaran Mark
Published: June 2010

Long‐term (1977–2000) ecological biomass data for total phytoplankton, diatoms, and cyanobacteria, based on plankton samples from Himmerfjärden Bay (Baltic proper), were compared to the historical sediment record using diagnostic plant pigment biomarkers. Radionuclides (210Pb and 137Cs) were used to determine chronology, sedimentation, and carbon burial rate. Despite high resolution sampling, using crust‐freeze sampling, of sediment layers representing annual varves, no significant correlations between pigments preserved in sediments and phytoplankton biomass in plankton samples were detected for periods of 1–4 yr. This lack of correspondence was probably at least partly due to the importance of resuspension events in Himmerfjärden. When sedimentary pigments were averaged over longer time intervals (5 yr) averages of annual diatom biomass in the Himmerfjärd inlet were positively correlated to down‐core concentrations of fucoxanthin (r2 = 0.98) and diatoxanthin (r2 = 0.62). This indicates that pigment biomarkers can still be used to interpret longer term development of eutrophication‐related blooms in such estuarine systems. In contrast, zeaxanthin concentrations were not significantly correlated (p > 0.05, r2 = 0.05) to cyanobacterial biomass as a 5–yr average. Using fossil pigments to determine relative differences in phytoplankton biomass composition in the absence of historical ecological patterns of phytoplankton composition can be misleading due to selective losses of pigments such as the epoxy‐carotenoids. However, while the use of fossil pigments in laminated sediments alone may not allow for detailed interpretations of past phytoplankton communities, it does allow for the simple determination of the presence of significant biomass of phytoplankton classes, for which unique biomarker pigments exist.
Does competition for nanomolar phosphate supply explain the predominance of the cyanobacterium Synechococcus?
Authors: Moutin Thierry , Thingstad Tron F. , Van Wambeke France , Marie Dominique , Slawyk Gerd , Raimbault Patrick , Claustre Herveé
Published: June 2010

Experimental work during a cruise along a W‐E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6–2ߝµm size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (<5 nM) as well as at relatively high (5–25 nM) concentrations could explain both general Synechococcus abundance in P‐depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur.
The function of plastids in the deep‐sea benthic foraminifer, Nonionella stella
Authors: Grzymski Joe , Schofield Oscar M. , Falkowski Paul G. , Bernhard Joan M.
Published: June 2010

Curiously, the benthic foraminifer, Nonionella stella, found in the upper 3 cm of sediments collected off California at a depth of ;600 m, retains chloroplasts. We examined the origin and physiological function of the organelles within the host cell. Transmission electron micrographs, fluorescence and absorption spectra, Western blots, and enzyme assays revealed that the chloroplasts were intact and retained functionality for up to 1 year after sample collection. 16S rDNA gene sequences established that the plastids were derived from diatoms closely related to Skeletonema costatum and Odontella sinensis. Western blots of three major chloroplast proteins (ribulose bisphosphate carboxylase oxygenase [RuBisCO], the D1 protein, and the fucoxanthin chlorophyll a protein complex) confirmed that the organelle retained both nuclear and chloroplast encoded proteins, which suggests that the turnover of the plastid machinery is extremely low. Moreover, the two carboxylating enzymes examined, RuBisCO and phosphoenol pyruvate carboxylase, retained catalytic activity. Three hypotheses regarding the function of sequestered chloroplasts in deep‐sea foraminifera were considered: (1) the organelles are photosynthetic under extremely low irradiance levels, (2) the organelles utilize exogenous substrates to generate an electrochemical gradient that permits chemoautrophy in the dark, and (3) they are used for the assimilation of inorganic nitrogen. Our results suggest that the chloroplasts are used to meet the nitrogen requirements of the host. Immunolocalization of the nuclear encoded protein, nitrate reductase, supports this hypothesis. This protein is widely distributed in photoautotrophs but is not encoded in eukaryotic protists such as foraminifera.
Cobalt limitation and uptake in Prochlorococcus
Authors: Saito Mak A. , Moffett James W. , Chisholm Sallie W. , Waterbury John B.
Published: June 2010

Processes that enable marine phytoplankton to acquire trace metals are fundamental to our understanding of primary productivity and global carbon biogeochemical cycling. Here we show that the abundant marine cyanobacterium, Prochlorococcus strain MED4–Ax, has an absolute cobalt requirement and that zinc cannot substitute for cobalt in the growth medium, as is the case in some other phytoplankton species. When resuspended into fresh medium, uptake of cobalt into the cell occurs as free cobalt (Co2+). In contrast, cultures augmented with conditioned medium assimilated cobalt significantly faster than those in fresh medium, leading to the hypothesis that Prochlorococcus produced organic cobalt ligands in the conditioned medium. This work suggests that the availability of cobalt might influence the composition of phytoplankton assemblages in the open ocean.
Experimental assessment of UV effects on temperate marine phytoplankton when exposed to variable radiation regimes
Authors: Barbieri Elena S. , Villafanñ Virginia E. , Helbling E. Walter
Published: June 2010

Phytoplankton samples were collected at Bahéa Engaño, Chubut, Argentina (43°S, 65°W) at different times of the year to assess the combined effect of ultraviolet radiation (UVR, 280–400 nm) and vertical mixing (i.e., the depth of the upper mixed layer, UML) on photosynthesis. Samples were exposed to fixed and fluctuating radiation regimes in an illuminated chamber at 15°C (photosynthetically available radiation [PAR] = 66 W m−2; UV‐A = 15.3 W m−2; UV‐B = 0.7 W m−2), receiving either PAR 1 UVR or PAR only. A comparison between fixed and rotating systems showed that when ZUML/ZEu = 0.6 (i.e., 60% of the euphotic zone [Eu] was mixed), only postbloom assemblages (codominated by nanoplanktonic flagellates and diatoms [Chaetoceros spp.]) were affected significantly by UVR. Integrated carbon fixation values during pre‐ and postbloom periods were higher under mixed conditions than under fixed irradiances. However, during the bloom (dominated by the microplanktonic diatom Odontella aurita), phytoplankton exposed to fluctuation radiation regimes had lower integrated carbon fixation. When postbloom samples were exposed to different mixing conditions, integrated UVR‐induced inhibition reduced carbon fixation by 11–13% when ZUML/ZEu = 0.6, whereas when ZUML/ZEu = 0.91, carbon fixation increased by 7–12%. The differences in responses observed between prebloom, bloom, and postbloom samples can be attributed to a number of factors, such as the light history of cells, taxonomic composition, and size structure of the community and most probably reflect the different inhibition kinetics of these assemblages.
Pathways of organic carbon utilization in small lakes: Results from a whole‐lake 13C addition and coupled model
Authors: Cole Jonathan J. , Carpenter Stephen R. , Kitchell James F. , Pace Michael L.
Published: June 2010

In many small aquatic ecosystems, watershed loading of organic C exceeds autochthonous primary production. Although this allochthonous organic C has long been thought of as refractory, multiple lines of evidence indicate that substantial portions are respired in the receiving aquatic ecosystem. To what extent does this terrestrial C support secondary production of invertebrates and fish? Do current models adequately trace the pathways of allochthonous and autochthonous C through the food web? We evaluated the roles of allochthonous and autochthonous organic C by manipulating 13C content of dissolved inorganic C in a small, softwater, humic lake, thereby labeling autochthonous primary production for about 20 d. To ensure rapid and sufficient uptake of inorganic 13C, we enriched the lake with modest amounts of N and P. We constructed a carbon flow model based on the ambient and manipulated levels of 13C in C compartments in the lake, along with information on key rate processes. Despite the short nature of this experiment, several results emerged. (1) Fractionation of photosynthetically assimilated 13C‐CO2 by phytoplankton (ɛ) is lower (~6‰) than physiologic models would estimate (~20‰). (2) Bacteria respire, but do not assimilate, a large amount of terrestrially derived dissolved organic C (DOC) and pass little of this C to higher trophic levels. (3) The oxidation of terrestrial DOC is the major source of dissolved inorganic C in the lake. (4) Zooplankton production, a major food of young‐of‐year fishes, is predominantly derived from current autochthonous carbon sources under the conditions of this experiment.