bluuebunny/crossref_metadata_2025 huggingface dataset:
Testing zooplankton food limitation across gradients of depth and productivity in small stratified lakes
Authors: DeMott William R. , Edington Jennifer R. , Tessier Alan J.
Published: July 2010

We examined the significance of vertical structure in lake plankton to grazer‐resource coupling by quantifying how the intensity and mechanism of grazer food limitation varies with depth. We used Daphnia growth assays and also sampled resources (chlorophyll, particulate carbon, and phosphorus) and the day/night depth distributions of Daphnia spp. during summer in four depth strata of lakes that differ in trophic state. The growth assays included treatments with natural seston and seston supplemented with phosphate or Synechococcus, an energy‐ and P‐rich cyanobacterium highly deficient in essential fatty acids and sterols. Results from growth assays and measurements of Daphnia P balance indicate strong energy limitation and weak and sporadic P limitation, despite seston C: P ratios in the upper depth strata above the predicted threshold for Daphnia P limitation. Growth improved with depth and was very high with resources collected from a deep layer of sulfur bacteria that was partially protected from grazing by low dissolved oxygen. Strong positive responses to Synechococcus supplements and high growth with resources from the sulfur bacteria layer are both contrary to limitation by essential lipids. The intensity of food limitation was independent of lake trophic state but was influenced by the quality of the deep‐water refuge. Across depths and lakes, food limitation was dependent on Daphnia density; strong food limitation was associated with high Daphnia biomass (>100 µg L−1) and Daphnia grazing >;20% per day.
Allelopathic interactions between phytoplankton species: The roles of heterotrophic bacteria and mixing intensity
Authors: Hulot Florence D. , Huisman Jef
Published: July 2010

Toxin‐producing phytoplankton species may compensate for competitive disadvantages by secreting chemicals that affect toxin‐sensitive phytoplankton species. Heterotrophic bacteria, however, may, in turn, degrade the toxins produced by allelopathic phytoplankton, thus confounding allelopathic interactions between phytoplankton species. Moreover, recent theoretical studies suggest that incomplete mixing of the water column might also modify allelopathic interactions. Here we analyze a model where phytoplankton species, bacteria, nutrients, and a toxin are linked through material cycling. The model considers a toxin‐producing and a toxin‐sensitive phytoplankton species and two species of heterotrophic bacteria. The model is analyzed for two scenarios: a simple well‐mixed aquatic ecosystem is contrasted with an aquatic ecosystem with low mixing intensity. The results show that (1) the winner of competition between toxin‐producing phytoplankton and toxin‐sensitive phytoplankton species may depend on the species that becomes dominant first, (2) heterotrophic bacteria able to degrade allelopathic toxins will facilitate dominance of toxin‐sensitive phytoplankton species, (3) heterotrophic bacteria unable to degrade allelopathic toxins may to some extent counter the facilitating effect of toxin‐decomposing bacteria, due to competition between the bacterial species, and (4) there is a major effect of mixing intensity on these species interactions: when turbulent mixing rates are low, toxin concentrations are less diluted whereas degradation activities of heterotrophic bacteria are more localized. As a result, this model study predicts that weak mixing, especially when combined with the presence of bacteria unable to degrade allelopathic compounds, will favor the development of allelopathic phytoplankton populations.
Small‐scale spatial and temporal variability in coastal benthic O2 dynamics: Effects of fauna activity
Authors: Wenzhöfer Frank , Glud Ronnie N.
Published: July 2010

In situ measurements in a shallow water sediment were performed using three different modules—a microprofiling unit, a transparent benthic chamber, and a planar optode periscope. The combined data set revealed an extremely patchy and variable benthic O2 distribution primarily due to temporal variations in fauna activity and photosynthesis. A distinct diel pattern in the fauna activity, dominated by Hediste diversicolor, resulted in strongly elevated O2 uptake rates of ~5.3 mmol m𠈒2 h𠈒1 at the onset of darkness. The activity gradually diminished during the night, and the O2 uptake decreased to less than half the maximum rate just before sunrise. The volume of oxic sediment around burrow structures was influenced by changing environmental conditions (benthic photosynthesis and fauna activity) but grossly exceeded that below the primary sediment surface. The volume specific respiration rate around burrows was more than seven times higher than the equivalent value at the sediment surface. A budget of the O2 consumption revealed that the O2 uptake through the burrow walls just after sunset accounted for the major part of the total O2 uptake on a diel scale. The study demonstrates that light‐driven variations in fauna activity can have great effects on the total benthic O2 consumption rate with large implications for estimated benthic mineralization rates.
Denitrification and anammox activity in Arctic marine sediments
Authors: Rysgaard Søren , Glud Ronnie Nøhr , Risgaard-Petersen Nils , Dalsgaard Tage
Published: July 2010

We measured rates of N2 production through anaerobic NH4+ oxidation with NO2−(anammox) and denitrification in permanently cold (from −1.7°C to 4°C) sediments off the east and west coasts of Greenland. The investigated sites (36– to 100–m water depth) covered sediments in which carbon contents ranged from 0.3 to 3.2 dry weight %, O2 uptake rates ranged from 3.4 to 8.3 mmol m−2 d−1, O2 penetration depths ranged from 0.25 to 1.70 cm, and bottom‐water NO3−concentrations ranged from 0.3 to 15.3 µmol L−1. Total N2 production was 34–344 µmol N m−2 d−1, of which anammox accounted for 1–92 µmol N m−2 d−1 (1–35% of total) and denitrification for 33–265 µmol N m−2 d−1. At one of the high‐Arctic sites, anammox activity had an optimum temperature (Topt) of 12°C, while that of bacterial denitrification was 24°C. According to the classical temperature scheme for metabolic growth, the anammox response was psychrophilic, while denitrification was psychrotrophic. Although Topt was considerably higher than in situ temperatures, rates of denitrification and anammox were still high at −1.3°C, reaching 17% and 40%, respectively, of those found at Topt. The activation energies, Ea, of anammox and denitrification were 51.0 and 60.6 kJ mol−1, respectively, and the corresponding Q10 values were 2.2 and 2.4. Rates of anammox were linearly correlated with bottom‐water NO3−concentrations (r2 = 0.96, p < 0.0001, n = 11) at the investigated sites. We suggest that the slow‐growing anammox bacteria are favored in sediments with high and stable NO3−conditions. This may be a general pattern in deeper waters at other latitudes as well.
Sediment‐water exchange of total mercury and monomethyl mercury in the San Francisco Bay‐Delta
Authors: Choe Key‐Young , Gill Gary A. , Lehman Ronald D. , Han Seunghee , Heim Wesley A. , Coale Kenneth H.
Published: July 2010

Five field trips were conducted in the San Francisco Bay‐Delta between May 2000 and October 2001 to investigate the sediment‐water exchange of total mercury (Hg) and monomethyl mercury (MMHg). Solid‐phase Hg averaged ~1 nmol g−1 and did not show any variability with depth or time or among sites. In contrast, solid‐phase MMHg showed considerable vertical, temporal, and spatial variability (0.4–66 pmol g−1), with the highest values occurring at a peat‐rich environment in May 2001, suggesting that MMHg production was largely controlled by temporal factors and habitat type. In pore water, both Hg and MMHg concentrations were generally elevated near the sediment‐water interface during warm months. Sediment‐water exchange flux of MMHg, determined with benthic chamber deployments, ranged from −92 to 850 pmol m−2 d−1, with higher values occurring in May. In most cases, diffusional fluxes of Hg and MMHg, estimated with the use of interfacial concentration gradients, constituted only a minor portion of the measured fluxes, suggesting the importance of advective processes on sediment‐water exchange. Surface‐water transect and time series studies conducted in Franks Tract support the commonly held belief that wetland and marsh regions are major sources for MMHg within the Delta. The integrated sediment‐water fluxes of Hg and MMHg in the study area were estimated to be 130 and 6 mmol d−1, respectively, and the benthic input was as important a source of Hg and MMHg as the riverine input within the Delta during low‐flow months.
Sedimentary geochemical record of human–induced environmental changes in the Lake Brunnsviken watershed, Sweden
Authors: Routh Joyanto , Meyers Philip A. , Gustafsson Örjan , Baskaran Mark , Hallberg Rolf , Schöldström Anna
Published: July 2010

Environmental changes in Lake Brunnsviken, its watershed, and the greater Stockholm region since the middle of the nineteenth century have left interpretable geochemical imprints in the bottom sediments. These human‐induced perturbations within the lake’s watershed included agriculture, urbanization, sewage and industrial disposal, and water column aeration. Smaller δ15Ntotal values, high organic carbon mass accumulation rates, low C:N ratios, and larger δ13Corg values identify periods of increased nutrient delivery and elevated primary productivity in the lake. C: S ratios that change from high to low trace the transition from an oxic hypolimnion to an anoxic one during the periods of high productivity. Accumulations of redox‐sensitive trace elements increase during the anoxic period and are further magnified during a time of industrial waste discharge into the lake. A recent decrease in black carbon concentrations in sediments reflects the conversion from wood and coal to cleaner forms of energy.
Growth rate of the major phylogenetic bacterial groups in the Delaware estuary
Authors: Yokokawa Taichi , Nagata Toshi , Cottrell Matthew T. , Kirchman David L.
Published: July 2010

The phylogenetic composition of bacterial communities varies along the salinity gradient of estuaries, one notable pattern being the prevalence of alpha‐ and beta‐proteobacteria in salt‐ and freshwater regions, respectively. We tested the hypothesis that bottom‐up forces (substrate supply) control these and other changes in bacterial community composition in the Delaware estuary. We measured the biomass and growth rate of four major phylogenetic groups of bacterioplankton (alpha‐, beta‐, and gamma‐proteobacteria and the Cytophaga‐Flavobacter cluster) in low‐ and high‐salinity regions of the estuary by the dilution culture approach combined with fluorescent in situ hybridization with rRNA‐targeted probes. Group‐specific growth rates were highly variable depending on location and season and could far exceed (up to nearly fourfold) the growth rates of total bacteria. The phylogenetic groups that exhibited the highest growth rates included alpha‐ and gamma‐proteobacteria at both low‐ and high‐salinity stations and betaproteobacteria at a low‐salinity station. These data help to explain the high abundance of beta‐proteobacteria in the freshwater region, but bottom up controls appear to account only partly for the variability in bacterial communities. The relationship between production and biomass suggested that bacterial communities at the low‐salinity site were controlled by substrate supply, whereas bacterial mortality appeared to be more important at the high‐salinity site. Our data demonstrate that group‐specific growth rates are useful parameters for examining the competitive advantages of individual groups and the mode of regulation of bacterial communities in estuarine environments.
Early diagenesis of plant‐derived dissolved organic matter along a wetland, mangrove,estuary ecotone
Authors: Scully Norman M. , Maie Nagamitsu , Dailey Susan K. , Boyer Joseph N. , Jones Ronald D. , Jaffé Rudolf
Published: July 2010

We studied the role of photochemical and microbial processes in contributing to the transformation of dissolved organic matter (DOM) derived from various plants that dominate the Florida Everglades. Plant‐derived DOM leachate samples were exposed to photochemical and microbial degradation and the optical, chemical, and molecular weight characteristics measured over time. Optical parameters such as the synchronous fluorescence intensity between 270 and 290 nm (Fnpeak I), a strong indicator of protein and/or polyphenol content, decreased exponentially in all plant leachate samples, with microbial decay constants ranging from ‐1.0 d−1 for seagrass to ‐0.11 d−1 for mangrove (half‐life [t1/2] = 0.7‐6.3 d). Similar decreases in polyphenol content and dissolved organic carbon (DOC) concentration also occurred but were generally an order of magnitude lower or did not change significantly over time. The initial molecular weight composition was reflected in the rate of Fnpeak I decay and suggests that plant derived DOM with a large proportion of high molecular weight structures, such as seagrass derived DOM, contain high concentrations of easily microbially degradable proteinaceous components. For samples exposed to extended simulated solar radiation, polyphenol and Fnpeak I photochemical decay constants were on average −0.7 d−1 (t1/2 1.0 d). Our data suggest that polyphenol structures of plant‐derived DOM are particularly sensitive to photolysis, whereas high molecular weight protein‐like structures are degraded primarily through physical‐chemical and microbial processes. Furthermore, microbial and physical processes initiated the formation of recalcitrant, highly colored high molecular weight polymeric structures in mangrove‐derived DOM. Thus, partial, biogeochemical transformation of plant‐derived DOM from coastal areas is rapid and is likely to influence carbon and nutrient cycling, especially in areas dominated by seagrass and mangrove forests.
Assessing sources and ages of organic matter supporting river and estuarine bacterial production: A multiple‐isotope (Δ14C, 㬔C, and δ15N) approach
Authors: McCallister S. Leigh , Bauer James E. , Cherrier Jennifer E. , Ducklow Hugh W.
Published: July 2010

We used radiocarbon (Δ14C) and stable isotopic (Δ13C, Δ15N) signatures of bacterial nucleic acids to estimate the sources and ages of organic matter (OM) assimilated by bacteria in the Hudson River and York River estuary. Dual‐isotope plots of Δ14C and Δ13C coupled with a three‐source mixing model resolved the major OM sources supporting bacterial biomass production (BBP). However, overlap in the stable isotopic (Δ13C and Δ15N) values of potential source end members (i.e., terrestrial, freshwater phytoplankton, and marsh‐derived) prohibited unequivocal source assignments for certain samples. In freshwater regions of the York, terrigenous material of relatively recent origin (i.e., decadal in age) accounted for the majority of OM assimilated by bacteria (49–83%). Marsh and freshwater planktonic material made up the other major source of OM, with 5–33% and 6–25% assimilated, respectively. In the mesohaline York, BBP was supported primarily by estuarine phytoplankton‐derived OM during spring and summer (53–87%) and by marsh‐derived OM during fall (as much as 83%). Isotopic signatures from higher salinity regions of the York suggested that BBP there was fueled predominantly by either estuarine phytoplankton‐derived OM (July and November) or by material advected in from the Chesapeake Bay proper (October). In contrast to the York, BBP in the Hudson River estuary was subsidized by a greater portion (up to ~25%) of old (~24,000 yr BP) allochthonous OM, which was presumably derived from soils. These findings collectively suggest that bacterial metabolism and degradation in rivers and estuaries may profoundly alter the mean composition and age of OM during transport within these systems and before its export to the coastal ocean.
Seasonal bioavailability of dissolved organic carbon and nitrogen from pristine and polluted freshwater wetlands
Authors: Wiegner Tracy N. , Seitzinger Sybil P.
Published: July 2010

We examined the chemical composition and bioavailability of dissolved organic carbon (DOC) and nitrogen (DON) from two pristine and two polluted cedar bog wetlands across three seasons. Pristine and polluted wetlands differed in DOC and DON concentrations, chemical characteristics, and bioavailability. DOC and DON concentrations were higher in the polluted than in the pristine wetlands. In contrast, a higher percentage of the dissolved organic matter (DOM) was more aromatic in the pristine (54% ± 19) than in the polluted (27% ± 4) wetlands. A higher percentage of DOC was bioavailable in the pristine (22% ± 9) than the polluted (12% ± 4) wetlands. A similar percentage of DON was consumed in both the pristine (33% ± 25) and polluted (28% ± 25) wetlands. Seasonally, the bioavailability of DOC and DON varied and differed between the pristine and polluted wetlands. The availability of phosphate appeared to affect the amount of DOC incorporated into bacterial biomass, whereas inorganic nutrient availability did not affect the assimilation of DON. Bioavailable DOC primarily fueled bacterial respiration, whereas DON supported bacterial growth. Overall, our results demonstrate that anthropogenic activities and season affect the quantity and quality of wetland DOM exported to rivers and that different factors control the utilization and fate of DOC and DON within the bacterial community.
Relationships among photoperiod, carbon fixation, growth, chlorophyll a, and cellular iron and zinc in a coastal diatom
Authors: Sunda William G. , Huntsman Susan A.
Published: July 2010

We conducted culture experiments with the diatom Thalassiosira pseudonana to determine the interactive effects of day length and biologically available concentrations of iron and zinc on cellular iron (Fe), zinc (Zn), chlorophyll a (Chl a), and fixed carbon (C) throughout the light period. Specific rates of C‐fixation and growth were also measured. Specific C‐fixation rates showed a single linear relation with the cellular Fe :C ratio regardless of the photoperiod. Decreasing the photoperiod from 14 to 7 h increased the mean daytime cellular Fe :C ratio by 40%, the specific C‐fixation rate by 34%, and the Chl a :C ratio by 91% in mildly iron‐limited cultures. These changes reflect a cellular acclimation to the shortened light period. The higher cellular iron level apparently allowed for synthesis of additional iron‐rich proteins (e.g., those utilized in photosynthetic electron transport) needed to support the increased rate of C‐fixation. Mean cellular Chl a concentration decreased linearly with decreasing specific growth rate under iron and zinc limitation, thereby allowing the cells to maintain a balance between light harvesting and biosynthesis. Cellular concentrations of carbon, Chl a, zinc, and iron typically varied during the light period because of the day‐night differences in rates of C‐fixation, Chl a synthesis, growth, and metal uptake. Cell carbon concentrations increased by 36–96% during the light period, reflecting daytime storage of fixed carbon to support nighttime respiration and growth. Cellular zinc concentrations decreased by 25% during the light period owing to higher daytime specific growth rates and resulting higher rates of biodilution. By contrast, the direction of change in cellular iron concentrations was dependent on the extent of photochemical redox cycling of iron chelates, which increased iron uptake rates during the day. The direction and magnitude of daytime changes in cellular zinc and iron were also dependent on the parameter (cell volume, cell numbers, or carbon) to which the cellular metal was normalized, as each of these parameters exhibited its own unique diurnal pattern.
Food quality controls egg quality of the zebra mussel Dreissena polymorpha: The role of fatty acids
Authors: Wacker Alexander , von Elert Eric
Published: July 2010

We investigated the investment of adult Dreissena polymorpha to the eggs by sampling mussels monthly from 4‐ and 15−m water depth. The fatty acid composition of eggs differed significantly between depths and over time. To assess whether temperature and food conditions led to the differences observed for mussels sampled from the two depths, mussels were reared in the laboratory under two different 3‐month temperature regimes, simulating the temperature of the lake at 4− and 15−m depth. Possible effects of food quality were tested in each simulation using four diets differing in fatty acid composition: Cryptomonas erosa, Nannochloropsis limnetica [rich in polyunsaturated fatty acids (PUFAs) and long‐chained PUFAs (≫C18)], Scenedesmus obliquus, and the cyanobacterium Aphanothece sp. (deficient in long‐chained PUFAs). In newly released eggs, specific (n‐3) and (n‐6) long‐chained PUFAs increased when these fatty acids were available in the natural seston or in the laboratory diets. Mussels fed organisms deficient in long‐chained PUFAs were still able to allocate arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid into eggs, which suggests that minimum levels of particular fatty acids were maintained in eggs by transfer from internal reserves of the female mussels to oocytes. In contrast to the diet, there were no effects of the temperature on the fatty acid composition of eggs.
The stoichiometry of dissolved organic carbon, nitrogen, and phosphorus release by a planktonic grazer, Daphnia
Authors: Frost Paul C. , Xenopoulos Marguerite A. , Larson James H.
Published: July 2010

We examined the relationship between Daphnia release rates and ratios of dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P) and the elemental composition of algal food. We constructed a three‐element (C, N, and P) mass‐balance model to explore the relationships between food elemental composition and Daphnia release of DOC, N, and P. The model predicts that Daphnia DOC:N and DOC: P release ratios will increase with increasing food C:N and C: P ratios, respectively. These effects on DOC:N and DOC: P ratios are dependent, however, on the availability of the third element (P and N respectively) in our model. In addition, we experimentally measured the mass‐specific release of DOC, ammonia (NH4), and soluble reactive phosphorus (SRP) from Daphnia magna fed food of contrasting C:N: P ratios. Daphnia release rates of DOC were not significantly affected by algal C:N or C: P ratios. However, high C:N and C: P ratios in algal food raised the DOC:NH4 and DOC:SRP ratios (respectively) released from Daphnia. Our results, experimental and modeling, show a strong dependence of DOC: N and DOC: P release ratios by Daphnia on food C:N and C: P ratio. Thus, elevated C:N or C: P ratios in food can cause imbalanced release of DOC and NH4 or SRP, respectively, by Daphnia with potentially strong feedback on microbial heterotrophs in planktonic food webs.
Aeration of corals by sleep‐swimming fish
Authors: Goldshmid Rotem , Holzman Roi , Weihs Daniel , Genin Amatzia
Published: July 2010

The morphology of branching corals effectively blocks impinging currents. Weak flows in the colony’s inner sections can hinder some of the corals’ basic physiological functions. At night, high respiration by the coral and the absence of photosynthesis by the symbiotic algae sometimes generate severe hypoxia near coral branches. Nevertheless, branching corals thrive in flow‐protected areas. Many of them host resident fish that seek shelter between their branches during the night. In situ observations using an infrared‐sensitive video camera revealed that, unlike crevice‐dwelling fishes, which sleep motionless, fish (Dascyllus marginatus, D. aruanus, and Chromis viridis) that spend the night in living hard corals (Stylophora pistillata, Acropora spp., Pocillopora spp., and Seriatopora spp.) exhibit a unique sleep‐swimming mode that is characterized by energetic, high‐frequency fin motions. In the reef, the presence of fish in the coral S. pistillata increased gypsum dissolution and, hence, mass transfer, by ~17% compared with colonies without fish. The extent to which the fish enhanced water replenishment in the inner zone of the coral was assessed in the laboratory using oxygen measurements. In the absence of fish, oxygen levels near the coral tissues (~1 mm) rapidly declined, reaching 10–30% of ambient levels, which was much lower than the nearly steady levels of 60–80% when sleep‐swimming fish were present. Individual sleep‐swimming fish were spatially separated across the coral, so that most of the coral’s inner zone was frequently ventilated. This common, yet so‐far overlooked mutualistic relationship is unique by the virtue of its effect, the active modulation of hydrodynamic conditions, and its operation during the sleep of the active animal. The modulation of hydrodynamic conditions by the fish can mitigate flow limitation on the growth and survival of branching corals.
Testing a selective tidal‐stream transport model: Observations of female blue crab (Callinectes sapidus) vertical migration during the spawning season
Authors: Hench James L. , Forward Richard B. , Carr Sarah D. , Rittschof Daniel , Luettich Richard A.
Published: July 2010

With field, laboratory, and modeling approaches, we examined the interplay among habitat structure, intraguild predation (IGP), and parasitism in an ongoing species invasion. Native Gammarus duebeni celticus (Crustacea: Amphipoda) are often, but not always, replaced by the invader Gammarus pulex through differential IGP. The muscle‐wasting microsporidian parasite Pleistophora mulleri infects the native but not the invader. We found a highly variable prevalence of P. mulleri in uninvaded rivers, with 0–91% of hosts parasitized per sample. In addition, unparasitized natives dominated fast‐flowing riffle patches of river, whereas parasitized individuals dominated slower‐flowing, pooled patches. We examined the survivorship of invader and native in single and mixed‐species microcosms with high, intermediate, and zero parasite prevalence. G. pulex survivorship was high in all treatments, whereas G. duebeni subsp. celticus survivorship was significantly lower in the presence of the invader. Further, parasitized G. duebeni subsp. celticus experienced near‐total elimination. Models of the species replacement process implied that parasite‐enhanced IGP would make invasion by G. pulex more likely, regardless of habitat and parasite spatial structure. However, where heterogeneity in parasite prevalence creates a landscape of patches with different susceptibilities to invasion, G. pulex may succeed in cases where invasion would not be possible if patches were equivalent. The different responses of parasitized and unparasitized G. duebeni subsp. celticus to environmental heterogeneity potentially link landscape patterns to the success or failure of the invasion process.
Fluorescent dissolved organic matter in lakes: Relationships with heterotrophicmetabolism
Authors: Cammack W. K. Levi , Kalff Jacob , Prairie Yves T. , Smith Erik M.
Published: July 2010

Characterizing dissolved organic matter (DOM) composition remains a major unresolved problem in aquatic ecology and is one of the key impediments to developing a good understanding of DOM production and consumption by heterotrophic bacteria. Fluorescence spectroscopy has been proposed as a promising method for characterizing DOM, but few links have been demonstrated between DOM fluorescence and DOM composition or the processes affected by DOM composition. In 28 southern QueÂbec lakes, tryptophan‐like DOM fluorescence (TFDOM) was found to be a much better descriptor of rates of heterotrophic bacterial metabolism than dissolved organic carbon, describing 52%, 44%, 51%, and 55% of the variability in bacterial production, bacterioplankton respiration, total bacterial carbon consumption, and total plankton community respiration, respectively. In addition, evidence from a series of bacterial regrowth cultures suggests that T‐FDOM represents a product of bacterial activity as well as, to a lesser extent, a bioavailable substrate. Our results instead raise the intriguing possibility that TFDOM concentration reflects a balance between its production and consumption by bacteria. We demonstrate here that fluorescence spectroscopy can be used to identify a highly dynamic fraction of DOM related to bacterial metabolism in lakes.
The heterotrophic bacterial response during the Southern Ocean Iron Experiment (SOFeX)
Authors: Oliver Jacques L. , Barber Richard T. , Smith, Walker O. , Ducklow Hugh W.
Published: July 2010

We studied heterotrophic bacterial dynamics as part of the Southern Ocean Iron Experiment (SOFeX), January‐February 2002. Two phytoplankton blooms were monitored following infusions with iron sulfate (FeSO4). The first bloom was initiated north of the Antarctic Polar Front Zone (APFZ) in silica‐poor waters (North Patch) and was observed sporadically over 42 d, whereas the second was south of the APFZ in silica‐rich waters (South Patch) and was continuously observed for 30 d. In both experiments, iron additions resulted in increased chlorophyll a (Chl a), particulate organic matter (POC + PN), and a drawdown of inorganic nutrients. Heterotrophic bacteria responded by increasing their abundance (110% and 60% increases in bacterial abundance in the North and South Patch, respectively, relative to nonenriched waters). Thymidine (TdR) and leucine (Leu) incorporation rates in the North Patch increased by 400% and 120%, respectively, with more modest increases in the South Patch (80% and 70%, respectively). In the South Patch, bacterial production (BP) was significantly correlated with net particulate primary production (PP) and Chl a. Bacterial abundance was also significantly correlated with Chl a. Net bacterial accumulation rate in the South Patch was 0.02 d−1 over 17 d, excluding physical dilution by mixing with water outside the South Patch. The evidence suggests that bacterial growth during Southern Ocean Iron Experiment (SOFeX) was limited by labile carbon rather than by iron. Despite the close association of BP to PP, BP remained a small fraction of PP (<10%).
Biological and photochemical production of dissolved gaseous mercury in a boreal lake
Authors: Poulain A. J. , Amyot M. , Findlay D. , Telor S. , Barkay T. , Hintelmann H.
Published: July 2010

We used in situ experiments and measured depth profiles of dissolved gaseous mercury (DGM) to investigate the relative contribution of photochemical versus biological processes on the production of DGM in an oligomesotrophic lake of the Canadian Shield. At the surface, DGM production was mainly photomediated, with reduction rates being twice as high in the wetland than in the lake. In the water column, the distribution of DGM concentrations was not strictly related to light but followed a multimodal distribution, with peaks encountered below the epilimnion at depths receiving <5% of the incident light. Those peaks were recorded in the middle and at the bottom of the metalimnion during the ice‐free season, as well as under ice cover and at the bottom of an anoxic hypolimnion. Rather than being a consequence of the bacterial mercuric reductase activity, metalimnetic DGM peaks were associated with the intensity and duration of phytoplankton blooms. In situ incubation experiments also showed that DGM production ceased when samples were kept in the dark, filtered, or when an inhibitor of photosynthesis was added. Our results illustrate the important role of phytoplankton on Hg redox dynamics in the water column of lakes.
Thiols in wetland interstitial waters and their role in mercury and methylmercury speciation
Authors: Zhang Jinzhong , Wang Feiyue , House James D. , Page Bryan
Published: July 2010

Organic sulfhydryl compounds, or thiols, are ligands that strongly complex class B metals such as Hg and methylmercury (MeHg). We determined the concentration profiles of five low‐molecular‐weight thiols (cysteine, thioglycolic acid, glutathione, N‐acetyl‐L‐cysteine, and 3‐mercaptopropionic acid) in sediment interstitial waters, at a vertical resolution of 1 cm, in three contrasting freshwater and brackish wetlands in Canada. All five thiols were detected in the porewaters, with concentrations ranging from nanomolar to submicromolar. In one of the wetlands (Baie St. Francois) the profiles of Hg and MeHg were also obtained at the same vertical resolution. Thermodynamic calculations revealed that at these levels thiols play a negligible role in inorganic Hg speciation in sediment interstitial waters, but they can dominate the MeHg speciation. Consistent with recent findings that intracellular MeHg in fish is dominated by MeHg‐thiol complexes, this suggests that thiols also play a significant role in MeHg speciation in the extracellular environment.
Synchronized hatch and its ecological significance in rainbow smelt Osmerus mordax in St. Mary's Bay, Newfoundland
Authors: Bradbury Ian R. , Campana Steven E. , Bentzen Paul , Snelgrove Paul V. R.
Published: July 2010

Early life history stages in most marine animals are subject to high mortality through predation, starvation, and dispersal. Accordingly, the potential exists for the selection of behavioral mechanisms that reduce mortality. We examined the ecological significance of synchronization in hatch and the initiation of larval drift in rainbow smelt, Osmerus mordax, populations in St. Mary's Bay, Newfoundland. Larval abundances from six 24‐h ring net surveys (2‐h intervals) in Colinet and Salmonier Rivers during 2002/2003 suggest synchronized hatch following dusk (~2200 h). Monitoring of egg hatching in situ confirmed synchrony was at hatch and notemergence. Larval abundance showed no relationship with temperature or flow rates, and the consistency in hatch pattern suggested a light/dark cue. In experimental manipulations in which eggs were exposed to light and dark conditions for 2‐h periods, hatch percentages were up to five times higher (p < 0.005) in dark treatments. We hypothesized that the linkage of hatch to low light levels represents a mechanism to avoid elevated larval predation in daylight conditions. Egg predation determined from predator gut content analysis suggested that extreme predation risk from small (<20 cm) salmonids peaked during the day, prior to dusk, and was lowest during night (2200–0400 h). Microcosm experiments demonstrated that newly hatched larvae exposed to predators in dark conditionsdid not change in number, but mortality averaged 60% in light conditions. Our results suggest that predation pressure during the early life history of aquatic organisms might play a strong role in the optimization of aquatic life histories.
Control of silica production by iron and silicic acid during the Southern Ocean Iron Experiment (SOFeX)
Authors: Brzezinski Mark A. , Jones Janice L. , Demarest Mark S.
Published: July 2010

We examined the role of Si limitation in mediating the response of siliceous biomass and silica production to mesoscale Fe fertilization in the high‐silicic acid high‐nitrate waters of the Antarctic and in the low‐silicic acid high‐nitrate waters of the Subantarctic during austral summer. Iron fertilization stimulated biogenic silica production and silicic acid depletion in both regions. Si limitation significantly curtailed the response of silica production to Fe in the Subantarctic, but not in the Antarctic. Additions of silicic acid to Fe‐enriched waters of the Subantarctic more than doubled specific silica production rates beyond the increase caused by Fe alone. This result, combined with the presence of Si limitation both inside and outside of the fertilized patch in the Subantarctic, indicates that silica production in the Subantarctic is regulated by both Fe and Si during austral summer. Fe increased the ability of Subantarctic diatom assemblages to take up low concentrations of Si over an order of magnitude by increasing maximum uptake rates and lowering half‐saturation constants for silicic acid uptake. Our observations indicate that the dramatic gradient in Si availability across the Antarctic Circumpolar Current exerts a strong control on the contribution of diatoms to new production following Fe enrichment in the Southern Ocean during austral summer. With sufficient Fe to allow nitrate depletion, the abundant silicic acid in the Antarctic would allow diatoms to dominate nitrate use; however, low silicic acid concentrations in the Subantarctic would restrict the fraction of available nitrate that would be consumed by diatoms to ca. 5%.
Effect of UV‐B and different PAR intensities on the primary production of the mixotrophic planktonic ciliate Stentor araucanus
Authors: Modenutti Beatriz E. , Balseiro Esteban G. , Callieri Cristiana , Bertoni Roberto , Queimaliños Claudia P.
Published: July 2010

Stentor araucanus is a mixotrophic ciliate that, in Andean lakes, inhabits the upper epilimnetic levels, which are commonly avoided by other planktonic organisms. This freshwater heterotrich has dark pigmented cortical granules and lives autotrophically with endosymbiotic algae. The effect of photosynthetically active radiation (PAR) and ultraviolet (UV)‐B radiation on primary production was analyzed during summer 2003–2004 in Lake Moreno Oeste, a highly transparent ultraoligotrophic lake (mean summer Kd = 0.16 m−1). Primary production (PP) was measured in the field in the euphotic zone during both static and variable‐depth incubations. Static exposure of the organisms was examined at different depths (0.30, 10, and 20 m), and the variable depth exposure involved experimental containers moved continuously up and down the epilimnion (0–15 m). In the static exposure closest to the surface and in the mobile incubation, quartz tubes were incubated with and without a UV‐B screen (Mylar™). Additionally, PP was measured in the laboratory with and without previous exposure to a UV‐B lamp (290–315 nm). S. araucanus was present throughout the summer with highest abundances at or above 15 m in depth. A high proportion of the ciliate population (80%) was, therefore, exposed to UV radiation, and between 30% and 60% of the population occupied depths at which UV‐B (305 nm) exceeded 1% of surface incidence. PP values were higher in the epilimnion than below it and were not reduced by exposure to high irradiances of PAR+UV‐A and PAR+UV‐A+UV‐B. The laboratory experiments showed no difference between UV‐B and PAR preexposure treatments. The variable‐depth epilimnetic incubations gave similar PP values and did not differ from the static incubations. The average PAR irradiance of the epilimnion was high, around 600 µmol photons m−2 s−1, which was the value at which S. araucanus reached a saturation level in the laboratory. In contrast, the incubations at 20 m differed significantly from those in the epilimnion, exhibiting lower values, except when PAR irradiance was higher than 100 µmol photons m−2 s−1. These results indicate that pigmented mixotrophs like S. araucanus achieve high population densities in the epilimnion because they receive sufficient irradiance (PAR between 100 and 1,600 µmol photons m−2 s−1) to allow endosymbiotic algae to produce.
Subitaneous eggs of freshwater copepods pass through fish guts: Survival, hatchability, and potential ecological implications
Authors: Bartholmeé Silvia , Samchyshyna Laryssa , Santer Barbara , Lampert Winfried
Published: July 2010

We measured the survival of subitaneous eggs of two calanoid (Eudiaptomus gracilis, E. graciloides) and two cyclopoid (Cyclops abyssorum, Macrocyclops albidus) freshwater copepods after they had been consumed by fish. Unexpectedly, over 80% of the calanoid eggs and 30–59% of the cyclopoid eggs were morphologically intact in fish feces. Subitaneous eggs of E. graciloides showed similar proportions of gut passage as dormant resting eggs. About 70–80% of the calanoid eggs and 35–50% of the cyclopoid eggs released nauplii within 3 d. Hence, a total of 50–70% of the calanoid eggs and 11–29% of the cyclopoid eggs survived ingestion and gut passage. Survival was slightly higher because of shorter gut passage time when the fish had been prefed natural plankton compared with hungry fish. We interpret digestion resistance of subitaneous eggs in copepods as an adaptation to fish predation on egg‐carrying females.
Weak coupling between community composition and functioning of aquatic bacteria
Authors: Langenheder Silke , Lindström Eva S. , Tranvik Lars J.
Published: July 2010

We performed a batch culture experiment with a factorial design in which sterile water from four lakes and bacterial assemblages (size‐fractionated lake water) from the same lakes were set up in all possible combinations. The functional performance (biomass yield, respiration, growth rates, and growth efficiency) of bacterial communities growing in the cultures depended primarily on the type of the medium and to a much lesser extent on the origin of the bacterial assemblage. Functional changes were only partly paralleled by changes in community composition, as indicated by terminal restriction fragment length polymorphism analysis. Similar bacterial communities developed in different cultures as a result of receiving either the same medium or the same inoculum, indicating that bacterial communities are comprised of populations of generalists that can grow under most conditions as well as populations with the life strategy of specialists. However, bacteria originating from a slightly acidic polyhumic lake failed to grow, grew unsteadily, or exhibited an extended lag phase when exposed to media originating from other lakes, indicating that the bacterial community in the polyhumic lake was not able to adapt rapidly to changes in environmental conditions.
Linking bacterial richness with viral abundance and prokaryotic activity
Authors: Winter Christian , Smit Arjan , Herndl Gerhard J. , Weinbauer Markus G.
Published: July 2010

We measured relative bacterial and archaeal richness and its covariation with physical and other biological parameters during six cruises in the North Sea over prokaryotic productivity levels ranging over two orders of magnitude. Relative bacterial and archaeal richness was estimated as the number of peaks detected by terminal‐ restriction fragment length polymorphism (T‐RFLP) analysis of polymerase chain reaction‐amplified prokaryotic 16S rRNA gene fragments in unfiltered (total community) and 0.8‐µm filtered seawater (free‐living community). Relative bacterial richness ranged from 20 to 57 peaks in the total community and from 16 to 56 peaks in the freeliving community. Relative archaeal richness varied between 2 and 14 peaks in the total community and between 2 and 21 peaks in the free‐living community. Coamplified plastid DNA might have influenced relative bacterial richness in unfiltered but not in 0.8‐µm filtered seawater. Relative bacterial richness decreased with viral abundance and total and cell‐specific prokaryotic production in the free‐living and the total community. Stepwise multiple regression analysis revealed that temperature also influenced relative bacterial richness. Relative archaeal richness was not related to any other parameter. The data suggest that high prokaryotic productivity was sustained by a relatively small number of highly active bacterial populations that also maintained high viral abundance.
Hydrophobicity and reactivity of trace metals in the low‐salinity zone of a turbid estuary
Authors: Turner Andrew , Mawji Edward
Published: July 2010

We chemically fractionated water samples from the low‐salinity zone of a turbid estuary (Tamar, UK) using C18 columns to examine the controls on the complexation, hydrophobicity, and reactivity of dissolved trace metals (Co, Cu, Mn, and Pb). The hydrophobic (C18‐retained) fraction of metal was related to the solubility of metal in the amphiphilic solvent, n‐octanol, and in acid‐base titrations and for all metals this fraction exhibited a peak centered around pH 7–8 in both freshwater and brackish water. In freshwater, hydrophobicity decreased in the order Pb > Cu > Mn > Co, and in brackish water, hydrophobicity was either reduced (Cu and Pb) or enhanced (Co and Mn) compared to freshwater, but in all cases peaks were more distinct. Salinity distributions of dissolved trace metals in the estuary indicated significant removal of Co, Cu, and Mn, and a source of Pb that was coincident with the onset of the turbidity maximum, and all distributions could be accounted for empirically by end‐member water mixing and sediment resuspension. Salinity distributions of the hydrophobic fraction of metal were not consistent with any variation in water composition (including pH), but revealed that this fraction was preferentially removed from (Co, Cu, and Mn) or added to (Pb) the aqueous phase in the vicinity of the turbidity maximum. Removal implies that hydrophobic species are more reactive than hydrophilic species, and that either re‐equilibration between these fractions is not attained within the hydrodynamic timescales of the upper estuary, or new equilibria are established by the buffering effects of additional anions and ligands in the water column. Addition of hydrophobic species of Pb coincided with minimum total and hydrophobic concentrations of Co, Cu, and Mn, and could, therefore, be explained in terms of reduced competition for nonspecific hydrophobic ligands from these and other (e.g., Al) metals in this region of the estuary.
Responses of estuarine and coastal marine phytoplankton to nitrogen and phosphorus enrichment
Authors: Smith Val H.
Published: July 2010

A cross‐ecosystem comparison of data obtained from 92 coastal zone ecosystems worldwide revealed a strong positive response of marine phytoplankton biomass to nutrient enrichment that is highly consistent with the general patterns reported previously in the limnological literature for freshwater lakes and reservoirs. Average concentrations of chlorophyll a in estuarine and coastal marine systems were strongly dependent on the mean concentrations of total nitrogen and total phosphorus in the water column. Moreover, as is true of freshwater ecosystems, the identity of the primary growth‐limiting nutrient for marine phytoplankton appeared to be generally predictable from watercolumn total nitrogen : total phosphorus (TN : TP) ratios. This similarity in physiological response to nutrients likely derives from the shared evolutionary histories of marine and freshwater phytoplankton.
Phytoplankton response to nutrient enrichment in an urbanized estuary: Apparent inhibition of primary production by overeutrophication
Authors: Yoshiyama Kohei , Sharp Jonathan H.
Published: July 2010

Phytoplankton response to nutrients was examined with a 26‐yr database from the Delaware Estuary. Biomassnormalized primary production did not increase linearly with increasing nutrient concentrations and instead showed saturation at comparatively low nutrient concentrations and decreased at high concentrations. To separate the effects of light availability and temperature on primary production from those of other environmental variables, we developed an empirical model of areal primary production. The model equation was obtained for the entire dataset and the effect of the residual variables was expressed as correction factors of observed primary production to the model estimates. The model accounted for 67% of variability of observed primary production overall, indicating that production of the estuary was mainly controlled by light availability and temperature. In contrast, a similar model applied to a Chesapeake Bay database had shown a poorer fit, indicating consistent light limitation in the Delaware Estuary and varying strengths of light and nutrient limitation in the Chesapeake Bay. The relationships between nutrients and correction factors for the Delaware Estuary showed that the model underestimates primary production at low and mid nutrient concentrations and overestimates it at high concentrations. The model fit and correction factors for five regions of the estuary indicate a high‐nutrient, low‐growth situation in the Delaware Estuary because of varying influences of light limitation, proportions of nutrients, and probably toxic contaminants in areas with large anthropogenic inputs—including high nutrients.