Showing papers in "Limnology and Oceanography in 2012"

Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers

Abstract: Scaling is an integral component of ecology and earth science. To date, the ability to determine the importance of air – water gas exchange across large spatial scales is hampered partly by our ability to scale the gas transfer velocity and stream hydraulics. Here we report on a metadata analysis of 563 direct gas tracer release experiments that examines scaling laws for the gas transfer velocity. We found that the gas transfer velocity scales with the product of stream slope and velocity, which is in alignment with theory on stream energy dissipation. In addition to providing equations that predict the gas transfer velocity based on stream hydraulics, we used our hydraulic data set to report a new set of hydraulic exponents and coefficients that allow the prediction of stream width, depth, and velocity based on discharge. Finally, we report a new table of gas Schmidt number dependencies to allow researchers to estimate a gas transfer velocity using our equation for many gasses of interest.

The Pacific oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: Implications for near-term ocean acidification effects

Abstract: We report results from an oyster hatchery on the Oregon coast, where intake waters experienced variable carbonate chemistry (aragonite saturation state , 0.8 to . 3.2; pH , 7.6 to . 8.2) in the early summer of 2009. Both larval production and midstage growth (, 120 to , 150 mm) of the oyster Crassostrea gigas were significantly negatively correlated with the aragonite saturation state of waters in which larval oysters were spawned and reared for the first 48 h of life. The effects of the initial spawning conditions did not have a significant effect on early-stage growth (growth from D-hinge stage to , 120 mm), suggesting a delayed effect of water chemistry on larval development.

The spectral slope coefficient of chromophoric dissolved organic matter (S275–295) as a tracer of terrigenous dissolved organic carbon in river‐influenced ocean margins

Abstract: The present study demonstrates that the spectral slope coefficient of chromophoric dissolved organic matter (CDOM) between 275 nm and 295 nm (S275–295) can be used as a tracer of the percent terrigenous dissolved organic carbon (%tDOC) in river-influenced ocean margins, where rivers exert an important control on carbon dynamics and CO2 fluxes. Absorption coefficients of CDOM and concentrations of dissolved organic carbon (DOC) and dissolved lignin were measured on a seasonal basis in the Mississippi and Atchafalaya rivers and in surface waters of the northern Gulf of Mexico (NGoM). A strong, linear relationship between lignin concentrations and CDOM absorption coefficients indicated lignin is an important chromophore in this environment. The dual nature of lignin as an important chromophore in CDOM and as a terrigenous component of DOC facilitated development of the tracer. The applicability of the tracer relies on the existence of a strong, nonlinear relationship between S275–295 and the DOC-normalized lignin yield in rivers and along the freshwater– marine continuum in the NGoM. Physical mixing and the effects of photodegradation on S275–295 and dissolved lignin were largely responsible for maintaining this relationship, suggesting the tracer is applicable to surface waters of most river-influenced ocean margins. The spectral slope coefficient (S275–295) provides new capabilities to trace tDOC on synoptic scales of relevance to ocean margins and represents an important tool for improving ocean carbon budgets. The key processes controlling carbon transformations in ocean margins remain poorly quantified, thereby limiting our understanding of how the coastal ocean affects the ocean carbon cycle and atmospheric CO2. Ocean margins account for , 10% of the global ocean surface area but

Allometric scaling and taxonomic variation in nutrient utilization traits and maximum growth rate of phytoplankton

Abstract: Nutrient utilization traits can be used to link the ecophysiology of phytoplankton to population dynamic models and the structure of communities across environmental gradients. Here we analyze a comprehensive literature compilation of four traits: maximum nutrient uptake rate; the half-saturation constant for nutrient uptake; the minimum subsistence quota, measured for nitrate and phosphate; and maximum growth rate. We also use these traits to analyze two composite traits, uptake affinity and scaled uptake affinity. All traits tend to increase with cell volume, except for scaled uptake affinity and maximum growth rate, which tend to decline with cell volume. Most scaling relationships are the same for freshwater and marine species, although important differences exist. Most traits differ on average between major taxa, but between-taxon variation is nearly always due to between-taxon variation in volume. There is some evidence for between-trait correlations that could constrain trait evolution, but these correlations are difficult to disentangle from correlation driven by cell volume. These results should enhance the parameterization of models that use size or taxonomic group to structure physiological variation in phytoplankton communities.

A size-structured food-web model for the global ocean

Abstract: We present a model of diverse phytoplankton and zooplankton populations embedded in a global ocean circulation model. Physiological and ecological traits of the organisms are constrained by relationships with cell size. The model qualitatively reproduces global distributions of nutrients, biomass, and primary productivity, and captures the power-law relationship between cell size and numerical density, which has realistic slopes of between −1.3 and −0.8. We use the model to explore the global structure of marine ecosystems, highlighting the importance of both nutrient and grazer controls. The model suggests that zooplankton : phytoplankton (Z : P) biomass ratios may vary from an order of 0.1 in the oligotrophic gyres to an order of 10 in upwelling and high-latitude regions. Global estimates of the strength of bottom-up and top-down controls within plankton size classes suggest that these large-scale gradients in Z : P ratios are driven by a shift from strong bottom-up, nutrient limitation in the oligotrophic gyres to the dominance of top-down, grazing controls in more productive regions.

Anammox and denitrification in the oxygen minimum zone of the eastern South Pacific

Abstract: We quantified the removal of fixed nitrogen as N2 production by anammox and N2 and N2O production by denitrification over a distance of 1900 km along the coasts of Chile and Peru, using short-term incubations with 15N-labeled substrates. The eastern South Pacific contains an oxygen minimum zone (OMZ) characterized by an anoxic, nitrate- and nitrite-rich layer of , 200-m thickness below 30‐90 m of oxic water. Anammox and denitrification were almost exclusively recorded when the in situ O2 concentration was below detection, indicating that the induction of these processes is highly oxygen sensitive. Anammox was detected in 70% of the samples from anoxic depths. Denitrification was detected in fewer samples, but maximum rates were an order of magnitude higher than those of anammox. In our incubations denitrification was responsible for 72% of the total N2 production and 77% of the total removal of fixed nitrogen including N2O production. However, at the individual depths it could be one or the other process that was responsible for all of the nitrogen removal. Anammox activity was highest just below the oxic‐anoxic interface and declined exponentially with depth, whereas no depth dependence was discerned for denitrification. Denitrification resulted in net production of N2O in some of the samples and consumption of added 15N2O in others. Together with the accumulation of NO { this indicates that denitrification must be seen as a sequence of individually regulated reactions, each of which may start and stop depending on the electron donor input, while anammox is much less variable. The highly patchy distribution of denitrification contributes to explain the apparent imbalances between ammonium sources and sinks suggested by previous 15 N-based studies in OMZs.

Biological lability of streamwater fluorescent dissolved organic matter

Abstract: We investigated the biological lability of fluorescent dissolved organic matter (FDOM) from a temperate Piedmont stream. Plug-flow bioreactors, colonized and maintained with natural stream water, were used to determine the concentrations of stream-water biodegradable dissolved organic carbon (BDOC) and relative concentrations of FDOM within operationally defined biodegradability classes. Labile molecules with turnover times of hours are metabolized within the stream reach where they originated; semi-labile molecules with turnover times of days travel out of the reach and are transported downstream before being metabolized; and a more recalcitrant class with a longer but undetermined turnover time flows through the river network without being metabolized. Between 26% and 31% of the DOC was biodegradable, with 8.6% labile and the balance semi-labile. Humic-like FDOM was a proxy for more recalcitrant DOM, exhibiting a ± 2% change as a function of increased bioreactor residence time. Humic-like FDOM represents only the more recalcitrant and perhaps the more hydrophobic constituents and not the ecologically important semi-labile humic substances within the BDOC pool. Tyrosine-like and tryptophan-like FDOM constituents included not only labile DOM, but also semi-labile, and more recalcitrant moieties. For example, 13% of the tryptophan-like FDOM was labile, 14% was semi-labile, and 73% was more recalcitrant, while tyrosine-like FDOM was 100% biodegraded and the majority (44–69%) was classified as labile. Collectively our results challenge some previous assessments of FDOM lability classifications and highlight the need to connect fluorescence characteristics of DOM to residence times of different carbon pools that spiral through a river network.

In situ variability of mass‐specific beam attenuation and backscattering of marine particles with respect to particle size, density, and composition

Abstract: This study analyzes relationships between concentration of suspended particles represented by dry mass, [SPM], or area, [AC], and optical properties including particulate beam attenuation (cp), side scattering (bs), and backscattering (bbp), obtained from an intensive sampling program in coastal and offshore waters around Europe and French Guyana. First-order optical properties are driven by particle concentration with best predictions of [SPM] by bbp and bs, and of [AC] by cp. Second-order variability is investigated with respect to particle size, apparent density (dry weight-to-wet-volume ratio), and composition. Overall, the mass-specific particulate backscattering coefficient, b m (5bbp:[SPM]), is relatively well constrained, with variability of a factor of 3–4. This coefficient is well correlated with particle composition, with inorganic particles having values about three times greater (b m5 0.012 m2 g21) than organic particles (b m5 0.005 m 2 g21). The mass-specific particulate attenuation coefficient, c m (5cp:[SPM]), on the other hand, varies over one order of magnitude and is strongly driven (77% of the variability explained) by particle apparent density. In this data set particle size does not affect c m and affects b m only weakly in clear (case 1) waters, despite size variations over one order of magnitude. A significant fraction (40–60% )o f the variability inb m remains unexplained. Possible causes are the limitation of the measured size distributions to the 2–302-mm range and effects of particle shape and internal structure that affect bbp more than cp and were not accounted for.

Nutrient dynamics and biological consumption in a large continental shelf system under the influence of both a river plume and coastal upwelling

Abstract: National Basic Research Program of China (973 Program) [2009CB421204, 2009CB421201]; National Natural Science Foundation of China [90711005, 40821063]; National Natural Science Foundation of China-Research Grants Council Hong Kong Special Administrative Region Government [40731160624N, N_HKUST623/07]; South China Sea Coastal Oceano graphic Process Experiment project

Temperature, resources, and phytoplankton size structure in the ocean

Abstract: We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world’s oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values . 80% when chlorophyll a concentration is . 2 m gL 21 or primary production is . 100 m gCL 21 d21, irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (. 40%), at all temperatures, only when chlorophyll a concentration is , 1 m gL 21 or primary production is , 50 m gCL 21 d21. The temperature–size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon- and sizedependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure. Phytoplankton size structure largely determines the trophic organization of pelagic ecosystems and thus the efficiency with which organic matter produced by photosynthesis is channeled towards upper trophic levels or

Terrestrial organic matter support of lake food webs: Evidence from lake metabolism and stable hydrogen isotopes of consumers

Abstract: We quantified the utilization of terrestrial organic matter (OM) in the food web of a humic lake by analyzing the metabolism and the consumers' stable isotopic (C, H, N) composition in benthic and pelagic habitats. Terrestrial OM inputs (3 g C m(-2) d(-1)) to the lake greatly exceeded autochthonous OM production (3 mg C m(-2) d(-1)) in the lake. Heterotrophic bacterial growth (19 mg C m(-2) d(-1)) and community respiration (115 mg C m(-2) d(-1)) were high relative to algal photosynthesis and were predominantly (> 85%) supported by terrestrial OM in both habitats. Consequently, terrestrial OM fueled most (85%) of the total production at the base of the lake's food web (i.e., the sum of primary and bacterial production). Despite the uncertainties of quantitatively estimating resource use based on stable isotopes, terrestrial OM clearly also supported around half the zooplankton (47%), macrozoobenthos (63%), and fish (57%) biomass. These results indicate that, although rates of terrestrial OM inputs were around three orders of magnitude greater than that of autochthonous OM production, the use of the two resources by higher trophic levels was roughly equal. The disproportionally low reliance on terrestrial OM at higher trophic levels, compared with its high rates of input and high support of basic biomass production in the lake, suggests that autochthonous resources could not be completely replaced by terrestrial resources and indicates an upper limit to terrestrial support of lake food webs.

Quantification of ammonia oxidation rates and ammonia‐oxidizing archaea and bacteria at high resolution in the Gulf of California and eastern tropical North Pacific Ocean

Abstract: Ammonia-oxidizing microorganisms compete with phytoplankton for reduced nitrogen in the euphotic zone and provide oxidized nitrogen to other microbes present in the sea. We report 15NH z oxidation rate measurements made at 5–20-m resolution using an in situ array and quantification of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in corresponding samples from the upper water column and oxygen minimum zone (OMZ) of the Gulf of California (GOC) and eastern tropical North Pacific Ocean (ETNP). 15NH z oxidation rates varied substantially with depth and between stations: they were greatest at the base of the euphotic zone, and maximum rates were up to 28-fold greater than rates measured within 5–10 m. Pyrosequencing and quantitative polymerase chain reactions (QPCR) indicated that AOA were present throughout the water column at all latitudes and always outnumbered AOB. AOB constituted only 39 of 432,240 16S ribosomal ribonucleic acid gene sequences produced via pyrosequencing but were more abundant at greater depths and higher latitudes. 15NH z oxidation rates were correlated with AOA abundance at some stations and were detectable in 96% of samples, including depths where oxygen concentrations were , 5 mmol kg21 and depths within the euphotic zone, where up to 42% of ammonia oxidation occurred. Ammonia is rapidly oxidized within discrete depth intervals in the GOC and ETNP; while pyrosequencing and QPCR demonstrate that AOB are confined to deeper portions of the water column, AOA appear to be active within the euphotic zone, where they may quickly respond to nitrogen inputs.

Iron-light interactions differ in Southern Ocean phytoplankton

Abstract: In laboratory experiments we examined the interplay of light and iron availability on the intracellular iron concentrations, specific growth rates, and photosynthetic physiology of Southern (S.) Ocean diatoms (Eucampia antarctica and Proboscia inermis) and the haptophyte Phaeocystis antarctica. Intracellular iron concentrations and iron : carbon (Fe : C) molar ratios increased with decreasing irradiance in temperate coastal (Thalassiosira weissflogii) and oceanic (Thalassiosira oceanica) diatoms, in support of the well-established antagonistic iron–light relationship. In contrast, S. Ocean species required lower cellular iron concentrations and Fe : C ratios than temperate diatoms to grow at comparable rates, and their iron requirements decreased or remained relatively constant with decreasing light. These results suggest that the current paradigm that low light increases algal cellular iron requirements (supplied through ‘‘biodilution’’) is not applicable to S. Ocean phytoplankton. Although iron use efficiencies decreased at sub-saturating light in all species, these reductions were due primarily to lower growth rates, but not higher intracellular Fe : C ratios, in S. Ocean species. We propose that S. Ocean species have overcome the antagonistic iron–light relationship by increasing the size, rather than the number, of photosynthetic units under low irradiances, resulting in an acclimation strategy that does not increase their cellular iron requirements.

Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean

Abstract: We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N2O, and O2, as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Biophysical interactions in the plankton: A cross‐scale review

Abstract: In plankton ecology, biological and physical dynamics are coupled, structuring how plankton interact with their environment and other organisms. This interdisciplinary field has progressed considerably over the recent past, due in large part to advances in technology that have improved our ability to observe plankton and their fluid environment simultaneously across multiple scales. Recent research has demonstrated that fluid flow interacting with plankton behavior can drive many planktonic processes and spatial patterns. Moreover, evidence now suggests that plankton behavior can significantly affect ocean physics. Biophysical processes relevant to plankton ecology span a range of scales; for example, microscale turbulence influences planktonic growth and grazing at millimeter scales, whereas features such as fronts and eddies can shape larger-scale plankton distributions. Most research in this field focuses on specific processes and thus is limited to a narrow range of spatial scales. However, biophysical interactions are intimately connected across scales, since processes at a given scale can have implications at much larger and smaller scales; thus, a cross-scale perspective on how biological and physical dynamics interact is essential for a comprehensive understanding of the field. Here, we present a review of biophysical interactions in the plankton across multiple scales, emphasizing new findings over recent decades and highlighting opportunities for cross-scale comparisons. By investigating feedbacks and interactions between processes at different scales, we aim to build cross-scale intuition about biophysical planktonic processes and provide insights for future directions in the field.

Meiofauna enhances organic matter mineralization in soft sediment ecosystems

Abstract: We investigated the influence of meiofauna on the benthic decomposition of a radiolabeled diatom bloom by measuring the production of 14CO2 in a laboratory microcosm. Mineralization of the diatom bloom material in the sediment was significantly enhanced in the treatment with high meiofauna abundance, with cumulative mineralization values, on average, 50% greater in the treatment with high meiofaunal abundance after 17 d, compared to sediments with low meiofauna abundance. In addition, bacteria species composition in the treatment with high meiofauna abundance was significantly different from the treatment with low meiofauna abundance, indicating that the activities of meiofauna in the sediments had an effect on the bacterial community composition. Meiofauna can enhance the mineralization of organic matter, probably by stimulating the activity of sediment bacterial community, indicating that positive biological interactions such as facilitation from meiofauna are important for ecosystem processes in soft sediments.

Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone

Abstract: Benthic fluxes of dissolved ferrous iron (Fe2+) and phosphate (TPO4) were quantified by in situ benthic chamber incubations and pore-water profiles along a depth transect (11°S, 80–1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O2 levels were < 2 µmol L-1 down to 500-m water depth, and increased to ~40 µmol L-1 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m-2 yr-1), moderate (15.4 mmol m-2 yr-1) between 250 m and 600 m, and negligible at deeper stations. In the persistent OMZ core, continuous reduction of Fe oxyhydroxides results in depletion of sedimentary Fe :Al ratios. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO4 flux indicate excess release of P over C when compared to Redfield stoichiometry. Most likely, this is caused by preferential P release from organic matter, dissolution of fish debris, and/or P release from microbial mat communities, while Fe oxyhydroxides can only be inferred as a major P source on the shallow shelf. The benthic fluxes presented here are among the highest reported from similar, oxygen-depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean. The shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom-water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of Fe2+ and TPO4.

Does warming enhance the effect of microzooplankton grazing on marine phytoplankton in the ocean

Abstract: We evaluated a hypothesis derived from the metabolic theory of ecology (MTE) that the ratio of microzooplankton herbivory (m) to phytoplankton growth (m) will arise in a warming ocean because of the different temperature dependencies of autotrophic and heterotrophic organisms. Using community-level growth and grazing data from dilution experiments, generalized additive models (GAMs) were constructed to describe the effects of temperature and chlorophyll on m:m. At low chlorophyll levels, m:m decreases with increasing temperature, whereas at high chlorophyll levels, m:m increases initially with temperature before reaching a peak and then declines. These complex responses of m:m result from mixed effects of temperature and chlorophyll on microzooplankton biomass (Bz), biomass-specific microzooplankton grazing rate (m:Bz), and phytoplankton growth rate (m). Bz decreases with rising temperature and increases with rising chlorophyll. m:Bz increases with temperature and decreases with chlorophyll. Nutrient-enriched growth rate of phytoplankton (mn) and m increase with increasing temperature and chlorophyll. Holding chlorophyll constant, the calculated activation energies of m:Bz and mn are 0.67 6 0.05 and 0.36 6 0.05 eV, respectively, both consistent with previous MTE estimates for heterotrophs and autotrophs. Our study indicates that warming may enhance phytoplankton losses to microzooplankton herbivory in eutrophic but not in oligotrophic waters. The GAM analysis also provides important insights into underlying system relationships and reasons why community-level responses in natural systems may depart from theory based on laboratory data and individual species.

Effects of CO2 and their modulation by light in the life-cycle stages of the coccolithophore Emiliania huxleyi

Abstract: The effects of ocean acidification on the life-cycle stages of the coccolithophore Emiliania huxleyi and their modulation by light were examined. Calcifying diploid and noncalcifying haploid cells (Roscoff culture collection strains 1216 and 1217) were acclimated to present-day and elevated CO2 partial pressures (PCO2; 38.5 vs. 101.3 Pa, i.e., 380 vs. 1000 µatm) under low and high light (50 vs. 300 µmol photons m-2 s-1). Growth rates as well as cellular quotas and production rates of C and N were measured. Sources of inorganic C for biomass buildup were determined using a 14C disequilibrium assay. Photosynthetic O2 evolution was measured as a function of dissolved inorganic C and light by means of membrane-inlet mass spectrometry. The diploid stage responded to elevated PCO2 by shunting resources from the production of particulate inorganic C toward organic C yet keeping the production of total particulate C constant. As the effect of ocean acidification was stronger under low light, the diploid stage might be less affected by increased acidity when energy availability is high. The haploid stage maintained elemental composition and production rates under elevated PCO2. Although both life-cycle stages involve different ways of dealing with elevated PCO2, the responses were generally modulated by energy availability, being typically most pronounced under low light. Additionally, PCO2 responses resembled those induced by high irradiances, indicating that ocean acidification affects the interplay between energy-generating processes (photosynthetic light reactions) and processes competing for energy (biomass buildup and calcification). A conceptual model is put forward explaining why the magnitude of single responses is determined by energy availability.

The regional abundance and size distribution of lakes and reservoirs in the United States and implications for estimates of global lake extent

Abstract: We analyzed complete geospatial data for the 3.5 million lakes and reservoirs larger than 0.001 km2, with a combined surface area of 131,000 km2, in the contiguous United States (excluding the Laurentian Great Lakes) and identified their regional distribution characteristics. For Alaska, we also analyzed (1) incomplete data that suggest that the state contains 1–2.5 million lakes larger than 0.001 km2 covering over 50,000 km2 and (2) localized high-resolution (5 m) data that suggest that the number of very small water bodies ( 0.001 km2 in some areas. The Pareto distribution cannot accurately describe the lake abundance-size relationship across the entire size spectrum, and extrapolation of this density function to small size classes has likely resulted in the overestimation of the number of small lakes in the world. While small water bodies dominate in terms of numbers, they are not numerous enough to dominate in terms of surface area, as has been previously suggested. Extending our results to the global scale suggests that there are on the order of 64 million water bodies larger than 0.001 km2 in the world, with a total surface area of approximately 3.8 million km2.

Oxygen isotopic composition of nitrate and nitrite produced by nitrifying cocultures and natural marine assemblages

Abstract: The d18O value of nitrate produced during nitrification (d 18 ONO3,nit) was measured in experiments designed to mimic oceanic conditions, involving cocultures of ammonia-oxidizing bacteria or ammonia-oxidizing archaea and nitrite-oxidizing bacteria, as well as natural marine assemblages. The estimates of d 18 ONO3,nit ranged from 21.5% 6 0.1% to +1.3% 6 1.4% at d18O values of water (H2O) and dissolved oxygen (O2 )o f 0% and 24.2% vs. Vienna Standard Mean Ocean Water, respectively. Additions of 18O-enriched H2O allowed us to evaluate the effects of oxygen (O) isotope fractionation and exchange on d 18 ONO3,nit. Kinetic isotope effects for the incorporation of O atoms were the most important factors for setting overall d 18 ONO3,nit values relative to the substrates (O2 and H2O). These isotope effects ranged from +10% to +22% for ammonia oxidation (O2 plus H2O incorporation) and from +1% to +27% for incorporation of H2O during nitrite oxidation. d 18 ONO3,nit values were also affected by the amount and duration of nitrite accumulation, which permitted abiotic O atom exchange between nitrite and H2O. Coculture incubations where ammonia oxidation and nitrite oxidation were tightly coupled showed low levels of nitrite accumulation and exchange (3% 6 4%). These experiments had d 18 ONO3,nit values of 21.5% to +0.7%. Field experiments had greater accumulation of nitrite and a higher amount of exchange (22% to 100%), yielding an average d 18 ONO3,nit value of +1.9% 6 3.0%. Low levels of biologically catalyzed exchange in coculture experiments may be representative of nitrification in much of the ocean where nitrite accumulation is low. Abiotic oxygen isotope exchange may be important where nitrite does accumulate, such as oceanic primary and secondary nitrite maxima.

Phytoplankton niches estimated from field data

Abstract: We combine phytoplankton occurrence data for 119 species from the continuous plankton recorder with climatological environmental variables in the North Atlantic to obtain ecological response functions of each species using the MaxEnt statistical method. These response functions describe how the probability of occurrence of each species changes as a function of environmental conditions and can be reduced to a simple description of phytoplankton realized niches using the mean and standard deviation of each environmental variable, weighted by its response function. Although there was substantial variation in the realized niche among species within groups, the envelope of the realized niches of North Atlantic diatoms and dinoflagellates is mostly separate in niche space.

Physical Environments of the Caribbean Sea

Abstract: The Caribbean Sea encompasses a vast range of physical environmental conditions that have a profound influence on the organisms that live there. Here we utilize a range of satellite and in situ products to undertake a region-wide categorization of the physical environments of the Caribbean Sea (PECS). The classification approach is hierarchical and focuses on physical constraints that drive many aspects of coastal ecology, including species distributions, ecosystem function, and disturbance. The first level represents physicochemical properties including metrics of satellite sea surface temperature, water clarity, and in situ salinity. The second level considers mechanical disturbance and includes both chronic disturbance from wind-driven wave exposure and acute disturbance from hurricanes. The maps have a spatial resolution of 1 km2. An unsupervised neural network classification produced 16 physicochemical provinces that can be categorized into six broad groups: (1) low water clarity and low salinity and average temperatures; (2) low water clarity but average salinity and temperature, broadly distributed in the basin; (3) low salinity but average water clarity and temperature; (4) upwelling; (5) high latitude; and (6) offshore waters of the inner Caribbean. Additional mechanical disturbance layers impose additional pattern that operates over different spatial scales. Because physical environments underpin so much of coastal ecosystem structure and function, we anticipate that the PECS classification, which will be freely distributed as geographic information system layers, will facilitate comparative analyses and inform the stratification of studies across environmental provinces in the Caribbean basin.

Rates of dinitrogen fixation and the abundance of diazotrophs in North American coastal waters between Cape Hatteras and Georges Bank

Abstract: We coupled dinitrogen (N2) fixation rate estimates with molecular biological methods to determine the activity and abundance of diazotrophs in coastal waters along the temperate North American Mid-Atlantic continental shelf during multiple seasons and cruises. Volumetric rates of N2 fixation were as high as 49.8 nmol N L(sup -1) d(sup -1) and areal rates as high as 837.9 micromol N m(sup -2) d(sup -1) in our study area. Our results suggest that N2 fixation occurs at high rates in coastal shelf waters that were previously thought to be unimportant sites of N2 fixation and so were excluded from calculations of pelagic marine N2 fixation. Unicellular N2-fixing group A cyanobacteria were the most abundant diazotrophs in the Atlantic coastal waters and their abundance was comparable to, or higher than, that measured in oceanic regimes where they were discovered. High rates of N2 fixation and the high abundance of diazotrophs along the North American Mid-Atlantic continental shelf highlight the need to revise marine N budgets to include coastal N2 fixation. Integrating areal rates of N2 fixation over the continental shelf area between Cape Hatteras and Nova Scotia, the estimated N2 fixation in this temperate shelf system is about 0.02 Tmol N yr(sup -1), the amount previously calculated for the entire North Atlantic continental shelf. Additional studies should provide spatially, temporally, and seasonally resolved rate estimates from coastal systems to better constrain N inputs via N2 fixation from the neritic zone.

Constrained microbial processing of allochthonous organic carbon in boreal lake sediments

Abstract: We investigated sediment bacterial metabolism in eight lakes with different inputs of allochthonous and autochthonous organic carbon in south-central Sweden. Sediment bacterial production, minerali ...

Coastal fronts set recruitment and connectivity patterns across multiple taxa

Abstract: We show that ocean fronts set recruitment patterns among both community-building invertebrates and commercially important fishes in nearshore intertidal and rocky reef habitats. Chlorophyll concentration and recruitment of several species of intertidal invertebrates (Balanus spp., Chthamalus spp., Mytilus spp.) and rockfishes (Sebastes spp.) are positively correlated with front probability along the coast of the California Current Large Marine Ecosystem. Abundances of recent settlers and adults for nearshore rockfish species are also positively correlated with front probability. The interaction of coastal topography and bathymetry sets spatial scales of fronts and consequently recruitment at approximately 50 km during active upwelling, compared to 200 km or greater during non-upwelling periods. Such relationships between fronts and recruitment are likely to be consistent across other marine ecosystems—from coastal waters to the open ocean—and provide a critical link between adults and widely dispersing young. Ocean fronts, already known as features with high biodiversity and resilience in pelagic habitats, also set recruitment and connectivity patterns across multiple taxa for intertidal and rocky reef communities, thus linking biodiversity and resilience in coastal and benthic habitats as well.

Cyanobacteria as a carbon source for zooplankton in eutrophic Lake Taihu, China, measured by 13C labeling and fatty acid biomarkers

Abstract: Using a combined stable-isotope and fatty-acid approach, we examined carbon-transfer routes from the cyanobacterium Microcystis to zooplankton in eutrophic Lake Taihu, China. Microcystis is generally considered poor food for zooplankton, and we hypothesized that most Microcystis carbon flows to zooplankton via dissolved organic matter (DOM)–bacteria and detritus–bacteria pathways rather than via direct grazing. The hypothesis was tested by analyzing 13C isotopes at natural abundance in field samples and in tracer experiments with 13Cenriched Microcystis. 13C-enriched Microcystis was added as live Microcystis, Microcystis detritus, or Microcystis DOM to lake-water incubations with Bosmina sp. and Daphnia similis as the dominant species. The 13C isotope signatures of Microcystis, heterotrophic bacteria, and eukaryotic algae in seston were determined from isotope analyses of specific fatty acids, and the presence and labeling of these fatty acids were also analyzed in zooplankton consumers. Bosmina and Daphnia consumed carbon via all pathways, but the amount of carbon transfer from the Microcystis DOM was the highest, followed by the Microcystis detritus. Bosmina consumed relatively more live Microcystis than Daphnia. The presence and high 13C enrichment of bacteria-specific fatty acids in the zooplankton consumers showed that heterotrophic bacteria were an important link between Microcystis and zooplankton. Microbial pathways dominate the energy flow from cyanobacteria to zooplankton in eutrophic lakes with heavy cyanobacteria blooms, such as Lake Taihu.

Grazer-induced chain length plasticity reduces grazing risk in a marine diatom

Abstract: We show that Skeletonema marinoi suppresses chain formation in response to copepod cues. The presence of three different copepod species (Acartia tonsa, Centropages hamatus, or Temora longicornis) significantly reduced chain length. Furthermore, chain length was significantly reduced when S. marinoi was exposed to chemical cues from caged A. tonsa without physical contact with the responding cells. The reductions in chain length significantly reduced copepod grazing; grazing rates on chains (four cells or more) were several times higher compared to that of single cells. This suggests that chain length plasticity is a means for S. marinoi to reduce copepod grazing. In contrast, chain length was not suppressed in cultures exposed to the microzooplankton grazer Gyrodinium dominans. Size-selective predation may have played a key role in the evolution of chain formation and chain length plasticity in diatoms.

Carbon dynamics and community production in the Mississippi River plume

Abstract: Dissolved inorganic carbon (DIC), total alkalinity (TAlk), pH, and dissolved oxygen (DO) were determined in the Mississippi River plume during five cruises conducted in the spring, summer, and fall. In contrast to many other large rivers, both DIC and TAlk were higher in river water than in seawater. Substantial losses of DIC, relative to TAlk, occurred within the plume, particularly at intermediate salinities. DIC removal was accompanied by high DO, high pH, and nutrient depletion, and was attributed to high phytoplankton production. As a result, the carbonate saturation in the plume became much higher than in ocean and river waters. A mixing model was used to determine DIC removal. We provide evidence that the use of a two–end-member (river and ocean) mixing model was valid during late summer and fall (low discharge period). However, for other periods we used salinity and TAlk to delineate a mixing model that included two river end members and an ocean end member. Net community production rates in the plume, estimated using a box model, peaked in the summer and were among the highest reported to date for large river plumes. In the summer and fall, biological production in the river plume consumed a majority of the available nutrients, whereas during the spring only a small fraction of the available nutrients were consumed in the plume. Biological production was the dominant process influencing pH and carbonate saturation state along the river–ocean gradient, whereas physicochemical dynamics of mixing played an important role in controlling the TAlk and DIC distributions of this large river plume.

The “salt wedge pump”: convection-driven pore-water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Abstract: Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over-enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high-resolution radon (a natural groundwater and pore-water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced pore-water exchange at this area. We use the term “salt wedge pump” to describe convection-driven advective pore-water exchange at the sediment–water interface during the upstream propagation of the salt wedge. Radon-derived convection-driven pore-water exchange rates within the salt wedge were estimated at 2.8 cm d−1, a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore-water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes ∼ 10-fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5-fold higher than oxygen uptake related to advective pore-water exchange. Estimated fluxes, associated with the nonconservative DIC, δ13C-DIC, and ammonium behavior within the estuary support convective pore-water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems.