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  • 1
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    Delving deeper: Metabolic processes in the metalimnion of stratified lakes (2017)
    Wiley-Blackwell
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    Publication Date: 2017-03-10
    Description: Many lakes exhibit seasonal stratification, during which they develop strong thermal and chemical gradients. An expansion of depth-integrated monitoring programs has provided insight into the importance of organic carbon processing that occurs below the upper mixed layer. However, the chemical and physical drivers of metabolism and metabolic coupling remain unresolved, especially in the metalimnion. In this depth zone, sharp gradients in key resources such as light and temperature co-occur with dynamic physical conditions that influence metabolic processes directly and simultaneously hamper the accurate tracing of biological activity. We evaluated the drivers of metalimnetic metabolism and its associated uncertainty across 10 stratified lakes in Europe and North America. We hypothesized that the metalimnion would contribute highly to whole-lake functioning in clear oligotrophic lakes, and that metabolic rates would be highly variable in unstable polymictic lakes. Depth-integrated rates of gross primary production (GPP) and ecosystem respiration (ER) were modelled from diel dissolved oxygen curves using a Bayesian approach. Metabolic estimates were more uncertain below the epilimnion, but uncertainty was not consistently related to lake morphology or mixing regime. Metalimnetic rates exhibited high day-to-day variability in all trophic states, with the metalimnetic contribution to daily whole-lake GPP and ER ranging from 0% to 87% and 〈 1% to 92%, respectively. Nonetheless, the metalimnion of low-nutrient lakes contributed strongly to whole-lake metabolism on average, driven by a collinear combination of highlight, low surface-water phosphorous concentration and high metalimnetic volume. Consequently, a single-sensor approach does not necessarily reflect whole-ecosystem carbon dynamics in stratified lakes.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
    Published by Wiley-Blackwell on behalf of The Association for the Sciences of Limnology and Oceanography (ASLO).
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  • 2
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    Stable isotope estimates of evaporation : inflow and water residence time for lakes across the United States as a tool for national lake water quality assessments (2015)
    Wiley-Blackwell
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    Publication Date: 2015-01-06
    Description: We used water δ 2 H and δ 18 O from ca. 1000 lakes sampled in the 2007 U.S. Environmental Protection Agency's National Lakes Assessment (NLA) to assess two hydrological variables—evaporation as a percentage of inflow ( E : I ) and water residence time ( τ ) for summer 2007. Using a population survey design, sampled lakes were distributed across the conterminous U.S., and results were scaled to the inference population (∼50,000 U.S. lakes). These hydrologic variables were related to lake nutrients and biological condition to illustrate their usefulness in national water quality monitoring efforts. For 50% of lakes, evaporation was 〈 25% of inflow, with values ranging up to 113% during the 2007 summer. Residence time was 〈 0.52 yr for half of the lakes and 〈 1.12 yr for 75% of lakes. Categorizing lakes by flow regime, 66.1% of lakes were flow-though lakes (60% or more of the water flows through the lake, E : I 〈 0.4), 33.6% were restricted-basin lakes (40% or more of the lake inflow evaporates, 0.4 〈 E : I 〈 1), and 〈 0.3% were closed basin (all water entering the lake leaves through evaporation, E : I 〉 1). While climate patterns drove some of the spatial patterns of E : I and τ , variation in lake depth and watershed size (influencing precipitation volume) were also significant drivers. Lake hydrochemistry was strongly correlated to E : I and more weakly related to τ . Lakes in poor biological condition (based on a predictive model of planktonic taxa) were significantly more evaporated than lakes in good biological condition.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
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  • 3
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    Nitrogen sources and net growth efficiency of zooplankton in three Amazon River plume food webs (2015)
    Wiley-Blackwell
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    Publication Date: 2015-11-13
    Description: The plasticity of nitrogen specific net growth efficiency (NGE) in marine mesozooplankton is currently unresolved, with discordant lines of evidence suggesting that NGE is constant, or that it varies with nitrogen source, food availability, and food quality in marine ecosystems. Specifically, the fate of nitrogen from nitrogen fixation is poorly known. We use 15 N : 14 N ratios in plankton in combination with hydrological data, nutrient profiles, and nitrogen fixation rate measurements to investigate the relationship between new nitrogen sources and the nitrogen specific NGE in three plankton communities along the outer Amazon River plume. The NGE of small (200–500 μm) mesozooplankton was estimated from the δ 15 N differences between particulate nitrogen and zooplankton using an open system Rayleigh fractionation model. The transfer efficiency of nitrogen among larger (〉 500 μm) mesozooplankton was estimated from the change in δ 15 N as a function of zooplankton size. The Amazon River was not a significant source of bioavailable nitrogen anywhere in our study region, and subsurface nitrate was the primary new nitrogen source for the outer shelf community, which was dominated by diatoms. N 2 fixation was the principal new nitrogen source at sites of high diatom diazotroph association abundance and at oceanic sites dominated by Trichodesmium spp. and Synechococcus spp. Although we found clear spatial differences in food quantity, food quality, and diazotroph inputs into mesozooplankton, our data show no significant differences in mesozooplankton nitrogen transfer efficiency and NGE (for latter, mean ± SD: 59 ± 10%) among sites.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
    Published by Wiley-Blackwell on behalf of The Association for the Sciences of Limnology and Oceanography (ASLO).
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  • 4
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    Amazon River influence on nitrogen fixation and export production in the western tropical North Atlantic (2016)
    Wiley-Blackwell
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    Publication Date: 2016-11-15
    Description: As part of the ANACONDAS program, we investigated the role of the Amazon plume in stimulating offshore nitrogen fixation and export production during the river's high-discharge period (May–June 2010). Using the shipboard underway system, we performed high-resolution sampling of over 450,000 km 2 of surface waters, characterizing the distribution of nutrients, phytoplankton, particulate organic matter (POM), and stable carbon and nitrogen isotopes of POM in the offshore plume. We found distinct regional variations in diazotroph communities, with the Diatom-Diazotroph Associations (DDA) Hemiaulus hauckii – Richelia intracellularis dominating the low N : P mesohaline waters to the northwest of the plume axis and Trichodesmium spp. primarily occupying oceanic waters to the east. Nutrient availability broadly shaped diazotroph distributions along the salinity gradient, but habitat longevity may also play a role in the finer-scale distributions of communities, particularly of DDAs. H. hauckii and Trichodesmium spp. affected the nitrogen and carbon budgets in fundamentally different ways within the plume-influenced regions, with H. hauckii making much greater contributions to the particulate nitrogen pool and to CO 2 drawdown than Trichodesmium spp., leading to much higher export fluxes. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
    Published by Wiley-Blackwell on behalf of The Association for the Sciences of Limnology and Oceanography (ASLO).
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  • 5
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    Deep Water Horizon oil and methane carbon entered the food web in the Gulf of Mexico (2016)
    Wiley-Blackwell
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    Publication Date: 2016-11-09
    Description: The Deep Water Horizon (DWH) incident caused the release of an unprecedented amount of 13 C- and 14 C-depleted oil and gas to the Gulf of Mexico (GoM), which formed surface slicks and deep oil/gas plumes that extended laterally at 1000–1200 m. We conducted three research cruises between 2010 and 2012 to study the potential assimilation of petrocarbon (C derived from oil and/or gas) into the GoM microbial food web. In 2010, we found low δ 13 C (−30 to −25‰) and Δ 14 C (−603 to −55‰) values for suspended particles at 1000–1200 m depth as far as 289 km SW of the wellhead, providing direct evidence of the spatial extent of the subsurface plumes. At those depths in 2010, methane and oil carbon accounted for up to 28% and 62% of total particulate carbon (C sp ), respectively. In the total area affected by the DWH, 80 ± 56 to 104 ± 91 tonnes (t) of methane-derived and 216 ± 174 to 292 ± 165 t of oil-derived carbon were incorporated into C sp . In 2011 and 2012, the δ 13 C values were distributed throughout the water column indicating that petrocarbon was still present and recycling, especially in the section closest to the DWH, where oil supplied up to 53% and 75% of C sp , respectively. Relatively low δ 15 N (〈 4‰) values in suspended particles at 1000–1200 m in 2010 indicate stimulation of nitrogen fixation linked to methane oxidation in the months after the spill, which accounted for up to 40% of the particulate nitrogen in the water column at those depths.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
    Published by Wiley-Blackwell on behalf of The Association for the Sciences of Limnology and Oceanography (ASLO).
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  • 6
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    De novo amino acid synthesis and turnover during N2 fixation (2017)
    Wiley-Blackwell
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    Publication Date: 2017-11-25
    Description: Cyanobacteria are the main autotrophs and N 2 -fixing (diazotrophic) organisms in large parts of the oligotrophic global ocean, where generally all heterotrophic production depends on their activity. Amino acids (AAs) from cyanobacteria are essential macronutrients for these heterotrophic food webs, yet little is known about the de novo synthesis of AAs during N 2 fixation. Through a combination of bulk and amino acid nitrogen (AAN) specific analyses of field based N 2 fixation experiments, we demonstrate that the de novo synthesis of 13 AAs accounted for the majority of bulk N 2 fixation rates at four stations in the central Baltic Sea in July 2015. Slow AA turnover times of 87 ± 14 d coincided with low phosphate concentrations and high cell-carbon biomasses of unicellular cyanobacteria. Very fast turnover times of 17 ± 3 d coincided with high phosphate concentrations and undecayed Nodularia spumigena cells, but unexpectedly also with phosphate depletion and decayed N. spumigena cells. In a decayed bloom, volumetric N 2 fixation rates into AAN provided a much better estimate of the net incorporation of N 2 into biomass than fixation into bulk nitrogen that rather reflected gross N 2 fixation. In an undecayed bloom, the turnover times of 13 AAs can be predicted from a single bulk N 2 fixation rate. This is the first direct evidence that the very late, decayed stage of a cyanobacteria bloom can be a flashpoint of very fast AA turnover during N 2 fixation with hitherto uncharacterized consequences for heterotrophic food webs and diazotroph N inputs to the global ocean.
    Print ISSN: 0024-3590
    Electronic ISSN: 1939-5590
    Topics: Biology , Geosciences , Physics
    Published by Wiley-Blackwell on behalf of The Association for the Sciences of Limnology and Oceanography (ASLO).
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