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  • 1
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    Baltic Sea diazotrophic cyanobacterium is negatively affected by acidification and warming (2018)
    Inter Research
    In:  Marine Ecology Progress Series, 598 . pp. 49-60.
    Details
    Publication Date: 2021-02-08
    Description: Nitrogen fixation is a key source of nitrogen in the Baltic Sea which counteracts nitrogen loss processes in the deep anoxic basins. Laboratory and field studies have indicated that single-strain nitrogen-fixing (diazotrophic) cyanobacteria from the Baltic Sea are sensitive to ocean acidification and warming, two drivers of marked future change in the marine environment. Here, we enclosed a natural plankton community in twelve indoor mesocosms (volume ~1400 L) and manipulated pCO2 to yield six CO2 treatments with two different temperature treatments (16.6°C and 22.4°C, pCO2 range = 360 – 2030 μatm). We followed the filamentous, heterocystous diazotrophic cyanobacteria community (Nostocales, primarily Nodularia spumigena) over four weeks. Our results indicate that heterocystous diazotrophic cyanobacteria may become less competitive in natural plankton communities under ocean acidification. Elevated CO2 had a negative impact on Nodularia sp. biomass, which was exacerbated by warming. Our results imply that Nodularia sp. may contribute less to new nitrogen inputs in the Baltic Sea in future.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/meps12632
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Organic matter partitioning and stoichiometry in response to rising water temperature and copepod grazing (2015)
    Inter Research
    In:  Marine Ecology Progress Series, 522 . pp. 49-65.
    Details
    Publication Date: 2017-04-13
    Description: Rising ocean temperature is expected to change the balance between production and degradation of organic matter due to different temperature sensitivities of auto- and heterotrophic processes. Copepods are the most prominent zooplankton group, and elevated temperature increases their growth and grazing rates. So far, it is unknown to what extent copepods affect the partitioning and stoichiometry of organic matter in a warmer surface ocean. We therefore conducted a mesocosm experiment with 3 copepod densities and 2 temperature scenarios to determine effects on the pools of dissolved and particulate organic matter and their C:N:P ratios. Here we show that particulate organic C (POC) concentrations decreased with increasing copepod abundance. This effect was more pronounced at elevated temperature, yielding a decrease in the POC to particulate nitrogen ratio (POC:PN) from 26 to 13 and in the POC:particulate organic phosphorus (POP) ratio from 567 to 257, from low to high copepod density. Dissolved organic carbon (DOC) accumulation was positively affected by temperature. However, increasing copepod abundance decreased the accumulation of DOC at elevated temperature. Copepod grazing and egestion enhanced the recycling of N and P, thereby increasing the availability of these nutrients for autotrophs. In concert with temperature-induced shifts in the phytoplankton community composition and size, changes in copepod abundance may therefore have contributed to altering the elemental composition of seston. Our findings suggest combined effects of zooplankton grazing and temperature on the composition and recycling of organic matter that should be taken into account when simulating biogeochemical cycles in a future ocean.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/meps11148
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Dissolved N:P ratio changes in the eastern tropical North Atlantic: effect on phytoplankton growth and community structure (2016)
    Inter Research
    In:  Marine Ecology Progress Series, 545 . pp. 49-62.
    Details
    Publication Date: 2019-09-23
    Description: Previous bioassays conducted in the oligotrophic Atlantic Ocean identified availability of inorganic nitrogen (N) as the proximate limiting nutrient control of primary production, but additionally displayed a synergistic growth effect of combined N and phosphorus (P) addition. To classify conditions of nutrient limitation of coastal phytoplankton in the tropical ocean, we performed an 11 d nutrient-enrichment experiment with a natural phytoplankton community from shelf waters off northwest Africa in shipboard mesocosms. We used pigment and gene fingerprinting in combination with flow cytometry for classification and quantification of the taxon-specific photoautotrophic response to differences in nutrient supply. The developing primary bloom was dominated by diatoms and was significantly higher in the treatments receiving initial N addition. The combined supply of N and P did not induce a further increase in phytoplankton abundance compared to high N addition alone. A secondary bloom during the course of the experiment again displayed higher primary producer standing stock in the N-fertilized treatments. Bacterial abundance correlated positively with phytoplankton biomass. Dominance of the photoautotrophic assemblage by N-limited diatoms in conjunction with a probable absence of any P-limited phytoplankton species prevented an additive effect of combined N and P addition on total phytoplankton biomass. Furthermore, after nutrient exhaustion, dinitrogen (N-2)-fixing cyanobacteria succeeded the bloom-forming diatoms. Shelf waters in the tropical eastern Atlantic may thus support growth of diazotrophic cyanobacteria such as Trichodesmium sp. subsequent to upwelling pulses.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/meps11600
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Transparent exopolymer particles and dissolved organic carbon production by Emiliania huxleyi exposed to different CO2 concentrations: a mesocosm experiment (2004)
    Inter Research
    In:  Aquatic Microbial Ecology, 34 (1). pp. 93-104.
    Details
    Publication Date: 2016-05-26
    Description: The role of transparent exopolymer particles (TEP) and dissolved organic carbon (DOC) for organic carbon partitioning under different CO2 conditions was examined during a mesocosm experiment with the coccolithophorid Emiliania huxleyi. We designed 9 outdoor enclosures (similar to11 m(3)) to simulate CO2 concentrations of estimated 'Year 2100' (similar to710 ppm CO2), 'present' (similar to410 ppm CO2) and 'glacial' (similar to190 ppm CO2) environments, and fertilized these with nitrate and phosphate to favor bloom development. Our results showed fundamentally different TEP and DOC dynamics during the bloom. In all mesocosms, TEP concentration increased after nutrient exhaustion and accumulated steadily until the end of the study. TEP concentration was closely related to the abundance of E. huxleyi and accounted for an increase in POC concentration of 35 2 % after the onset of nutrient limitation. The production of TEP normalized to the cell Abundance of E. huxleyi was highest in the Year 2100 treatment. In contrast, DOC concentration exhibited considerable short-term fluctuations throughout the study. In all mesocosms, DOC was neither related to the abundance of E. huxleyi nor to TEP concentration. A statistically significant effect of the CO2 treatment on DOC concentration was not determined. However, during the course of the bloom, DOC concentration increased in 2 of the 3 Year 2100 mesocosms and in 1 of the present mesocosms, but in none of the glacial mesocosms. It is suggested that the observed differences between TEP and DOC were determined by their different bioavailability and that a rapid response of the microbial food web may have obscured CO2 effects on DOC production by autotrophic cells.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/ame034093
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    High levels of solar radiation offset impacts of ocean acidification on calcifying and non-calcifying strains of Emiliania huxleyi (2017)
    Inter Research
    In:  Marine Ecology Progress Series, 568 . pp. 47-58.
    Details
    Publication Date: 2020-02-06
    Description: Coccolithophores, a globally distributed group of marine phytoplankton, showed diverse responses to ocean acidification (OA) and to combinations of OA with other environmental factors. While their growth can be enhanced and calcification be hindered by OA under constant indoor light, fluctuation of solar radiation with ultraviolet irradiances might offset such effects. In this study, when a calcifying and a non-calcifying strain of Emiliania huxleyi were grown at 2 CO2 concentrations (low CO2 [LC]: 395 µatm; high CO2 [HC]: 1000 µatm) under different levels of incident solar radiation in the presence of ultraviolet radiation (UVR), HC and increased levels of solar radiation acted synergistically to enhance the growth in the calcifying strain but not in the non-calcifying strain. HC enhanced the particulate organic carbon (POC) and nitrogen (PON) productions in both strains, and this effect was more obvious at high levels of solar radiation. While HC decreased calcification at low solar radiation levels, it did not cause a significant effect at high levels of solar radiation, implying that a sufficient supply of light energy can offset the impact of OA on the calcifying strain. Our data suggest that increased light exposure, which is predicted to happen with shoaling of the upper mixing layer due to progressive warming, could counteract the impact of OA on coccolithophores distributed within this layer.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.3354/meps12042
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Chromophoric dissolved organic matter in experimental mesocosms maintained under different pCO2 levels (2004)
    Inter Research
    In:  Marine Ecology Progress Series, 272 . pp. 25-31.
    Details
    Publication Date: 2018-05-30
    Description: Chromophoric dissolved organic matter (CDOM) represents the optically active fraction of the bulk dissolved organic matter (DOM) pool. Recent evidence pointed towards a microbial source of CDOM in the aquatic environment and led to the proposal that phytoplankton is not a direct source of CDOM, but that heterotrophic bacteria, through reprocessing of DOM of algal origin, are an important source of CDOM. In a recent experiment designed at looking at the effects of elevated pCO2 on blooms of the coccolithophorid alga Emiliania huxleyi, we found that despite the 3 different pCO2 levels tested (190, 414 and 714 ppm), no differences were observed in accumulation of CDOM over the 20 d of incubation. Unlike previous mesocosm experiments where relationships between CDOM accumulation and bacterial abundance have been observed, none was observed here. These results provide some new insights into the apparent lack of effect of pCO2 on CDOM accumulation in surface waters, and question the previously proposed mechanisms and rates of CDOM production in natural phytoplankton blooms.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/meps272025
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Carbon isotope fractionation by a marine diatom: Dependence on the growth rate limiting resource (2000)
    Inter Research
    In:  Marine Ecology Progress Series, 193 . pp. 295-303.
    Details
    Publication Date: 2018-05-09
    Description: The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (ε p) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variability requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potentially influence ε p, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate (µ) and CO2 concentration ([CO2aq]) in controlling ε p. Experimental examination of the relationship of ε p with µ and [CO2aq] in studies using different experimental approaches, however, has yielded inconsistent results. Here we directly compare new and previously published data on ε p as a function of CO2 concentration and growth rate for the marine diatom Phaeodactylum tricornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), ε p of P. tricornutum decreases with increasing growth rate. In contrast, under N-replete conditions ε p values are considerably lower at comparable growth rates and CO2 concentrations and are largely insensitive to a 3-fold increase in growth rate due to increasing photon flux density. In both experimental approaches, ε p shows a relatively small CO2 sensitivity in the range of CO2 concentrations naturally occurring in the ocean (8 to 25 µmol kg-1). Below ca 5 µmol CO2 kg-1, a strong decline in ε p with decreasing [CO2aq] is observed. The apparent difference in ε p responses between nitrate-limited and light-controlled cultures of P. tricornutum suggests a principal difference in carbon acquisition for different growth-rate-limiting resources. A mechanistic explanation is proposed and potential implications for the interpretation of phytoplankton carbon isotope fractionation are discussed.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3354/meps193295
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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