ALBERT

Description: Strontium isotopic compositions of the interstitial water and carbonate from marine sediments sampled during IODP 308, in two basins (the normally-pressured Brazos-Trinity Basin IV and the over-pressured Ursa Basin) on the northern slope of the Gulf of Mexico, are present in this study. In the Brazos-Trinity Basin IV, the 87Sr/86Sr ratios of the interstitial water range from 0.70917 to 0.70954, with carbonates sharing similar or slightly lower values from 0.70851 to 0.70952. The interstitial water above 31 m shows similar 87Sr/86Sr ratio close to that of seawater, whereas the interstitial water below 31 m shows more radiogenic 87Sr/86Sr ratios, possibly indicating a water/rock interaction between the fluid and silicate component in the deep basin sediments. The Sr-isotope ratios of the carbonates are less radiogenic than the seawater, which may reflect a terrestrial carbonate input (such as limestone) transported through the Brazos and Trinity rivers. In the Ursa Basin, the 87Sr/86Sr ratios of the interstitial water range from 0.70887 to 0.70999, those of the carbonate vary from 0.70808 to 0.70930. Both the interstitial water and carbonate show a similar trend of 87Sr/86Sr ratios throughout the depth. The 87Sr/86Sr ratios of the interstitial water decrease from the sea floor surface to the minimum at the Seismic Reflector S10. A lateral fluid incursion with less radiogenic 87Sr/86Sr ratios along the Seismic Reflector S10 can be inferred in the Ursa Basin. It is suggested that the lateral fluid incursion shows a seawater origin, modified by diagenetic reactions including the dissolution of halite. Between the Seismic Reflector S10 and S40, 87Sr/86Sr ratios of the interstitial water increase linearly to the maximum. Along the Seismic Reflector S40, a lateral fluid incursion with more radiogenic 87Sr/86Sr ratios is again recognized. The fluid may have also originated from seawater, but modified by the diagenesis of terrigenous sediments, likewise characterized by highly radiogenic 87Sr/86Sr compositions. A two-dimension fluid-flow model in the Ursa Basin is established.

Description: A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated pCO2 on bloom formation by phytoplankton species previously studied in laboratory-based ocean acidification experiments, to determine if the indoor-grown species performed similarly in mesocosms under more realistic environmental conditions. We measured biomass, primary productivity and particulate organic carbon (POC) as well as particulate organic nitrogen (PON). Phaeodactylum tricornutum outcompeted Thalassiosira weissflogii and Emiliania huxleyi, comprising more than 99% of the final biomass. Mainly through a capacity to tolerate nutrient-limited situations, P. tricornutum showed a powerful sustained presence during the plateau phase of growth. Significant differences between high and low CO2 treatments were found in cell concentration, cumulative primary productivity and POC in the plateau phase but not during the exponential phase of growth. Compared to the low pCO2 (LC) treatment, POC increased by 45.8–101.9% in the high pCO2 (HC) treated cells during the bloom period. Furthermore, respiratory carbon losses of gross primary productivity were found to comprise 39–64% for the LC and 31–41% for the HC mesocosms (daytime C fixation) in phase II. Our results suggest that the duration and characteristics of a diatom bloom can be affected by elevated pCO2. Effects of elevated pCO2 observed in the laboratory cannot be reliably extrapolated to large scale mesocosms with multiple influencing factors, especially during intense algal blooms.

Description: There is increasing concern about the effects of ocean acidification on marine biogeochemical and ecological processes and the organisms that drive them, including marine bacteria. Here, we examine the effects of elevated CO2 on the bacterioplankton community during a mesocosm experiment using an artificial phytoplankton community in subtropical, eutrophic coastal waters of Xiamen, southern China. Through sequencing the bacterial 16S rRNA gene V3-V4 region, we found that the bacterioplankton community in this high-nutrient coastal environment was relatively resilient to changes in seawater carbonate chemistry. Based on comparative ecological network analysis, we found that elevated CO2 hardly altered the network structure of high-abundance bacterioplankton taxa but appeared to reassemble the community network of low abundance taxa. This led to relatively high resilience of the whole bacterioplankton community to the elevated CO2 level and associated chemical changes. We also observed that the Flavobacteria group, which plays an important role in the microbial carbon pump, showed higher relative abundance under the elevated CO2 condition during the early stage of the phytoplankton bloom in the mesocosms. Our results provide new insights into how elevated CO2 may influence bacterioplankton community structure.

Description: Increases of atmospheric CO2 concentrations due to human activity and associated effects on aquatic ecosystems are recognized as an environmental issue at a global scale. Growing attention is being paid to CO2 enrichment effects under multiple stresses or fluctuating environmental conditions in order to extrapolate from laboratory-scale experiments to natural systems. We carried out a mesocosm experiment in coastal water with an assemblage of three model phytoplankton species and their associated bacteria under the influence of elevated CO2 concentrations. Net community production and the metabolic characteristics of the phytoplankton and bacteria were monitored to elucidate how these organisms responded to CO2 enrichment during the course of the algal bloom. We found that CO2 enrichment (1000 μatm) significantly enhanced gross primary production and the ratio of photosynthesis to chlorophyll a by approximately 38% and 39%, respectively, during the early stationary phase of the algal bloom. Although there were few effects on bulk bacterial production, a significant decrease of bulk bacterial respiration (up to 31%) at elevated CO2 resulted in an increase of bacterial growth efficiency. The implication is that an elevation of CO2 concentrations leads to a reduction of bacterial carbon demand and enhances carbon transfer efficiency through the microbial loop, with a greater proportion of fixed carbon being allocated to bacterial biomass and less being lost as CO2. The contemporaneous responses of phytoplankton and bacterial metabolism to CO2 enrichment increased net community production by about 45%, an increase that would have profound implications for the carbon cycle in coastal marine ecosystems.

Description: There is a growing consensus that the ongoing increase in atmospheric CO2 level will lead to a variety of effects on marine phytoplankton and ecosystems. However, the effects of CO2 enrichment on eutrophic coastal waters are still unclear, as are the complex mechanisms coupled to the development of eutrophication. Here, we report the first mesocosm CO2 perturbation study in a eutrophic subtropical bay during summer by investigating the effect of rising CO2 on a model artificial community consisting of well-characterized cultured diatoms (Phaeodactylum tricornutum and Thalassiosira weissflogii) and prymnesiophytes (Emiliania huxleyi and Gephyrocapsa oceanica). These species were inoculated into triplicate 4 m**3 enclosures with equivalent chlorophyll a (Chl-a) under present and higher partial pressures of atmospheric CO2 (pCO2 = 400 and 1000 ppmv). Diatom bloom events were observed in all enclosures, with enhanced organic carbon production and Chl-a concentrations under high CO2 treatments. Relative to the low CO2 treatments, the consumption of the dissolved inorganic nitrogen and uptake ratios of N/P and N/Si increased significantly during the bloom. These observed responses suggest more extensive and complex effects of higher CO2 concentrations on phytoplankton communities in coastal eutrophic environments.

Description: Solid phase extraction (SPE) has become a widespread method for isolating dissolved organic matter (DOM) of diverse origin such as fresh and marine waters. This study investigated the DOM extraction selectivity of 24 commercially available SPE sorbents under identical conditions (pH = 2, methanol elution) on the example of Suwannee River (SR) water and North Sea (NS) water by using DOC analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Proton nuclear magnetic resonance (1H NMR) spectroscopy was employed to assess leaching behavior, and HLB sorbent was found to leach substantially, among others. Variable DOC recoveries observed for SR DOM and NS DOM were primarily caused by the respective molecular composition, with subordinated and heterogeneous contributions of relative salinity. Scatter of average H/C and O/C elemental ratios and gross alignment in mass-edited H/C ratios according to five established coarse SPE characteristics was near identical for SR DOM and NS DOM. FTMS-based principal component analysis (PCA) provided essentially analogous alignment of SR DOM and NS DOM molecular compositions according to the five established groups of SPE classification, and corroborated the sorption-mechanism-based selectivity of DOM extraction in both cases. Evaluation of structural blanks and leaching of SPE cartridges requires NMR spectroscopy because FT-ICR mass spectrometry alone will not reveal inconspicuous displacements of continual bulk signatures caused by leaching of SPE resin constituents.

Description: This paper proposes improved guidelines for dissolved organic matter (DOM) isolation by solid phase extraction (SPE) with a styrene–divinylbenzene copolymer (PPL) sorbent, which has become an established method for the isolation of DOM from natural waters, because of its ease of application and appreciable carbon recovery. Suwannee River water was selected to systematically study the effects of critical SPE variables such as loading mass, concentration, flow rate, and up-scaling on the extraction selectivity of the PPL sorbent. High-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) and proton nuclear magnetic resonance (1H NMR) spectroscopy were performed to interpret the DOM chemical space of eluates, as well as permeates and wash liquids with molecular resolution. Up to 89% dissolved organic carbon (DOC) recovery was obtained with a DOC/PPL mass ratio of 1:800 at a DOC concentration of 20 mg/L. With the application of larger loading volumes, low proportions of highly oxygenated compounds were retained on the PPL sorbent. The effects of the flow rate on the extraction selectivity of the sorbent were marginal. Up-scaling had a limited effect on the extraction selectivity with the exception of increased self-esterification with a methanol solvent, resulting in methyl ester groups. Furthermore, the SPE/PPL extract exhibited highly authentic characteristics in comparison with original water and reverse osmosis samples. These findings will be useful for reproducibly isolating DOM with representative molecular compositions from various sources and concentrations and minimizing potential inconsistencies among interlaboratory comparative studies.