ALBERT

Description: Environmental context: Halocarbons are trace gases important in atmospheric ozone chemistry whose biogenic production – among other factors – depends on light-induced stress of marine algae. Several studies have confirmed this effect in laboratory experiments but knowledge in natural systems remains sparse. In mesocosm experiments, which are a link between field and laboratory studies, we observed that the influence of natural levels of ultraviolet radiation on halocarbon dynamics in the marine surface waters was either insignificant or concealed by the complex interactions in the natural systems. Abstract: The aim of the present study was to evaluate the influence of different light quality, especially ultraviolet radiation (UVR), on the dynamics of volatile halogenated organic compounds (VHOCs) at the sea surface. Short term experiments were conducted with floating gas-tight mesocosms of different optical qualities. Six halocarbons (CH3I, CHCl3, CH2Br2, CH2ClI, CHBr3 and CH2I2), known to be produced by phytoplankton, together with a variety of biological and environmental variables were measured in the coastal southern Baltic Sea and in the Raunefjord (North Sea). These experiments showed that ambient levels of UVR have no significant influence on VHOC dynamics in the natural systems. We attribute it to the low radiation doses that phytoplankton cells receive in a normal turbulent surface mixed layer. The VHOC concentrations were influenced by their production and removal processes, but they were not correlated with biological or environmental parameters investigated. Diatoms were most likely the dominant biogenic source of VHOCs in the Baltic Sea experiment, whereas in the Raunefjord experiment macroalgae probably contributed strongly to the production of VHOCs. The variable stable carbon isotope signatures (δ13C values) of bromoform (CHBr3) also indicate that different autotrophic organisms were responsible for CHBr3 production in the two coastal environments. In the Raunefjord, despite strong daily variations in CHBr3 concentration, the carbon isotopic ratio was fairly stable with a mean value of –26 ‰. During the declining spring phytoplankton bloom in the Baltic Sea, the δ13C values of CHBr3 were enriched in 13C and showed noticeable diurnal changes (–12 ‰ ± 4). These results show that isotope signature analysis is a useful tool to study both the origin and dynamics of VHOCs in natural systems.

Description: Environmental context: Once released to the atmosphere, halocarbons are involved in key chemical reactions. Stable carbon isotope measurements of halocarbons can provide valuable information on their sources and fate in the atmosphere. Here, we report δ13C values of 13 polyhalomethanes released from brown algae, which may provide a basis for inferring their sources and fate in future studies. Abstract: Halocarbons are important vectors of reactive halogens to the atmosphere, where the latter participate in several key chemical processes. An improved understanding of the biogeochemical controls of the production–destruction equilibrium on halocarbons is of vital importance to address potential future changes in their fluxes to the atmosphere. Carbon stable isotope ratios of halocarbons could provide valuable additional information on their sources and fate that cannot be derived from mixing ratios alone. We determined the δ13C values of 13 polyhalomethanes from three brown algae species (Laminaria digitata, Fucus vesiculosus, Fucus serratus) and one seagrass species (Zostera noltii). The δ13C values were determined in laboratory incubations under variable environmental conditions of light, water levels (to simulate tidal events) and addition of hydrogen peroxide (H2O2). The δ13C values of the polyhalomethanes ranged from –42.2 ‰ (±3.5 s.d.) for CHCl3 to 6.9 ‰ (±4.5) for CHI2Br and showed a systematic effect of the halogen substituents that could empirically be described in terms of linear free energy relationships. We further observed an enrichment in the δ13C of the polyhalomethanes with decreasing polyhalomethane yield that is attributed to the competing formation of halogenated ketones. Though variable, the isotopic composition of polyhalomethanes may provide useful additional information to discriminate between marine polyhalomethane sources.

Description: The subtropical northeast Atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production. To better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of N2 fixation in providing new nitrogen (N), we investigated the depth distribution of nitrate δ15N and δ18O at six stations across the Azores Front in the NE Atlantic. In addition, we measured the δ15N of dissolved organic N (DON) in surface waters and of sinking particulate N collected in sediment traps at 2000 m depth between 2003 and 2005 at Station KIEL276. The nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the δ15N from depth toward low-nitrate surface waters, concomitant with an increase in δ18O. Given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate δ15N and δ18O, the observed surface water nitrate isotope anomalies (Δ(15;18) up to −6‰) indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed N that is remineralized in surface and subsurface waters. Both the concentration of DON and its δ15N in surface water were spatially invariant, showing mean values of 4.7 ± 0.5 μmol L−1 and 2.6 ± 0.4‰ (n = 35), respectively, supporting the conjecture of a mostly recalcitrant DON pool. The weighted biannual mean δ15N of sinking particulate N (1.8 ± 0.8‰, n = 33) was low with respect to thermocline nitrate. The anomalous dual nitrate isotope signatures together with the low δ15N of export production and elevated nitrate-to-phosphate ratios in surface and subsurface waters strongly suggest that N2 fixation represents a substantive source of N in this part of the subtropical northeast Atlantic. Simple isotope mass balance suggests that, locally, N2 fixation supplies between 56 and 259 mmol N m−2 a−1 for phytoplankton growth in the photic zone, accounting for up to ∼40% of the estimated export production.

Description: In order to investigate temporal changes in combination with the influence of different environmental parameters on the concentration and the composition of volatile organic compounds (VOCs), seawater samples from the coastal Baltic Sea were weekly measured from January to November 2008. In most cases, concentrations of VOCs varied seasonally and were influenced by changes in temperature and light conditions or biological species composition. A nearly two-fold increase in the mean concentration was noticed for isoprene, iodomethane and bromoform in the season with higher water temperature. The strongest flux of dimethylsulfide to the atmosphere appeared in May and July. Its high production was related to the presence of Prymnesiophyceae. The highest concentrations of diiodomethane and chloroiodomethane were observed with the spring and autumn phytoplankton bloom; their distribution was strongly controlled by light intensity. Flux calculations showed that coastal regions can affect local atmosphere, especially during biologically active periods. The strongest emission of bromoform and iodomethane was in July and August. The data presented here highlights the need to include seasonal cycles when calculating the global budgets and modelling sea-air fluxes of trace gases.