Following publications have been announced by our Institute of Carbon Cycles. For further information please contact the linked authors and co-authors of the publications:
Francescangeli, F., Milker, Y., Bunzel, D., Thomas, H., Norbisrath, M., Schönfeld, J., & Schmiedl, G. (2021): Recent benthic foraminiferal distribution in the Elbe Estuary (North Sea, Germany): A response to environmental stressors. Estuarine, Coastal and Shelf Science, Volume 251, 107198, doi:10.1016/j.ecss.2021.107198
For the past 200 years, estuarine environments experienced intense and rapid environmental degradations due to human interventions. In addition, Global Changes are modifying the estuarine physiography, leading to a re-structuration of marginal marine benthic communities. The aim of this study is to document, the modern assemblage composition and the species-environment relations of benthic foraminifera upstream the Elbe Estuary (southern North Sea) and to observe what has changed since the first survey in 1981. For this purpose, a surface sampling was carried out from 22 stations along the transitional area of the Elbe Estuary. Living (rose-Bengal stained) and dead foraminiferal assemblages were analysed as well as hydrological and sedimentological parameters (such as salinity, pH, grain-size, and organic matter). Living faunas are characterized by very low densities and largely dominated by Ammonia species. Dead assemblages are more diverse and dominated by Ammonia aomoriensis, Haynesina germanica, and Cribroelphidium selseyense. Salinity and grain-size seem to be the major factors influencing foraminiferal distributions in the transitional area. Under the ongoing climate changes, future strategies will be taken to foster the application of benthic foraminifera as biomonitoring tool in the Elbe Estuary, via this baseline investigation.
Callies, U. (2021): Sensitive dependence of trajectories on tracer seeding positions – coherent structures in German Bight backward drift simulations. Ocean Sci., 17, 527–541, doi:10.5194/os-17-527-2021
Backward drift simulations can aid the interpretation of in situ monitoring data. In some cases, however, trajectories are very sensitive to even small changes in the tracer release position. A corresponding spread of backward simulations implies attraction in the forward passage of time and, hence, uncertainty about the probed water body’s origin. This study examines surface drift simulations in the German Bight (North Sea). Lines across which drift behaviour changes non-smoothly are obtained as ridges in the fields of the finite-time Lyapunov exponent (FTLE), a parameter used in dynamical systems theory to identify Lagrangian coherent structures (LCSs). Results closely resemble those obtained considering two-particle relative dispersion. It is argued that simulated FTLE fields might be used in support of the interpretation of monitoring data, indicating when simulations of backward trajectories are unreliable because of their high sensitivity to tracer seeding positions.
Van Dam, B., Polsenaere, P., Barreras‐Apodaca, A., Lopes, C., Sanchez‐Mejia, Z., Tokoro, T., Kuwae, T., Gutiérrez Loza, L., Rutgersson, A., Fourqurean, J., & Thomas, H. (2021): Global trends in air‐water CO2 exchange over seagrass meadows revealed by atmospheric Eddy Covariance. Global Biogeochemical Cycles, 35, doi:10.1029/2020GB006848
Coastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2 as “blue carbon” (BC). Already, some coastal ecosystems are actively managed to enhance BC storage, with associated BC stocks included in national greenhouse gas inventories. However, the extent to which BC burial fluxes are enhanced or counteracted by other carbon fluxes, especially air‐water CO2 flux (FCO2) remains poorly understood. In this study, we synthesized all available direct FCO2 measurements over seagrass meadows made using atmospheric Eddy Covariance, across a globally representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO2 sources, with average FCO2 equivalent to 44%–115% of the global average BC burial rate. At the remaining sites, net CO2 uptake was 101%–888% of average BC burial. A wavelet coherence analysis demonstrated that FCO2 was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing was a key driver of global‐scale patterns in FCO2, likely due to a combination of lateral carbon exchange, bottom‐driven turbulence, and pore‐water pumping. Lastly, sea‐surface drag coefficients were always greater than the prediction for the open ocean, supporting a universal enhancement of gas‐transfer in shallow coastal waters. Our study points to the need for a more comprehensive approach to BC assessments, considering not only organic carbon storage, but also air‐water CO2 exchange, and its complex biogeochemical and physical drivers.
Plain Language Summary:
Carbon storage is a valuable ecosystem service of seagrass meadows, serving as a possible pathway to draw down atmospheric carbon dioxide (CO2) levels. However, this approach may be unsuccessful if carbon storage in sediments is exceeded by the release of CO2 from the water. To better understand the scope of this problem, we compiled all available measurements of air‐water CO2 exchange over seagrass meadows. We found that rates of CO2 release or uptake were indeed large, even when compared with potential rates of carbon storage in seagrass soils. However, these large air‐water exchanges of CO2 did not occur for the same reason everywhere. While light availability was sometimes a strong predictor of air‐water CO2 exchange, tidal mixing and temperature were also very important, revealing a much more complex network of drivers than previously thought. Despite these diverse conditions, we found one key similarity across all sites, in that rates of air‐water gas transfer appear to always be greater than would be expected for the open ocean. Taken together, the results of our study show that assessments of carbon storage in coastal seagrass ecosystems will be incomplete if they do not consider exchanges of CO2 between the water and air.