Coupling and decoupling of biogeochemical cycles in marine ecosystems

• Main Author: Hohn, Sönke
• Other Authors: Wolf-Gladrow, Dieter, Völker, Christoph, Oschlies, Andreas
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2008
• Subjects: ecological stoichiometry; ecosystem model; diatom; silicon; nitrogen; carbon; biogeochemistry; modelling; 570; 570 Life sciences; biology; ddc:570; Southern Ocean
• Online Access: https://media.suub.uni-bremen.de/handle/elib/2604; https://nbn-resolving.org/urn:nbn:de:gbv:46-diss000112787

The biogeochemical cycles of biologically important elements are coupled to each other via the formation of biomass. Many ecosystem models assume this coupling to follow fixed stoichiometric ratios, even though, under certain environmental conditions, the stoichiometric composition of marine phytoplankton can deviate strongly from fixed Redfield ratios. This thesis investigates the effect of variable phytoplankton stoichiometry on large scale biogeochemical fluxes in different marine biological systems. In the first study, an ecosystem model is developed for a shallow coastal tidal basin in the Danish-German Wadden Sea and the adjacent North Sea. The model allows for variations in the cellular quotas of carbon (C), nitrogen (N), and chlorophyll (Chl) of the simulated phytoplankton biomass. The phytoplankton C:N ratio in the tidal basin is found to vary from 5 to 15 between light-limited winter conditions and nitrogen-limited summer growth conditions, respectively. Different water depths between the North Sea and the shallow tidal inlet lead to differences in phytoplankton C:N ratios that can also induce a decoupling of carbon and nitrogen fluxes in the budgeting of the annual tidal transport between the North Sea and the Wadden Sea. The second study extends the parameterization of phytoplankton growth by inclusion of the elements silicon (Si) and iron (Fe) to obtain a parameterization for diatom growth that can be applied in diatom-dominated high-nutrient low-chlorophyll (HNLC) ocean regions like the Southern Ocean. The parameterization considers separate pools of cellular chlorophyll, carbon, nitrogen, and silicon and reproduces the elevated Si:N uptake ratios of diatoms growing under iron-limitation. In the third study, the parameterization of diatom growth is applied to an ecosystem model that is coupled to a global setup of the ocean general circulation model of the Massachussets Institute of Technology (MITgcm). The model is adjusted to the Southern Ocean ecosystem and analysed for the biogeochemical fluxes ...