Work Package 16: Biological feedbacks

Leader: Marion Gehlen

Objectives
To estimate the strength of feedback processes between increasing CO2 and air/sea fluxes induced
by biogeochemical processes (marine biota, ecosystem structure, particle flux).

Description of work
Mesocosm experiments: Perturbations in seawater carbonate chemistry and temperature will be
applied separately and in combination and will be tested on natural phytoplankton populations
consisting of a mixed population of calcifying and non-calcifying phytoplankton, as well as on
benthic calcareous algae. Expected results include the effect of increasing pCO2 on phytoplankton
species succession, calcification rates, carbonate dissolution, composition of export fluxes (partners
2, 4, 20, 35)
Laboratory studies will assess the roles of N, P and Fe limitation of N2 fixation, as well as the effect
of increasing temperature on the production and fate of dissolved organic matter. (partners 3, 6, 20)
Field work will assess the stimulation or inhibition of diazotrophic activity (N2-fixation) by
additions of inorganic N, P and Fe will be determined in experiments conducted on one research
cruise (Atlantic Meridional Transect). (partner 3, 46)
Modeling studies (links to WPs 9, 17) will be used to quantify the impacts of processes studied
during experimental work. The effects of plankton assemblage shifts on export and
remineralisation of particulate organic carbon at depth will be evaluated by means of 3D
biogeochemical circulation models (BOGCMs ORCA/PISCES and HAMOCC5) including particle
dynamics (exploitation of tools created by EU FP5 project ORFOIS). The feedback of decreased
calcification rates and modified export fluxes through decreased ballasting on surface ocean pCO2
will be quantified and the sensitivity of surface ocean pCO2 to remineralisation depth will be
evaluated. A global model including at least three phytoplankton functional types (the ‘Dynamical
Green Ocean Model’ - DGOM) will be further developed and used to assess the impact of
competition between different plankton types in a changing environment, and related integrated
changes in marine ecosystems on atmospheric CO2. (partners 3, 2, 9, 12, 17, 35, 46)