Coordinator: Prof. Dr Ulf Riebesell (Deputy: Prof. Dr. Andreas Oschlies)

Mission:

WP2 investigates the mechanisms driving biogeochemical cycling in pelagic ecosystems and changes in element fluxes in response to anthropogenic perturbations. 

Scientific questions

Research conducted in WP2 addresses biogeochemically-relevant processes from the cellular to the ecosystem level applying molecular, physiological, ecological and geochemical approaches in laboratory and in situ experiments, field observations and numerical modelling. Key questions addressed in this WP include:

• How do key marine organisms and processes respond to global change and how does this translate to the ecosystem level?
• What are the mechanisms underlying biotic sensitivities to ocean change?
• How do biological responses to global change impact marine biogeochemical cycling and climate system feedbacks?

Contents and goals

A major challenge in this WP is to unravel the chain from individual organism responses and their underlying mechanisms, through ecosystem effects, to biogeochemical consequences and climate system feedbacks. To pave the way for a more encompassing assessment of future biological responses to ocean change, research in this WP will:

• Develop and apply advanced analytical instrumentation [link with WP1],
• Allow for more realistic community-level experimentation, including multiple stressors [links with Topic 3-WP2],
• Develop and assess ecosystem and biogeochemical models,
• Address oceanic regions potentially vulnerable to global change,
• Improve the integration between experimentation, field observation and modelling,
• Foster the transfer of know-how for socio-economic assessments.

Key processes and tasks to be addressed in this WP:

Biogenic calcification: The shells and skeletons produced by calcifying organisms do not only have a vital function for the physiology and ecology of their producers, they also play a key role in biogeochemical cycling. Research in this WP will cover a wide range of topics, including studies on (i) the subcellular mechanisms of calcification in model organisms, (ii) the physiological responses to ocean change in a variety of key taxa, (iii) community level responses in systems dominated by calcifying organisms, and (iv) impacts of calcification changes on biogeochemical processes [links with Topic 3 and collaboration with PACES II programme (AWI+HZG)].

Microbial turnover of organic matter: The net productivity and carbon sequestration in the oceans are determined by a balance between autotrophic and heterotrophic processes. Because both processes are known to be highly sensitive to ocean change, our research aims to determine how this balance may shift under projected future conditions [Topic 3 and PACES II].

Dissolved organic matter (DOM): The amount of CO₂ stored in marine DOM equals the amount of atmospheric CO₂, but the vast majority of DOM components in the ocean are still uncharacterized and their cycling is largely unknown. Carbohydrates and peptides are among the reactive DOM components and their molecular signatures provide valuable information on the diagenetic status of DOM. To better trace DOM during biological, chemical and physical processes, our studies aim to reveal the molecular and size structure of reactive DOM in coastal, polar and open ocean areas [Topic 1-WP1, PACES II].

Particle aggregation: Aggregation processes cascade from the nano-scale to the size of fast settling marine snow and are often initiated by the process of gel particle formation from organic polymers. Minerals in association with organic matter act as ballast, thereby increasing sinking velocity and enhancing the export of organic matter to depth, but also retarding microbial degradation. Our research aims for a process-based understanding of particle aggregation and sinking in the context of ocean change [Topic 2-WP3].

Nitrogen fixation: The marine nitrogen inventory is determined by a balance between nitrogen fixation by cyanobacteria and nitrogen loss via microbially-mediated processes. Our research in this area will focus on the distribution and productivity of N-fixing cyanobacteria in relation to environmental conditions [Topic 2-WP1 & 3].

Ecological stoichiometry considers how the balance of energy and elements affects and is affected by organisms and their interactions in ecosystems. Ocean change can instigate stoichiometric changes in organisms, nutrient pools and the fluxes between them. How these changes balance out in multi-trophic communities and how these impact nutrient reservoirs and carbon sequestration is the focus of on-going research in our Topic [link with Topic 3].

Marine oxygen cycle: Ocean deoxygenation is now recognized as a global phenomenon with potentially high biogeochemical relevance due to expanding and strengthening Oxygen Minimum Zones (OMZ). This may give rise to multiple cascading impacts on biogeochemical processes with high feedback potential to the climate system. Particular scientific emphasis is placed on how O₂ is provided to OMZ in the NE Atlantic and SE Pacific Oceans and how changes therein affect biogeochemical processes [Topic 2-WP1].

Developing new ecosystem modelling concepts: In order to account for likely adaptation and evolution of marine ecosystems to environmental change, novel numerical biological and biogeochemical models are required that go beyond the historically employed concept of static model state variables and parameter values. Using information obtained from laboratory experiments and field observations, we will continue the development of optimality-based models and apply these to IPCC-type scenario simulations with global Earth system models [Topic 3].

Data-based calibration and assessment of global biogeochemical models: Despite several decades of biogeochemical model development, a quantitative evaluation and assessment of model performance is not normally done. Using numerically efficient novel methods to accelerate the spin-up and the parameter optimization of biogeochemical models, a model assessment platform will be established. A range of state-of-the-art biogeochemical models coupled to circulation fields derived from different ocean general circulation models will be assessed. Research will include the development of adequate assessment metrics and the exploitation of residual model-data misfits for model improvement [Topic 1-WP1, PACES II].

WP2 research highlight: The Kiel off-shore Mesocosms for Future Ocean Simulations (KOSMOS) provide a unique opportunity to study the drivers and controls of pelagic biogeochemical cycling in situ. This mobile, sea-going experimental facility, which was developed at GEOMAR, allows us to investigate the pelagic ecosystem and biogeochemical responses to ocean change in a wide range of marine environments (see 2.2.1). Deployed from mid-sized research vessels, the KOSMOS facility can be used in moored or free-floating mode.

KOSMOS CO₂ perturbation experiments were successfully conducted in the high Arctic off Svalbard (2010), in temperate waters off Bergen (2011), in subtropical, oligotrophic waters off Hawaii (2011), and in the Baltic Sea off Finland (2012). Each of these multidisciplinary experiments generated comprehensive data sets on community responses to ocean acidification and their biogeochemical impacts. They revealed major changes in plankton community composition and biogeochemical cycling at high CO₂, several of which would not have been predicted based on the knowledge gained from organism-based laboratory studies. A new set of KOSMOS mesocosms are presently being designed which can be easily containerized and shipped to remote places around the globe for in situ studies of ocean change processes in locations considered as biogeochemical hotspots. Future campaigns are planned in waters influenced by the West African dust plume, above the oxygen minimum zones off South America and in upwelling waters of the California Current System.