Project GB2

The availability of the nutrients nitrogen (N), phosphorus (P), and iron (Fe) limits primary productivity in the ocean. Variations in their relative abundance affect the species composition of phytoplankton communities in surface waters.

Project GB2 will investigate the fate of these nutrients in the benthic boundary layer (BBL), the lower 2-3 m of the water column. Little is known about the biogeochemical and geomicrobial processes controlling the abundance of N, P, dissolved organic matter (DOM), and Fe in the BBL. It is the transition zone for particulate OM sinking down from surface waters to the sediment and for dissolved species of N, P, and Fe diffusing out of the sediment into the water column. Water column mixing will eventually return these nutrients back into surface waters. The BBL thus intimately links fluxes of N, P, and Fe from surface sediments to primary production in the ocean.

The objectives in this project build upon from former projects B1 (Bacterial Control of Nutrient Exchange in Continental Margin Sediments and Bottom Water) and B2 (Biogeochemical Processes in Subsurface Sediments) during 2005 and 2009.

• How important is the release of nutrients N and P upon organic matter mineralization in the BBL relative to nutrient release from the underlying sediments?
• How is oceanic N-loss affected by microbial processes in the BBL? Which processes are responsible for this N-loss - denitrification and/or anaerobic ammonium oxidation (anammox)?
• Do N-loss, Fe-respiration/chelation and P-mineralization/dissolution in this zone alter nutrient N/P/Fe-ratios, thereby affecting oceanic phytoplankton growth and species composition?
• Does organic carbon decomposition transform sedimentary organic matter into refractory soluble molecules that sequester nutrients on decadal to millenial time scales?
• How much do microbial processes in the BBL drive phosphate availability and can we use oxygen isotopes in phosphates to monitor these processes?