Work Package 6

Fluxes of solutes across the benthic boundary layer in response to variable bottom water redox regimes

Involved Scientists


Oxygen minimum zones (OMZ) in eastern boundary upwelling systems are hotspots for benthic biogeochemical carbon and element turnover. The underlying sediments are typically associated with high rates of release of N, P and trace metals (Bohlen et al. 2011, Dale et al. 2015, Noffke et al. 2012) and are linked to feedbacks on the maintenance or even expansion of OMZs. Fluctuating external forcings (e.g. hydrodynamics, climate, primary productivity, and bottom water redox conditions) cause major regime shifts in benthic element turnover and their net source/sink terms (Sommer et al. 2016). During periods of low bottom water oxygen, nitrate and nitrite variability, for instance, the sulfide buffering capacity of the benthos becomes exhausted, resulting in a massive seabed release of sulfide with adverse conditions on local fisheries and aquaculture. Large sulfur oxidizing bacteria thus represent key organisms in the detoxification of sulfide whilst at the same time modulate benthic N cycling (Sommer et al. 2016) as well as sequestration and release of P species (Schulz & Schulz, 2005). This work package addresses the following question:

General Questions and Research topics:

  • What is the response (tipping points, timing and magnitude) of benthic fluxes in the BUS to fluctuating bottom water redox conditions and what are the major microbial controls?

Natural in situ benthic flux measurements of O2, CO2, DIC, nutrients, sulfide, iron and trace gases across the benthic boundary layer (BBL) will be made parallel and perpendicular to the coastline in relation to different bottom water redox-conditions. Key activities of SP6 will also encompass in situ as well as ex situ experiments involving the manipulation of bottom water redox conditions (e.g. O2, NO3-, NO2-, labelled tracers). This will resolve tipping points beyond which different pathways of microbial P and N turnover are activated, affecting the magnitude and direction of benthic solute fluxes. Both natural fluxes as well as experiments will allow accurate constraints on the boundary conditions of the biogeochemical modelling planned in SP8. The natural flux measurements in SP6 will further contribute to reconstruct the net source-sink function of the Benguela upwelling area for CO2 and to determine its controls by correlating benthic and pelagic fluxes of DIC and CO2 in conjunction with carbon burial and benthic-pelagic carbon turnover.