Small-scale oxygen fluxes and remineralization in sinking aggregates

Sinking aggregates are the major component of the vertical particulate flux in most regions of the ocean. Controlling factors for aggregate remineralization rates and solute exchange with the surrounding water, however, are poorly quantified because of few empirical data. To study the role of flow and diffusion on aggregate remineralization rates, oxygen distributions were mapped within and around aggregates by use of microelectrodes in a flow field similar to that experienced by sinking aggregates. The oxygen distribution was asymmetrical with a wake of undersaturated water at the rear (downstream) of the aggregates. Oxygen concentrations within the aggregates were .80% of air saturation. The diffusive fluxes of oxygen at the aggregate-water interface were similar along the equator and at the downstream pole for a wide range of different aggregate sources (field-sampled diatom aggregates, lab-made diatom aggregates, aggregates formed from freeze-thawed diatoms, and zooplankton detritus aggregates) measured at various temperatures. Remineralization rates were reaction limited and, hence, determined by substrate quality and quantity rather than by transport-limited oxygen supply during sedimentation at ambient oxygen concentrations above ;25 mM. Aggregates of phytoplankton and detritus colonized by microorganisms are common features in the pelagic environment of lakes, rivers, and the ocean (Alldredge and Silver 1988; Eisma 1993; Grossart and Simon 1993). Bacteria, flagellates, and ciliates are up to 10,000-fold more concentrated on aggregates compared to a similar volume of ambient water (Caron et al. 1986). Aggregate sinking velocities range between 20 and 200 m d21 (Alldredge and Gotschalk 1988), and sinking aggregates comprise a significant component of the flux of organic matter formed in the euphotic zone and exported to sediments in most regions of the ocean, i.e., the draw down of atmospheric CO2 into the deep ocean and sediments (Shanks and Trent 1980; Fowler and Knauer 1986). However, the vertical flux of organic matter below the euphotic zone decreases significantly with increasing water depth, and the organic matter that reaches the sediment is partly solubilized and remineralized through microbial degradation during the sedimentation process (Lee and Wakeham 1988). Solubilization and remineralization processes of the organic substrates often imply that the concentrations of oxygen, silicic acid, and dissolved organic carbon (DOC) within aggregates are significantly different from those in the bulk water (Alldredge and Cohen 1987; Brzezinski et al. 1997; Alldredge 2000), and the measured influx of oxygen and efflux of dissolved combined amino acids between the aggregates and the surrounding water suggest an efficient mass transfer at the aggregate-water interface (Smith et al. 1992; 1 Present address: Max Planck Institute for Marine Microbiology, Celsiusstr. 1, D-28359 Bremen, Germany ([email protected]).