Ballast, sinking velocity, and apparent diffusivity within marine snow and zooplankton fecal pellets: Implications for substrate turnover by attached bacteria

We analyzed size-specific dry mass, sinking velocity, and apparent diffusivity in field-sampled marine snow, laboratory-made aggregates formed by diatoms or coccolithophorids, and small and large zooplankton fecal pellets with naturally varying content of ballast materials. Apparent diffusivity was measured directly inside aggregates and large (millimeter-long) fecal pellets using microsensors. Large fecal pellets, collected in the coastal upwelling off Cape Blanc, Mauritania, showed the highest volume-specific dry mass and sinking velocities because of a high content of opal, carbonate, and lithogenic material (mostly Saharan dust), which together comprised ,80% of the dry mass. The average solid matter density within these large fecal pellets was 1.7 g cm23, whereas their excess density was 0.25 6 0.07 g cm23. Volume-specific dry mass of all sources of aggregates and fecal pellets ranged from 3.8 to 960 mg mm23, and average sinking velocities varied between 51 and 732 m d21. Porosity was .0.43 and .0.96 within fecal pellets and phytoplankton-derived aggregates, respectively. Averaged values of apparent diffusivity of gases within large fecal pellets and aggregates were 0.74 and 0.95 times that of the free diffusion coefficient in sea water, respectively. Ballast increases sinking velocity and, thus, also potential O2 fluxes to sedimenting aggregates and fecal pellets. Hence, ballast minerals limit the residence time of aggregates in the water column by increasing sinking velocity, but apparent diffusivity and potential oxygen supply within aggregates are high, whereby a large fraction of labile organic carbon can be respired during sedimentation. Marine snow and fecal pellets comprise a significant fraction of the sinking carbon flux in the ocean (Alldredge and Silver 1988; Simon et al. 2002; Turner 2002). Hence, sedimentation of these particles into the bathypelagic zone is important for the ocean’s capacity to sequester CO2 from the atmosphere, i.e., the ocean’s biological carbon pump (De La Rocha and Passow 2007). The recent observation that carbonate and organic carbon fluxes show close correlations in the bathypelagic zone of the ocean has led to the hypothesis that biominerals in phytoplankton, e.g., carbonate and opal, promote carbon preservation of the sinking flux because these biominerals increase sinking velocity because of their high densities and/or protect a fraction of the organic matter in the cells from being degraded in the deep ocean (Armstrong et al. 2002; Francois et al. 2002; Klaas and Archer 2002). The effect of ballast minerals on sinking velocity relative to that on small-scale oxygen fluxes and degradation rates in sinking particles, however, is largely unknown. The physical and chemical microenvironment of sinking particles is significantly different from that of the surrounding water. High concentrations of inorganic and organic matter, ecto-enzymatic activities, and remineralization rates by attached bacteria lead to oxygen and pH gradients within sinking marine snow and fecal pellets (Alldredge and Cohen 1987; Smith et al. 1992; Ploug et al. 1999). The observations that dissolved organic carbon (DOC), silicic acid, ammonium, and phosphate concentrations are higher inside marine snow compared to the surrounding water have led to the hypothesis that diffusion within marine snow is significantly slower than in sea water (Shanks and Trent 1979; Brzezinski et al. 1997; Alldredge 2000). The flux of a solute within an aggregate equals the product of the apparent diffusivity and the radial concentration gradient of the solute, i.e., Fick’s first law of diffusion: