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Scientific findings

From grazing experiments and visual observations of the feeding behavior of the copepods Calanus helgolandicus, Pseudocalanus elongates, and Oithona similis it was observed that most encountered pellets were rejected by the calanoid copepods (C. helgolandicus and P. elongatus) (Iversen & Poulsen 2007). No pellet encounters were observed for the cyclopoid copepods O. similis (Iversen & Poulsen 2007), though O. similis have been suggested to be an effective pellet grazer (see Turner 2002). Pellet rejections often caused damage to the pellets and occasionally cut them in halfs. It was therefore concluded that the main impact of copepods on pellet degradation is via coprorhexy (pellet fragmentation) and that copepods are not the main pellet degraders (Iversen & Poulsen 2007). From experiments with different size fractions of a natural plankton community from Øresund (Denmark) incubated with a known amount of pellets, large protozooplankton (20 to 100 μm) was found to be the main degraders of fecal pellets (Poulsen & Iversen 2008). Thus, the main impact from mesozooplankton organisms, apart from pellet production, seemed indirect via grazing on the protozooplankton (potentially increasing export) and via coprorhexy (potentially decreasing export) (Iversen & Poulsen 2007; Poulsen & Iversen 2008). The protozooplankton organisms formed an effective ´protozoan filter´ which could remove fecal pellets from the vertical flux (Poulsen & Iversen 2008). It was further found that pellets produced from phytoplankton containing biominerals became ballasted and experienced elevated sinking speeds (Ploug et al. 2008a; Ploug et al. 2008b). However, no indications of protection against microbial degradation as a function of biominerals were observed in freshly produced pellets. Thus, it was demonstrated that fresh fecal pellets produced from phytoplankton containing biominerals potentially had 10-fold higher carbon preservation than pellets produced without biominerals (Ploug et al. 2008a). Further, the oxygen supply to the pellets was potentially increased via the elevated sinking speeds of ballasted pellet, since ballasting had no influence on the apparent diffusivities in the pellets (Ploug et al. 2008b). Hereby, high amounts of oxygen were available for respiration of labile organic carbon during sedimentation.
The presence of biominerals and lithogenic material was indicated as an important factor for the sinking speeds in marine snow and phytoplankton-derived aggregates (Ploug et al. 2008b; Iversen et al. 2010). Sinking speeds of aggregates were found dependent on aggregate source, density, and age, rather than on the size of the aggregates (Ploug et al. 2008b). However, sinking speeds measured in laboratories indicate the potential maximum sinking speeds of aggregates and the residence times of aggregates in situ will likely be much longer in the upper ocean than estimated from laboratory experiments (Alldredge and Gotschalk 1988). This is due to the increased retention times caused by physical processes, e.g., turbulence, and biological processes, e.g., consumption, dissolution, and disaggregation. The biological retention processes in the upper ocean was estimated from changes in carbon fluxes with increasing
depths (Iversen et al. 2010). These estimates revealed the main carbon removal occurred in the upper 220 m off Cape Blanc, and could be divided into two important processes, one between 20 and 80 m and one between 80 and 220 m. Mesozooplankton organisms dominated the carbon removal in the depth layer between 20 and 80 m, and below 80 m the carbon removal was dominated by bacterial respiration and hydrolysis. At depths between 220 and 2500 m very low carbon removal rates were observed and assumed via bacterial activity which seemed limited potentially due to temperature decrease, decrease in bacterial abundance and/or activity, or detachment of bacteria (Iversen et al. 2010). This indicated that carbon escaping the upper 220 m was likely to settle to the deep ocean where it can be sequestered. Therefore, a short residence time in the upper ocean seems important for carbon export and deep-ocean carbon sequestration. Thus, ballast minerals was concluded an important factor controlling aggregate sinking speed and carbon export in the ocean (Ploug et al. 2008a; Ploug et al. 2008b; Iversen et al. 2010).
Experimental investigations of fecal pellet:
1. Measurements of carbon remineralization in copepod fecal pellets (using microsensors).
2. Sinking rates of fecal pellets; comparing of measurements versus estimations via model calculations.
3. The role of zooplankters in particle remineralization. (e.g.
fragmentation (coprorhexy) or ingestion (coprophagy) of particles).

Experimental investigations of marine snow aggregates:
4. Organic carbon degradation in marine snow aggregates (using microsensors and Winkler titration).
5. The importance of scavenging of ballast material for sinking velocities.
6. Observations of coccolithophore in aggregates to estimate the importance of formations of micro environments in marine snow (potentially leading to carbonate dissolution).
9. Estimates of field abundance, content, and activity of marine snow aggregates.