From microstructures to macroscale carbon export : influences of marine snow composition on settling velocity and microbial colonization

Sinking marine aggregates are an integral component of the marine biological carbon pump. The specific focus of this thesis was on techniques to study the composition and sinking behavior of single aggregates collected in situ. Thin-sectioning of aggregates in soft and hard embedding media was used to examine the structure and colonization patterns of aggregates at high spatial resolution. This enabled detailed characterization of the aggregate matrix and localization of selected bacterial colonizers. To study the effect of aggregate composition on sinking velocity, I used Particle Image Velocimetry to analyze the fluid flow around settling aggregates in a flow chamber. Aggregate flow fields strongly resembled those of impermeable, porous spheres which suggested that Stokes law is applicable to settling marine aggregates. The influence of aggregate composition on export flux was studied in a laboratory experiment through incorporation of plastic microfibers into settling aggregates. Microfiber addition decreased potential export flux by 30-60% and, if confirmed in situ, may substantially reduce the efficiency of the biological carbon pump.