Interactive aggregation and sedimentation of diatoms and clay-sized lithogenic material

Monospecific diatom cultures (Thalassiosira punctigera and Skeletonema costatum) were incubated in rotating cylinders together with clay suspensions, present in a range of concentrations (5–100 mg kaolinite L21) and as different minerals (50 mg L21 kaolinite, smectite, illite, and clay-sized quartz powder). The addition of lithogenic suspensions to diatom cultures accelerated the formation of visible aggregates in the roller tanks by a factor of .3. Aggregate size decreased and density increased proportionally to the amount of kaolinite added to the diatom cultures. In the presence of kaolinite and illite, aggregate sizes were smaller and sinking rates lower than in the presence of smectite and quartz. The influence of lithogenic matter on the sinking velocities of aggregates was ambiguous. Compound aggregates sank faster with increasing amounts of lithogenic matter present in cultures of T. punctigera until a certain ratio between lithogenic and biogenic material was reached; further increasing the amount of lithogenic matter did not increase sinking rates significantly. In contrast, increasing the concentration of kaolinite added to cultures of S. costatum could decrease sinking velocities of the evolving compound aggregates. This nonlinear behavior is argued to be primarily a function of aggregate composition on aggregate sizes and excess densities. Although the possibility of a mutual acceleration of vertical flux of algae and clay is confirmed, the results show that the presence of lithogenic material could also decrease the downward flux of phytoplankton biomass. Rapid sinking of phytoplankton-derived organic carbon via sedimentation of aggregates contributes significantly to the vertical transport of marine particulate organic carbon (POC), a process known as the biological pump (Volk and Hoffert 1985). Rapid sinking of clay-sized lithogenic material can be inferred from sediment trap data (Wefer 1989). Laboratory studies on marine aggregate formation and properties have focused on pure phytoplankton aggregates (Kiørboe et al. 1990; Riebesell et al. 1991; Kiørboe and Hansen 1993) or on monomineralic clay aggregates (Kranck 1973). However, coaggregation of phytoplankton and clay is a familiar phenomenon in aquatic environments. First drawings of marine snow show sundry marine organisms and mineral grains embedded in a mucous matrix, which dominates the aggregates by volume and appears to hold the solid compounds together (Tsujita 1953). Avnimelech et al. (1982) described enhanced floc formation and sedimentation of freshwater algae in the presence of clay and vice versa. The fact that lithogenic and biogenic particles are often found in deep sediment traps during the same time interval in spite of independent occurrence in the surface water also suggests coaggregation and cosedimentation of these particles (Deuser et al. 1983; Wefer 1989; Ittekkot 1993). While small amounts of both biogenic and lithogenic particles are present in all marine environments, areas with elevated concentrations of lithogenic particles often show a high phytoplankton abundance at the same time and are thus particularly interesting in terms of interaction between phytoplankton and lithogenic material. Examples for marine en-