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

Abstract

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 L Ϫ1) and as different minerals (50 mg L Ϫ1 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 Ͼ3. 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.

Aggregation as a function of and mineral particles

“…In our main experiment here, illite helped to drive coagulation, measured as total area of aggregated material or as aggregate number, which was highly significantly positively correlated with illite concentration (n 5 14, p 5 0.0005). That the concentration of illite would affect the size and number as well as characteristics of aggregates was expected from previous experiments (Hamm 2002;Passow and De La Rocha 2006).…”

“…The relatively lower TEP content (Engel and Schartau 1999;Azetsu-Scott and Passow 2004) and the higher illite content (Ploug et al 2008a,b;Fischer and Karakas 2009;Iversen and Ploug 2010) of the aggregates formed at high initial suspended concentrations of illite should have increased excess density and hence sinking velocity of aggregates in the high-clay treatments. But incorporation of minerals also reduces the size of aggregates (Passow and De La Rocha 2006;De La Rocha et al 2008), which diminishes sinking velocity (McCave 1975;Hamm 2002;De La Rocha and Passow 2007). In previous experiments, the increase in sinking speed related to additional excess density has generally predominated over the loss of sinking speed due to loss of size, but the overall outcome of mineral addition in a given situation will certainly vary with a number of factors (the size, density, and charge of the minerals, the composition and character of the organic matter, the relative and absolute concentrations of the minerals and the organic matter, and so forth).…”

“…1 mm-TEP and clay have both been suggested as effective catalysts of coagulation ( [Alldredge et al 1993;Logan et al 1995] and [Avnimelech and Troeger 1982;Hamm 2002;Lee et al 2009]) and are important for the formation of the large (. 1 mm) aggregates with high sinking velocities that play a key role in the ocean's biological carbon pump.…”

Ballast minerals and the sinking carbon flux in the ocean: carbon-specific respiration rates and sinking velocity of marine snow aggregates

“…Previous studies of diatom aggregates mixed with minerals, e.g., clays and carbonate, have demonstrated that, on average, these aggregates are smaller than those formed in pure diatom cultures (Hamm, 2002;Passow and De La Rocha, 2006). In those studies, however, sinking velocity was not directly measured, but applying Stoke's law it was argued that the smaller size of mixed aggregates may lead to lower sinking velocities despite their higher content of ballasting minerals as compared to those composed of diatoms, only.…”

Sinking rates and ballast composition of particles in the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean

“…off NW Africa, the role of dust as ballast should be considered. However, laboratory experiments have shown that lithogenic material could even decrease the downward flux of phytoplankton biomass (Hamm, 2002). According to Berelson (2002), particle sinking rates estimated from sediment trap studies are not controlled by the lithogenic contents of particles.…”