Effect of biopolymer concentration on the kinetics of marine snow formation

Abstract

Marine snow floc refers to coagulation of microbes and marine debris in the upper ocean layers, bound together by bio-polymers such as transparent exopolymer particles (TEP) secreted by microbes. The stickiness of TEP plays a crucial role in determining the rate of marine snow floc formation. Additionally, the effect of TEP on the size distribution of marine snow influences the sinking velocity of the flocs. Using a surrogate material system, we study the kinetics of marine snow using a custom-built experimental setup, which allows direct measurement of floc size, floc number density, and floc sinking velocity as a function of TEP concentration. By comparing the experimental floc size with Smoluchowski coagulation theory, we obtain stickiness index, which increases with TEP concentration first, reaches maximum around 0.3 g/L of TEP and decreases upon further increase in TEP concentration. The experimental sinking velocity scales with floc size as $w_s=a{d}^b$, with $b$ ranging from 0.57 to 0.68. The exponent is slightly higher than that of 0.5 expected in the Stokes limit. This study establishes a clear link between stickiness index, sinking velocity, and polymer concentration, providing valuable insights for modelling of marine snow dynamics in deep ocean conditions. These findings contribute to a better understanding of the kinetics of marine snow formation, essential for predicting carbon sequestration within the biological carbon pump.