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Abstract

Abiotic fragmentation of large, rapidly sinking aggregates into smaller, suspended particles by fluid shear has been suggested as an important process governing the particle size spectrum in the ocean and as one explanation for the exponential decrease of particulate flux with depth below the euphotic zone. We investigated this process by quantifying the small-scale energy dissipation rates required to disaggregate marine snow settling through a gradient of turbulent kinetic energy in a laboratory tank.Aggregates of detrital debris, gelatinous houses of larvacean tunicates, and aggregates of living bacteria did not break apart even at energy dissipation rates > 1 cm2 ss3. The rate of energy dissipation required to disaggregate fragile diatom floes up to 25 mm long ranged from 10m3 to > 1 cm2 se3 and increased exponentially with decreasing maximum aggregate diameter. Aged diatom aggregates were significantly stronger than otherwise identical but unaged particles. These results indicate that only the highest shears associated with storm events or flows in tidal channels would be able to fragment even the most fragile organic aggregates in the upper ocean. Biological processes of disaggregation, such as animal grazing, appear far more likely to mediate the size spectrum of aggregated particulate matter in the ocean than abiotic fragmentation due to fluid motion.The magnitude of particulate flux to the determined by the abundance and sinking ocean interior and the sea floor is largely characteristics of particles in the larger size categories of the particle size spectrum (see of smaller particles of algae, microorganAcknowledgments We thank S. Bernstein and S. Collard for assistance preliminary research which stimulated our initiation with image analysis, T. Deitrich and Instructional De-of this study and S. MacIntyre for comments on the velopment at UCSB for loan of a fresnel spotlight, B. manuscript and for bringing the existence of the USC E. Logan for supplying Zoogloea ramigera cultures, grid-turbulence tank to the attention of A.L.A. and D. S. Parker for comments on the manuscript. We This research was supported by ONR contract especially thank P. McGillivary for collaboration on NOOO14-85K-0771.