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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.

Fine-scale spatial effects of a seagrass meadow on suspended particle transport were assessed from current speeds, orbital wave velocities, turbulent Reynolds stress, in situ particle concentrations, and sedimentation rates for a horizontal grid in a coastal seagrass (Posidonia oceanica) meadow at 2 depths and during low-and high-energy periods. For the low-energy period, the vertical reduction of the total kinetic energy, from 100 cm to ≈10 cm above the bottom, was larger in the meadow (up to 95%) than over the sand (35 to 75%). Velocity maps suggest that a recirculating flow formed in the meadow with a higher Reynolds stress at the edge of the meadow. Near the bed, concentrations of small particles (<10 µm diameter) were lower inside the meadow than over barren sand, while concentrations of large particles (>10 µm) were lower over the barren sand. For the period of stronger current and wave activity following a storm, nearbed turbulence and orbital wave velocity were elevated, though still lower inside the meadow than over the sand. For this high energy period, particle concentrations increased over the whole study area, but were still lowest deep inside the meadow. Overall, the horizontal spatial distribution of plants in the study area had a profound effect on the flow field and on vertical transport, even during the high-energy period. The reduced nearbed turbulence and lower sedimentation rate below the canopy confirms it as a calm zone with lower mixing compared to unvegetated areas. KEY WORDS: Seagrass meadow · Flow fields · Particle transportResale or republication not permitted without written consent of the publisher

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A Lagrangian analysis of a particle sinking through a random mesoscale eddy field is used to evaluate the effects of horizontal diffusion and particle sinking rates on particulate fluxes sampled by an idealized sediment trap. The analysis indicates that the spatial region where collected particles are formed (