Distribution and transport of marine snow aggregates in the Panama Basin

Abstract

Depth profiles of marine snow aggregate abundance were acquired using a photographic technique at several stations in the Panama Basin in order to look at regional and temporal variations in aggregate abundances. Concentrations were generally highest in the surface water and adjacent to the sea floor, with maximum abundances varying from 4 to 6 aggregates l −1. In areas adjacent to the basin margin, subsurface maxima of aggregate concentrations were observed. Time-series samples from sediment traps show that sediment flux (mg m −2 day −1) varied by a factor of 6 over the 28 day deployment. Aggregate abundance, in contrast, varied only by a factor of 3 with depth and little over time scales of hours to weeks. Mass fluxes and suspended mass concentrations estimated from aggregate abundances using published conversion parameters were orders of magnitude larger than those measured by sediment traps and submersible pumps. These results suggest that aggregates detected by the survey technique do not directly produce the flux of particles collected by the sediment traps. In the proposed particle transport model, suspended aggregates (produced in situ or resuspended from the sea floor and transported laterally) settle at negligible rates and produce the depth-varying signal in the abundance profiles. Fast-sinking (and consequently rare) aggregates fall from the surface, collide with suspended aggregates, increase their sinking speed and scavenge them from the water column. The flux signal observed in the sediment trap collections results from a combination of the time-varying direct surface-derived flux of fast-sinking aggregates and the depth-increasing concentration of suspended (scavenged) aggregates. These results indicate that aggregate size, as determined by the photographic technique, may be a poor indicator of sinking speed and that fluxes calculated from profiles of suspended aggregates may show poor relationships to actual fluxes.