In situ characteristics of suspended particles as determined by the floc camera assembly FCA

Abstract

The Floc Camera Assembly, or FCA, provides for the in situ size, shape, concentration and settling velocity of marine particles. Through use of floc-attribute theory, FCA data also provide approximations of the in situ excess-density, porosity and mass of flocs, their population characteristics, and aggregation and sedimentation rate. Image analysis using photoCD transfer routines and recently developed image-analysis techniques make possible daily interpretation of hundreds of digital FCA images. FCA studies have demonstrated a wide variety of suspended sediment characteristics and transport phenomena. The character of marine snow is found to vary at a variety of time-scales: near-instantaneous, daily and seasonally. For example a floc's excess density also varies with floc size, but the relationship for flocs within a coastal water mass may change daily. In mid-latitude coastal basins, small flocs form when primary production is low and river input is high; conversely large flocs form when river input is very low. In Antarctic ice margins, flocculation fronts are identified beneath cold water tongues emanating out from glacier margins. In Greenland, an intermediate nephloid layer is identified, filled with large flocs formed from sediment released from melting icebergs. In the Gulf of St. Lawrence, boundary-layer turbulence near the seafloor is shown to break up flocs after their long descent through the water column and create a 50 m thick bottom turbid zone.