In-situ characteristics of particles settling within a deep-water estuary

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 1995, Vol.42 (1), p.223-256
• Main Authors: Syvitski, James P.M., Asprey, K.W., Leblanc, K.W.G.
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/0967-0645(95)00013-G

A Floc Camera Assembly is used to obtain the in-situ size, shape, concentration and settling velocity of marine particles, providing insight into the in-situ excess-density, porosity and mass of flocs, their population characteristics, and aggregation and sedimentation rate. Data highlight the time-dependent character of marine snow, including near-instantaneous, daily and seasonal influences for a mid-latitude estuary, Halifax Inlet. Methods are developed for estimating the mean size of the constituent particles comprising flocs, for determining the number spectra of non-flocculated and flocculated particles, and for determining the mass concentration and mass flux spectra for suspended flocs. The most active aggregation level is just below the estuarine surface layer. The largest aggregation rates involve the finest (< 10 μm) suspended particles colliding by both differential settling and turbulent shear. During plankton blooms, large floes become attached to the mucoid stringers. The settling velocity of stringers is, in turn, controlled by these large floes attached to their base. Floc settling velocity varies with floc size, and as a seasonally-variable power function. A floc's excess density also varies with floc size, but the relationship changes daily. Floes account for between 36% and 100% of the total suspended mass within Halifax Inlet, and >90% of the mass below the surface layer. Floc concentrations may change by a factor of four in just a few minutes, affected by the flux of larger floes. When river input of suspended load is high, floc concentration is also high, but the portion of total particulate matter composed of floc is low. Mean floc size varies considerably between seasons and sometimes between estuarine location. Small floes form when primary production is low and river input is high. Large flocs form when river input is very low. Suspended stringers are highest during the spring freshet and associated plankton bloom.