Fractal dimensions of small (15-200 μm) particles in Eastern Pacific coastal waters

Abstract

Particles 3-300 μm (average length) in seawater include single cells, non-viable particles of identifiable origin (such as fecal pellets), aggregated particles formed from water column debris, and aggregated mixtures of all of these materials. While macroscopic marine snow-sized aggregates (>0 5 mm in average length) have been shown to be fractal, relatively less is known about the average characteristics of smaller particles. We calculated the fractal dimensions of microscopic particles 15-200 μm in length through simultaneous measurements of particle size distributions as a function of solid equivalent diameter (from solid volumes measured using a Coulter Counter) and average length (from image analysis of acridine-orange stained filtered particles) Particle size distributions were measured at two eastern Pacific coastal areas, one in Monterey Bay, CA, and the other in East Sound, WA Average fractal dimensions of particles indicated that D was highest in East Sound (D = 2.59 ± 0 17) during a phytoplankton bloom that did not appear to be aggregating, and lowest at one site in Monterey Bay (D = 1 77 ± 0 34), where old diatom flocs and marine snow-size aggregates were observed. There was no direct relationship between D and total particle concentration, chlorophyll a, or transparent exopolymer particles (TEP) concentration, although the highest concentration of TEP was found at the site with the lowest fractal dimension. Particles with low fractal dimensions are produced through coagulation. Our subjective assessment of the importance of aggregate formation at these sites, based on diving and microscopic observations, indicated that aggregates were more abundant at sites where particles had lower fractal dimensions. Thus, we attribute the low fractal dimensions of these small particles to be the result of their formation through coagulation processes.