Fractal geometry of marine snow and other biological aggregates

Fractal dimensions of aggregates can potentially be used to classify aggregate morphology as well as to identify coagulation mechanisms. Microbial aggregates of Zoogloeu rumigera have a cluster fractal dimension of 1.8kO.3 (+ SD), suggesting that these aggregates are formed through cluster-cluster coagulation. An analysis of size-porosity correlations for two types of marine snow aggregates yielded fractal dimensions of 1.39kO.06 and 1.52kO.19, which were lower than values describing inorganic colloidal aggregation. Large, amorphous aggregates, known as marine snow, compose the largest component of mass sedimenting through the water column (Fowler and Knauer 1986). These aggregates consist of diatoms, bacteria, fecal Acknowledgments This work was supported by the Donors of The Petroleum Research Fund (administered by the American Chemical Society) and ONR contract N00014-88K0387. pellets, cast houses of appendicularians, and nearly all other microscopic organic and inorganic matter present in the ocean. The manner in which these larger aggregates are formed is not well understood. The growth of microaggregates (< 0.5 mm) and marine snow-sized aggregates (> 0.5 mm) can result from Brownian motion, eddy diffusion, shear coagulation, and differential sedimentation, as well as through growth of organisms within the aggregate microhabitat. McCave (1984) has calculated that shear rates in the ocean are too low to account for observed, steady state particle size distributions. Although this conclusion implies that physical mechanisms of aggregate formation are less important than biological mechanisms in marine systems, marine snow aggregates have macroscopic morphologies that are characteristic of inorganic aggregates formed through physical coagulation mechanisms. This similarity, which