Comparing observed changes in particle size spectra with those predicted using coagulation theory

The increasing appreciation of the importance of aggregates in transport of material through the ocean makes understanding their formation and movement crucial. Coagulation theory has the potential to explain key aspects of oceanic particle dynamics. Unfortunately, there are uncertainties about the best way to formulate coagulation theory in marine systems. A mesocosm experiment in which members of the SIGMA (Significant Interactions Generating Marine Aggregates) program grew marine phytoplankton and observed the changes in particle size spectra provides information needed to test different formulations of coagulation rates. Computer simulations incorporating coagulation theory were able to predict aspects of the particle size spectral evolution. The rectilinear formulation of coagulation kernels over-predicted coagulation rates, particularly because of high interaction rates between particles of much different sizes. The curvilinear formulation worked better, especially if used in conjunction with a disaggregation model. Disaggregation was a very important process regulating particle size spectra that was necessary to include in the simulations. The derived disaggregation rates were consistent with past observations of marine snow disaggregation. The simulations in which coagulation and disaggregation predicted the observed results best were those for which the dominant aggregation was between similar sized particles. The largest discrepancies between observed and predicted particle size spectra occurred when theory predicted that dominant interactions were between widely different particle sizes. While coagulation theory was useful in predicting particle size spectra for this experiment, there are still improvements needed in the theory.