Simple coagulation models, such as those for the critical concentration and for determining particle stickiness, have proven remarkably useful by providing simple relationships that can be easily applied to interpret environmental data. Unfortunately, the simple relationships do not work as well when applied to more realistic conditions or accurate coagulation mechanisms.72,73 As a result, the simple mechanisms should be considered semi-quantitative at best.

The number of models of planktonic systems that incorporate coagulation is surprisingly large (Table 13.4). Unfortunately, the range in their formulations is so large that it is difficult to compare and interpret their results. Given the differences resulting from different coagulation kernels, as illustrated above, it is difficult to interpret them together. One response has been to abandon the theoretical structure and instead use values for the interaction kernels determined by fitting results from laboratory experiments.69 While such an approach does fit the laboratory system, it is unclear how to extrapolate the results to different environmental conditions. Despite the importance that the choice of model formulation can have, there has been remarkably little discussion or consensus on the best one to use.

Finding the best way to incorporate disaggregation into coagulation models for marine systems remains an outstanding problem. There have been attempts to do so,34,62,74 but more needs to be done. Among the factors that need to be included are the potential roles of zooplankton75 and any other organisms29 in weakening and sundering marine aggregates.

One of the outstanding questions in aquatic systems is what is the precise role of transparent exopolymeric particles (TEPs). These are organic particles that have been associated with marine particle coagulation.76-78 There have been several different roles assigned to them: extra particles to participate in collisions,63 agents for the changing of particle stickiness,79 and a separate system of coagulating particles.76 Unfortunately for the resolution of the role of TEP on coagulation in natural waters, most TEP studies have focused on the ecological aspects of the material and have not accompanied them with the size spectral and stickiness measurements that could be used to test the various possibilities. In future studies, measurements of particle size spectra in conjunction with observations of TEP concentrations would make it easier to test the role of TEP in coagulation.

Stemmann et al.70,71 have developed a promising approach to test the importance of coagulation, as well as biological processes, in determining particle distributions and fates. The method compares the particle size spectra measured through the water column over time with those expected from different particle transformation processes. This approach would be improved if there were a better quantitative understanding of how organisms, including bacteria and other microorganisms, change the particle properties.

The particle size spectra are probably the most useful measurements that could be made in systems where coagulation is believed to be important. Such measurements should use multiple techniques in order to cover the range of important reactions.34,35 In addition, multiple measurements on the same particles that could be used to test models that invoke multidimensional particle size spectra would also help.44,45

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