Maximum phytoplankton concentrations in the sea

A simplification of plankton dynamics using coagulation theory provides predictions of the maximum algal concentration sustainable in aquatic systems. These predictions have previously been tested successfully against results from iron fertilization experiments. We extend the test to data collected in the North Atlantic as part of the Bermuda Atlantic Time Series program as well as data collected off Southern California as part of the Southern California Bight Study program. The observed maximum particulate organic carbon and volumetric particle concentrations are consistent with the predictions. The results imply that physical processes control maximum particle concentrations in planktonic systems. Planktonic systems in the ocean are controlled by a myriad of physical, chemical, and biological processes. Coagulation, known to be important in the dynamics of clouds, air pollution, and water treatment, is emerging as important in controlling particle dynamics in the ocean as well. Coagulation is the process by which small particles collide and stick to form new, larger particles. By many such collisions, material originally colloidal in size, as well planktonic algal cells, can become visible to the naked eye. Particle size is an important property because it controls the rate at which material falls out of the surface layer. Understanding the factors that control it is important for our understanding of the ocean. Physical coagulation theory describes the rates at which different mechanisms bring particles together to calculate the rates at which particle size distributions change and particles fall. Such mechanisms depend on the sizes and concentrations of the colliding particles. This concentration dependence has led to an emphasis on describing the role of coagulation in regulating the maximum concentration of an algal bloom: coagulation rate increases with particle concentration squared and will eventually balance algal growth rate and thus puts an upper limit on cell concentration. For such a rapidly growing, initially monodisperse system, the maximum algal concentration attainable can be simply estimated as a function of the cell growth rate, the turbulent shear rate, the probability of two colliding particles sticking (‘‘stickiness’’), and the algal diameter (initially derived in Jackson 1990, corrected in Jackson 2005). The applicability of the maximum algal concentration has been tested in natural phytoplankton blooms (Riebesell 1991a,b; Kiørboe et al. 1994) as well in more recent iron addition experiments (Boyd et al. 2002, 2005; Jackson et al. 2005), and coagulation theory has generally proved efficient in predicting maximum algal