EFFECT OF COAGULATION ON NUTRIENT AND LIGHT LIMITATION OF AN ALGAL BLOOM

Coagulation is the formation of large particles from multiple collisions of smaller ones. Because larger particles fall faster than smaller ones, coagulation can be important in accelerating the export of organic matter from the ocean's surface to the deep sea and has the potential to limit phytoplankton populations. We have developed a model of an algal bloom that includes nutrient and light limitation of algal growth rate and coagulation of single algal cells. The results show two effects of coagulation on the growth model. Loss of dividing cells to coagulating particles can occur when algal cells are growing at a fairly constant rate, placing a cap on the concentrations that algae can achieve. The second effect is enhancement of vertical particle flux from the surface mixed layer in aggregates. This enhanced transport moves algal biomass from the surface mixed layer over shorter periods at rates far greater than those associated with settling of single cells. For example, half the material associated with a bloom of 2-mu-m-radius algae, which would take 350 d to settle out as isolated cells, took 42 d as aggregates. Aggregation was not limited to the large algae, although the rate was slower for the smaller ones. Coagulation was enhanced by higher initial nutrient concentrations, deeper mixed layers, and higher shear rates. Vertical transport associated with aggregation has the potential to be an important mechanism for removing biological material from eutrophic regions.