Effect of mixed layer depth on phytoplankton removal by coagulation and on the critical depth concept

Coagulation theory predicts that there is a maximum particle concentration that is sustainable in an algal bloom; the critical depth theory of Sverdrup [Sverdrup, H.U., 1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil/Conseil International pour l’ Exploration de la Mer 18, 287–295.] predicts a different maximum particle concentration based on light-limitation of phytoplankton growth. This paper uses a model incorporating both coagulation and light-limited algal growth to predict the effect of mixed layer depth and wind velocity on maximum phytoplankton populations. As part of this process, the critical concentration parameter is compared to the results of full coagulation simulations. Including phytoplankton aggregation decreases the maximum phytoplankton concentration but increases the rate of particle export through sedimentation. The results help explain field observations made in the Antarctic Ocean. They also suggest that the accuracy of the critical concentration parameter depends on the average shear rate.