This is the third of a four day lesson plan in the Unit Plankton Ecology. This lesson explores the consequences of a heterogeneous ocean on phytoplankton. Spatial heterogeneity in the ocean arises as a consequence of distance from shore as well as depth: water column stratification greatly affects the ecology and species distribution of phytoplankton. The distribution, population growth rates and general ecology of phytoplankton is explored as a consequence of the heterogeneity of water masses. Students will learn how the environment determines a species distribution and persistence, with a final discussion and understanding that how the ocean changes will include drastic changes to the biological community it holds within.