Nine contributors bring both the phycological (i.e., botanical) and limnological (i.e., ecological) literature into focus to reveal the morphological, reproductive, and physiological characteristics of these microscopic organisms as adaptive evolutionary strategies for survival in stressful and seasonally changing aquatic habitats. The synthesis of these literatures from an organismal and evolutionary perspective is unique. The following important groups of planktonic algae are considered: cyanobacteria, green algae, diatoms, dinoflagellates, cryptomonads, and chrysophyte flagellates. For each algal group, the importance of both growth and loss processes influencing biogeography and seasonality is emphasized.