In this article the potential paleoecologic boundary conditions are discussed favoring prasinophyte prosperity commonly associated with sediments rich in organic carbon, as exemplified by palynologic case studies from the Northwest European Basin (Lower Jurassic Posidonia shale, Cretaceous Cenomanian-Turonian Boundary Event). Based on the literature of nutrient requirements of modern green algae and the evolution of photic-zone water chemistry throughout earth history, it is suggested that the enhanced availability of reduced nitrogen chemospecies, especially ammonium, in photic-zone waters is the ultimate cause for stimulating prasinophyte productivity. It is also suggested that the erosional suspension of suboxic-to-anoxic pore waters from shelf sediments or the vertical expansion of the redox zones from within sediments or the oceanic oxygen minimum zone are the main mechanisms for introducing reduced nitrogen chemospecies into these waters. Thus, changes in intensity and duration of suboxic-to-anoxic water conditions within the photic zone may be reflected by the corresponding presence and change respectively of significant proportions of prasinophyte phycomata within associated sediments. In addition, with respect to general water mass character, several arguments for a principal cooler water affinity of prasinophytes are suggested: (1) the accumulation of fossil—almost rock-building—prasinophyte deposits in higher paleolatitudes or in close temporal proximity to major global glaciation episodes; (2) the enhanced biochemical stability of ammonium within cooler waters; and (3) the principal modern main distribution of prasinophytes in cool-temperate waters and aquatic ice-covered regions.