The sources of species diversity in plankton samples take it non-clossing nets down to 140 m during two cruises in the southern part of the California Current were examined, and it was found that these sources were various : advection, by the mingling of originally separtmet separate plankton ecosystems, caused species assemblages to occur in the samples that were apparently more diverse than the original ecosystems; when the plankton net crossed ecological transitions the samples resembled those caused by the advective processes; however, these sources of diversity did not obscure the basic relationship between environmental and trophic stability of the ecosystems and their species diversity. The zooplankton of coastal upwelling situations was found to resemble that of oceanic situation in high lattidues by its very low diversity and its predominance of herbivorous forms.