A literature review demonstrates that crude oil spills can affect phytoplankton, favoring the growth of some while inhibiting the growth of others. Subsequently, the phytoplankton assemblage can change as a result of exposure to crude oil. Studies of phytoplankton responses to the Macondo (Deepwater Horizon) oil spill indicate that the phytoplankton may have been stimulated by the oil spill, although the presence of low-salinity water in the region makes it difficult to discount the importance of riverine-borne nutrients as a factor. A few studies suggest that the oil spill was toxic to some phytoplankton species, whereas others indicate that the degree of tolerance to the oil or to dispersants differs among species. These results generally comply with findings of previous studies, but a lack of published field data analyses prevents further assessment of the impacts of the Deepwater Horizon oil spill on phytoplankton population dynamics in the northern Gulf of Mexico.