Temporal patterns and variations in phytoplankton community organization and abundance in Narragansett Bay during 1959–1980

Incident irradiance, surface water temperature and phytoplankton species abundances were measured at weekly intervals in Narragansett Bay from 1959 through 1980. Stepwise discriminant analyses (SDA) of this 22-year data set indicate that fundamental ecosystem changes occurred between the 1960s and 1970s, with 1969 being the key transitional year in these decadal shifts in phyto plankton taxonomic structure and seasonal abundance. This decadal shift was accompanied by the increased summer abundance of small Thalassiosira spp., which first appeared in 1966 and by 1969 became the sixth most important phytoplankton component in this bay. Decadal trends in phyto plankton community organization and abundance were also accompanied by distinct long-term climatological gradients of temperature and light. The 1960s were generally colder and brighter than the 1970s. Prior to 1969, the annual phytoplankton maximum occurred most commonly during winter; in the 1970s, the annual maximum generally shifted to a summer event. Three 5-year phytoplankton cycles occurred between 1959 and 1974. During each pentade, the phytoplankton community returned to a similar taxonomic organization and abundance cycle after diverging in the intervening years. Pentade cycles did not occur after 1974; the phytoplankton community thereafter diverged significantly from each preceding year. Five species [Skeletonema costatum, Detonula confervacea, Asterionellopsis glacialis, Hererosigma akashiwo (= Olisthodiscus lureus) and Thalassiosira nordenskioeldii] dominated the phytoplankton over the 22-year period. SDA revealed a high degree of similarity and constancy in the annual occurrence patterns of these taxa. The decadal shifts revealed by SDA were more directly related to the considerable interannual variability that characterized the abundance and seasonality of the less abundant species.