Temperature and seasonal occurrence patterns of 30 dominant phytoplankton species in Narragansett Bay over a 22-year period (1959-1980)

A 22-yr series of quantitative, weekly phytoplankton samples collected in Narragansett Bay, RI (USA) established patterns in temporal occurrence and abundance of 138 taxa. Patterns exhibited by the 30 species most abundant numerically were evaluated by principal components analysis. Their seasonal occurrence ordinated along seasonal gradients which corresponded to annual trends in water temperature and, secondarily, light intensity. Ordination along an in situ temperature gradient was generally consistent with the expected influence of temperature on cell division rates, based on laboratory experiments. Several lines of evidence contest, however, that their succession in Narragansett Bay was determined primarily by temperature: annual field maxima of the individual species occurred at mean temperatures (22 yr data set) 3 O to 14 OC lower than their laboratory growth optima; several species usually disappeared in situ at temperatures below their laboratory optima; annual maxima for the various species occurred over a wide temperature range, with the minimum and maximum range being 7 and > 23OC, respectively; temperature accompanying the first detectable annual occurrence of the species (= entry temperature) varied considerably between years; and resting spores of winter species apparently do not survive summer bottom water temperatures in Narragansett Bay. These observations raise a number of questions as to the actual role of temperature in regulating phytoplankton species succession in Narragansett Bay. Other environmental factors (biotic and/or abiotic) may exert a more direct influence on this ecological process, or through interaction with temperature.