Weekly measurements of Chl a were carried out between 1973 and 1990 in Narragansett Bay, Rhode Island, a temperate estuary along the north-eastern US coast. There was only a threefold variation in mean annual chlorophyll values but a considerably greater variation in the period when the annual maximum was reached. The inter-annual variation in annual means was relatively modest compared with monthly means. For a given month, the inter-annual variability in monthly mean chlorophyll varied from 3- to 30-fold; it was least during the summer months and maximal during the winter–spring bloom period. The long-term variability patterns characterizing individual months differed in their trends, cycles, and irregular fluctu-ations. Major blooms occurred during most of the year, and were not restricted to the annual winter–spring bloom. However, the frequency and magnitude of blooms were higher during winter–spring months than during summer months. A significant finding is that the annual mean chlorophyll decreased by more than 1 mg m−2 year−1 over the 18 year time series. This decrease apparently was under multifactorial control, with zooplankton grazing, sea surface temperature and windspeed being the most important factors.