Temporal variability of chlorophyll in Narragansett Bay, 1973–1990

Abstract

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.