Analysis and simulation of Skeletonema costatum (Grev.) Cleve annual abundance patterns in lower Narragansett Bay 1959 to 1996

A 38-year time series (1959 to 1996) of weekly observations of abundance of the marine diatom Skeletonema costatum and related plankton habitat parameters in lower Narragansett Bay, Rhode Island was analyzed. A phytoplankton growth model was then developed for investigation of Skeletonema annual abundance pattern regulating mechanisms. A ca. 45% decline in Skeletonema abundance was detected, from 2,292 cells ml−1 (prior to September 1974) to 1,263 cells ml−1 (after August 1980). Winter-spring decreases were greatest, with declines in March Skeletonema abundance from near 3,400 cells ml−1 before a 1977 change-point to mean abundance of ca. 1,000 cells ml−1 after 1977. Three types of Skeletonema annual abundance patterns (winter-spring [w-s], summer, or autumn bloom dominated) were characterized, with the frequency of w-s dominated annual cycles decreasing in the later portion of the time series.; Winter-spring Skeletonema bloom years were bright, windy and cold, with reduced Acartia hudsonica abundance in the first quarter. Summer and fall Skeletonema bloom years were dark, warm, and had calm winds and elevated A. hudsonica abundance in the first quarter. Years in which an index of winter weather pattern, the North Atlantic Oscillation Index (NAOI), was low had colder winter water temperature and w-s bloom dominated Skeletonema annual cycles. In elevated NAOI years (mainly during the 1980s and 1990s), warmer winters occurred and summer-fall blooms dominated. Linear regression models suggest that Narragansett Bay Skeletonema annual patterns are a function of both nearfield and farfield control. Initial (week 1) Skeletonema abundance, position of the Gulf Stream north wall and first quarter zooplankton abundance could differentiate between high and low abundance Skeletonema years. Initial Skeletonema abundance and the NAOI distinguished w-s from summer bloom dominated years.; The growth model simulated major features of Skeletonema annual cycles, replicating bloom timing and magnitude. Grazing (Acartia -based zooplankton was dominant) and flushing losses offset Skeletonema growth. In response to warmer winter water temperatures characteristic of the 1980s and 1990s, simulated zooplankton grazing rate and quahog grazing rate increased 25–50 percent during 1959–96. Model results implied that increased winter-spring grazing and reduced immigration were responsible for the observed Skeletonema costatum decline.