Multidecadal (1959–1997) changes in Skeletonema abundance and seasonal bloom patterns in Narragansett Bay, Rhode Island, USA

A 38-year time series (January 1959 to May 1997) of weekly observations of abundance of the marine diatom Skeletonema spp. and related plankton habitat parameters in lower Narragansett Bay, Rhode Island was compiled and analyzed. A statistical change point test identified two different abundance regimes characterized by a ca. 45% decline in Skeletonema abundance. In the first 260 months of the time series (January 1959 to August 1980), the mean deseasonalized Skeletonema abundance was 2137 cells ml− 1, which declined to 1128 cells ml− 1 in the final 201 months (August 1980 to May 1997) of the time series. The decline was greatest during the winter–spring bloom period; Skeletonema abundance in March declined from a mean of ca. 3300 cells ml− 1 prior to the change-point to ca. 700 cells ml− 1 after the change point. onema exhibited three types of annual abundance patterns: winter–spring, summer and autumn bloom peaks. A decline in winter–spring Skeletonema abundance was part of a shift away from winter–spring bloom dominated annual cycles in the 1960s to summer bloom dominated annual cycles in the 1990s. Of 25 years suitable for analyses, Skeletonema winter–spring bloom dominated in 12 years, summer blooms dominated in ten years and autumn blooms dominated three years. Winter–spring Skeletonema bloom years tended to be bright, windy, cold, and have lower copepod (Acartia hudsonica) abundance in the first quarter, and were cool and had high A. hudsonica abundance in the fourth quarter. In contrast, during summer and fall Skeletonema bloom years the first quarters were darker, warmer, less windy and accompanied by higher first quarter A. hudsonica abundance. In summer and fall bloom years the fourth quarters were warm and had above-mean river flow and low A. hudsonica abundance. The observed first quarter environmental differences between winter–spring and summer–fall bloom years (i.e., water temperature, wind, light) may be partially regulated by changes in weather induced by large-scale atmospheric circulation patterns. Years in which the North Atlantic Oscillation (NAO) index was relatively low (mean = − 1.4) tended to have colder winters, and winter–spring bloom dominated Skeletonema annual cycles; years with high NAO index (> + 1.1) featured warmer winters and summer or autumn Skeletonema blooms.