Gulf Stream position and winter NAO as drivers of long-term variations in the bloom phenology of the diatom Skeletonema costatum "species-complex" in Narragansett Bay, RI, USA

Abstract

Bloom behavior of the Skeletonema costatum species-complex, a keystone diatom cluster, in Narragansett Bay (NBay), USA, during a 39 - year (1959-1997) time series was statistically related to variations in the path and latitudinal position of the north wall of the Gulf Stream (GSNW). Winter-spring abundance of the Skeletonema complex was ca. 10-fold greater in years when the GSNW was displaced to the south; summer abundance was 3-fold higher in years of southerly displaced GSNW flow. The relationships with the North Atlantic Oscillation (NAO), the influence of temperature on growth and grazing of Skeletonema and the advective connectivity of NBay with contiguous regional waters potentially impacted by GSNW dynamics are evaluated. The two major, regionally coupled ocean-atmosphere systems, the NAO and the GSNW, differ in their seasonal and interannual impacts and do not directly explain the specific causes of the associated variations in bloom behavior of the Skeletonema complex. Summer bloom behavior was correctly forecast in 14 of 17 years based on advanced predictions of GSNW position at its extreme northern (GSNWI > + 0.5) and southern (GSNW < - 0.5) latitudinal limits.