The Phytoplankton of Narragansett Bay1

Abstract

The phytoplankton of Narragansett Bay is described on the basis of 927 unpreserved samples taken at a series of stations from 1952 to 1956. The annual cycle is divided into four natural periods: 1 (winter and spring) with diatoms dominant; 2 (usually late spring, early summer), flagellates dominant; 3 (usually late summer), diatoms dominant; 4 (fall), flagellates dominant. The major features of this cycle, with maxima in the first and third seasons, are largely due to changes in the concentrations of diatoms, principally Skeletonema costatum. Although the concentrations of diatoms and of flagellates in any given season are predictable only within wide limits, their average abundance for the entire annual cycle is relatively constant. Average standing crops in the East and West Passages are approximately the same, but the Upper Bay is more than three times as rich as the lower in both diatoms and flagellates. The highly estuarine upper West Passage is characterized by certain neritic dinoflagellates, while the lower East Passage is marked by oceanic diatoms. Microflagellates and Skeletonema costatum are outstanding in frequency of occurrence and as dominants. Each of the four seasons is distinguished by certain dominant species. Correlations of the dominance of the most important species with prevailing temperatures reveal apparent temperature preferences, some species being eurythermal as dominants while other dominants are restricted to narrow temperature ranges. From the first to the third or fourth period there is a progressive increase in the number of species represented, the rate of change in species composition, and the spatial heterogeneity of the population. The average annual standing crop is more than two times as great as that in adjacent coastal waters. This difference is explained by the unusual length of the winter–spring diatom flowering in the Bay (January–May), which results from the combined effects of shallowness, moderate nontidal drift, and turbulence generated by tidal activity. The secondary diatom maximum in the Bay and in the coastal waters of southern New England is typically a summer event and usually precedes the major annual breakdown of stratification in the fall.