OLISTHODISCUS LUTEUS (CHRYSOPHYCEAE). V. ITS OCCURRENCE, ABUNDANCE AND DYNAMICS IN NARRAGANSETT BAY, RHODE ISLAND1

Abstract

ABSTRACTThe distribution and abundance of the flagellate Olisthodiscus luteus Carter were examined over a 17 yr period (1959–75) at one station in lower Narragansett Bay, Rhode Island, and at many stations throughout the Bay during the 1974, 1975 summer bloom periods. A bimodal pattern of maximum abundance characterized the 17 yr data set. Maxima (103–104 cells·ml−1) occurred during June–July and October–November separated by a mid summer (August) minimum of <102 cells·ml−1. This species was also absent from surface waters during the winter months (January–February) of each year. Regional abundances reached maximal densities of >105 cells·ml−1 during June 1974 and were confined to the northern and western regions of Narragansett Bay. Incident light, temperature, salinity and nutrient (NH4+, NO3−, NO2−, PO43‐) levels were also determined. The abundance cycle appears to be strongly effected by temperature, life cycle, motility, excreted substances and, to lesser extent, by light intensity, salinity, and nutrient dynamics. Regional distribution is affected by motility, wind forced water movements and benthic stage formation. The autecological data for O. luteus, pertinent observations of herbivorous zooplankton and the field observations are used to assess causative factors regulating the dynamics of O. luteus populations in Narragansett Bay. Pratt's hypothesis on the interaction between this species and Skeletonema costatum (Grev.) Cleve is reexamined.