Long-term trends in water clarity revealed by Secchi-disk measurements in lower Narragansett Bay

Observations of Secchi-disk depth and related parameters were made weekly, from 1972 through 1996, in lower Narragansett Bay, Rhode Island. A significant increase in Secchi depth. at a linearized rate of 0.05 m yr(-1), was detected during this 25-year time series. Most of the increase in water clarity occurred between 1984 and 1994, when loading of total suspended solids (TSS) to the Bay via waste-water treatment plants decreased ca. 75%, and Secchi depths increased ca. 25%. Decreased annual TSS loading explained 43% of the variance in mean annual Secchi depth between 1983 and 1995. In addition to a long-term increase, there was also a strong seasonal pattern in water clarity. The rate of increase in Secchi depth in Narragansett Bay between 1972 and 1996 was similar to rates observed in other coastal waters that have had recent reductions in anthropogenic TSS loading. (C) 1998 International Council for the Exploration of the Sea.