Coincident patterns of waste water suspended solids reduction, water transparency increase and chlorophyll decline in Narragansett Bay

Dramatic changes occurred in Narragansett Bay during the 1980s: water clarity increased, while phytoplankton abundance and chlorophyll concentration decreased. We examine how changes in total suspended solids (TSS) loading from wastewater treatment plants may have influenced this decline in phytoplankton chlorophyll. TSS loading, light and phytoplankton observations were compiled and a light- and temperature-dependent Skeletonema-based phytoplankton growth model was applied to evaluate chlorophyll supported by TSS nitrogen during 1983-1995. TSS loading declined 75% from ~ 0.60 × 106 kg month− 1 to ~ 0.15 × 106 kg month− 1 during 1983-1995. Model results indicate that nitrogen reduction related to TSS reduction was minor and explained a small fraction (~ 15%) of the long-term chlorophyll decline. The decline in NBay TSS loading appears to have increased water clarity and in situ irradiance and contributed to the long-term chlorophyll decline by inducing a physiological response of a ~ 20% reduction in chlorophyll per cell.