David G. Borkman

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 ~0.60×10(6)kgmonth(-1) to ~0.15×10(6)kgmonth(-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.

Changing Acartia spp. phenology and abundance during a warming period in Narragansett Bay, Rhode Island, USA: 1972–1990

The abundance of dominant zooplankton, the copepods Acartia hudsonica and Acartia tonsa, was observed for 19 years (1972–1990) as part of a long-term weekly monitoring program in a north temperate estuary, lower Narragansett Bay, Rhode Island, USA. Acartia abundance data were analyzed via time series and phenology analyses to detect changes in abundance and timing (phenology) of the seasonal appearance, disappearance and duration of the cryophilic (A. hudsonica) and thermophilic (A. tonsa) Acartia congeners. No long-term changes in A. hudsonica abundance (long-term mean abundance of 9244 animals m) or phenology were detected during 1972–1990. However, deseasonalized mean A. tonsa abundance increased 2-fold from near 2500 animals m in the early 1970s to near 5000 animals m in the late 1980s. Acartia tonsa also displayed changes in the timing of its annual appearance and departure as observed at the lower Narragansett Bay monitoring station. During 1972–1990, the time of A. tonsa first appearance in the spring advanced by ~3 weeks moving from Week 25 (mid-June) in the early 1970s to Week 22 (mid-May) in the late 1980s. Additionally, A. tonsa tended to depart from the plankton later in the year in the late 1980s (departure in late February) compared to the departure time observed in the 1970s (mid-January). Together, these changes resulted in an increased duration of A. tonsa presence in the water column, with A. tonsa duration increasing from 31 weeks in the early 1970s to 38 weeks in the late 1980s. These changes in thermophilic A. tonsa abundance and phenology appear to have been initial, stimulatory responses to warming that were subsequently modified by changes in the phenology and abundance of gelatinous zooplankton predators.