Time-series observations during the low sub-surface oxygen events in Narragansett Bay during summer 2001

Abstract

A series of automated water column time-series measurement systems has been established in Narragansett Bay, Rhode Island. These systems measure near surface and near bottom temperature, salinity, oxygen, pH, chlorophyll and pressure at 15-min intervals. The data obtained from two buoy sites during the period of July through September 2001 reveal the occurrence of episodic surface phytoplankton blooms followed by subsurface hypoxic events, particularly in the upper portions of the estuary including the area known as the Providence River. Three hypoxic events occurred at monthly intervals in July, August and September. Their timing, and that of the phytoplankton blooms that preceded them, is linked to the periodic weak neap tidal cycles that occur alternately with somewhat stronger neap tidal cycles. The connection between surface blooms/subsurface hypoxia and tidal range is attributed to water column stratification and the ability of moderate changes in tidal amplitude to reduce stratification through vertical mixing. During the summers of 2002 and 2003 we predicted and subsequently observed hypoxic events in the Upper Bay. Based on these findings we can project into future years the times when summer blooms and hypoxia are most likely to occur in the upper portions of Narragansett Bay. The observations from summer 2001 suggest that the oxygen consumption and renewal in the subsurface waters is delicately balanced. Further increases in inputs of nutrients, organic matter, or oxygen-consuming substances could shift this balance from hypoxic to anoxic with substantial impacts on fish and other marine organisms.