Summer formation of a high‐nutrient low‐oxygen pool in Cape Cod Bay, USA

Abstract

The Boston Harbor, Massachusetts Bay (MB), and Cape Cod Bay (CCB) system (MBS) is a semienclosed coastal embayment located in the Gulf of Maine. As the southern portion of the MBS, CCB is connected to MB by the coastal transport that varies seasonally. Observations have revealed that during every summer, nutrients accumulate below the thermocline in central CCB associated with relatively low dissolved oxygen (DO) concentrations. To understand the remote and local causes in the formation of this pool of high‐nutrient and low‐oxygen (HNLO) bottom water, a modeling study was conducted using a coupled physical‐biogeochemical model. A 4‐year simulation (1998–2001) indicated the persistent presence of this HNLO feature. A detailed analysis of modeled and observed results for 2000 suggested that three factors controlled the formation of the HNLO pool in the summer: (1) southward coastal transport of organic matter from MB into CCB in winter and spring, (2) a relatively long residence time in CCB favoring sedimentation of organic matter throughout the year, and (3) high temperature and strong stratification resulting in intensive regeneration (sediment and water column) and accumulation of nutrients below the thermocline in summer and fall. The magnitude and spatiotemporal scales of this HNLO pool were determined by the available amount of organic matter, stability of stratification, isolation of CCB, and wind‐induced lateral transport and mixing. This HNLO pool may significantly impact the biogeochemical processes and ecosystem dynamics in CCB.