Thirty‐One Years of Warming and Oxygen Decline in Massachusetts Bay, a Well‐Flushed Non‐Eutrophic Temperate Coastal Waterbody

Abstract Warming temperatures and declining oxygen are well‐studied in eutrophic coastal systems. To help broaden understanding we investigate non‐eutrophic Massachusetts Bay, a representative well‐flushed temperate embayment with seasonal thermal stratification, minor river inputs, and residence time of about a month due to exchange with the Gulf of Maine. Long‐term trends are computed from de‐seasoned measurements over 31 years (1992–2022) at nine bay sites, each sampled at five depths from sea surface to seafloor six times annually. For averages across all stations and depths, the mean warming and oxygen decline rates are +0.0582 (95% C.I. +0.0410 to +0.0759) °C yr −1 and −0.750 (−0.985 to −0.524) μmol kg −1 yr −1 [−0.0246 (−0.0323 to −0.0171) mg L −1 yr −1 ]. The observed warming reduces oxygen solubility, per seawater properties, at a rate about half the observed oxygen decline. Rates do not differ significantly by site, including near a seafloor outfall releasing treated wastewater effluent, nor at different depths. Because warming is comparable at all depths there is no significant trend in stratification. There is a significant trend of increasing salinity, similarly uniform across sites and depths. A strong hypothesis to explain the warming and salinity trends is advection of Gulf of Maine water into the bay, rather than local influences that could be expected to cause spatially differing rates contrary to those observed. Advection is also a plausible explanation for the observed oxygen decline beyond that due to the solubility decrease. This study highlights potential influence of external ocean forcing on long‐term trends in a coastal waterbody.