21 Feb 2022
21 Feb 2022
Status: this preprint is currently under review for the journal BG.

Unprecedented Summer Hypoxia in Southern Cape Cod Bay: An Ecological Response to Regional Climate Change?

Malcolm E. Scully1, W. Rockwell Geyer1, David Borkman2, Tracy L. Pugh3, Amy Costa4, and Owen C. Nichols4 Malcolm E. Scully et al.
  • 1Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
  • 2Rhode Island Department of Environmental Management, Providence, RI, 02908, USA
  • 3Massachusetts Division of Marine Fisheries, New Bedford, MA, 02744, USA
  • 4Center for Coastal Studies, Provincetown, MA, 02657, USA

Abstract. In late summer 2019 and 2020 bottom waters in southern Cape Cod Bay (CCB) became depleted in dissolved oxygen (DO), with documented benthic mortality in both years. Hypoxic conditions formed in relatively shallow water where the strong seasonal thermocline intersected the sea floor, both limiting vertical mixing and concentrating biological oxygen demand (BOD) over a very thin bottom boundary layer. In both 2019 and 2020, anomalously high sub-surface phytoplankton blooms were observed, and the biomass from these blooms provided the fuel to deplete sub-pycnocline waters of DO. The increased chlorophyll fluorescence was accompanied by a corresponding decrease in sub-pycnocline nutrients, suggesting that prior to 2019 physical conditions were unfavorable for the utilization of these deep nutrients by the late summer phytoplankton community. It is hypothesized that significant alteration of physical conditions in CCB during late summer, which is the result of regional climate change, has favored the recent increase in sub-surface phytoplankton production. These changes include rapidly warming waters and significant shifts in summer wind direction, both of which impact the intensity and vertical distribution of thermal stratification and vertical mixing within the water column. These changes in water column structure are not only more susceptible to hypoxia, but also have significant implications for phytoplankton dynamics, potentially allowing for intense late summer blooms of Karenia mikimotoi, a species new to the area. K. mikimotoi had not been detected in CCB or adjacent waters prior to 2017, however increasing cell densities have been reported in subsequent years, consistent with a rapidly changing ecosystem.

Malcolm E. Scully et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-48', Anonymous Referee #1, 13 Mar 2022
    • AC1: 'Reply on RC1', M. E. Scully, 10 May 2022
  • RC2: 'Comment on bg-2022-48', Anonymous Referee #2, 25 Apr 2022
    • AC2: 'Reply on RC2', M. E. Scully, 10 May 2022

Malcolm E. Scully et al.

Malcolm E. Scully et al.


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Latest update: 26 May 2022
Short summary
For two consecutive summers, the bottom waters in southern Cape Cod Bay became severely depleted of dissolved oxygen. Low oxygen levels in bottom waters have never been reported in this area before, and this unprecedented occurrence is likely the result of a new algae species that recently began blooming during the late summer months. We present data suggesting that blooms of this new species are the result of regional climate change including warmer waters and changes in summer winds.