Award: OCE-1634319

In continental shelf environments, the bottom boundary layer (BBL) is the portion of the water column where water motions and properties are influenced significantly by the seabed. On the Oregon shelf, the BBL responds to a form of wind-driven circulation that transports water along the bottom predominantly onshore (upwelling) during summer and offshore (downwelling) during winter. In summer, dissolved oxygen concentrations in the BBL also decline to low levels that in recent decades have caused die-offs in groundfishes and invertebrates. This condition, called hypoxia, may be caused by multiple interacting or latent factors that vary between years. During this project, interactions between the physics and biogeochemistry of the Oregon shelf were studied over a span of 20 months. Dynamic rates of oxygen uptake (aka fluxes) were derived from high-frequency, near-seafloor measurements made from autonomous landers at a mid-shelf and an inner-shelf site during ten short research cruises. These sites are also locations where Ocean Observatories Initiative (OOI) infrastructure provides data streams of relevant physical and biogeochemical variables. The energetic oscillatory motions of waves which form sand ripples, pump oxygen into surface sediments and contribute to the generation of turbulence in the BBL, were found to prime the seabed for much higher oxygen utilization rates in winter than in summer. Since oxygen is consumed primarily in the oxidation of organic matter, the winter rates of oxygen utilization indicate that sources of organic matter are retained in, or introduced to, the BBL throughout the year. These findings counter former descriptions of this ecosystem as one where organic matter is largely transported off the shelf via downwelling and particle sinking during winter. This new understanding enables adding rates of local respiration and organic carbon retention, with circulation and stratification conditions, into model predictions of the seasonal cycle of oxygen. Other endeavors of this research included efforts to develop an underwater rotating disc microelectrode as an alternative sensor for sensitive, rapid-response dissolved oxygen measurements. A prototype was produced and taken through extensive laboratory tests, but it did not prove to be reliable during field trials. This experimental work did form the basis of research for a student's Masters thesis and led to a contract with a small business to build the prototype. Two Masters students were supported in part by the project and participated in research cruises. This project also provided unique experiences and professional development for a postdoctoral researcher who became active in a community of early-career scientists pursuing interdisciplinary research based on OOI data. Two undergraduate students did summer Research Experience for Undergraduates (REU) projects connected to the project, and a high school student became a valuable lab member and participated in several research cruises before starting college. Lastly, the repeat cruises executed from the R/V Oceanus for this project had broader impacts on the ocean sciences. These cruises facilitated the development, installation and testing of a ship-to-shore data sharing and logging system being designed for the next generation of coastal Regional Class Research Vessels being constructed by the National Science Foundation. Cruise data also served to either verify or identify issues with OOI sensors, and cruises facilitated additional sampling opportunities for several colleagues and their students including scientists from the National Oceanographic and Atmospheric Administration (NOAA). Data products are being made public through the Biological and Chemical Oceanography Data Management Office located at Woods Hole Oceanographic Institution and through publication in peer-reviewed journals. Last Modified: 01/04/2021 Submitted by: Clare E Reimers