Within the last decade, broad sections of the Oregon-Washington continental shelf located in the northern California Current System have been affected by severe hypoxia during the summer upwelling season. Mass balances of dissolved oxygen on the shelf indicate oxygen uptake of sediments is greater than estimated from benthic flux measurements using traditional benthic chambers and microprofiles. The most probable explanation for this inbalance is uncharacterized temporal and spatial variability in the major physical and biological processes contributing to on-shelf oxygen utilization. A scientist from Oregon State University (OSU) plans to determine the magnitude and variability of benthic oxygen fluxes on the inner and middle Oregon shelf and the contribution of wave-induced motions to these fluxes using the eddy correlation (EC) technique. This technique assumes that a direct vertical flux estimate can be obtained by measuring the covariance between fluctuations of oxygen and fluctuations of vertical velocity above the seabed. To attain the goal, both wave flume experiments and field measurements using the EC lander will be carried out. Initially, EC measurements in the presence of energetic waves will be studied in the large wave flume at OSU's Hinsdale Wave Research Facility which is the largest wave channel of its type in North America. The purpose of the wave flume experiments is to experimentally verify the best approaches to data collection, averaging, and coordinate rotation to derive unbiased fluxes in the presence of waves and a sandy bed. In addition, the wave-turbulence decomposition method will be applied to quantify wave contributions to seafloor oxygen exchange and to document the sequence of bedforms and pore water profiles that evolve in response to stepwise increases and decreases in wave height. Once the wave flume experiments have been completed, four 8-day research cruises will be carried out to make measurements on the Oregon shelf over 3 years during spring, summer and fall conditions. The sites to be targeted are characterized by permeable sands at 25 to 85 m water depth and can exhibit ripples. Ancillary measurements will include bottom water dissolved oxygen, nutrients, pigment concentration, temperature and salinity, whereas sediment cores will be subsampled for bulk permeability measurements and profiles of 210Pb, organic carbon, nitrogen, grain size, and pigments.
As regards broader impacts, the scientist plans to collaborate with U.S. and Chilean scientists involved in the Microbial Initiative in Low Oxygen waters off Concepcion and Oregon (MI-LOCO) project. One graduate student would be supported and trained as part of this project.
During the last decade, hypoxic waters (i.e., waters with low dissolved oxygen) have occupied significant areas of the Oregon-Washington continental shelf for much of the summertime. This study was motivated by the need to understand how much local biogeochemical processes contribute to the development of hypoxia compared to remote forcing by climate variables that regulate the upwelling of preformed oxygen-depleted waters onto the shelf. Specifically, oxygen consumption by seafloor sediments was quantified using a new eddy correlation "EC" method that is also ideally suited for determining relationships between oxygen uptake and hydrodynamic factors including oscillatory wave motions. The co-located velocity and oxygen sensors required for EC measurements were deployed on a seafloor lander, 25 times at 10 locations ranging in water depth from 40 - 85 m, during four coastal research cruises, with complementary measurements of sediment and water column properties. The lander was further evaluated in the OH Hinsdale Large Wave Flume (104 m long) at Oregon State University. The wave flume experiments and a model simulating the measurements under waves showed that as long as both the measured velocity and oxygen time series are in phase temporally, biases unique to wave influenced measurements do not compromise the methodology. Instead, turbulence stemming from surface and internal waves was observed to produce real and dramatic enhancements of shelf oxygen fluxes, so that oxygen consumption rates in the wave energy-rich inner shelf environment are on average 2-5 times greater than fluxes in the middle shelf. An integrated assessment of the contributions of individual biogeochemical and transport processes to oxygen utilization, and carbon oxidation within the benthic boundary layer of the Oregon shelf environment will be the final outcome of the project. The training of one graduate student (Masters Degree 2014) and a summer undergraduate intern, and the professional development of two faculty research assistants were supported. Related projects were spearheaded with colleagues from the US, Canada and Israel.
