Continental Shelf Benthic Oxygen Fluxes Determined by Eddy Correlation in the Presence of Wave Motions

Oregon State University, Corvallis OR

Investigators

Abstract

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.