Nitrate and oxygen flux across the sediment‐water interface observed by eddy correlation measurements on the open continental shelf

Chemical fluxes into and out of permeable seafloor sediments, such as those found on much of the continental shelf, are difficult to measure using conventional techniques. To overcome this difficulty, the eddy correlation method was adapted by oceanographers to determine oxygen flux across the sediment-water interface. In this article, we demonstrate that the eddy correlation method can also be used to measure nitrate flux across the sediment-water interface. A modified ISUS (In Situ Ultraviolet Spectrophotometer) optical nitrate sensor was used to measure dissolved nitrate at a frequency of 1.8 Hz. These observations were combined with vertical velocity measurements to calculate nitrate fluxes. Oxygen fluxes were also determined using a fast oxygen electrode system. The results for nitrate and oxygen flux compare well with simultaneous benthic flux chamber measurements made at a muddy sediment site at 95 m depth in Monterey Bay on the central California coast. The nitrate fluxes determined by the eddy correlation technique were considerably higher than benthic flux chamber values at sandy sediment sites at similar depths to the north and south of Monterey Bay. This difference is expected in permeable sediments where lateral flow through sediments will ventilate chamber water.