OCE-PRF Track 1 (Broadening Participation): Internal wave-generated turbulent mixing and vertical nitrate flux during spring and neap tides along the Mid-Atlantic Bight shelf break

Haskell William, Los Angeles CA

Investigators

Abstract

The marine biological carbon pump transforms carbon dioxide into organic carbon and transports it from the surface ocean to the abyss. Biological productivity in the oceans is dependent on the rate at which deep water nutrients mix upward into the surface ocean. Tides are believed to contribute to this mixing in the coastal ocean, but we do not know the extent to which they influence the biological carbon pump. In this project, the fellow will address the fundamental question: Do tides significantly impact biological production and organic carbon export in the coastal ocean by increasing the rate at which essential nutrients enter the sunlit surface ocean? In addition, the fellow will broaden participation of underrepresented groups in the ocean sciences by leading hands-on educational activities teaching introductory robotics to students of Baltimore City public schools; presenting findings to the local community during the an open house at Horn Point Laboratories, including students in the Maryland public school system; and collaborating with the University of Maryland's Louis Stokes Alliance for Minorities Participation program to mentor an undergraduate researcher. The marine biological carbon pump, a critical process in reducing atmospheric carbon dioxide concentration, is the fixation of carbon dioxide into organic material (Corg) during primary production and subsequent transport from the surface ocean into the abyss. Over the continental shelves, the proportion of total Corg that is exported is much greater than that in the open ocean, so even small fluctuations in biological productivity over the shelves can have a large impact on the global carbon budget. In much of the surface ocean, biological productivity is limited by nutrient availability and the vertical transport of nutrients is controlled by turbulent mixing. During interaction with the shelf/slope topography, a portion of tidal energy is lost to turbulence during conversion into internal waves, which lose further energy to turbulence as they move onshore. This mechanism likely increases the rate vertical nutrient flux and biological productivity over the continental shelves. Given the ubiquity and frequency (hours to months) of tides worldwide, mixing caused by internal tide-shelf break interaction may be a globally important mechanism of nutrient supply to surface ocean ecosystems. The goal of this study is to quantify the effect of internal waves on vertical nutrient flux. Using a propelled Autonomous Underwater Vehicle (AUV), the fellow will make simultaneous in-situ measurements of vertical turbulent mixing rate and nitrate concentration profiles over two semidiurnal tidal cycles during both a spring and neap tide over the southern New England shelf. These measurements will be used to estimate the change in vertical nitrate flux on unprecedented temporal and spatial scales. The fellow test the hypothesis that tidal energy dissipation caused by the interaction with the continental shelf edge will increase the rate of vertical turbulent mixing, and thus vertical nutrient flux, near the base of the euphotic zone in the Mid-Atlantic Bight.