UC Santa Cruz

Eastern Boundary Current systems, running along the west coasts of Africa and the Americas, are among the most biologically productive oceanic ecosystems. Their disproportionately large contributions to global marine primary productivity (photosynthesis) and fish catch are supported by upwelling of deep, nutrient rich water, a process driven by the interaction of surface winds and Earth's rotation. Upwelling in these systems may be forced by two mechanisms: equatorward winds at the coastal boundary (coastal divergence), or a cross-shore gradient in the magnitude of winds (wind stress curl). Though bulk estimates of upwelled volume and the individual contributions of coastal divergence and wind stress curl have been estimated, their roles in regulating productivity remain unclear. Similarly, while nutrient supply from upwelling has been measured or modeled in specific regions, its dependence on variability in physical factors including topography, stratification, and latitude has not been adequately addressed. The measurement of primary productivity itself is laborious and complicated in situ, and satellite-borne ocean color sensors currently represent our best option for obtaining the large-scale estimates needed to constrain global carbon budgets. However, after over a quarter century of satellite primary productivity model development, performance has improved little.

The research in this dissertation employs a suite of cutting edge oceanographic tools - computer models, satellite sensors, and autonomous underwater platforms - to elucidate controls on primary productivity and its measurement in coastal upwelling systems. First, an idealized numerical model is used to evaluate the respective roles of stratification, continental shelf topography, latitude, and wind stress magnitude on upwelling source depth and nutrient delivery to the sunlit surface layer (Chapter 1). Next, this analysis is extended to investigate the impact of nearshore reduction in surface wind stress, a poorly constrained process with significant implications for bottom-up control of phytoplankton growth (Chapter 2). Finally, an extensive record of shipboard observations off the California Coast is analyzed to evaluate limitations on primary productivity estimation, and synergistic use of satellites with autonomous underwater gliders is identified as fertile ground for improvement (Chapter 3).