Frontogenesis and the Creation of Fine‐Scale Vertical Phytoplankton Structure

Author(s): de Verneil, A; Franks, PJS; Ohman, MD | Abstract: ©2019. American Geophysical Union. All Rights Reserved. Fine-scale spatial structuring of phytoplankton patches has significant consequences for the marine food web, from altering phytoplankton exposure to surface light and limiting nutrients, to influencing the foraging of zooplankton, modifying carbon export, and impacting patterns of diversity. Hence, it is important to identify these fine-scale features and determine what generates their variability. Here we present evidence of fine-scale, tilted, interleaved layers in salinity and chlorophyll-a fluorescence observed in free-fall Moving Vessel Profiler surveys across a frontal system west of Point Conception, California. The observed covariability of hydrographic and biological properties allows for decomposition of the features into different water histories. Our analyses suggest that recently upwelled coastal water subsequently advected and intermingled with surrounding water masses from farther offshore. Orientations of the fine layers found in the filament are consistent with restratification and downwelling due to an ageostrophic secondary circulation brought on by frontogenesis. Finite size Lyapunov exponents, a Lagrangian diagnostic calculated from remote sensing data, provide positive evidence for frontogenetic convergence occurring upstream of the feature and allow for direct comparisons with in situ data to gauge their general utility in defining dynamical boundaries. These results highlight how frontal systems not only horizontally compress the biological niches represented by formerly disparate water masses but also create vertical structure and patchiness that can rapidly change over submesoscales.