Frontogenesis and the creation of fine-scale vertical phytoplankton structure

TitleFrontogenesis and the creation of fine-scale vertical phytoplankton structure
Publication TypeJournal Article
Year of Publication2019
Authorsde Verneil A., Franks PJS, Ohman MD
JournalJournal of Geophysical Research-Oceans
Date Published2019/03
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000464656900009
Keywordscoastal transition zone; frontogenesis; Fronts; instabilities; mesoscale; mixed-layer; northern california; oceanography; patchiness; physical-biological interactions; phytoplankton; responses; subduction; submesoscale; thermohaline intrusions

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. Plain Language Summary Like weather in the atmosphere, frontal systems form in the ocean at the boundary of two different bodies of water. Phytoplankton, the organisms at the base of the marine food web, can find themselves brought into fronts by surface currents. When this happens, patches of phytoplankton are stretched along the front. The circulation at these fronts also causes the patches to tilt across the front, leading to stacked layers which we observed from a ship in the waters off California. When this happens, it changes how much light they receive for photosynthesis and how much of a target they are for the zooplankton that eat them. Besides taking up the greenhouse gas CO2, phytoplankton growth ultimately determines how much food is available for commercial fish to eat. Therefore, the layering process we describe shows how frontal currents can impact the functioning of the marine food web.

Short TitleJ Geophys Res-Oceans
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