Cross-shore fluxes in the California Current Ecosystem LTER site: a Lagrangian Approach

(on behalf of the CCE-LTER group)

Empirical evidence and modeling studies suggest that the classical assumption that New Production is balanced by Export Production (expressed as f-ratio = e-ratio) does not hold in the California Current System. There is an excess of New Production nearshore and excess of vertical Export flux offshore. The California Current Ecosystem Long-Term Ecological Research (CCE-LTER) site is testing the hypothesis that this spatial imbalance is resolved by lateral fluxes that transport C, N, nutrients, and organisms to the offshore. We also hypothesize that the long-term dynamics of the CCE region may be influenced by secular changes in the magnitude of these lateral fluxes. We are presently concentrating on the role of energetic mesoscale filaments in mediating cross-shore transport, together with Ekman and eddy-related fluxes. Evidence also suggests that (sub)mesoscale fronts can be particularly important sites of carbon export, due to gravitational fluxes and Active Transport, augmented by advective fluxes of DOM (e.g., Stukel et al. 2017. PNAS). Here we present an overview of 2 major process studies (June 2017 and August 2019) that adopt a quasi-Lagrangian approach to understanding the ecological and biogeochemical transformations that occur in water parcels while they are advected from nearshore to offshore. We conduct experiments to assess growth, grazing (by micro- and mesozooplankton), viral lysis, Fe limitation, primary production, and secondary production by bacteria and by a dominant copepod. We measure microbially-mediated geochemical transformations between particulate and dissolved phases. Lagrangian in situ sampling characterizes temporal changes in microbial and metazoan communities using a combination of molecular, morphological, digital imaging, and acoustic sensing. Continuous underway measurements assess changes in Net Community Production (O2:Ar), carbonate system variables (pH, pCO2), and Advanced Laser Fluorometry. Vertical export fluxes are quantified by Th:Ur disequilbrium, drifting sediment traps, and size spectra, across the euphotic zone and into the mesopelagic ocean.