California Coastal Upwelling Onset Variability: Cross- Shore and Bottom-Up Propagation in the Planktonic Ecosystem Fanny Chenillat 1,2 *, Pascal Rivie `re 1 , Xavier Capet 3 , Peter J. S. Franks 4 , Bruno Blanke 2 1 Laboratoire des Sciences de l’Environnement Marin (LEMAR), CNRS/UBO/IRD/IFREMER, Institut Universitaire Europe ´en de la Mer (IUEM), Plouzane ´, France, 2 Laboratoire de Physique des Oce ´ans (LPO), CNRS/IFREMER/IRD/UBO, Universite ´ de Bretagne Occidentale, Brest, France, 3 Laboratoire d’Oce ´anographie et du Climat (LOCEAN), CNRS/ UPMC/IRD/MNHN, Institut Pierre Simon Laplace (IPSL), Paris, France, 4 Integrative Oceanography Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, United States of America Abstract The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/ predation involving both large and small plankton species. Citation: Chenillat F, Rivie `re P, Capet X, Franks PJS, Blanke B (2013) California Coastal Upwelling Onset Variability: Cross-Shore and Bottom-Up Propagation in the Planktonic Ecosystem. PLoS ONE 8(5): e62281. doi:10.1371/journal.pone.0062281 Editor: Steven J. Bograd, National Oceanic and Atmospheric Administration/National Marine Fisheries Service/Southwest Fisheries Science Center, United States of America Received October 5, 2012; Accepted March 19, 2013; Published May 15, 2013 Copyright: ß 2013 Chenillat et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: fannychenillat@gmail.com Introduction The high biological productivity and fisheries activity of coastal upwelling systems is characterized by long-term (decadal) variability. Correlations of this biological variability in the California Current System (CCS) with North Pacific climate modes such as PDO (Pacific Decadal Oscillation) and ENSO (El Nin ˜ o Southern Oscillation) [1,2,3,4] have failed to capture the decadal variability of salinity, nutrients and chlorophyll (Chl-a); the mechanisms driving the biological changes remain unclear. The recently introduced NPGO (North Pacific Gyre Oscillation) [5] is the second dominant mode of variability of sea surface height anomalies (SSHa) in the Northern Pacific and is associated with changes in strength of the central and eastern parts of the North Pacific Gyre. Interannual changes in the NPGO have been shown to explain a significant fraction of the long-term variability of salinity, nutrient and Chl-a in the CCS [5,6,7]. Variations in the NPGO correlate with changes in the strength and timing of wind-driven upwelling in the CCS. Upwelling in the central and northern CCS exhibits a strong seasonal cycle in response to seasonally varying equatorward winds. At the spring transition – the onset of upwelling – winds become predominantly upwelling-favorable, usually sometime between January and April, with some latitudinal and interannual variability. Chenillat et al. [8] showed that a positive phase of the NPGO is characterized by strong alongshore winter winds leading to an early spring transition, with the opposite patterns during a negative NPGO. These changes in the onset of upwelling change the nutrient input and consequent phytoplankton bloom, which in turn influence the rest of the coastal trophic web [9,10,11,12]. Only a few studies have focused on the mechanisms that communicate changes in the timing of the upwelling onset across trophic levels (e.g., Dorman et al. [13] for krill; Ji et al. [14], and references therein). A bottom-up process can certainly be anticipated, but its expression in time and space and the way it affects trophic links are unknown. Building on new findings about the seasonal expression of the NPGO and its effect on the onset of upwelling [8], we investigate the influence of changes in the timing of the spring transition on the structure and functioning of the California Current planktonic ecosystem. In particular, we examine the cross-shore dependence of the ecosystem response to a time lag in the onset of wind-driven coastal upwelling. We use a regional hydrodynamic model forced by two synthetic wind climatologies that only differ in winter, corresponding to early or late upwelling onset, such as those typically arising in NPGO+ or NPGO- conditions [8]. This model is coupled to an ecosystem model composed of several phytoplankton and zooplankton size classes. In principle, this model has enough complexity to reproduce the two types of food chains observed in the CCS: a short coastal chain characterized by large organisms, and a longer offshore chain composed of a more diverse set of organisms [15]. This allows us to explore how the perturbation in the onset of upwelling propagates both in space (across shore) [16] and through the food chain. PLOS ONE | www.plosone.org 1 May 2013 | Volume 8 | Issue 5 | e62281