Seasonal and synoptic oceanographic changes influence the larval biodiversity of a retentive upwelling shadow

•Oceanographic conditions and the larval assemblage varied seasonally in Monterey Bay, CA, USA.•Upwelling and relaxation dynamics affected the phytoplankton and larval assemblages.•Nearshore larval taxa and diatoms were abundant during strong upwelling conditions.•Offshore larval taxa and dinoflagellates were more abundant with the influx of offshore water.•Source water variation and meroplankton composition may provide insight into larval supply dynamics.