Upwelling events, depth varying succession of phytoplankton assemblages and vertical connectivity: A conceptual model

• New insights into phytoplankton succession from depth differentiated data analyses. • Distinct phytoplankton assembly successions co-occur in response to vertical structure. • Conceptual model of seasonal and event scale succession of adaptive strategies. • Pycnocline acting as a vertical pelagic seed bank. • Connectivity between layers by entrainment/detrainment coupled to upwelling cycles. Phytoplankton succession in temperate upwelling bays is subject to intermittent wind-driven events. Improved understanding of the phytoplankton response to these environmental disruptions is needed for prediction and modelling affecting coastal ecosystem services. Here we perform a depth-resolved analyses of phytoplankton assemblages and succession with fifteen months (March 2017 − May 2018) weekly data collected at a reference station in Ria de Vigo. Analyses by depth revealed co-occurrence of different assemblages and successional patterns in response to upwelling-driven changes in water column structure and an “emerging functional group” of potential Diatom-Diazotroph Associations (DDA). These assemblages include species with distinct traits but sharing strategies favouring persistence during upwelling-downwelling cycles at seasonal and subseasonal scales. A simplified plot of succession of strategies (C, S, R, and a new “winter stress tolerant”, WS) in a seasonal (X axis) and event-scale (Y) template illustrates the complexity of overlapped adaptive strategies operating at different scales. Interactions between upwelling circulation and species behaviour help to explain reset succession after each event and the selection of characteristic species for each assemblage. The apparent vertical connectivity, through entrainment /detrainment of species between depth layers, leads to propose a role of the subsurface chlorophyll maximum (SCML), located at the base of the pycnocline, as a “pelagic seed bank” and hub of phytoplankton species. Results and conceptual models here advance knowledge on phytoplankton succession and may be useful for early warning and operational models designed to protect extensive fish (cages) and shellfish (rafts and suspended long-lines) aquaculture in highly productive upwelling bays.