Small-scale topographic fronts along an exposed coast structure plankton communities

Fronts are boundaries between distinct water masses that can aggregate or segregate flotsam, detritus, phytoplankton and zooplankton. Studies of large-scale offshore fronts (widths 100-1000 m) that persist for days show that frontal aggregations affect ecological processes by concentrating prey for diverse planktivores and delivering larval recruits to settlement habitats. However, less is known about small-scale (widths 1-10 m), ephemeral (persisting <1 d) fronts that occur commonly along exposed coasts. While propagating fronts associated with river plumes or internal waves have been shown to be important for the many animals that live or forage in nearshore and tidal waters, fixed nearshore fronts associated with shoreline topography have received little attention. We characterized hydrodynamics and the plankton community across a nearshore front attached to a small headland on the northern California coast using a Tucker trawl, CTD profiler, satellite imagery, and timelapse photography between June and October over two years. The front, made visible by a foam line, forms during separation of northward flow, primarily during periods of relaxation following upwelling events. Aggregation of plankton was not observed along the front, consistent with weak cross-frontal density difference. However, the front acted as a boundary separating plankton, including the larvae of bivalves, barnacles (Chthamalus spp.), and copepods as well as adult cladocerans. The front occurred regularly north of the headland and similar fronts have been observed at numerous other small headlands along Californian and other upwelling coasts. In general, small-scale topographic fronts may play an unrecognized role in structuring plankton com-munities and influencing dispersal where alongshore flow is disrupted by coastal promontories.