Phytoplankton subsidies to the inter-tidal zone are strongly affected by surf-zone hydrodynamics

The size of bottom-up subsidies, food and larvae, from the ocean has a profound impact on inter-tidal communities and populations. Alongshore variations in subsidies are attributed to variation in coastal conditions, but also might be due to variations in surf-zone hydrodynamics. We tested this hypothesis by comparing surf-zone phytoplankton concentrations to surf-zone hydrodynamics as indicated by the width of the surf zones. To minimize the potential effects of alongshore variation in phytoplankton abundance in the coastal ocean, we sampled closely spaced sites (median separation 1km) over short periods (3days). Surf-zone concentrations of coastal phytoplankton taxa (e.g. Chaetoceros spp., Pseudo-nitzschia spp. and dinoflagellates) varied with surf-zone width; 65% to 94% of the variability in their concentration is explained by surf-zone width. Where surf zones were narrow and more reflective, phytoplankton concentrations were one to several orders of magnitude lower than in wider, more dissipative surf zones. The most closely spaced stations were 30m apart, but represented distinct habitats - a reflective and a more dissipative surf zone. Phytoplankton concentrations in the reflective surf zone were an order of magnitude or more lower than in the adjacent more dissipative surf zone. A reanalysis of a published study on phytoplankton subsidies found similar results. Alongshore variation in surf-zone hydrodynamics appears to be an important driver of phytoplankton subsidies to the inter-tidal zone.