Physical and biological processes underlying the sudden surface appearance of a red tide in the nearshore

Abstract

The sudden appearance at the surface of an alongshore‐parallel band of red tide near Huntington Beach, California, is described in high spatial and temporal resolution using novel instrumentation including a global positioning system–tracked jet‐ski. The scale of the surface chlorophyll a (Chl a ) band was small (~ 200 m cross‐shore) and ephemeral (3 h) compared with the subsurface extent of the red tide (~ 2 km, > 7 d). The red tide was dominated by the regionally common dinoflagellate Lingulodinium polyedrum (F. Stein) and had developed as a subsurface Chl a layer during the 7 d prior to the surface appearance. A few hours before the surface appearance, a subsurface patch of elevated Chl a (Chl a > 30 μg L −1 ) was observed in 13‐m total depth in the trough of a shoreward‐propagating internal wave, consistent with dinoflagellate vertical swimming interacting with the internal wave‐driven convergence. Internal wave‐breaking–induced vertical mixing in ~ 8‐m water depth vertically spread the Chl a patch to the surface, creating the alongshore surface band ~ 500 m from shore. Both the subsurface Chl a patch and the surface Chl a band were prevented from entering the surf‐zone by a density barrier of warm water adjacent to the beach. These high‐resolution observations emphasize the role of nearshore physical dynamics in controlling the duration and intensity of red tide exposure to coastal habitats.

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