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

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 (similar to 200 m cross-shore) and ephemeral (3 h) compared with the subsurface extent of the red tide (similar to 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 mu g 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 similar to 8-m water depth vertically spread the Chl a patch to the surface, creating the alongshore surface band similar to 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.