Dye tracing of a subsurface chlorophyll maximum of a red-tide dinoflagellate to surface frontal regions

Abstract

Rhodamine dye (3701) was injected into a 22-m subsurface chlorophyll maximum of the red-tide forming dinoflagellate Prorocentrum mariae-lebouriae var. minimum in the northern Chesapeake Bay (USA) and traced for a sixday period as it spread over a 600 km² area. The precise physical mechanisms, which resulted in the transfer of dye and organisms to the surface, are documented. The major component of the dye and organisms was transported from the central bay into major tributary estuaries via net nontidal flow of bottom waters and surfaced upstream in frontal regions. Once in surface waters, the dye and organisms flowed downriver toward the bay. Due to the three-layer flow of the rivers at this time, the dinoflagellate and the rhodamine re-entered the bay proper at middepth below the fresher Susquehanna plume, thus forming a near-surface chlorophyll maximum (4–6 m) flowing in an opposite direction to the deep subsurface chlorophyll maximum (18–26 m). Current meter arrays verified the opposite flows of these two lenses. The near-surface, southward-flowing lens was followed downstream to an area where the influence of the Susquehanna begins to subside as indicated by isopycnals inclined to the surface. Here the near-surface lens is mixed upward to the surface forming massive red tides (25 000 cells ml⁻¹) delineated by a frontal region. In addition to the predominant along channel flow, major cross stream Ekman transport and upwelling of dye and organisms was detected in response to wind forcing resulting in localized surface patch formation along the western shore shoaling regions of the bay proper. Thus, annual variations in the locations of surface red tides can be correlated to streamflow and windinduced variations in the locations of frontal regions.