Physical controls on the formation, development, and transport of Cochlodinium blooms in
lower Chesapeake Bay and its tributaries
Ryan E. Morse1, Jose Blanco2, Will Hunley3, and Margaret R. Mulholland1
1
Old Dominion University Department of Ocean, Earth, & Atmospheric Sciences, Norfolk, VA
Old Dominion University Center for Coastal and Physical Oceanography, Norfolk, VA
3
Hampton Roads Sanitation District, Virginia Beach, VA
2
Persistent blooms of the harmful alga Cochlodinium polykrikoides Margalef occurred in lower
Chesapeake Bay and its tributaries from August to late September of 2007 and from late July
through early September 2008. During the 2008 bloom, dissolved oxygen concentrations
decreased to hypoxic levels following the collapse of the bloom, resulting in fish and invertebrate
mortality in the Elizabeth and Lafayette Rivers. The Lafayette and Elizabeth Rivers appear to act
as the initiation grounds for Cochlodinium blooms to the lower James River and the lower
Chesapeake Bay. Bloom formation appears to be correlated with brief but intense localized
rainfall events from late July to early August. Stratification increases following these rain events,
and nutrient concentrations also increase. The timing and duration of the rainfall appears
correlated with bloom development. VIMS HEM-3D model predictions for the James River
provide evidence for the tidally driven formation of a counter-clockwise flowing eddy adjacent to
and downstream from a strong tidal front located off Newport News Point. The eddy flow is
centered on an area directly across from the mouth of the Elizabeth River. Water exiting the
Elizabeth River flows into the lower James River, but rather than exiting into the Bay, the water
becomes entrained in the eddy flow centered on Hampton Flats. The eddy flow brings saltier,
denser water from Hampton Flats into the region of the strong tidal front. The salty, dense water
submerges below the fresher, less dense water at the frontal boundary. Particles suspended in the
water from Hampton Flats become entrained in bottom water, where they are transported upestuary due to typical net estuarine flow. A similar mechanism may be at work during the
Cochlodinium blooms. These blooms are initially observed near the mouth of the Elizabeth
River, then in the lower James River near Hampton Flats, and later observed in the upper James
River estuary. Vertical migration of Cochlodinium to deep waters at night may also contribute to
up-estuary advection as a result of entrainment into bottom waters. A confluence of physical
controls, including seasonal rainfall patterns, stratification, tidal flushing, spring-neap variability,
and estuarine mixing and circulation, allows Cochlodinium to out-compete other taxa and to form
massive blooms over large portions of the lower Bay.