Date of Award

Summer 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

Margaret R. Mulholland

Committee Member

Alexander Bochdansky

Committee Member

Harold G. Marshall

Committee Member

Hans Paerl

Abstract

Algal blooms occur seasonally in the Chesapeake Bay and its tributaries, and while the consequences of algal blooms have been qualitatively and quantitatively assessed, the causes of algal blooms and mechanisms of bloom initiation are still not well understood despite decades of research. In order to understand nutrient dynamics and other factors that promote the initiation of algal blooms, the Lafayette River, a tidal sub-estuary of Chesapeake Bay that experiences seasonal algal blooms, was sampled daily in the fall of 2005. Three phytoplankton blooms (Chlorophyll a concentrations exceeding twice the average of monthly measurements from 2000-2009) occurred during this period, a mixed bloom of Akashiwo sanguinea and Gymnodinium sp., a Skeletonema costatum bloom, and a monospecific Gymnodinium sp. bloom. Over the sampling period, nutrient concentrations increased following precipitation events and were elevated between bloom periods but low during blooms. All measured forms of nitrogen were positively lag-correlated with dinoflagellate abundance between 3 and 5 days in reverse time. Concentrations of NO2- reached 10 μM between September and October, indicative of incomplete nitrification. Over a 24-h period, nutrient concentrations and chlorophyll a biomass varied by an order of magnitude and were strongly linked to the tidal phase.

Massive blooms of the harmful alga Cochlodinium polykrikoides Margalef occurred in the lower Chesapeake Bay and its tributaries during the summers of 2007, 2008, and 2009. The Lafayette River appears to act as initiation grounds for these blooms. However in 2008 there were also localized sites of initiation and growth of C. polykrikoides populations within the mesohaline portion of the James River. In 2008, bloom initiation appeared to be correlated with intense, highly localized rainfall events during neap tides. During 2009, bloom formation occurred when water temperatures had stabilized at 26°C during a period of calm winds, neap tides, high positive tidal residuals, higher salinity, low nutrient concentrations and a low dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorous (DIP) ratio (DIN:DIP). Tidal flushing transported the C. polykrikoides bloom from the Lafayette River into the lower James River where it was transported upriver by local estuarine circulation. A combination of physical factors including, seasonal rainfall patterns, increased stratification, nutrient loading, spring-neap tidal modulation, and complex estuarine mixing and circulation allowed C. polykrikoides to spread and form massive blooms over large portions of the tidal James River and lower Chesapeake Bay. The primary control on the formation of algal blooms in the Lafayette River was water column stability, and bloom formation occurred during neap tides, when there was low wind-driven mixing, and increased buoyancy from rainfall and runoff.

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DOI

10.25777/180q-ac88

ISBN

9781124973081

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