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Massive blooms of the dinoflagellate Cochlodinium polykrikoides occur annually in the Chesapeake Bay and its tributaries. The initiation of blooms and their physical transport has been documented and the location of bloom initiation was identified during the 2007 and 2008 blooms. In the present study we combined daily sampling of nutrient concentrations and phytoplankton abundance at a fixed station to determine physical and chemical controls on bloom formation and enhanced underway water quality monitoring (DATAFLOW) during periods when blooms are known to occur. While C. polykrikoides did not reach bloom concentrations until late June during 2009, vegetative cells were present at low concentrations in the Elizabeth River (4 cells ml−1) as early as May 27. Subsequent samples collected from the Lafayette River documented the increase in C. polykrikoides abundance in the upper branches of the Lafayette River from mid-June to early July, when discolored waters were first observed. The 2009 C. polykrikoides bloom began in the Lafayette River when water temperatures were consistently above 25 °C and during a period of calm winds, neap tides, high positive tidal residuals, low nutrient concentrations, and a low dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorous (DIP) ratio. The pulsing of nutrients associated with intense but highly localized storm activity during the summer months when water temperatures are above 25 °C may play a role in the initiation of C. polykrikoides blooms. The upper Lafayette River appears to be an important area for initiation of algal blooms that then spread to other connected waterways.

Massive blooms of the harmful alga Cochlodinium polykrikoides Margalef occurred in the lower Chesapeake Bay and its tributaries during the summers of 2007 and 2008. The Lafayette and Elizabeth Rivers appeared to act as initiation grounds for these blooms during both years. However, in 2008 there were also localized sites of initiation and growth of populations within the mesohaline portion of the James River. Bloom initiation appeared to be correlated with intense, highly localized rainfall events during neap tides. Subsequent spring tides increased tidal flushing and transport of C. polykrikoides from the Lafayette and Elizabeth Rivers into the lower James River where it was transported upriver by local estuarine circulation. Blooms dissipated in response to increased wind-driven mixing associated with frontal systems moving through the region. 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 the lower Chesapeake Bay.

Phytoplankton communities are constantly changing with variable spatial distributions and shifting species composition in response to numerous biological and abiotic fluctuations. The most dynamic response is the formation of algal blooms; when a taxon is able to take advantage of a particular set of conditions, and quickly grow in numbers to become dominant in the algal community. Some blooms can be associated with negative impacts on the rest of the aquatic community, and may even cause human health concerns in the case of toxin production. Due to the transient nature of plankton, monitoring of algal blooms and the phytoplankton community in general is difficult, and often accomplished using a network of fixed stations, which may be limited in their spatial coverage of these events. Alternatively, higher throughput data such as the measurement of chlorophyll a, can deliver a larger spatial and temporal area, but not provide information on species composition vital for describing the biology and potential health impacts associated with certain taxa. This paper describes the use of a hybrid approach integrating continual chlorophyll measurements over a large area along with the collection of water samples for microscopic analysis. The Dataflow system and taxonomic identifications were used in conjunction during 2011 and 2012 to monitor algal blooms on the James River, Virginia, USA. During this time, the region experienced significant blooms of a variety of dinoflagellates, most notably Cochlodinium polykrikoides and Heterocapsa triquetra. Using these methods allowed for high resolution description of the initiation, transport, expansion, and subsidence of multiple algal blooms within the tributary, including the impact of tropical storms which occurred in 2011. This technique shows potential for providing research scientists, managers and modelers with the quality and quantity of data necessary to address questions relating to river-wide quantities of algal composition and abundance and could be applicable to other water bodies, including coastal systems.

Due to the unpredictable nature of intense storm events, little is known about event-driven impacts on water quality, nutrient dynamics, nutrient demand, and phytoplankton community composition in estuarine and riverine environments such as the James River estuary. In summer 2011 we monitored nutrient concentrations, physical conditions, and conducted nutrient uptake and primary productivity experiments before, during, and after two major storm events to evaluate the extent to which storms can affect phytoplankton communities in the James River. After the storms, we observed changes in phytoplankton community composition, nutrient concentrations, and nutrient uptake. There was an uncoupling between primary productivity and chlorophyll biomass subsequent to storm events likely due to washout of algal communities. Rapid response sampling ("storm chasing") may be key to determining the triggers, sustenance, and demise of algal blooms. With climate models predicting increases in both total precipitation as well as storm frequency, these questions are of growing importance.