Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but they may be increasing in frequency, and in the types of bloom events. For example, during 1997, outbreaks of Pfiesteria piscicida were observed in several Chesapeake Bay tributaries, including the Pocomoke and the Chicamicomico River, while in 1998, Pfiesteria related events were not found, but massive blooms of Prorocentrum minimum occurred in the Pocomoke and other tributaries. In 1999, Aureococcus anophagefferens developed in the coastal bays in early summer in sufficient densities to cause a brown tide. Toxic Pfiesteria was responsible for fish kills at relatively low densities in the plankton population. Prorocentrum minimum and A. anophagefferens, on the other hand, were not toxic, but reached sufficiently high densities to displace much of the other phytoplankton and have ecological consequences. These 3 years differed in the amount and the timing of rainfall events, and in resulting nutrient loading from runoff from the largely agricultural land basin. Nutrient loading to the eastern tributaries of Chesapeake Bay have been increasing over the past decade. Much of this nutrient delivery is in organic form. The sites of the Pfiesteria outbreaks ranked among those with the highest organic loading of all sites monitored Bay-wide. The relative availability of the dissolved organic constituents DOC, DON, and DOP were also higher at sites experiencing A. anophagefferens blooms relative to those not. Dense blooms, like brown tides, pose other physiological challenges for the cells, and the ability to supplement photosynthesis with organic substrates may provide an advantage in maintaining blooms. For P. minimum and A. anophagefferens, urea is used preferentially over nitrate, and both increase their net growth rate in response to organic additions. Pfiesteria is a grazer, but has the ability to take up nutrients directly. Dissolved organic nutrients appear to play an important role in the development and/or maintenance of these HAB species.