Harmful algal blooms in the Chesapeake and coastal bays of Maryland, USA: Comparison of 1997, 1998, and 1999 events

Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but may be increasing in frequency and diversity. Outbreaks of Pfiesteria piscicida (Dinophyceae) were observed during 1997 in several Chesapeake Bay tributaries, while in 1998, Pfiesteria-related events were not found but massive blooms of Prorocentrum minimum (Dinophyceae) occurred. In 1999, Aureococcus anophagefferens (Pelagophyceae) developed in the coastal bays in early summer in sufficient densities to cause a brown tide. In 1997, toxic Pfiesteria was responsible for fish kills at relatively low cell densities. In 1998 and 1999, the blooms of P. minimum and A. anophagefferens were not toxic, but reached sufficiently high densities to have ecological consequences. These years differed in the amount and timing of rainfall events and resulting nutrient loading from the largely agricultural watershed. Nutrient loading to the eastern tributaries of Chesapeake Bay has 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 availability of dissolved organic carbon and phosphorus were also higher at sites experiencing A. anophagefferens blooms than at those without blooms. The ability to supplement photosynthesis with grazing or organic substrates and to use a diversity of organic nutrients may play a role in the development and maintenance of these species. For P minimum and A. anophagefferens, urea is used preferentially over nitrate. Pfiesteria is a grazer, but also has the ability to take up nutrients directly. The timing of nutrient delivery may also be of critical importance in determining the success of certain species.