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Sporadic algal bloom development within a 10 year monitoring program in Virginia tidal tributaries of Chesapeake Bay is reviewed. These blooms were common events, characteristically producing a color signature to the surface water, typically short lived, occurring mainly from spring into autumn throughout different salinity regions of these rivers, and were produced primarily by dinoflagellates. The abundance threshold levels that would identify bloom status from a non-bloom presence were species specific, varied with the taxon's cell size, and ranged from ca. 10 to 104 cells mL-1. Among the most consistent sporadic bloom producers were the dinoflagellates Akashiwo sanguinea, Cochlodinium polykrikoides, Heterocapsa rotundata, Heterocapsa triquetra, Karlodinium veneficum, Prorocentrum minimum, Scrippsiella trochoidea, the cyanobacterium Microcystis aeruginosa, and two categories containing several species of often unidentified Gymnodinium spp. and Gyrodinium spp. Additional bloom producers within these tributaries are also discussed.