Field ecology of toxic Pfiesteria complex species and a conservative analysis of their role in estuarine fish kills

Within the past decade, toxic Pfiesteria outbreaks have been documented in poorly flushed, eutrophic areas of the largest and second largest estuaries on the U.S. mainland. Here we summarize a decadal field effort in fish kil l assessment, encompassing kills related to Pfiesteria (49 major kills in N orth Carolina estuaries since 1991 and 4 in Maryland estuaries in 1997) and to other factors such as low oxygen stress (79 major fish kills in North C arolina estuaries). The laboratory and field data considered in developing our protocols are described, including toxic Pfiesteria behavior, environme ntal conditions conducive to toxic Pfiesteria activity, and impacts of toxi c clonal Pfiesteria on fish health. We outline the steps of the standardize d fish bioassay procedure that has been used since 1991 to diagnose whether actively toxic Pfiesteria was present during estuarine fish kills. Detaile d data are given for a 1998 toxic Pfiesteria outbreak in the Neuse Estuary in North Carolina to illustrate of the full suite of diagnostic steps compl eted. We demonstrate that our conservative approach in implicating toxic Pf iesteria involvement in fish kills has biased in favor of causes other than Pfiesteria. Data are summarized from experiments that have shown stimulati on of toxic Pfiesteria strains by nutrient (N, P) enrichment, supporting fi eld observations of highest abundance of toxic strains in eutrophic estuari es. On the basis of a decade of research on toxic Ffiesteria, we present a conceptual model of the seasonal dynamics of toxic strains as affected by c hanging food resources and weather patterns. We also recommend protocols an d research approaches that will strengthen the science of fish kill assessm ent related to Pfiesteria and/or other causative factors.