Maryland's 1998 Pfiesteria Experience:
Introduction and Methods
Rivers labeled in Red were the river systems that experienced outbreaks in 1997 and river systems labeled in yellow were added to the 1998 monitoring activities.
The goals of this monitoring program were
threefold; first, to protect public health by identifying potential outbreaks as soon as
possible, second, to refine our understanding of the environmental factors that contribute
to the growth and toxic activity of this very complex organism, and third, to track our
progress at improving habitat conditions. Fish health was monitored bi-weekly at 8 to 10
sites, and habitat quality and phytoplankton were monitored monthly at 8 to 12 sites in
each river April through October. Additionally, plans were made for intensive, rapid
response monitoring of fish, water quality, and phytoplankton if and when unexplainable
fish kills or high percentages of fish with lesions were found.
Preliminary Results from 1998
in early August, and the other was on the Chicamacomico River (same site as 1997's outbreak) in early September. (see map below)
Additional fish, water quality, and phytoplankton sampling was initiated at both sites in response to the findings.
During 1997, fish bioassays and scanning electron microscopy (SEM) confirmed the presence of toxic Pfiesteria at the time and place of all four fish health events. To date, fish bioassays conducted with samples collected during the two 1998 events have failed to demonstrate that Pfiesteria was active in its toxic state. (see table below)
*: Toxicity was demonstrated on Chicamacomico 1998 samples after 8 ½ weeks incubation, suggesting that Pfiesteria was present, but not in an actively toxic state, when collections were made.
Light microscopy found low levels of a dinoflagellate
similar in size to Pfiesteria present at both 1998 events, and researchers
working on the development of molecular probes specific to Pfiesteria have had
positive results with water samples collected at the events, suggesting that Pfiesteria
may have been present at low densities. Toxic activity was eventually demonstrated in a
fish bioassay on a sample from the Chicamacomico River after a long incubation, suggesting
that Pfiesteria was present, but not in an actively toxic state when collections
were made. Analysis by SEM to confirm Pfiesteria's presence is underway.
These interannual differences in flow, salinity, and nutrients, in turn, may have altered the timing and location of algal blooms. In 1997 there was a brief July algal bloom upriver of Shelltown (see below)
and then, during the remainder of the year, the largest concentration of algae remained in the vicinity of Shelltown. In 1998, an April bloom occurred downriver of Shelltown, and then the center of the algal population gradually moved upriver. (see below)
Dissolved oxygen levels in the river were also different between the two years. The change in dissolved oxygen levels could be due to many factors, including differences in the location of algal concentrations (see above), bacterial activity, and flushing, all of which can be influenced by flow. Hypoxia (low dissolved oxygen) upriver of Shelltown was more prolonged and severe in 1997 than 1998.(see below)
The yellow dashed lines on the above space-time plots indicates a sampling cruise and the dots indicate station locations.
As a result, menhaden may have been more densely concentrated in the Shelltown region in 1997 than 1998 due to a more extensive habitat "blockage". Dense concentrations of fish are necessary to trigger Pfiesteria to transform into its toxic stages.
In conclusion, differences in rainfall and flow appear to have changed a number of potentially important environmental factors between 1997 and 1998. The resulting changes to ecological dynamics in the river may have been sufficient to prevent a toxic Pfiesteria outbreak from reoccurring in 1998.
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