Maryland's 1998 Pfiesteria Experience:
Preliminary Analysis of Data
and Comparison to 1997

Introduction and Methods
During the summer and fall of 1997, Maryland experienced four separate toxic outbreaks of the dinoflagellate, Pfiesteria piscicida on three Eastern Shore rivers; the Pocomoke River, the Manokin River, and the Chicamacomico River. These outbreaks resulted in the deaths of thousands of fish, a high percentage of fish with lesions, and negative impacts to the health of some individuals who came in contact with the affected waterways. During 1998, Maryland DNR instituted a monitoring program on the three rivers affected in 1997 and five additional rivers that exhibit similar habitat conditions. (see map below)

map of river systems sampled in 1998

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
Fortunately, Maryland experienced no fish kills or lesion outbreaks in 1998 similar in magnitude to the four 1997 events. During 1998, investigations were made of several, small fish health events. In two cases, a substantially higher than natural percentage of fish were observed with the type of anomalies similar to those associated with toxic Pfiesteria outbreaks. One event was at Shiles Creek on the Wicomico River (see map below)

wicomico river station map

in early August, and the other was on the Chicamacomico River (same site as 1997's outbreak) in early September. (see map below)

map of the chicamacomico river system and the monitoring stations on the river

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)

Maryland Fish Kill and Lesion Events, 1997 and 1998:
Dinoflagellate Species Present and Toxicity


"+": positive bioassay,
"-": negative bioassay
Pfiesteria piscicida Cryptoperidiniopsis


Gyrodinium galatheanum


"Shepherd's Crook"


SEM (presumptive count - light micro.) Oldach molec. probe Rublee molec. probe
1997 x x x x x x x
Pocomoke River
(August 6-13)
+ (<900 cells/ml) na na x x x
Pocomoke River
(August 26-28)
+ (<300 cells/ml) na na x x x
Kings Creek
+ (<315 cells/ml) na na x x x
Chicamacomico River (September
+ (<315 cells/ml) na na      
1998 (preliminary and incomplete, 2/18/99) x x x x x x x
Wicomico River
(August 5-6)
- (<130 cells/ml) x x x x x
Chicamacomico River (September 3-9) -* (<100 cells/ml SEM underway) x x x x x

*: Toxicity was demonstrated on Chicamacomico 1998 samples after 8 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.

Two confirmed toxic outbreaks of Pfiesteria occurred on the Pocomoke River during August, 1997. (see map below)

 legend of pocomoke river monitoring stationspocomoke river map with all the water quality monitoring stations

Habitat quality data collected before, during, and after the outbreaks suggests that this could have been the result of a combination of nutrient enrichment, biological activity, and physical dynamics. In the vicinity of Shelltown, the Pocomoke River dramatically changes from a relatively deep and narrow river to a wide, shallow, and slow moving river.

pocomoke river dynamics that led to the 1997 fish kills and lesion events

High loads of nitrogen and phosphorus resulted in elevated algal levels at and immediately upstream of the Shelltown area from mid-June through early August, 1997.

August 1997 Algal levels in the Pocomoke River

The Shelltown area and site of the 1997 events take place around River Mile 0, which is where the peak concentrations in algal abundance occurred in 1997.

Pfiesteria feeds on algae during some of its non-toxic stages, and these elevated algal levels could have provided the food supply for large populations of non-toxic Pfiesteria to develop. Algae also serve as prey for menhaden, and the blooms may have acted to concentrate schools in this region. Finally, a sharp drop in dissolved oxygen levels immediately upstream of Shelltown may have further concentrated menhaden in this area by restricting their movement upriver. This model suggests the possibility that a combination of 1) a large Pfiesteria population and, 2) dense concentrations of menhaden in a slow moving portion of the river, was ideal for Pfiesteria to transform to its toxic state and result in the observed fish lesions and kills during August, 1997.

Differences in the timing and amount of rainfall likely prevented the combination of factors that set the stage for toxic outbreaks in 1997 from repeating themselves in 1998. Mid winter through July 1998 was generally wetter on the Lower Eastern Shore than in 1997. As a result, Pocomoke River flow was about two and a half times greater from January through March,

pocomoke river flow from january through september for 1997 and 1998

These flow values are supplied by the United States Geological Survey.

1998 and more than two times greater from May through the third week in July 1998 than during the same periods the previous year. These 1998 differences in flow, especially in the May through late July period, may have resulted in sufficient changes in the timing and location of nutrient concentrations, algal blooms, and low dissolved oxygen events to prevent a repeat of 1997's toxic outbreaks.

pocomoke river flow from may through september

These flow values are supplied by the United States Geological Survey.

August 1998 habitat conditions in the Shelltown area were generally similar to those during the August 1997 fish kills and lesion events, but habitat conditions in the months prior to August were different between the two years. The high Spring to mid-summer 1998 flows reduced salinity throughout the river relative to 1997.(see graphs below) Mid June through July 1997 salinities in the Shelltown area ranged around 8-9 ppt, compared to between 5-6 ppt during the same period in 1998. Changes in salinity may act to influence the activities and distribution of many estuarine organisms.

salinity legend1998 salinity values
The yellow dashed lines on the above space-time plots indicates a sampling cruise and the dots indicate station locations.

Although total concentrations of nutrients in the Pocomoke River appear to be roughly similar between 1997 and 1998, differences in flow may have resulted in differences in when and where on the river these nutrient concentrations appeared. High concentrations of phosphate (PO4) upstream of Shelltown, which were observed in June through August 1997, did not appear until early August in 1998. (see graphs below) Phosphate is normally chemically bound to particles in the sediment. If the amount of dissolved oxygen in the water drops to very low levels, a chemical reaction takes place that releases phosphate from the sediment into the water column. These low oxygen events frequently occur in slow moving, poorly flushed tributaries when the water temperature warms during the summer. These low dissolved oxygen events occurring in small tributaries and ditches throughout the watershed may explain the high phosphate concentrations upstream of Shelltown in 1997. The high flows and resulting flushing observed in early 1998, however, may have delayed the establishment of these low oxygen conditions and, in turn, prevented phosphate from entering the water column. It was not until flow returned to more typical levels in late July that high phosphate levels were observed upstream in 1998.

phosphate legend

1997 phosphate levels

1998 phosphate levels

The black dashed lines on the above space-time plots indicates a sampling cruise and the dots indicate station locations.

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)

August 1997 Algal levels in the Pocomoke River
The Shelltown area and site of the 1997 events take place around River Mile 0,which is where the peak concentrations in algal abundance occurred in 1997.

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)

1998 Algal levels in the Pocomoke River

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)

dissolved oxygen legend1997 dissolved oxygen values

1998 dissolved oxygen values

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.

Search Maryland DNR


Restoration and Protection | Bay Grasses | Harmful Algae | Bay Monitoring
Bay Life Guide | Bay Education

Return to the Maryland DNR Home Page.
Your opinion counts! Take a