An Ecosystem in Transition

A healthy stand of rockweed (Fucus gardneri) growing on a boulder in Prince William Sound. This site was oiled during the Exxon Valdez spill and has since been recolonized by these plants. (Photo credit: OR&R, NOAA)

The graph below shows changes in the area covered by one of the most abundant alga on the shorelines of Prince William Sound, Fucus gardneri, commonly called "rockweed" or "popweed." Because this alga favors the middle part of the intertidal zone where much of the heavy oiling and cleanup occurred, its abundance declined in many areas of the Sound. Beginning in 1990, scientists saw the cover of rockweed increase steadily at oiled sites—until 1994, that is. From 1994 through 1995, there appeared to be a noticeable decline in cover, especially at sites that had been oiled.

	Mean Percent Cover per 0.25 Sq Meter Quadrat by Year
Comparison of percent cover of rockweed in unoiled and oiled/washed rocky sites, 1989-1996. Note the steep decrease in percent cover at oiled/washed sites in 1989 after treatment, and the steady decline in percent cover at oiled/washed sites beginning in 1993. The washed sites were cleaned with hot water in the months after the spill.

What caused the decline in 1994 and 1995?

Scientists don't know for certain. Here are some possibilities:

• It is possible that the oiling or cleanup (or both) in 1989 killed the original stands of rockweed, so that the normal mix of plants of different ages was not present. Instead, the areas became dominated by plants of all the same age because they were reestablished all at the same time after the spill. In 1994, all these plants would have reached the end of their life cycle at the same time, leading to the decrease evident in the graph. The die-off of plants of all the same age would not be noticeable under normal circumstances because plants of all different ages would be present.
  
  
• Alternatively, an explosion in the population of grazers (such as periwinkle snails) that feed on algae two and three years after the spill may have had something to do with the decline in rockweed (see graph below).
  
  
• Perhaps it is a subtle, longer-term toxic effect of oiling. Or, it might have nothing at all to do with the spill or cleanup. Data collected in the coming years will help shed light on these conditions.

	Mean Abundance per 0.25 Sq Meter Quadrat by Year
Abundance of periwinkle snails (Littorina scutulata) at rocky sites, 1989-1996. The large increase in snails at oiled/washed sites in 1991 and 1992 was due to increased numbers of juveniles. Periwinkle snails are grazers that feed on algae, including Fucus.

Close-up of periwinkle snails (Littorina scutulata), an algal grazer, and a newly-laid egg mass on a boulder in Prince William Sound. (Photo credit: OR&R, NOAA)

What do these trends over time mean for recovery in Prince William Sound?

Scientists think they suggest that highs and lows in abundance of plants and animals will continue as the system adjusts itself. With time, natural controls will dampen the fluctuations in abundance of marine life. Most of these adjustments will not be noticeable, and to the casual observer, conditions will continue to look much as expected in an area that remains a very beautiful place to visit. However, these subtle changes may have implications for how scientists view the process of recovery from the one-time stress of an oil spill. Eventually, the changes  also may affect other parts of the ecosystem that are commercially and aesthetically important, such as the fisheries and tourist destinations. NOAA scientists will continue to study the sound until 2005 in an effort to answer these questions.

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