Hurricane Isabel's impact on the Coastal Bays

by Cathy Stokes-Wazniak.(Wazniak is a scientist for the Maryland Department of Natural Resources and a member of the Coastal Bays Scientific and Technical Advisory Committee.)

Although hurricane Isabel was downgraded to a tropical storm when it reached Maryland, its effects were still severe in the region.

Flooding, power outages, and downed trees were common after the storm. While IsabelÆs high winds and heavy rains were felt by millions of people, the storm also had an impact on the plants and animals that live in the coastal bays. As damage assessments on the shore continue, scientists are assessing the tempest's ecological impacts.

The effects of Isabel in the coastal bays began several days before the tropical storm reached Maryland. As the hurricane pushed ocean water towards the Mid-Atlantic coast, water levels began to rise 12 to 24 hours prior to the storm reaching Delmarva. As a result of storm surge, water levels in the bays went up at least two feet. This is significant since the total tidal range in Chincoteague Bay is generally no more than that. Additionally, a US Geologic Survey stream gauge on Birch Branch recorded a water level rise of almost 4.5 feet.

Immediate impacts in the bays included decreased salinity in some areas as well as large spikes in sediment inputs from the land and re-suspension of the bay bottom. A Department of Natural Resources continuous monitor on Bishopville Prong recorded salinity levels dropping to almost zero. Such large decreases in salinity may have stressed species unable to move into more preferred habitat. Monitors on Turville Creek recorded large turbidity spikes as a result of the storm and again with the heavy rainfall five days later. Sediment and nutrient pollution (as indicated by increased turbidity) from farm fields, suburban lawns, and streets cloud local waters and stress fish, crabs, oysters, and underwater grasses.

Since Isabel struck near the end of the growing season, short-term impacts will likely be minimized. Much of the SAV annual growth is complete, and oysters, crabs, and many fish species have finished their reproductive periods. However, much of the soil in the watershed is near saturation, which could result in more nutrients and sediments washing into the bays than if the storm had struck after a dry period when the ground could absorb more rainfall. Nutrient (nitrogen and phosphorus) enrichment is the primary threat to the health of the Coastal Bays.

Nutrients promote the excessive growth of algae which, in turn, can:
a) damage bay grasses,
b) substantially reduce oxygen levels resulting in fish and shellfish kills, and
c) produce toxins that may pose a threat to fish, shellfish, and even
humans.

Any additional nutrients flushed into the bays as a result of Isabel will likely exacerbate these problems. However, Isabel struck as water temperature and algal growth were declining, perhaps minimizing the short-term impacts of additional nutrients in the bays.

Like on land, the true effects of such a strong storm are not realized until sometime after the storm is gone. The additional nutrients washed into the bays by Isabel are not likely to disappear over the winter and may result in greater than typical algal growth next spring and summer. Isabel's contribution could be relatively small since 2003 is proving to have unusually high amounts of rainfall.

Over the next year, ongoing water quality monitoring programs will provide more information on Isabel's long-term impacts.