Drought Effect On Estuaries

When the drought flows down to the bay

Lower flows of streams and rivers into Texas' bays have a number of effects on the creatures and plants that live in and by the bay and on the people that depend on or enjoy the bay. In general, lower flows mean:

 

  • Less flushing of part of the bay
     
  • higher salinities
  • lower inputs of nutrients ordinarily delivered by streams
  • less freshwater for gulls, ducks, herons, coyotes, etc. to drink
  • invasion of oyster reefs by marine disease organisms
  • less bay area favorable for fish and shrimp larvae to grow
  • possible invasion of the bay by red tide
     

Climatic cycles and meteorological events which bring flooding or drought stressed conditions to upland plant and animal communities likewise bring equivalent hard times to the aquatic and wetland organisms of the estuaries. Most bay plants and animals do best within a range of moderate salinities. When floods flush salt water from the bay, they may suffer, possibly starve, or may swim to the Gulf. When drought does not bring freshwater to replace what is lost to the Gulf and to evaporation, salinities in the bay can rise to high levels. Actually, as salinities in the bay become closer to Gulf salinities, Gulf plants and animals may move in. However, when salinities are high, the bay conditions are not optimal for high productivity of the whole system. Though many factors influence fisheries production, graphical comparisons of freshwater inflow and fish harvest show the general relationship between adequate inflows and harvest.

What are normal salinities in the bays? How high can salinities go in the bays during a drought?

Standard seawater is around 35 ppt salt (parts per thousand: about 1.2 oz salt in a quart). Off the Texas shore, the Gulf can be from 28 - 36 ppt, depending on how far the local currents sweep Mississippi River and other freshwater flows along our coast. Depending on the cycle of flood and drought, our bays can be almost completely fresh or saltier than seawater. The bays can get saltier than seawater because evaporation removes much water in the summer, leaving salt behind. Historically, during droughts, salinities in the upper Laguna Madre have reached 50 - 60 ppt. These are salinities high enough to kill most fish. (Since the construction of the Gulf Intracoastal Waterway, increased circulation of water between the bays has reduced the occurrence of high salinities in the Upper Laguna, and provided an escape route for fish.)

The Texas Water Development Board, Texas Department of Health, Texas Natural Resource Conservation Commission, and Texas Parks & Wildlife Department, as well as some local agencies routinely measure salinity and other water quality parameters in each bay.