Excerpt from “ARANSAS  A Naturalist’s Guide”

By Wayne H. McAlister and Martha K. McAlister

© University of Texas Press 1995

 Because these bays are shallow, their fine sediments are easily roiled by the wind, keeping them too turbid and unstable to support much rooted vegetation. Diatoms, dinoflagellates, widgeon grass, shoalgrass, and detritus form the basis for the marine food chains. Salinity in the bays fluctuates from virtually fresh to hypersaline but averages about 22 parts per thousand (ppt), lower than the open Gulf’s nearly constant salinity of about 33 ppt. At the height of a drought in 1956, the lower part of San Antonio Bay registered a devastating 50 ppt. When the drought broke in 1957, the salinity of the bays plunged to less than 1 ppt ---- fresh enough for human consumption but lethal for most marine organisms. Water temperature in the bays is equally changeable and occasionally extreme. Sometimes the shallows are hot enough to drive out nearly all dissolved oxygen; now and then they freeze. To escape these extremes, marine creatures swim into deeper channels or move through the passes into the Gulf, or they sink into the mud. All vicissitudes in the bays eventually affect the well - being of the surrounding communities of life on land.

 The presence of Matagorda and nearby St. Joseph* islands about 5 miles at sea renders the bays a sequestered, somewhat independent marine system. Salt water flushes in through Pass Cavallo  30 miles up the coast at the north end of Matagorda Island and at Aransas Pass 25 miles down the coast at the south end of St. Joseph. Cedar Bayou, a narrow cut between the two islands directly opposite the refuge, can be an important exchange point, but it is commonly filled with sand. This natural pass was purposely plugged in 1979 to protect the bays from the Ixtoc oil spill in Mexico. Hurricane Allen partly opened the waterway the following year, but by 1985 it had filled with sand. In 1988, Cedar Bayou was dredged clear, and it has remained open to date**. When this small pass is open, the abundance of marine life in the adjacent bays increases and more birds enjoy nesting success on the nearby oyster shell reefs.  

NOTE:

  * St. Joseph’s Island is also commonly known as San Jose Island.
  ** This was written in 1995, and Cedar Bayou has since silted closed again.