Alan Jassby -- associate research ecologist, UC Davisenvironmental studies division, (916) 752-3938 (Thursdays) or (510)654-3201 (home), adjassby@ucdavis.edu.Central to the final federal regulations protecting the complex bay ecology is one simple indicator of habitat health calculated by Jassby -- an estuarine transition zone known as the "X2" marker -- a salinity index that ebbs and flows with tides and weather. A member of the science advisory group for the Interagency Ecological Program for the Bay/Delta, Jassby studies water quality management and long-term biological and chemical variability in estuaries and inland waters.