There's still a lot to fight about when it comes to determining how much fresh water is needed for a healthy San Francisco Bay, but research by a UC Davis scientist has advanced the controversy far enough so that final federal regulations protecting the bay ecology could be issued Dec. 15. Central to the forthcoming rules is a simple measurement, known as the "X2" mark, of a complex ecosystem. Scientists have long suspected that fresh-water diversions were harmful to certain fish in the bay. Yet, skeptical agricultural and urban water users did not want to lose any of their share of river water to the fish. So UC Davis limnologist Alan Jassby and his colleagues analyzed available data on all organisms ranging from the microscopic phytoplankton to the striped bass and calculated one simple indicator of habitat health -- an estuarine transition zone where the near-bottom salinity measures 2 percent, or the X2 mark. On a map, the X2 salinity mark would resemble a wavy topographic line that ebbs and flows with tides and weather. It also offers the easiest and most precise way to index water flow, which cannot be measured directly.