Color-coded for ease of visualization, these charts of the Chesapeake show two very different days at the end of April. In 2000, a year with high river flow from January through April, relatively dense algal concentrations (shown in green) are detected by aircraft-borne sensors. The next year, with lower flows from January to April, those same sensors see much clearer water, represented here in blue and purple. Like a biological bellwether, the southern Bay in particular responds quickly to shifts in nutrient loads. In low-flow years the Bay begins to resemble its past, when fewer nutrients left waters clearer and when light penetrated down to rooted underwater plants.