One of the larger and more productive estuaries in the world is the Chesapeake Bay. Because many characteristics of the bay and its watershed have been well documented, it represents a good case study for further elaboration of some key concepts in estuarine science and management, and in particular, elaboration of issues of predictability, scale, and diversity. Perhaps the most distinctive feature that contrasts estuaries from other biomes is the nature and variability of the physical forces which influence these ecosystems. Within small geographic regions, many estuaries experience widely varying conditions of temperature, salinity, concentrations of a wide variety of chemicals, and plant and animal densities, much of which is mediated by water movement over relatively short time scales. For example, in the Chesapeake Bay, temperatures range from near 30 C in surface waters in late summer to zero by late winter.
Ice cover is extensive in severe winters, but of only local importance in normal winters.
Salinity also varies widely, from near zero in the upper reaches of tributaries to 30 ppt near the capes. Superimposed on these general gradients of temperature and salinity, significant, unpredictable, interannual variability also occurs related to normal climatic shifts from wet to dry periods. As a result, the spatial extent of various estuarine habitats undergoes periods of significant expansion and contraction. Since these elements are essential for plant growth, their availability determines, at least in part, the amount of organic matter available to support food webs. Recent measurements indicate that nutrient loading rates and algal production rates vary among years by at least a factor of two in the Chesapeake Bay.