222 WOODWELL, RICH, AND HALL 



accept the mixing of freshwater and salt water as a principal criterion identifying 

 an estuary, but few have gone as far as McHugh in defining estuarine waters 

 simply on the basis of salinity. Using the 3 3.5% isopleth of salinity, he showed 

 that substantial segments of the coastal oceans would be estuarine (Fig. 1). This 

 definition makes good sense if one wishes to emphasize the continuity of biotic, 

 physical, and chemical functions from coastal wetlands into the oceans. Most 

 definitions, however, are more restrictive. Certainly in considering the carbon 

 budget of the world, the carbon budget of oceans is to be differentiated from 

 that of enclosed coastal waters. 



The most common use is with reference to the mouths of rivers upstream to 

 the limits of tidewater, to the enclosures behind baymouth bars, and to brackish 

 lagoons. We use "estuary" in this more restrictive sense, making no attempt to 

 differentiate bays that are not fed by freshwater in which there may be an 

 accumulation of salt by evaporation. We include also marshlands, which in 

 middle latitudes may accrue to 20 to 30% of the total area of large estuaries. We 

 include also mangroves and estuarine swamp forests of the lower latitudes. We 

 do not include coral reefs. 



AREA OF ESTUARIES 



We know of no detailed tabulation of the estuaries of the world beyond the 

 map prepared by McHugh. The area and composition of estuaries are available 

 for the United States from two recent comprehensive studies, the National 

 Estuary Inventory 2 and the National Estuarine Pollution Study. 3 Table 1 was 

 compiled from the National Estuarine Pollution Study. In the absence of other 

 data, we have used the data for the United States to make an estimate of the 

 area of estuaries of the world. The estimate was made by applying to the 

 coastlines of the world the ratio of the area of estuary to the oceanic coast 

 determined for the coasts of North America. We recognize that the area of 

 estuaries varies greatly from one region to another. The east and Gulf coasts of 

 North America have much greater areas of estuaries than the west coast, Alaska, 

 or Hawaii (Table 1). The mean ratios of estuarine area per oceanic coastline for 

 the United States, however, are probably not greatly different from mean ratios 

 for North America or for other continents. An attempt to match topography 

 and precipitation made no change in estimates for other continents on the basis 

 of the average ratios for the United States. Such major estuaries as Chesapeake 

 Bay, the Baltic Sea, and the mouths of major rivers were treated separately. We 

 have estimated that on a worldwide basis 20% of the area of estuaries is marsh. 

 Results are summarized in Table 2. 



The total area of estuaries of the world appears by this estimate to be about 

 1.75 X 10 6 km 2 . Of this, about 1.4 X 10 6 km 2 is open water and about 

 3.8 X 10 s km 2 is marsh or mangrove. It would be surprising if estimates derived 

 in this way were accurate within ±50%. 



