60 • Wetlands: Their Use and Regulation 



Analysis of the stomach contents of estuarine fish 

 and shellfish shows a wide variety of foods. For in- 

 stance, the stomach contents of menhaden include 

 primarily algae, but also detritus, small crustaceans, 

 and even small fish and fish eggs (50). Commer- 

 cial shrimp seem to have an even broader diet, con- 

 sisting of single-celled algae, algal filaments, detri- 

 tus, bacteria, protozoa, and easily captured ani- 

 mals, including very small worms and crustaceans 

 (25). Analysis of the stomach contents of oysters 

 and hard clams often shows both detritus from vas- 

 cular plants and phytoplankton, probably from the 

 open estuary. However, there is evidence that most 

 of the food value comes from the phytoplankton 

 (37,69,84). 



While commercially and recreationally impor- 

 tant fish may not directly consume detritus as their 

 major food source, they may feed on invertebrates 

 that use detritus as a major food source. Newly 

 hatched Adantic croaker, for instance, eat the small 

 crustaceans found in the water column, particularly 

 various copepods commonly found in the tidal 

 creeks dissecting grassy salt marshes (2). As they 

 grow, they add larger items to their diets, such as 

 amphipod crustaceans, mysid shrimp, small crabs, 

 worms of all sorts, mollusks, and smaller fish (69, 

 84). Also, opposum shrimp, a common marsh in- 

 vertebrate, is a major component of the diet of 

 striped bass on both the east and west coasts. Chi- 

 ronomid midge larvae were found to account for 

 over 80 percent of the diet of juvenile chum and 

 chinook salmon (24). 



Most coastal marshes export detritus to adjacent 

 coastal waters. While estuarine fish and shellfish 

 may direcdy and indirectly use detritus when avail- 

 able, the quantitative significance of wetlands- 

 derived detritus to the food supply of the estuary 

 relative to contributions of detritus from other ter- 

 restrial or open-water food sources generally is not 

 known, but probably varies widely with both species 

 and estuary. If the estuary has very few marshes 

 and much open water, such as in the North and 

 Middle Atlantic States and most areas in the Pacif- 

 ic, the likelihood is increased that the ultimate 

 source of organic matter for fish is not the marsh 

 grass, but the phytoplankton. For example, Chesa- 

 peake Bay is the source of a great deal of commer- 

 cially valuable seafood, but its ratio of marsh to 

 open water is only 0.04; the ratio at Sapelo Island, 



Ga., is nearly 2.0. Given what is known about the 

 phytoplankton production in the Chesapeake Bay, 

 the annual contribution of salt marshes to total 

 available energy is only around 2 to 5 percent (61). 

 In fact, the scientific literature lacks convincing 

 evidence, at least for Atlantic and Pacific coasts, 

 supporting the belief that coastal marshes play a 

 significant role in supporting fish and shellfish pro- 

 ductivity through the export of detritus (68). 



Climatic and Atmospheric Functions 



Although there has been little research related 

 to these functions, some wetland scientists have 

 hypothesized that large wetlands help to maintain 

 lower air temperatures in the summer and prevent 

 extremely low temperatures in the winter. They also 

 are a source of water to the atmosphere, leading 

 to the formation of cumulus clouds, thunderstorms, 

 and precipitation. Finally, wedands, through proc- 

 esses of microbial decomposition, either may store 

 or emit gaseous byproducts important to global 

 atmospheric stability. 



Moderation of Local Temperatures 



Water warms and cools slowly in comparison 

 with land areas; thus, wetlands will have a moder- 

 ating influence on daily atmospheric temperatures. 

 Drained agricultural areas in Florida, for instance, 

 were found to be 5° F colder in the winter than 

 were surrounding, undrained areas (35). It has been 

 suggested that wetland drainage of the Everglades 

 may have increased frost action (87). Because 

 deeper water bodies contain more water than wet- 

 lands with the same area, lakes will have a more 

 moderating influence on atmospheric temperature 

 than will wetlands (35). 



Maintaining Regional Precipitation 



Wedands contribute to rainfall through processes 

 of evaporation and the release of water vapor from 

 plants (evapotranspiration). In a study of Florida 

 cumulus clouds, for instance, lakes larger than 1 

 mile in diameter exerted a noticeable effect on 

 clouds in the area (35). It has been hypothesized 

 that wetland drainage could reduce summer thun- 

 derstorm activity in Florida by reducing evapo- 

 transporation, leading in turn to regional rainfall 

 deficits (22). 



