vironment and the estuary's fertility is due to that 

 freshwater system. But there's a fine line between an ade- 

 quate amount of nutrients and an excess," says Paerl. An 

 excess of nutrients can cause undesirable levels of algal 

 growth. 



North Carolina's Pamlico Sound is 

 one of the most productive estuaries 

 in the world. 



If Pamlico Sound were a bank, it would have one of the 

 richest stockpiles of assets in the world. But the assets in 

 Pamlico Sound are measured in terms of pounds of fish, 

 bushels of shellfish and acres of marsh grass. The dividends 

 from the production of fish and shellfish are easy to 

 calculate. But dividends also accrue from more obscure 

 things like benthic worms, seagrass beds, phytoplankton 

 and bacteria. They are all part of a simple, but lucrative 

 food chain that adds up to a system rich in productivity. 



Sea Grant researcher Charles Peterson wanted to com- 

 pare productivity in two different estuarine systems to see 

 how they stacked up. He chose a creature common to both 

 systems — the clam — to test in estuaries located in southern 

 California and in North Carolina's Back Sound, part of the 

 overall Pamlico Sound estuarine system. 



Using wire-mesh cages to exclude predators, Peterson laid 

 one layer of clams "cheek to jowl." On the shoulders of the 

 first layer, he laid another layer of clams to double the den- 

 sity. Peterson wanted to learn if a change in density would 

 affect the clam's growth rate. "I wanted to get an idea 

 whether the availability of resources were a limiting factor 

 in growth," Peterson says. 



In similar studies in California, Peterson had found that 

 the growth rate of clams was cut in half when their density 

 was doubled. But in Back Sound, double density meant 

 double production. Why the difference? Food availability. 

 The North Carolina clams had all the food they could eat. 

 The California clams didn't. 



The physical properties of the estuary help make it so 

 productive. Nutrients are supplied from upstream rivers 

 and the ocean. But North Carolina's shallow estuarine basin 

 traps the nutrients and recycles them for continued use. In 

 the California system, fewer nutrients are supplied from up- 



stream and the estuarine basin is deeper, allowing less 

 nutrient recycling. 



If food is so plentiful, why aren't we waist-high in hard 

 clams in Back Sound? The answer is simple. What's good 

 for the hard clam is also good for the blue crab, the conch, 

 the ray and the snapping shrimp, all predators of the hard 

 clam. The food chain works to balance the account of the es- 

 tuary's depositors. 



The estuary plays an important role 

 as a feeding area for birds. 



For migrating birds preparing to make a pit stop for food, 

 the estuary is the best restaurant in town. Birds require a 

 lot of food to fuel their rapid metabolisms. Naturally, they 

 head for the place that offers the most food for the least 

 amount of effort — the estuary. Whether the bird is a short- 

 billed dowitcher probing for benthic worms or a least tern 

 diving for fish, it can find large quantities of food in a 

 relatively small area in the estuary. 



Researchers have noticed that fall and spring migrations 

 of shallow-probing birds such as the red knot or the 

 American golden plover differ in length and intensity. 

 Shallow-probing birds dine along intertidal flats, eating 

 polychaete worms and small crustaceans. 



During the spring migration, larger concentrations of 

 shallow-probing birds spend less time in the estuary feeding 

 than during fall migration. Why the difference? Food is 

 more abundant during the spring when the estuary is at 

 peak productivity. And spring tides are lower, offering the 

 birds a larger area to scavenge. 



Photo by Scott Taylor 



Even seasonal and full-time residents like terns, gulls and 

 egrets benefit from the estuary. While some species of terns 

 and gulls are ocean-feeding species, others feed from the es- 

 tuary. The shallow estuaries bring even bottom-feeding 

 fishes close to the surface for birds to feed upon. And nearby 

 land masses tend to block the wind, creating a flat water 

 surface where prey are more easily seen from the air. 



Because birds feed heavily on a wide variety of estuarine 

 organisms, they tend to serve as a barometer for the es- 

 tuary's health. Birds come at the end of the estuary's short 

 food chain. And if production were limited or changed in the 

 estuary, either because of man-made or natural reasons, the 

 effects would almost immediately be seen in the birds that 

 feed there. 



— Nancy Davis and Kathy Hart 



