significantly increase habitat diversity 

 and provide substrate for epifauna, de- 

 composers, and small nursery species (at 

 least during flood tides). The 3 x 10"^ 

 kcal/m2/yr would require the total net 

 production of about 5 m^ of marsh estuary 

 for each square meter of reef if total 

 production were usable by the community. 

 If only phytoplankton production were 

 usable, the reef community would require 

 at least 50 m^ of marsh estuary for nutri- 

 tional support (see Section 1.3). 



A final point should be made about 

 oyster reef energy requirements: the met- 

 abolic rate of this community ranks high 

 among the values measured for the macro- 

 fauna! metabolism of benthic communities, 

 exceeding even such systems as kelp beds. 

 Table 7 summarizes the results of some 

 representative benthic community metabolic 

 measurements. Of particular interest is 

 the 1974 study by Lehman, in which total 

 reef community metabolism from gulf coast 

 oyster reefs (Crystal River, Florida) was 

 measured at 16 to 21 g 02m^/day at 31.7°C. 

 Lehman's values for biomass were lower 

 than those measured from Georgia reefs 

 (119.5 g/m^ dry wt vs. 970 g afdw/m^), and 

 his experimental temperature was about the 

 same as the maximum experimental tempera- 

 ture used by Bahr (1974). 



The increasing number of metabolic 

 studies in which partitioning has been at- 

 tempted have well established that macro- 

 fauna usually play a relatively minor role 

 in total benthic community energy flow. 

 Smith (1971), for example, determined that 

 the proportion of total respiration rate 

 attributable to macrofauna of a sublitto- 

 ral community was equal to only 12.1%. 

 Therefore, the oyster reef community is 

 unique among benthic subsystems in that 

 the oysters and other macrofauna conspicu- 

 ously dominate community metabolism as 

 well as community structure. Intertidal 

 oyster reefs may be thought of as hetero- 

 trophic "hot spots" in the marsh-estuarine 

 system. 



3.4 REEF PREDATION 



No quantitative information is avail- 

 able on the rate at which salt marsh con- 

 sumers prey on the inhabitants of the 

 intertidal reef community. From a quali- 

 tative standpoint, the predators include 



53 



three groups: (1) small reef residents 

 such as mud crabs; (2) strictly aquatic 

 forms that migrate onto the reefs to feed 

 during flood tides, e.g., the blue crab 

 ( Cal linectes sapidus ) and the sheepshead 

 minnow ( Cyprinodon variegatus ); and (3) 

 terrestrial animals that prey on exposed 

 reefs during ebb tides, e.g., raccoons and 

 wading birds. This "time sharing" arrange- 

 ment by both aquatic and terrestrial pred- 

 ators, representing a "coupling" between 

 the reef and adjacent ecosystems, would 

 appear to wreak havoc on the reefs; but 

 relatively little evidence of predation 

 was ever detected in the reefs examined by 

 Bahr (1974). Blue crabs were observed 

 feeding on small oysters on partially 

 exposed reefs; raccoon tracks were seen 

 around reefs; and the most commonly ob- 

 served reef predators were boat-tailed 

 grackles ( Cassidix mexicanus ), seen pick- 

 ing unidentified organisms (probably small 

 crustaceans, insects, and polychaetes) 

 from recently exposed reefs. 



Drinnan (1957) estimated that the 

 European oystercatcher ( Haematopus ostra- 

 lequs ) preyed on between 28.5 and 51 cock- 

 les per hour during active feeding, each 

 cockle being between 23 and 30 mm in 

 length. He concluded that about 22% of 

 the total cockle population in his study 

 area in Nova Scotia were removed as a re- 

 sult of this predation. 



Butler and Kirbyson (1979) reported 

 that the black oystercatcher {H. bachmani ) 

 can eat up to nine large oysters per hour, 

 the oysters ranging from 80 to 160 mm. 

 These birds feed primarily on single oys- 

 ters, however, as opposed to American oys- 

 tercatchers (F[. palliatus ) that feed on 

 clumped or reef oysters (Tomkins 1947). 

 The latter author observed predation on 

 Crassostrea by oystercatchers on reefs 

 near Savannah, Georgia, but no attempt at 

 quantification was made. It was assumed 

 from Tomkins' description of the feeding 

 behavior of f[. palliatus that only about 4 

 hr/day are available for feeding on inter- 

 tidal oysters (2/hr/tide). Observations 

 on the density of oystercatchers at Sapelo 

 Island indicated fewer than one bird per 

 reef, perhaps one per eight reefs, result- 

 ing in an estimated maximum of 25 oysters 

 eaten by oystercatchers per reef per day 

 (4 hr/ day x 1/8 bird/reef x 50 oysters/ 

 hr/bird). If an average reef were approx- 

 imately 25 m^, a total loss of about 



