A-5). The entire oyster biomass of the 

 reef population is therefore considered 

 here as divided among 0.25-g oysters. 

 Bahr (1974) reported that the average bio- 

 mass of the reef oyster population was 970 

 g/m2 afdw (total wt); thus one can postu- 

 late a hypothetical reef populated by 

 60-rnni oysters at a density of about 4,000 

 oysters/m^. The dry meat weight of an 

 oyster of 0.25 g total afdw would equal 

 approximately 0.18 g (from Table A-1). 



Before one estimates the value of the 

 terms of Equation 1 for the "average" reef 

 oyster population, it is appropriate to 

 consider two independent studies that were 

 conducted at approximately the same time 

 and that attempted to measure certain 

 aspects of the energy budget of oyster 

 reefs. Bahr (1974, 1976) and Dame (1970, 

 1972a-, 1972b, 1976, 1979) studied oyster 

 reefs in Georgia and South Carolina, 

 respectively. Significant differences 

 between the studies are compared in Table 

 A-2. 



Some differences between the two sets 

 of conclusions are explainable on the 

 basis that test reefs in Dame's studies 

 were significantly lower in the intertidal 

 zone than were the reefs in Bahr's work, 

 although the absolute elevation of Dame's 

 reefs with respect to mean low water (MLW) 

 was not reported. This elevation dif- 

 ference perhaps indicates a significant 

 difference in inundation time, which could 

 explain the higher production reported by 

 Dame. In Dame's studies, oyster produc- 

 tion estimates for large oysters were 

 based on holding oysters in trays beneath 

 a pier (presumably shaded) and therefore 

 not in as stressful a setting as on a 

 natural reef. A real difference probably 

 existed in intertidal oyster reef produc- 

 tion (higher in South Carolina). The ac- 

 tive commercial harvest of South Carolina 

 reef oysters is proof that net production 

 of large oysters occurs there. Lunz (1943) 

 reported that oysters can grow to 3 inches 

 in 2 years in South Carolina reefs. Using 

 a calorific coefficient of 3.3 kcal/g Oj, 

 one can estimate that reef oysters respire 

 the equivalent of 13,000 kcal/m^/yr. The 

 implication of this high metabolic rate is 

 that the total biomass turns over on the 

 average about once every 0.38 yr, or 2.6 

 times per year (13,000 kcal/m2 /yr t 5,000 

 kcal/m2). 



Energy expended for gamete production 

 increases with the age of a particular 

 oyster but remains about half the respira- 

 tion rate (Figure A-1). Bernard (1974) 

 estimated that a subtidal population of C^. 

 gigas expended as much energy on gamete 

 production as on respiration (Figure A-6). 

 Thus, between 7,500 and 13,000 kcal/m^/yr 

 of the energy assimilated by reef oysters 

 would be converted to gametes and released 

 into the water column. At least 99% of 

 this energy "investment" would never reach 

 "maturity" but would be consumed by other 

 members of the salt marsh ecosystem. 



The rate of external work (W) per- 

 formed by oysters is the rate at which a 

 unit weight of shell material is elevated 

 above the mud surface, multiplied by its 

 elevated distance. In energy terms this 

 translates into the cost to oysters of 

 producing the shell protein that comprises 

 1.3% of the total shell dry weight or 

 about 400 g protein/m^ (2,000 kcal/m^). 

 The maximum elevated distance is 1.5 m 

 (see Section 3.1), but unfortunately we 

 have no reliable estimate of reef growth 

 rates. Bernard (1974) estimated that sub- 

 tidal oysters (C^. gigas ) in British Colum- 

 bia only expend about 10 kcal /m^/yr on 

 shell production. This is equivalent to 

 (30 kcal/m2/yr) for oysters in the study 

 area, calculated by using Bernard's data 

 but correcting for biomass differences 

 between the two different populations. 

 We suspect that this estimate is much too 

 low. The rate of predation on oyster 

 reefs is discussed in Section 3.4. 



Energy Budget Summary 



An energy budget for reef oysters is 

 presented in the following paragraphs, and 

 the rationale and values for the terms of 

 the equations are discussed. Because of 

 the method used in estimating net produc- 

 tion [P (net)^ '■'^ ^^^ studies discussed 

 above, we are inclined to agree with the 

 conclusions of the Georgia study. Charac- 

 teristically, net secondary production of 

 reef oysters is low in the upper portion 

 of high reefs and large oysters are quite 

 old, perhaps even 5 to 10 years or more. 

 In these reefs, somatic growth is balanced 

 by mortality. In lower "immature" reefs, 

 f'(netj ''^ undoubtedly significant. Because 

 the South Atlantic Bight includes large 

 areas of low "immature" reefs, especially 



101 



