ts 



of an oyster slows as its gamete produc 

 tion gradually begins to dominate its 

 energy budget and as its respiratory rate 

 "catches up" to its ingestion rate (Rod- 

 house 1978), as illustrated in Figure A-1. 



Ingestion by oysters (I) is usually 

 estimated by measuring the rate of clear- 

 ance of particles in a suspension to which 

 test animals are exposed for a unit of 

 time. Walne's (1972) experiments using 

 Crassostrea qigas and Ostrea edulis are 

 exemplary in that realistic food concen- 

 trations and a wide range of sizes of oys- 

 ters were used. In addition, Walne used 

 flowing water conditions rather than the 

 usual standing water experiments. Haven 

 and Morales-Alamo (1970) also measured 

 oyster ingestion in a flowing water system 

 but did not use a wide size range of oys- 

 ters. 



Egestion (E) is measured by holding 

 test oysters in trays in which feces and 

 pseudofeces are collected and measured 

 during a known time interval (Haven and 

 Morales-Alamo 1967; Bernard 1974). 



The respiration rate of oysters (R) 

 is usually measured by documenting the 

 rate of decline in dissolved oxygen in 

 water in which oysters are immersed or by 

 measuring the change of dissolved oxygen 

 in water as it flows over a population of 

 oysters. The rate of change of CO2 is not 

 as convenient to measure with oysters, 

 partly because an infrared CO2 analyzer is 

 required, partly because oysters can fix 

 CO2 (Hammen 1969), and partly because they 

 can respire anaerobically and release COj 

 from the dissolution of shell carbonate 

 (Hochachka and Mustafa 1972). 



One major problem in quantifying in- 

 dividual terms in the oyster energy budget 

 equation is that most terms change in a 

 nonlinear fashion as an oyster (or size 

 class) grows. Small animals operate at 

 higher metabolic rates than large animals. 

 Another problem is that at least five en- 

 vironmental variables affect each term: 

 (1) intertidal elevation, (2) water tem- 

 perature, (3) levels of food and other 

 suspended matter in the water column, (4) 

 dissolved oxygen levels, and (5) current 

 velocity. To further complicate the pic- 

 ture, the size of the animals and these 

 other variables are interrelated in com- 

 plex (nonlinear) ways. 



Energy budgets are invariably simpli- 

 fied models because of these problems, and 

 the present budget is no exception. Some 

 comments about the variables used and 

 assumptions made follow. 



VARIABLES 



Tide Stage 



Oysters obviously cannot pump water 

 to respire and feed unless they are im- 

 mersed. Intertidal reef oysters are 

 assumed to be inundated on the average of 

 only 50X of any 24-hr day. Other workers 

 have made similar assumptions on feeding 

 duration, even for subtidal oyster popula- 

 tions. Bernard (1974) assumed 50%; Rod- 

 house (1978) assumed 70% feeding time. 



Water Temperature 



Temperature affects all biochemical 

 reactions, including oyster energy con- 

 sumption. Intertidal oysters are exposed 

 to water temperatures that vary by a 

 factor of about three, from 9° to 31°C 

 (Dame 1970; Bahr 1974). The annual pat- 

 tern of water temperature variation in 

 coastal South Carolina is illustrated in 

 Figure A-2 (Dame 1970). Over this temper- 

 ature range oyster metabolism is estimated 

 to vary by a factor of about eight (Bahr 

 1976). 



Food and Other Suspended Matter 



Loosanoff (1962) showed that food and 

 other suspended matter significantly 

 altered oyster ingestion rate. Excess 

 turbidity, caused either by suspended 

 organic or inorganic matter, reduces "oys- 

 ter pumping." It can be assumed that sus- 

 pended matter in the study area is close 

 to optimum for intertidal oysters and that 

 they are exposed to about 0.01 gC/liter or 

 0.04 kcal/ liter when inundated (Odum and 

 de la Cruz 1967). 



Dissolved 0-, 



Oyster respiration rates are unaf- 

 fected by dissolved oxygen concentrations 

 unless the concentration decreases below 

 one-half saturation level (Ghiretti 1966). 

 In other words, dissolved oxygen in estu- 

 aries in the study area should normally 

 not affect respiration or feeding rates 



94 



