sediments are distinctly heterotrophic. Although no direct assays 

 have been conducted, the RQ value of 1.5 indicates a substantial com- 

 ponent of anaerobic metabolism in lagoon sediment communities. 

 Kinsey (1979) likewise derived high RQ values for sand-rubble systems, 

 also invoking anaerobic metabolism as an explanation. Net carbon 

 deficits for each depth interval are presented (Table 3) along with 

 depth distribution data from Emery, et al (1954) to derive a mean 

 carbon requirement for Enewetak sediment communities of 41 mgC m -2 

 day -1 . 



Lagoon sediment community carbon deficits 



Mean deficit 



41 mgC m" 2 day 



- 1 



Preliminary calculations based on organic carbon production of wind- 

 ward reefs (Smith and Marsh, 1973) indicate that excess production of 

 these reefs is probably more than sufficient to provide the carbon 

 requirement of the sediment community. However, in the absence of 

 detailed information on metabolism in the lagoon water column, 

 mechanisms and precise characteristics of the trophic relationship 

 between organic sources and sinks within Enewetak Atoll must remain 

 speculative . 

 Macroinf aunal influences. 



Inspection of the summarized results (Tables 1, 2) reveals substan- 

 tial variability in both metabolic and dissolved nutrient flux data. 

 Previous sediment metabolic surveys have related high levels of para- 

 meter variability to the presence of actively bioturbating infaunal 

 communities in interreef lagoonal systems. In Kaneohe Bay, Hawaii, 

 alpheid shrimps thoroughly mix the upper 30 cm of sediments, greatly 

 enhancing rates of sediment community metabolism and nutrient cycling 

 (Harrison, 1981). The lagoon floor at Enewetak contains an extensive 

 population of callianassid shrimps actively turning over large vol- 

 umes of sediment daily. Callianassid burrow systems penetrate in 

 excess of 2 m below the sediment surface (Colin, et al . , in prep.), 

 providing direct routes for transport of organic cletritus into the 

 sediment column, and suggesting mechanisms for stimulation of the 

 apparently high levels of anaerobic metabolism inherent in these com- 

 munities. In this respect, and in others, there is a remarkable cor- 

 respondence between both structural and functional characteristics of 

 widely disparate reef ecosystems. 



148 



