53 



Zone 1, represents the input of several kilograms of reactive organic matter m 2 /yr. The high flux of sulfides and 

 absence of oxygen in the benthic boundary layer eliminates all metazoa. The biological mixing depth is zero. 



Associated Processes and System Attributes 



The deep benthic mixing (bioturbation) in Zones 4 and 3 serves to prevent the build-up of reactive organic 

 matter in the sediment. Rather, organic matter is respired as CO^. Nitrogen, phosphorus, and silica are recycled 

 back to the water column where they may be used by plants (Aller 1982). The sediment therefore has a low 

 inventory of sulfides and methane (anaerobic metabolites) and the sediment has a low oxygen demand. 



The importance of physical stirring for digestion of organic matter is understood by sanitary engineers; physical 

 stirring and aeration are designed into tertiary treatment plants. A major attribute of deep bioturbation is that 

 it prevents the build-up of labile organic matter in sediments that might otherwise cause high sediment oxygen 

 demand. Bioturbation is a natural form of "tertiary" treatment. 



The sediment column in Zone 2 has a high inventory of labile organic matter and anaerobic metabolites. The 

 sediment oxygen demand is high and such areas are candidate sites for developing bottom hypoxia. The 

 inventory of organic matter is high both because the sedimentation rates are high and dense tube mats of 

 enrichment polychaetes or amphipods may serve to trap organic particles (Rhoads and Boyer 1982). High organic 

 content of the sediment also reflects the absence of deep biological mixing that would otherwise contribute to 

 efficient aerobic decomposition, i.e. "tertiary" treatment (see above). 



One positive attribute of Zone 2 type seres is that the density and productivity of small opportunistic polychaetes 

 and amphipods can be very high because of the high input of easily metabolized (labile) detrital food. These 

 seres can serve as important food sources for demersal species (many of commercial importance) (Becker and 

 Chew 1983). It is interesting that benthic fish or crustacean food resources may increase just before the benthic 

 system collapses (e.g., reverts to Zone 1) (Rosenberg et al. 1987). 



Zone 1, representing extreme organic loading, has a zero benthic mixing depth. All digestion is via anaerobic 

 bacterial pathways. Such areas are unproductive in terms of fisheries and are sources of anoxic, hypoxic, and/or 

 sulfide water. This water may outwell away from such sites and adversely affect far-field water quality. 



In summary, in situ field mapping of organism-sediment relationships (particularly benthic mixing depths) allows 

 one to identify successional mosaics on the seafloor. The mapped patterns can frequently identify the sources 

 of disturbance that formed them. For example, the above type of survey was successfully conducted off the 

 Louisiana coast around four production platforms to identify the effects of these platforms on benthic processes 

 (SAIC 1985). 



Once mapped, inferences about geochemical processes can be made in the context of the successional model 

 outlined here. Each sere is associated with benthic processes that have important implications for overall 

 ecosystem functions. This paradigm has particular utility for addressing management questions about how oil and 

 gas activities (or any other disturbance) may affect benthic succession, secondary productivity, and geochemical 

 processes. 



REFERENCES 



Aller, R. 1982. The effects of macrobenthos on chemical properties of marine sediment and overlying water, 

 pp. 53-102. In P.L. McCall and M.J.S. Tevesz, eds. Animal-Sediment Relations: The Biogenic Alteration 

 of Sediments. Plenum Press, New York and London. 



Becker, D.S., and K.K. Chew. 1983. Fish-benthos coupling in sewage enriched marine environments. NOAA 

 Final Report, Project NA80RAD00050, School of Fisheries, Univ. of Washington, Seattle, WA. 78 pp. 



Carney, R.S. 1989. Examining relationships between organic carbon flux and deep-deposit feeding, pp. 24-58. 

 In G. Taghon and J. Levinton, eds. Ecology of Marine Deposit Feeders, Lecture Notes on Coastal and 

 Estuarine studies. Springer- Verlag New York, N.Y. 



