eastern states (Meade 1969). Turbidity is signifi- 

 cant biologically in that sunlight penetration is so 

 reduced in extremely turbid waters that phyto- 

 plankton productivity is inhibited and subtidal 

 bcnthic algal production is nearly eliminated. 



Physical changes in the water column are 

 greatly buffered by the sediments, such that ben- 

 thic infauna (animals living buried within the sedi- 

 ments) are sheltered from the extensive environ- 

 mental variability of the overlying waters. This 

 buffering action occurs for temperature (Johnson 

 1965), salinity (Reid 1930, 1932, Sanders et al. 

 1965, Johnson 1967), and other physico-chemical 

 properties, and is especially significant for short- 

 term variation such as is generated by the tidal 

 cycle. The deeper one penetrates into the sedi- 

 ments, to a depth of approximately 10 cm, the 

 greater is the buffering effect (Johnson 1965). An 

 infaunal mode of living permits the avoidance of 

 many of the rigors f)f the estuarine physical envi- 

 ronment. 



The intertidal zone is a physically rigorous 

 place to live for any marine organism. Exposure 

 to air, the sun's heat, and the wind during a por- 

 tion of almost every tidal cycle is inevitable. 

 This exposure can cause desiccation, overheating, 

 and death for many marine organisms. Among the 

 marine animals (jn an intertidal flat, feeding must 

 cease whenever the overlying waters recede. Be- 

 cause the highest levels of the intertidal zone are 

 exposed for longer periods than the middle and 

 lower intertidal zones, there is a gradient of in- 

 creasing intensity of physical rigor which runs 

 from the subtidal to the top of the intertidal zone. 

 The effects of this gradient in physical stress have 

 been well described on rocky shorelines (Connell 

 1970), but there is very little information avail- 

 able on the impact of the varying degrees of aerial 

 exposure on soft-sediment organisms. It seems 

 likely that numerous species of plants and animals 

 in soft sediments are restricted to subtidal habi- 

 tats or at least to the lowest intertidal zones by 

 the rigors of exposure. For instance, the seagrasses 

 are almost certainly limited at the high margins of 

 their distribution by such increased physical harsh- 

 ness. Many epifaunal species, which as a group do 

 not possess a sedimentary buffer, are also limited 

 to subtidal zones or to the low intertidal by phy- 

 sical stress. Infauna with protective outer skeletons 

 or shells (such as clams) are probably not so 

 greatly affected by such exposure to air. 



Sandy sediments contain interstitial spaces 

 among the sand grains which permit a great deal 

 of diffusive exchange with the overlying water 

 column. For this reason, the buffering effect of 

 living at depth in the sediments is not quite as 

 great in sands as it is in muds, although it is still a 

 significant factor. Because of the large quantities 

 of oxidizable organic matter (detritus) contained 

 in the sediments of estuarine systems, the biolo- 

 gical oxygen demand (BOD) is extremely high 

 in the sediments. This demand produces anoxic 

 conditions at depth in the sediment column of 

 mud flats and sand flats. Because of the higher ex- 

 change rates with the overlying oxygenated waters 

 and the lower concentrations of detritus in sandy 

 sediments, the boundary between the oxygenated 

 layer at the surface and the deeper anoxic sedi- 

 ments (the so-called "redox layer") occurs at a 

 somewhat greater depth in sandy sediments than 

 in muddy sediments (Figure 4). 



Sandy and muddy sediments generally differ 

 radically in theirchemical environments as a direct 

 consequence of their differing BOD levels. In 

 muds, the supply rate and concentration of detri- 

 tus are sufficient to use up all the oxygen avail- 

 able so that a reducing environment exists below 

 the surface centimeter. Here anaerobic bacteria 

 dominate the sediment chemistry and will pro- 

 duce reduced compounds which accumulate in 

 the sediments (e.g., characteristically black iron 

 sulfide and distinctively odoriferous hydrogen sul- 

 fide). This smell of "rotten eggs" is what many 

 visitors best remember of a visit to a coastal mud 

 flat. In contrast, detritus is supplied to a sand flat 

 at a rate for which sufficient oxygen exists for 

 oxidative decomposition reactions, and the chem- 

 ical environment is far different. To emphasize 

 this difference, Fenchel (1969) has coined two 

 terms, defining the microbial community found 

 on mud flats as a sulfuretum system and the sand 

 flat community as an estuarine sand micro- 

 biocenosis. 



In shallow estuaries and sounds, the sediments 

 actually determine much of the water chemistry 

 because of their frequent rcsuspension and great 

 chemical activity. In some deeper areas, particu- 

 larly where there is seasonal stratification of the 

 water column and reduced mixing, the water col- 

 umn at depth can become anoxic as a result of 

 the BOD of the sediments. Such anoxic conditions 

 frequently occur during the summer in areas of 



