51 



2.2.4 



THE BENTHIC MIXED LAYER 



Dr. Donald C. Rhoads 



Science Applications International Corporation 



Maritime Technology Group 



89 Water Street 



Woods Hole, MA 02543 



INTRODUCTION 



The benthos are the focus of many monitoring studies because they are long-term integrators of water column 

 and benthic processes. My approach to benthic monitoring is similar to that of Pearson and Rosenberg (1978 i; 

 species assemblages are treated as seres in benthic succession (Rhoads and Germano 1986). In the words of 

 Johnson (1972): " The community is. ...a temporal mosaic, parts of which are at different levels of succession.. ..the 

 community is a collection of the relics of former disasters." 



Application of these ideas to real-world monitoring problems involves mapping of successional mosaics on the 

 seafloor and relating them to Johnson's "disasters". In addition, if we know the ecological functions of each sere, 

 this approach allows us to address certain management issues. For example, predicting how disposal activity 

 might affect successional correlates such as secondary production, geochemical cycling, etc. 



SUCCESSION AND THE BENTHIC MIXED LAYER 



As another example, I will chose organic enrichment as a variable that can cause successional reorganization of 

 benthic communities (Figure 4). First, we will look at the biological features of the seres that develop over 

 space and time along a hypothetical enrichment gradient and then describe the processes which accompany such 

 changes. The generalizations made in the following paragraphs are supported by more than 10 years of world- 

 wide coastal and shelf monitoring experience. 



Biological Features 



Zone 4 in Figure 4 represents the ambient bottom where organic sedimentation rates are on the order of 200 

 gm C/irr /yr or less. Most of the labile organic matter is consumed near the sediment surface by low population 

 densities of filter or deposit-feeding organisms. Most of the biomass is represented by deeper feeding infauna 

 that feed head-down. These infauna circulate water and advect particles over vertical distances of several 

 centimeters. This deep bioturbating assemblage feeds on the breakdown products of relatively refractory organic 

 matter by stimulating microbial decomposition through benthic mixing (Hylleberg 1975; Yingst and Rhoads 19S0; 

 Carney 1989; Rice and Rhoads 1989). 



It is important to note that deep head-down deposit feeding assemblages are best developed in relatively 

 oligotrophic sediments. They have relatively long mean life spans and conservative recruitment. Their presence 

 means that the seafloor has not experienced massive physical disturbance, extended hypoxia, or organic 

 enrichment in the recent past. The thickness of the biologically mixed layer will vary depending locally on the 

 profile (inventory) of refractory and labile detritus (Rice and Tenore 1982) but the mixing depth is usually greater 

 than 10 cm and may extend to several decimeters. 



Zone 3 is closer to the source of organic input and therefore sedimentation rates of organic matter are higher 

 (say, 300-400 gm/nr /yr). The increased inputs of detrital food result in increased biomass of all species, species 

 richness may increase, and bioturbation depths may also increase. 



In Zone 2, rates of organic input approach a kilogram or more of organic carbon/m 2 /yr. Above a critical 

 organic loading rate, (specific for the system of interest), the deep subsurface deposit-feeding faunal element is 

 lost. Only near surface feeding taxa (mainly a few species of enrichment polychaetes) remain, albeit in high 

 densities. The biologically mixed zone may be reduced to only one or two centimeters in thickness. Deep 

 feeding and mixing is absent. The gradient between Zone 3 and Zone 4 communities may be very sharp. 



