and phosphorite deposits may have a strong acoustic re- 

 flecting effect since they occur at or near the sediment- 

 water interface. How these concentrations arose is not 

 not clearly understood at the present time. 



III. BENTHONIC ORGANISMS 



Organisms living on or near the sea floor surface 

 are nearly always associated with soft, organically rich 

 clays, and nearshore silts and clayey silts. If burrowing 

 organisms such as worms and sea cucumbers are present, 

 the bottom materials are apt to be churned or disrupted by 

 feeding, exploratory, or escape activities. Surface feeders 

 such as sea urchins, crustaceans, and brittle stars account 

 for a large portion of the tracks, trails, scratches, and 

 marks found on bottom sediments. Brittle stars and sea 

 cucumbers engage in both burrowing and surface feeding 

 activities. Other organisms such as corals, sponges, sea 

 pens, and some worms hold fast to the bottom and filter- 

 feed directly from the water. Crustaceans and brittle 

 stars normally move about over many bottom environments 

 seeking detrital particles and modifying microstructures 

 such as mounds and pits formed by burrowing organisms. 



Fewer species of animals, but often of larger size, 

 are present in deeper environments where food is relatively 

 scarce. Animals are normally abundant in shoaler near- 

 shore areas where surface plankton blooms and subaerial 

 runoff from adjacent land areas provide large quantities of 

 organic matter. Benthonic organisms are not present in 

 high enough concentrations in deeper waters to greatly 

 affect directed sonar signals. 



IV. CHURNING OF SEA FLOOR SEDIMENTS 



The churning is related directly to the abundance of 

 burrowing organisms and the quantity of food matter present; 

 and it is most noticeable in clays and clayey silts, and in 

 shallow water. Water depth is related to the degree of 

 churning, amount of resultant microrelief, and the pre- 

 dominant type of microstructure present. Churning takes 



51 



