A depauperate benthic assemblage from the nutrient-poor sediments of the Puerto Rico Trench

The Puerto Rico Trench contains a low-density-biomass assemblage of macrofauna and meiofauna. The depauperate character of this assemblage is primarily the result of the low percentage and refractory nature of organic matter in the sediments. Recent turbidite flows may have contributed to the paucity of fauna. Sediments from this portion of the Puerto Rico Trench are the result of pelagic sedimentation interspersed with thin laminae that probably originated as turbidity flows from the northern and eastern, or deep-sea, walls of the trench. These recently deposited pelagic clays are low in available organic matter. Bioturbation and other mechanical and geochemical processes have not compacted these sediments into a stable, habitable substrate. The “oligotrophic” assemblages found in the Puerto Rico Trench contrast with “eutrophic” hadal assemblages which inhabit coarser-grained, organic-rich sediments. These types of sediments usually originate as turbidity flows from the walls of island-arc trenches proximal to shallow-water shelves and provide a substrate of higher shear strength and greater nutritive value.