Molluscan death assemblages on the Amazon Shelf: implication for physical and biological controls on benthic populations

Abstract

The Amazon Continental Shelf is an example of an environment impacted by frequent and intense physical disturbance with regions of high sedimentation rates. Cycles of extensive sediment erosion and deposition result in periods of sediment remobilization and instability in the seabed, and cause marked seasonal and spatial variations in the transport of water column organisms and benthic infauna. In this study, the molluscan death assemblages on the Amazon shelf are examined in an effort to understand how this physical regime is reflected in death assemblages as well as to provide insight into biological controls on mortality. Inner Shelf regions of <20 m water depth, comprise ∼50% of the total shelf area that extends to 100 m and have no relict shell beds or randomly dispersed shells, although a few living shell-bearing species are occasionally present. Extensive death assemblages are found deeper than 40 m. Bivalves were most numerous on the shelf, although gastropods dominate at the stations with the most extensive shell assemblages. The incidence of boring varied between 3 general groups of stations from 5, 12, and 30%, and tended to be similar for gastropods and bivalves. Evidence for significant exposure time on the sediment surface alternating with periods of burial was found. Fewer than one third of all specimens reached adult size suggesting settlement in a marginal habitat. In general the most abundant taxa in the living community were also dominant in death assemblages. The most abundant relict bivalve, Crassinella lunulata however, which reached densities of 5578 m−2, were never found alive on the shelf. This species along with old reworked fragments of coral and bryozoans, indicate onshore transport from south of the shelf or from the outer shelf, where relict coraline deposits are exposed, followed by deposition and downward mixing of shells into the deposit. In spite of the overwhelming influence of the physical regime, the incidence of boring is comparable to other environments suggesting significant biological control of benthic populations.