The distribution and seasonal characteristics of benthic communities on the Amazon shelf as indicators of physical processes

Characteristics of the benthic biological community at 11 stations on the Amazon continental shelf indicate strong seasonal and spatial patterns that covary with shelf-wide physical processes. Faunal abundances, size-frequency distributions, functional groupings and depths of burrowing and feeding respond to local depositional and erosional events that vary over time scales of several months, possibly less, as well as to shelf-wide seasonal changes. During February–March, the period of rising to peak riverine discharge and maximum trade-wind stress, shelf-wide minima occur in the sizes, numbers and vertical distributions of benthos. These minima reflect maximum instability of the seabed and apparent turnover of at least the upper ∼1 m of the deposits. In contrast, from August to October, corresponding to periods of falling to low riverine discharge and minimum wind stress, there are large influxes of burrowing infauna (polychaetes reach densities up to ∼6700 m−2 off the river mouth), meiofauna become abundant (particularly juvenile macrofauna up to ∼10,000 individuals per 100 cm2) and bacterial inventories increase up to two orders of magnitude relative to other periods (for example, 218 cells per 109 cm2 vs 4.4 cells per 109 cm2 over the upper 10 cm). The seabed appears to be most stable during falling to low flow, resulting in successful benthic recolonization. Superimposed on the significant shelf-wide seasonal variability are distinct nearshore to offshore and south to north trends in biological characteristics. Lack of sexually mature individuals indicates that it is unlikely that many of the macrofauna reproduce on this shelf. Epifauna are rare and suspension-feeding benthos are restricted to deeper-water stations off and to the south of the river mouth. As in other terrigenous tropical environments, bacterial biomass accounts for much of the total benthic biomass (up to ∼96%). Vertical and lateral distributions of sediment bacteria support the concept of rapid cross-shelf exchange and seasonal homogenization of surficial bottom sediments.