General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea

Abstract

Major patterns of bacteria, meiofauna, and macrobenthos distributions were examined from muddy sediments on the inner continental shelf near the Amazon River. The spatial distributions and functional groupings of the fauna were compared with faunal distributions on the East China Sea shelf off the Changjiang (Yangtze) River and used to assess the impact of long-term, large-scale physical processes on benthic community structure in shelf environments. Both areas are influenced by large discharges of fresh water and suspended solids, and have inner-shelf mud deposits. On the Amazon shelf, macrofauna were generally small in size or absent from highly mobile, muddy sediments on the inner shelf. Significant numbers occurred only in relatively stable sediments offshore or to the south or northeast of the mud belt. Macrobenthos reached greatest densities (3915 m −2) in a general area of firm muddy sediments (Stas 79 and 88) interbedded with sand and burrowed by the ghost shrimp Callianassa sp. At these stations, diversity was greatest, particularly in the dominant polychaete taxon. Macrofauna in the East China Sea, particularly epifaunal species, were reduced in abundance and biomass in inner-shelf areas of most active accumulation near the Changjiang River. However, macrofauna were 50–100 times more abundant (ranging to 10,000 m −2) than at sedimentologically comparable locations on the Amazon shelf. For areas of reduced deposition on the East China Sea shelf, diversity was also greater, with 50 species per sample as opposed to a maximum number of 19 per sample of comparable size on the Amazon shelf. Bacterial and meiofaunal abundances were low nearshore on the Amazon shelf compared with farther offshore or, in the case of meiofauna, other shelf environments. Average bacterial abundances for the top 10 cm of sediments ranged from 1.3 to 21 × 10 9 g −1 on the Amazon shelf and from 7.1 to 22 × 10 9 g −1 on the East China Sea shelf. Meiofauna on the Amazon shelf varied from none in the nearshore mobile mud belt to 2045 (10 cm −2) offshore on firm silty clay sediments. These densities compare with 66 (10 cm −2) near the Changjiang River mouth and 4870 (10 cm −2) farther offshore. Nematodes dominate the meiofauna on both shelves. The high degree of physical disturbance and unstable nature of the seabed coupled with reduced detrital food availability in bottom sediments appear to be the major factors limiting faunal abundances, controlling the taxonomic diversity of the meio- and macrofaunal species, and determining the functional groups of macrobenthos on the Amazon inner shelf, and, to a lesser extent on the East China Sea inner shelf.