The distribution of macrofauna on the inner continental shelf of southeastern Brazil: The major influence of an estuarine system

The environmental heterogeneity of the Campos Basin on the northern coast of Rio de Janeiro State was assessed by the benthic macrofauna on the platform adjacent to the Paraíba do Sul River (PSR) on the dry and rainy seasons. The samples were collected in triplicate from 33 sites using a van Veen grab during March 2009 – a period of higher precipitation and flow rate – and July 2009 – a period of lower precipitation and flow rate. The grab depths ranged from 12 to 97 m and were grouped into three strata: 1: <25 m, 2: 25–50 m and 3: >50 m. The particle size, total carbonate and total organic carbon in each sample were analyzed. Subsamples for the macrofauna analysis were washed, sieved with a 500 μm mesh and identified. The sediment was predominantly composed of sand, with mud pockets near the mouth of the river. The macrofauna included annelids, crustaceans, mollusks, echinoderms, cnidarians, nemerteans, cephalochordates, sipunculids and bryozoans. The density and richness were directly related to the depth, with both descriptors being higher during the rainy season and at depths greater than 50 m. This result is probably due to the higher availability of food in the river during this period and is corroborated by the predominance of deposit feeders in the deepest stratum. The number of individuals of each species was higher in the shallowest stratum, probably due to the higher productivity of this stratum. The rate of organic particulate matter flow from the coastal regions to the deeper regions can also be influenced by the material export dynamics of the river, which are more intense during the rainy season. These dynamics explain why a significantly higher number of individuals were observed in the rainy period in comparison to the dry one. Multivariate analyses identified differences between the sampling sites in the deepest stratum during both periods and revealed a stronger similarity between the shallow and intermediate strata, especially during the rainy season, which has a higher flow rate. These findings reinforce the influence of the Paraíba do Sul River on the structural pattern and composition of the benthic macrofauna of the internal platform (<50 m) on the northern coast of Rio de Janeiro.