Soft-sediment benthic assemblages of Bahia de Navidad (Jalisco, Mexico): spatial and seasonal patterns

Spatial and seasonal patterns of benthic assemblages in a semiexposed sandy bay were analyzed in relation to sedimentary variables and organic load. Four sampling stations with different environmental conditions; sediment size, wave exposure and proxmity of freshwater runoff; were selected along the bay. Monthly samplings were carried out during a year using a suction sampler. Sediment samples were also taken to analyze granulometry and organic load. For analysis purposes data were pooled according to climatic periods present in the area, dry season-winter and rainy season-summer. The north part of the bay showed fine sediment, silt and clay, with higher organic matter percentage, doubling the south percentages during summer, higher density of macrofauna, with a clear dominance of scaphopods and polychaetes; 88% of total abundance; and lower diversity and evenness. Scaphopods showed densities up to 9 788 ind.m(-2). The south part of the bay showed coarser sediment; mainly fine sand; with lower organic matter content, lower macrofaunal density and higher diversity and evenness. Scaphopod densities were much lower in this area. High significant correlation was found between macrofaunal assemblages and the percentage of silt and organic matter (Spearman r(s)=0.94).