Polychaetes are key components of marine ecosystems. Their diversity of ecological niches is enormous, playing a significant role in sedimentary habitats. This study broadens the biodiversity information for the west coast of Baja California and particularly for Bahía Magdalena. Polychaete diversity and structure were assessed at 21 stations. Polychaetes were the dominant macrofaunal group with 30 families and 102 species identified in samples collected during two cruises in December 1996 and October 1999.
Best-represented families were Capitellidae, Cirratulidae, Nephtyidae, Nereididae, Onuphidae, Paraonidae, and Spionidae. In 1996, densities ranged from 90 to 3460 individuals/m2 and Shannon index varied between 1.76–3.12 with the highest diversity values located in the northwestern area of Bahía Magdalena. In 1999, densities ranged from 70 to 1440 individuals/m2 and Shannon index was 1.72–2.83, and the highest values occurred in the north area of the bay. Organic matter content in sediments varied between 0.87–5.70%. Four functional feeding groups (carnivores, herbivores, deposit-feeders, and suspension-feeders) were found with deposit-feeders and carnivores as the most abundant. Multivariate analyses performed with abundance data showed different polychaete assemblages in 1999 compared to 1996. This work can serve as a base-line study to determine future changes in polychaete assemblages.