Annelid assemblages in soft bottoms subjected to human impact in the Urías estuary (Sinaloa, Mexico)

The annelid fauna inhabiting soft bottoms was analysed during February and April, 1997 in the Urías estuary which is mostly surrounded by mangrove swamps, but is also subjected to the effect of several anthropogenic activities. Classification and ordination techniques were used to perform the characterisation of the system based on the annelid assemblages according to environmental variables (mainly organic matter content and percentage of mud). Three zones were found: (1) ‘very polluted zone’, in front of a slaughterhouse and a fish processing factory with a total lack of macrofauna due to the strong enrichment of the sediment in organic matter; (2) ‘polluted zones’, characterised by the sporadic presence of oligochaetes in front of a domestic sewage effluent, by the dominance of Capitella capitata close to a thermoelectric plant, and by the dominance of Streblospio benedicti and oligochaetes in the surroundings of an upstream shrimp farm; (3) ‘unpolluted zones’, in the mouth of the estuary and an area surrounded by mangrove trees, with low organic matter content in sediment, high diversity and no dominant species.