Estructura de la comunidad de copépodos en Bahía Magdalena, México, durante El Niño 1997-1998

Seasonal changes in copepods taxocoenosis associated to environmental changes caused by El Niño 1997/98 were evaluated during the four most intensive months of the event. Sixty six copepod species were identified. In May, before the arrival of warm waters to the area, the taxocoenosis structure was characterized by a high dominance, low diversity and the predominance of two species of temperated affinity: Calanus pacificus and Labidocera trispinosa. During the warm period (August and November), the dominant species were of tropical affinity, such as Acartia clausi and A. lilljeborgii. In August, diversity increased in the stations influenced by oceanic waters whereas in November, diversity increased in the entire bay; in January, the dominance was relative low and the most abundant tropical species was Paracalanus aculeatus, and a general increase of the diversity was observed in the entire bay. Two environmental zones in the bay were identified: one characterized by internal waters of the bay, with resident species and with those species frequently entering the bay by tidal currents, and another zone characterized by the influence of outside oceanic waters, with the dominance of oceanic or neritic species. In this last area, it is more clearly reflected the warm water arrival and its associated fauna. The most remarkable changes in the copepods community were coincident with the occurrence of positive temperature anomalies. Changes included a 75% increase of species of tropical affinity and the presence of Pontella fera, which had not been previously recorded in the area, and the replacement of the temperate species P. parvus, by P. aculeatus, a species of tropical affinity.