Biological effects of El Niño 1997-98 on a shallow subtropical ecosystem: Bahía Magdalena, Mexico

Sea surface temperature (SST), zooplankton biomass and chlorophyll a concentration, along with abundance of four copepod species: Acartia clausi, Acartia lilljeborgii, Paracalanus parvus and Calanus pacificus, and eggs and larvae of small pelagic fishes: Sardinops caeruleus, Scomber japonicus, Opisthonema spp. and Anchoa spp., were measured and estimated from May 1997 to December 1998 in Bahía Magdalena, Mexico. In order to study response of these species to seasonal environmental conditions and to the anomalous warming conditions driven by the 1997-98 El Niño event. Positive SST anomalies were present during 15 months with a maximum value of +4.4 °C recorded during summer of 1997. Zooplankton biomass was lower by half than the previous El Niño 1982-83, but the seasonal pattern was maintained. A chlorophyll concentration maximum of 9.2 mg/m³ was observed in late spring to early summer, with a minimum of 0.2 mg/m³ in winter. Copepod abundance changes and an increase of tropical species were registered during the most intense phase of the warming. The temperate copepod Calanus pacificus associated with the California Current, was recorded only during the cold period before the onset of El Niño. The seasonal abundance of autochtonous species followed the pattern previously recorded in the bay, but decreased by about one third, compared to El Niño 1982-83. The abundance of temperate and tropical small pelagic fishes showed important changes related to the intense warming effect, with half the abundance of previous reports for temperate Sardinops caeruleus while, tropical Opisthonema spp abundance increased by up to an order of magnitud in 1997-98.