The 1997-98 El Nino and its effects on the coastal marine ecosystem off Peru

We summarize the important effects of the extraordinary El Nino 1997-98 oil the Peruvian marine ecosystem, and we compare these with what was observed during the extraordinary El Nino 1982-83. The SST anomalies at coastal sta tions. as well as data from marine explorations carried out during 1996-98, show among other things, that the preceding cold period ended in January-F ebruary 1997; that El Nino 1997-98 was clearly defined along all the Peruvi an coast in March-May 1997 (earlier in the north) and ended between April a nd June 1998, beginning from the south; and that El Nino was followed by a moderately cold period, evident since August 1998 (winter in the Southern H emisphere). The most dramatic alterations in chemical characteristics were observed in December 1997 and February 1998, when low concentrations of nit rate (0.15-1.1 mug-at/L) extended south from Callao. Apparently chemical co nditions were more extreme in the water column in 1997-98 than in 1982-83, because the oxycline and nutricline were located at greater depths. Anomalously small volumes of plankton were observed, and abundant dinoflage llates and copepods typical of warm waters entered the coastal waters with the equatorial surface waters (ESW) and subtropical surface waters (SSW), s imilar to what occurred during El Nino 1982-83. Anchovy were found near the coast ill the first phase, in the second phase they shifted toward the sou th and into deeper water, while sardine increased in the first phase, and t hen migrated to the south. Hake widened their distributional range to the s outh and also vertically, because this species inhabits the zone influenced by the southern extension of the Cromwell Current (SECC). On the other han d, subtropical, equatorial, and even tropical species widened their distrib ution, bring observed off the Peruvian coast. Of special relevance was the samasa, because its abundance and distribution reached 18 degreesS, which m ade it an important resource for the Peruvian fishery during El Nino 1997-9 8.