The effects of the 1997-99 El Nino/La Nina events on hydrography and zooplankton off the central Oregon coast

Hydrographic data and nutrient, chlorophyll and zooplankton samples were collected biweekly at a hydrographic station off Newport, Oregon during the 1997-98 El Nino and 1998-99 La Nina event. Our monitoring in 1997 showed that coastal upwelling was initiated in April, the usual time. However, a series of southwesterly storms in May and June 1997 led to a prolonged warming event and reductions in copepod biomass. Though El Nino-driven atmospheric teleconnections might have been responsible for these storms, the coastal ecosystem returned to normal with the resumption of coastal upwelling in mid-July 1997. A downwelling event began on 20 August 1997 resulting in onshore movement of offshore waters. This event appeared to mark the passage of the first Kelvin wave. At this time the shelf became flooded with low-nutrient waters and offshore, 'warm-water' copepod species were transported shoreward. The abundance of local endemic boreal neritic copepod species was reduced greatly, presumably because they were transported out of the system and to the north as a result of the strong poleward flows. Poleward flows strengthened during autumn, and the ocean off Oregon remained anomalously warm through the winter of 1997, spring, and early summer of 1998. Species composition of the coastal copepod assemblage remained anomalous for about a year; species with offshore and subtropical affinities dominated from late August 1997 through late July 1998. During this same period, nitrate concentrations were reduced by a factor of 4.5 (11-month mean of 1.5 muM for August 1997-July 1998 versus 6.9 muM for August 1998-July 1999), copepod biomass was reduced by a factor of 2.1 (4.9 mug carbon 1(-1) versus 10.5 for the same time periods), but chlorophyll concentrations were similar (2.0 versus 2.5 mug 1(-1)). Perhaps the most dramatic effect on copepods during the El Nino period was the nearly complete disappearance of the normally dominant boreal neritic copepod species (Pseudocalanus mimus, Calanus marshallae and Acartia longiremis) and their replacement by subtropical species such as Calanus pacificus, Paracalanus parvus, Ctenocalanus vanus, Corycaeus anglicus and several species of Clausocalanus. These species persisted in the coastal waters through the end of 1998. Although most of the subarctic species began to increase in numbers in August 1998, the impact of the El Nino on Pseudocalanus mimus, normally the numerical dominant in coastal waters, was longer lasting. The P. mimus population did not recover to normal abundance levels until summer of 1999. (C) 2002 Elsevier Science Ltd. All rights reserved.