US GLOBEC Newsletter 14: Zooplankton Abundance

| Changes in Zooplankton Abundance and Species Composition in the Oregon Coastal Zone by Bill Peterson and Julie Keister
Also in This Issue .... LTOP in the NEP Zooplankton in OR Coastal Zone El Niño Affects Marine Nekton GLOBEC Studies in the CGOA GoMex Workshop - Summary Cod & Climate Change Program Notes From the Field .... Sea Scallop Fishery Closures
Zooplankton abundance and species composition mirrored the physical observations discussed by Smith et al. The two copepod species which are most abundant in the Oregon upwelling zone, Pseudocalanus mimus and Calanus marshallae, began to decline in numbers commensurate with the positive sea level anomalies in May 1997. Numbers remained low throughout that summer. The populations did not begin to increase until the following summer, in August 1998, when sea level returned to normal (Fig. 1; Pseudocalanus mimus). Thus, densities of the common coastal copepod species were anomalously low during both the summers of 1997 and 1998. We attribute reduced numbers in 1997 to a combination of reduced upwelling and reduced secondary production. The failure of the populations to respond in 1998 may have been due to very low initial numbers in spring caused by removal of most individuals from the coastal zone by strong northward transport that persisted through the winter and spring of 1997/98. We hypothesize that when normal transport patterns were established in summer 1998, then members of the P. mimus and C. marshallae populations were returned to the Oregon upwelling zone and began to increase in numbers. "Three .... euphausiid species were collected that have NEVER been reported for coastal or oceanic waters off Oregon ...." FIGURE 1. Density of the Pseudocalanus mimus population at a station five miles off Newport, OR, in 1997 and 1998, compared to a five-year climatology record. In a "normal" year, numbers ca. 2000 individuals / cubic meter can be expected anytime between June and September (day 150-273). FIGURE 2. Density of the Calanus marshallae population at a station five miles off Newport, OR, in 1997 and 1998, compared to a five-year climatology record. In a "normal" year, numbers of 200-400 / cubic meter can be expected anytime between April and September (day 90-273). The El Niño also resulted in the appearance of a large variety of zooplankton species that ordinarily reside well offshore of Oregon and/or to the south. Copepods that became common in the coastal zone during the 15-month El Niño of 1997/98 included Calanus pacificus, Eucalanus californicus, Mesocalanus tenuicornis, Clausocalanus paululus and C. parapergens, Ctenocalanus vanus, Paracalanus parvus and Calocalanus styliremis. Unusual mollusks included Corolla spectabilis and Carinaria spp. Perhaps the most striking differences were seen in the euphausiid species complex: the coastal species Nyctiphanes simplex, which is common off southern and Baja California, appeared commonly in plankton samples collected between January and September 1998. This species was also common off Oregon during the 1982-83 El Niño. Three other euphausiid species were collected that have NEVER been reported for coastal or oceanic waters off Oregon: Euphausia recurva, E. mutica , and E. gibboides. These species are usually restricted to the northern portion of the north Pacific Central gyre, and to a zone several hundred miles offshore in the California Current south of 40 °N. All of these observations indicate: (a) reduced production during the summer of 1997, and (b) strong poleward and/or onshore flows during autumn-spring 1997/98 which transported the common species out of the area (and to the north?) and offshore and southern oceanic species into the area. See Gomez-Gutierrez & Peterson (1999) for a description of how anomalous conditions during the summer of 1997 affected copepod egg production. Literature Cited: Gomez-Gutierrez, J. and W.T. Peterson. 1999. Egg production rates of eight calanoid copepod species during the summer of 1997 off Newport, Oregon, USA. J. Plankton Res. 21: 637-657. Last updated: 14 October, 1999 Please address any comments, suggestions or questions to: GLOBEC Webmaster