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Abundance, distribution, and feeding aspects of selected zooplankton species in the subartic Pacific Wen, Marc Eric An-Ping


Changes in the abundance of zooplankton species from the North American west coast to the open northeastern subarctic Pacific are not well documented. This study examines the abundance and distribution of important zooplankton species in the subarctic Pacific from the continental slope region to the open ocean in relation to physical, chemical, and biological parameters. Zooplankton samples were collected to a depth of at least 100 m using a modified SCOR (WP-2) net along the Line P transect (about 49°N, 127°W to 50°N, 145°W), during March and May 1993, and February and May 1994. Despite recent efforts to quantify the grazing pressures imposed on phytoplankton by the predominant copepods species of the subarctic Pacific, there is little detailed information on the diets of the predominant copepod species in this region. A shipboard experiment was conducted at Station P in May 1994, to examine the feeding behaviour of a predominant subarctic copepod species, Neocalanus flemingeri (Miller), grazing on a natural assemblage of prey items. Single copepods were incubated in 1 L bottles for 24 h with controls, and laboratory enumeration of autotrophic and heterotrophic prey items was made using inverted transmission microscopy and epifluorescence microscopy. Zooplankton biomass (0-500 m) was generally predominated by copepods of the genus Neocalanus, although other zooplankton genera made significant contributions during all cruises. The development of some species of Neocalanus appears to occur progressively later seaward of the continental slope which may be the result of a coupling mechanism between copepod spawning and small blooms of large diatoms in the eastern subarctic Pacific. Zooplankton biomass was greatest during May cruises, but was also more variable between the two late spring cruises than between the two winter cruises. Zooplankton biomass (excluding salps) was more variable east of 134°40'W in late spring (< 100 m) and was generally lower when salps were present, except at Station P16 in May 1993, when salp numbers and zooplankton biomass were both at their highest during this cruise. Salps were observed only during the 1993 cruises and were never present at Station P. Although their presence along Line P in 1993 may be due to the encroachment of southern waters, their numbers do not appear to be directly associated with changes in sea surface temperature. Neocalanus flemingeri did not clear all size fractions of prey items at equal rates, and a calculated selectivity index indicates a feeding preference on autotrophic cells over heterotrophic cells. In addition, the selectivity index shows that cells < 10 iim were generally predated on preferentially to larger cells. N. flemingeri obtained about 69% of its metabolic requirements from ciliates alone. However, a comparison of ciliate growth rate and copepod clearance rate on ciliates suggests that N. flemingeri and Neocalanus plumchrus (Marukawa) cannot control the ciliate population. N. flemingeri consumed < 5 /um autotrophic cells at double the rate previously reported for N. flemingeri or N. plumchrus. Total ingested carbon was enough to allow substantial daily body growth. Despite this fact, estimates of community ingestion support the notion that copepods in the subarctic Pacific do not control the phytoplankton stock. N. flemingeri and N. plumchrus do not appear to contribute substantially to ammonium recycling in the mixed-layer, suggesting there exist other mechanisms for this process. Estimated biomass of fecal pellets produced by these species was approximately 61% of the downward vertical carbon flux at 100 m and may account for up to 28% of the metabolic requirements of N. cristatus residing at the bottom of the mixed-layer. Other sources of sinking particles are likely important for N. cristatus nutrition, and probably come from other major zooplankton species.

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