Parameters relating to the distributions of planktonic organisms, especially euphausiids in the eastern tropical Pacific

Zooplankton was sampled through eight depth intervals above about 500 m along a transect of the eastern tropical Pacific (ETP), 23°N to 3°S, encompassing four environments. (1) The California Current—ETP transition off Baja California and the mouth of the Gulf of California is inhabited by California Current species at their southern limits, and by the galatheid ‘red crab’ Pleuroncodes planipes together with euphausiids (e.g. Euphausia eximia) of an abundance-based recurrent group of species, distinguished using the criterion of > x abundance (Numbers under unit area of sea surface) at common localities, adapted to productive zones marginal to the O2-deficient part of the ETP. Tropical species appear here where water with surface temperature > 26°C and [O2] of < 0.1 ml l−1 beneath a shoaling thermocline replaces the upwelling environment off Baja California. (1) The zone 22° to 10°N harbors euphausiids of two groups: the vertically migrating tropical species (e.g. Euphausia diomedeae) which tolerate intense O2-deficiency at their daytime depths and enter the oxygenated mixed layer at night, and non-migrating Stylocheiron species which have vertical ranges extending up into the mixed layer. Nevertheless, most of these ‘ETP-adapted’ species are denser farther south, in the north equatorial countercurrent, but three ETP endemics (e.g. E. distinguenda), all vertical migrators belonging in one subgeneric division of Euphausia, are densest in the O2-deficient regions. (3) The zone of the North Equatorial Countercurrent maintains high densities of three groups: the widely-ranging, ETP-adapted tropical species, the four common Stylocheiron species which, while recurring in abundance at the same localities, differ in depth and the mesopelagic tropical-subtropical species, not tolerant of O2-deficiency, which occur here in easterly tongues of range. (4) At the equator (93°W), easterly ranging species (e.g. E. paragibba) and westerly Nyctiphanes simplex appear to migrate between equatorial currents which differ in direction with depth, thereby maintaining their narrow ranges along the equator. The ‘marginal proliferators’ such as E. eximia, prominent off Baja California, are again abundant here, availing of the equatorial divergence for high productivity and of the oppositely-directed currents for geographical stability. A second recurrent grouping of species, based on presence of their larvae at common localities, yielded groups also distinguishable by whether the larvae lived within or beneath the mixed layer. Ontogenetic strengthening of vertical migration capability is demonstrated by many species, with older larvae, juveniles and adults showing ranges, respectively, increasing from a few meters to up to 400 m. The pattern is the same in O2-deficient regions as elsewhere. Regional distribution of euphausiid volume (wet displacement biomass) tended to agree with zooplankton volume, with maxima at the equator, 8°N, and at some localities off Baja California and the Gulf of California where red crab volume peaked. The depth at which euphausiid volume is equal in amount day and night, across which vertical migration takes place, is designated the equilibrium depth (EqD) for euphausiid volume. EqD for euphausiids generally agreed with EqD for zooplankton volume, indicating that euphausiids play a role in determining depth of EqD for zooplankton volume. Euphausiids comprised 13% (x) of zooplankton volume. 80% (x) of euphausiid volume migrated across EqD, the value showing no significant regional differences. 37% (x) of zooplankton volume engaged in such migration, but in the region south of 14°N encompassing the broad O2-deficient zone, the value was 26%, which compares with 18% previously determined for biomass transferring in a comparable way between epiplankton and planktostad in the same region.