Active export of carbon and nitrogen at Station ALOHA by diel migrant zooplankton

With a high proportion of diel migratory zooplankton and low particle fluxes, the oligotrophic subtropical Pacific is a region where migrant-mediated export of materials across the base of the euphotic zone should be particularly important for elemental budgets. To assess the magnitude of this export flux at Stn. ALOHA (22°45′N, 158°W), we determined size-structured migrant biomass on 26 cruises (1994–1996) and used allometric equations to compute metabolic losses of respiratory carbon (DIC) and excreted ammonium (DIN) at the migrantʹs daytime depth (temperature). Depth-integrated (155 m) migrant biomass varied interannually from 286 to 457 mg DW m−2 and seasonally from 285 to 550 mg DW m−2 (fall versus spring), with an overall average of 394 mg DW m−2. The 2–5 mm size-fraction always dominated. Migrant fluxes varied from 0.22 to 0.37 mmol DIC m−2 d−1 and from 0.033 to 0.055 mmol DIN m−2 d−1 between years, with an overall average of 0.304 mmol C and 0.045 mmol N m−2 d−1 (15 and 20% of trap PC and PN fluxes). The fluxes were highest in the spring (0.405 mmol C and 0.060 mmol N m−2 d−1; 18 and 24% of trap PC and PN). Strong migratory taxa, defined as those that were virtually absent from the euphotic zone during the day, were dominated by Pleuromamma and Euphausia spp. and Aetideid copepods and accounted for 40% of the total biomass-derived estimates. Using mean cruise biomass structures and temperature information from Stn. ALOHA, migrant flux estimates with different literature-derived metabolic relationships and assumptions typically varied within 20% for carbon and 25% for nitrogen. When compared on the same basis, migrant flux estimates as % trap fluxes were higher for oligotrophic regions compared to the high-nutrient, low-chlorophyll equatorial Pacific. Long-term averages of migrant biomass were substantially (two- to three-fold) higher in the subtropical Pacific compared to the Atlantic, but migrant flux estimates were similar due to higher temperatures (18 versus 9°C) at daytime depths in the Atlantic. Including the effects of undersampled micronekton, dissolved organic excretion, mortality losses at depth and inter-zonal fecal transport, mean flux estimates were about doubled for carbon (30% of trap PC) and approximately tripled for nitrogen (57% of trap PN) relative to those for DIC and DIN metabolic losses.