@article{fdi:010074775,
  title = {{M}eso-zooplankton structure and functioning in the western tropical {S}outh {P}acific along the 20th parallel south during the {OUTPACE} survey ({F}ebruary-{A}pril 2015)},
  author = {{C}arlotti, {F}. and {P}agano, {M}arc and {G}uilloux, {L}. and {D}onoso, {K}. and {V}aldes, {V}. and {G}rosso, {O}. and {H}unt, {B}. {P}. {V}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he western tropical {S}outh {P}acific ({WTSP}) is one of the most understudied oceanic regions in terms of the planktonic food web, despite supporting some of the largest tuna fisheries in the world. {I}n this stratified oligotrophic ocean, nitrogen fixation may play an important role in supporting the plankton food web and higher trophic level production. {I}n the austral summer ({F}ebruary-{A}pril) of 2015, the {OUTPACE} ({O}ligotrophy to {U}l{T}ra-oligotrophy {PAC}ific {E}xperiment) project conducted a comprehensive survey of 4000 km along 20 degrees {S}, from {N}ew {C}aledonia to {T}ahiti, to determine the role of {N}-2 fixation on biogeochemical cycles and food web structure in this region. {H}ere, we characterize the zooplankton community and plankton food web processes at 15 short-duration stations (8 h each) to describe the large-scale variability across trophic gradients from oligotrophic waters around {M}elanesian archipelagoes ({MA}s) to ultra-oligotrophic waters of the {S}outh {P}acific gyre ({GY}). {T}hree long-duration stations (5 days each) enabled a more detailed analysis of processes and were positioned (1) in offshore northern waters of {N}ew {C}aledonia ({MA}), (2) near {N}iue {I}sland ({MA}), and (3) in the subtropical {P}acific gyre ({GY}) near the {C}ook {I}slands. {A}t all stations, meso-zooplankton was sampled with a bongo net with 120 mu m mesh size to estimate abundance, biomass, community taxonomy and size structure, and size fractionated delta {N}-15. {S}ubsequently, we estimated zooplankton carbon demand, grazing impact, excretion rates, and the contribution of diazotroph-derived nitrogen ({DDN}) to zooplankton biomass. {T}he meso-zooplankton community showed a general decreasing trend in abundance and biomass from west to east, with a clear drop in the {GY} waters. {H}igher abundance and biomass corresponded to higher primary production associated with complex mesoscale circulation in the {C}oral {S}ea and between 170-180 degrees {W}. {T}he taxonomic structure showed a high degree of similarity in terms of species richness and abundance distribution across the whole region, with, however, a moderate difference in the {GY} region, where the copepod contribution to meso-zooplankton increased. {T}he calculated ingestion and metabolic rates allowed us to estimate that the top-down (grazing) and bottom-up (excretion of nitrogen and phosphorous) impact of zooplankton on phytoplankton was potentially high. {D}aily grazing pressure on phytoplankton stocks was estimated to remove 19% to 184% of the total daily primary production and 1.5% to 22% of fixed {N}-2. {T}he top-down impact of meso-zooplankton was higher in the eastern part of the transect, including {GY}, than in the {C}oral {S}ea region and was mainly exerted on nano- and micro-phytoplankton. {T}he regeneration of nutrients by zooplankton excretion was high, suggesting a strong contribution to regenerated production, particularly in terms of {N}. {D}aily {NH}4+ excretion accounted for 14.5% to 165% of phytoplankton needs for {N}, whereas {PO}43- excretion accounted for only 2.8% to 34% of {P} needs. {F}rom zooplankton delta {N}-15 values, we estimated that the {DDN} contributed to up to 67% and 75% to the zooplankton biomass in the western and central parts of the {MA} regions, respectively, but strongly decreased to an average of 22% in the {GY} region and down to 7% in the easternmost station. {T}hus, the highest contribution of diazotrophic microorganisms to zooplankton biomass occurred in the region of highest {N}-2 fixation rates and when {T}richodesmium dominated the diazotrophs community ({MA} waters). {O}ur estimations of the fluxes associated with zooplankton were highly variable between stations and zones but very high in most cases compared to literature data, partially due to the high contribution of small forms. {T}he highest values encountered were found at the boundary between the oligotrophic ({MA}) and ultra-oligotrophic regions ({GY}). {W}ithin the {MA} zone, the high variability of the top-down and bottom-up impact was related to the high mesoscale activity in the physical environment. {E}stimated zooplankton respiration rates relative to primary production were among the highest cited values at similar latitudes, inducing a high contribution of migrant zooplankton respiration to carbon flux. {D}espite the relatively low biomass values of planktonic components in quasi-steady state, the availability of micro- and macronutrients related to physical mesoscale patterns in the waters surrounding the {MA}, the fueling by {DDN}, and the relatively high rates of plankton production and metabolism estimated during {OUTPACE} may explain the productive food chain ending with valuable fisheries in this region.},
  keywords = {{PACIFIQUE} {SUD}},
  booktitle = {},
  journal = {{B}iogeosciences},
  volume = {15},
  numero = {23},
  pages = {7273--7297},
  ISSN = {1726-4170},
  year = {2018},
  DOI = {10.5194/bg-15-7273-2018},
  URL = {https://www.documentation.ird.fr/hor/fdi:010074775},
}