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Abstract. The western tropical South Pacific (WTSP) is one of the most understudiedoceanic regions in terms of the planktonic food web, despite supporting someof the largest tuna fisheries in the world. In this stratified oligotrophicocean, nitrogen fixation may play an important role in supporting theplankton food web and higher trophic level production. In the austral summer(February–April) of 2015, the OUTPACE (Oligotrophy to UlTra-oligotrophyPACific Experiment) project conducted a comprehensive survey of 4000 kmalong 20 ∘  S, from New Caledonia to Tahiti, to determine the role of N2 fixation on biogeochemical cycles and food web structure in thisregion. Here, we characterize the zooplankton community and plankton food webprocesses at 15 short-duration stations (8 h each) to describe thelarge-scale variability across trophic gradients from oligotrophic watersaround Melanesian archipelagoes (MAs) to ultra-oligotrophic waters of theSouth Pacific gyre (GY). Three long-duration stations (5 days each) enabled amore detailed analysis of processes and were positioned (1) in offshorenorthern waters of New Caledonia (MA), (2) near Niue Island (MA), and (3) inthe subtropical Pacific gyre (GY) near the Cook Islands. At all stations,meso-zooplankton was sampled with a bongo net with 120  µ m mesh sizeto estimate abundance, biomass, community taxonomy and size structure, andsize fractionated δ15N . Subsequently, we estimated zooplanktoncarbon demand, grazing impact, excretion rates, and the contribution ofdiazotroph-derived nitrogen (DDN) to zooplankton biomass. Themeso-zooplankton community showed a general decreasing trend in abundance andbiomass from west to east, with a clear drop in the GY waters. Higherabundance and biomass corresponded to higher primary production associatedwith complex mesoscale circulation in the Coral Sea and between170–180 ∘  W. The taxonomic structure showed a high degree ofsimilarity in terms of species richness and abundance distribution across thewhole region, with, however, a moderate difference in the GY region, wherethe copepod contribution to meso-zooplankton increased. The calculatedingestion and metabolic rates allowed us to estimate that the top–down(grazing) and bottom–up (excretion of nitrogen and phosphorous) impact ofzooplankton on phytoplankton was potentially high. Daily grazing pressure onphytoplankton stocks was estimated to remove 19 % to 184 % of thetotal daily primary production and 1.5 % to 22 % of fixed N2 .The top–down impact of meso-zooplankton was higher in the eastern part ofthe transect, including GY, than in the Coral Sea region and was mainlyexerted on nano- and micro-phytoplankton. The regeneration of nutrients byzooplankton excretion was high, suggesting a strong contribution toregenerated production, particularly in terms of N. Daily NH 4 + excretion accounted for 14.5 % to 165 % of phytoplankton needs for N,whereas PO 4 3 - excretion accounted for only 2.8 % to 34 %of P needs. From zooplankton δ15N values, we estimated thatthe DDN contributed to up to 67 % and 75 % to the zooplankton biomassin the western and central parts of the MA regions, respectively, butstrongly decreased to an average of 22 % in the GY region and down to7 % in the easternmost station. Thus, the highest contribution ofdiazotrophic microorganisms to zooplankton biomass occurred in the region ofhighest N2 fixation rates and when Trichodesmium dominatedthe diazotrophs community (MA waters). Our estimations of the fluxesassociated with zooplankton were highly variable between stations and zonesbut very high in most cases compared to literature data, partially due to thehigh contribution of small forms. The highest values encountered were foundat the boundary between the oligotrophic (MA) and ultra-oligotrophic regions(GY). Within the MA zone, the high variability of the top–down andbottom–up impact was related to the high mesoscale activity in the physicalenvironment. Estimated zooplankton respiration rates relative to primaryproduction were among the highest cited values at similar latitudes, inducinga high contribution of migrant zooplankton respiration to carbon flux.Despite the relatively low biomass values of planktonic components inquasi-steady state, the availability of micro- and macronutrients related tophysical mesoscale patterns in the waters surrounding the MA, the fueling byDDN, and the relatively high rates of plankton production and metabolismestimated during OUTPACE may explain the productive food chain ending withvaluable fisheries in this region.

biogeosciences

Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)