Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)
Authors: Carlotti François , Pagano Marc , Guilloux Loïc , Donoso Katty , Valdés Valentina , Grosso Olivier , Hunt Brian P. V. 
Published: December 2018

Abstract. The western tropical South Pacific (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. In this stratified oligotrophic
ocean, nitrogen fixation may play an important role in supporting the
plankton food web and higher trophic level production. In the austral summer
(February–April) of 2015, the OUTPACE (Oligotrophy to UlTra-oligotrophy
PACific Experiment) project conducted a comprehensive survey of 4000 km
along 20∘ S, from New Caledonia to Tahiti, to determine the role of
N2 fixation on biogeochemical cycles and food web structure in this
region. Here, 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 Melanesian archipelagoes (MAs) to ultra-oligotrophic waters of the
South Pacific gyre (GY). Three long-duration stations (5 days each) enabled a
more detailed analysis of processes and were positioned (1) in offshore
northern waters of New Caledonia (MA), (2) near Niue Island (MA), and (3) in
the subtropical Pacific gyre (GY) near the Cook Islands. At all stations,
meso-zooplankton was sampled with a bongo net with 120 µm mesh size
to estimate abundance, biomass, community taxonomy and size structure, and
size fractionated δ15N. Subsequently, we estimated zooplankton
carbon demand, grazing impact, excretion rates, and the contribution of
diazotroph-derived nitrogen (DDN) to zooplankton biomass. The
meso-zooplankton community showed a general decreasing trend in abundance and
biomass from west to east, with a clear drop in the GY waters. Higher
abundance and biomass corresponded to higher primary production associated
with complex mesoscale circulation in the Coral Sea and between
170–180∘ W. The 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. The 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. Daily 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 N2.
The top–down impact of meso-zooplankton was higher in the eastern part of
the transect, including GY, than in the Coral Sea region and was mainly
exerted on nano- and micro-phytoplankton. The regeneration of nutrients by
zooplankton excretion was high, suggesting a strong contribution to
regenerated production, particularly in terms of N. Daily NH4+
excretion accounted for 14.5 % to 165 % of phytoplankton needs for N,
whereas PO43- excretion accounted for only 2.8 % to 34 %
of P needs. From zooplankton δ15N 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. Thus, the highest contribution of
diazotrophic microorganisms to zooplankton biomass occurred in the region of
highest N2 fixation rates and when Trichodesmium dominated
the diazotrophs community (MA waters). Our 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. The highest values encountered were found
at the boundary between the oligotrophic (MA) and ultra-oligotrophic regions
(GY). Within 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. Estimated 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.
Despite 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.