Mesozooplankton structure and functioning in the western tropical South Pacific along the 20° parallel south during the OUTPACE survey (February–April 2015)

Abstract

Abstract.This paper presents new results on spatial and temporal distribution patterns of mesozooplankton in the western 20 tropical South Pacific along the 20°S south visited during austral summer (February -April 2015). By contributing to the interdisciplinary OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project (Moutin et al., 2017), the specific aims of this study dedicated to mesozooplankton observations were (1) to document zooplankton density, species diversity, and biomass along the transect, and (2) to characterize the trophic pathways from primary production to mesozooplankton . Along a West-East transect of 4000 km from New Caledonia to the French Polynesia, 15 short-duration 25 stations (SD-1 to SD-15, 8hours each) dedicated to a large-scale description, and three long-duration stations (LD-A to LD-C, 5days each), respectively positioned (1) in offshore northern waters of New Caledonia, (2) near Niue Island, and (3) in the subtropical Pacific gyre near the Cook Islands, were sampled with a Bongo Net with 120 μm mesh size net for quantifying mesozooplankton abundance, biomass, community taxonomy and size structure, and size fractionated content of δ Daily grazing pressure on phytoplankton stocks and daily regeneration by zooplankton were as well estimated for the different regions showing contrasted impacts between MA and GY regions. For the 3 LD stations, it was not possible to find any relationship between the abundance and biomass in the water column and swimmers found in sediment traps. Diel vertical migration of zooplankton, which obviously occurs from observed differences in day and night samples, might 25 strongly influence the community of swimmers in traps.Biogeosciences Discuss., https://doi

Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the western tropical South Pacific Ocean

“…At the ecosystem level, even if the DDN transfer efficiency (∼ 15 %) to zooplankton from UCYN-B is higher than the one of Trichodesmium, the ultimate quantity of DDN transferred to secondary producers is higher when Trichodesmium dominates, as cell-specific N 2 fixation rates of Trichodesmium (∼ 250-340 fmol N cell −1 48 h −1 ) are far higher than those of UCYN-B (∼ 19 fmol N cell −1 48 h −1 ). This result is in agreement with Carlotti et al (2018) results based on 15 N isotopic data showing that ∼ 50-95 and ∼ 10-40 % of the zooplankton N content originates from N 2 fixation in MA and GY waters, respectively. Finally, the DDN transferred to zooplankton, either directly or indirectly, may be released in the dissolved pool as NH + 4 , providing additional NH + 4 from DDN in the environment that is likely assimilated by organisms in N-depleted waters.…”

“…Animals were recovered on a 120 µm sieve and placed into 4.5 L polycarbonate bottles filled with 0.2 µm filtered surface seawater in the dark for at least 6 h in order to allow them to empty their guts. Living animals were visually identified and the individuals belonging to the genus Clausocalanus, which largely dominated the zooplankton community at all stations (Carlotti et al, 2018), were handpicked and 12 animals were dispatched into each of the three 1 L bottles containing the 15 N pre-labelled plankton before being incubated in on-deck incubators for 24 h as described above. The three other bottles were immediately filtered after the introduction of animals, firstly through a 120 µm mesh in order to recover the animals and secondly through precombusted (4 h, 450 • C) GF/F filters, which were used to quantify the isotopic signature of the 15 N pre-labelled plankton at the beginning of the experiment, together with the initial NH + 4 concentrations in the incubation bottles.…”

“…At the global scale, N 2 fixation is the major source of new N to the ocean, before atmospheric deposition and riverine inputs (100-150 Tg N yr −1 : Duce et al, 2008;Gruber, 2008). N 2 fixation is performed by prokaryotic organisms termed diazotrophs, which include the non-heterocystous filamentous cyanobacterium Trichodesmium (Capone et al, 1997;Carpenter, 1983), heterocystous cyanobacteria living in symbiosis with diatoms (or diatom-diazotroph associations, termed DDAs; Villareal, 1994), unicellular cyanobacteria termed UCYN (subdivided into groups A, B and C based on the nifH gene sequence, (Zehr et al, 1998Zehr and Turner, 2001), diverse non-cyanobacterial bacteria (Bombar et al, 2015;Moisander et al, 2014;Riemann et al, 2010), and archaea (Loescher et al, 2014). Although considerable efforts have been made over the past decades to quantify N 2 fixation, identify the major players, and assess their biogeographical distribution in relation to environmental drivers, the fate of new N provided by N 2 fixation in the ocean and its role in the planktonic food web structure and large-scale biogeochemical fluxes are still poorly understood.…”

Silicon cycle in the tropical South Pacific: contribution to the global Si cycle and evidence for an active pico-sized siliceous plankton

“…They represent a strongly negligible fraction of surface Si stocks, implying no sedimentation at the time of sampling, and that active recycling and grazing occurred in the surface layer. Indeed, surface temperatures higher than 29 • C at all long-duration sites may favor intense dissolution in the upper layer, while active zooplankton grazing was also documented, removing between 3 % and 21 % of phytoplankton stocks daily (Carlotti et al, 2018). The virtual absence of silica export from the surface layer agrees well with the conclusion of Nelson et al (1995) that no siliceous sediment is accumulating beneath the central ocean gyres.…”

Nitrogen and phosphorus recycling mediated by copepods and response of bacterioplankton community from three contrasting areas in the western tropical South Pacific (20° S)

“…In addition, the strong thermal stratification, enhanced by current global warming, decreases the nutrient supply to the euphotic zone (Polovina et al, 2008). The western tropical South Pacific (WTSP) was recently recognized as a hotspot of N 2 fixation (Bonnet et al, 2017) and the input of new nitrogen to the surface ocean through this process sustains over 50 % of the primary productivity (Karl et al, 1997;Carpenter et al, 2004). However, it has been reported that predator-prey interactions can provide substantial amounts of nitrogen and phosphorus, and supply an alternative substrate for phytoplankton and heterotrophic microorganisms, stimulating the microbial loop in a variety of areas and contributing to the regenerated and new production (Richardot et al, 2001;Vargas et al, 2007;Arístegui et al, 2014;Valdés et al, 2017a, b).…”