Abstract

Abstract. Zooplankton play a key role in the regeneration of nitrogen and phosphorus in the ocean through grazing and metabolism. This study investigates the role of the organic and inorganic nitrogen and phosphorus compounds released by copepods on biogeochemical processes and on the microbial community composition during the OUTPACE cruise (18 February–3 April 2015) at three long-duration stations (LD). Two LD stations were located in the Melanesian Archipelago region (MA; LD A and LD B) and one in the South Pacific Gyre (SG; LD C), which represent oligotrophic and ultra-oligotrophic regions respectively. At each station, on-board microcosm experiments were performed with locally sampled organisms, comprising a mix of epipelagic copepods fed with their natural food and then incubated along with wild microbial assemblages. In the presence of copepods, ammonium and dissolved organic nitrogen showed a significant increase compared to a control in two situations: in ammonium concentration (rate: 0.29 µmol L−1 h−1 after 4 h of incubation) in LD C and in dissolved organic nitrogen concentration (rate: 2.13 µmol L−1 h−1 after 0.5 h of incubation) in LD A. In addition, during the three experiments, an enhanced remineralization (ammonification and nitrification) was observed when adding copepods compared to the controls. A shift in the composition of the active bacterial community was observed for the experiments in LD A and LD B, which were mainly characterized by an increase in Alteromonadales and SAR11, respectively, and linked with changes in nutrient concentrations. In the experiment performed in LD C, both groups increased but at different periods of incubation. Alteromonadales increased between 1 and 2 h after the beginning of the experiment, and SAR 11 at the end of incubation. Our results in near in situ conditions show that copepods can be a source of organic and inorganic compounds for bacterial communities, which respond to excretion pulses at different timescales, depending on the initial environmental conditions and on their community composition. These processes can significantly contribute to nutrient recycling and regenerated production in the photic zone of ultra-oligotrophic and oligotrophic oceanic regions.

Highlights

  • The tropical South Pacific is considered one of the most oligotrophic regions in the world ocean

  • The Melanesian Archipelago region (MA) region was characterized by a gradient of nutrient conditions, with the highest and the lowest nitrate concentrations in longduration stations (LD) A and LD B, respectively

  • The bacterioplankton community composition revealed a higher diversity in the MA region compared to South Pacific Gyre (SG), with a higher dominance of Alphaproteobacteria in both regions; SAR 11 increased their contribution in the ultraoligotrophic area (SG)

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Summary

Introduction

The tropical South Pacific is considered one of the most oligotrophic regions in the world ocean. Zooplankton may release dissolved organic and inorganic matter through sloppy feeding (Møller, 2004, 2007), leaching of fecal pellets (Hasegawa et al, 2000; Steinberg et al, 2002) and excretion (Saba et al, 2011) and along with bacterial remineralization, it determines the amount of regenerated nitrogen and phosphorus available for phytoplankton production (Steinberg and Landry, 2017). Despite the advances over the last decades, studies of the impact of copepod nitrogen and phosphorus excretion on microbial communities from oligotrophic and ultra-oligotrophic areas can provide valuable information about the recycling of these nutrients in larger areas of the ocean. We studied the role of organic and inorganic dissolved compounds released by copepods, and its potential effect on their recycling and structuring of the bacterioplankton community during late austral summer in distinct biogeochemical regions of the WTSP

Study area and sampling strategy
Sampling points
Preparation of the microcosm and experimental setup
Dissolved inorganic and organic nutrients measurements
Analysis of the bacterial community structure
Statistical analysis
Biogeochemical in situ conditions
Changes in bacterioplankton abundance during the experiment evolution
Active bacterial composition and their response during the incubations
Discussion
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