Elevated particle export fluxes facilitated by salps, east of New Zealand

Salps are regarded as an important vector for the rapid transfer of particulate organic carbon (POC) to the deep-sea via their dense, fast sinking fecal pellets. Using free-floating, surface-tethered sediment traps, with complementary biogeochemical measurements, including 234-Th, the export flux of POC associated with salps was quantified in late spring in Subantarctic waters and in the Subtropical Front (STF), east of New Zealand/Aotearoa. The traps were deployed for periods ranging from 3-4 days, with cylindrical particle-interceptor MULTI-traps, deployed at four depths (70, 100, 300 and 500 m). Total mass, POC, particulate nitrogen (PN), C and N stable isotopes and phytopigments were measured in the trap samples over five sampling cycles in surface waters that were characterised variously by no salps to abundant salps. The highest POC and PN fluxes were observed during Cycle 4 (400-800 mgC/m²/d and 60-100 mgN/m²/d, respectively), which was conducted on the Chatham Rise crest in productive STF waters, characterised by an advanced-stage bloom of Salpa thompsoni, with the lowest fluxes observed during the non-salp cycles 3 and 5. The cycles with the highest numbers of salps (cycles 1 and 2) were characterised by intermediate to high fluxes (200-500 mgC/m²/d) in the top 100 m, with the more diverse salp community during Cycle 2 resulting in relatively elevated export fluxes, compared to Cycle 1. The influence of salps on the export of POC is highlighted, but the species composition and stage of the salp ‘bloom’ are also significant factors in determining the amount of carbon sequestered to the deep-sea by zooplankton-related processes.