Pyrosoma atlanticum fecal pellet production in the California Current: implications for grazing and carbon export dynamics

Pyrosoma atlanticum, a colonial pelagic tunicate, has exhibited significant increases in abundance in the California Current Ecosystem (CCE) since anomalously warm temperatures emerged following the 2014-2016 Northeast Pacific heatwave. This study investigates the ecological role of pyrosomes in the Southern California Current region. We investigated the role of pyrosome fecal pellets by quantifying their production, morphology, sinking rates, and carbon, nitrogen and biogenic silica content. Shipboard experiments were conducted during the California Current Ecosystem Long-Term Ecological Research 2021 process cruise, across upwelling-influenced coastal and oligotrophic offshore waters. We conducted a total of 32 experiments, which showed pyrosomes produced small pellets (354 ± 111 µm), at high rates (19 ± 19 pellets min^-1), which resulted in the production of up to 500,000 pellets m^-2 d^-1 in locations with high densities. Fecal pellets had slow sinking rates: freshly produced pellets had average sinking rates of 47 ± 38 m d^-1, and pellets aged for 1-2 days h sank even slower (31 ± 28 m d^-1), suggesting that a large portion could be remineralized within the upper water column. We estimated their removal of primary production using an assimilation efficiency of 0.6, which indicated that ~20-90 of the stock could be grazed depending on the location. Finally, we present novel results based on our biogenic silica measurements to estimate their impact on diatom standing stocks. Our study indicates that P. atlanticum can play an important role in carbon cycling and food-web pathways and that their role might be enhanced in offshore oligotrophic waters.

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Primary Presenter: Grace Cawley, University of California: San Diego (gcawley@ucsd.edu)

Authors:

Grace Cawley, University of California: San Diego, Scripps Institution of Oceanography  (gcawley@ucsd.edu)

Stephanie Matthews, University of Washington  (samatthews@ucsd.edu)

Natalia Llopis Monferrer, Monterey Bay Aquarium Research Institute  (nllopis@mbari.org)

Minerva Padilla, University of California: San Diego, Scripps Institution of Oceanography  (mpadillavilla@ucsd.edu)

Katherine Barbeau, University of California: San Diego, Scripps Institution of Oceanography  (kbarbeau@ucsd.edu)

Moira Décima, University of California: San Diego, Scripps Institution of Oceaography  (mdecima@ucsd.edu)