Presenter: Jessica Luo (National Oceanic and Atmospheric Administration)

Description:
Pelagic tunicates (salps, doliolids, appendicularians) are key components of marine ecosystems. When present, they produce dense fecal pellets, carcasses, and discarded houses that comprise a significant source of particulate organic carbon (POC) to depth. As climate change is projected to decrease the average plankton size and POC export from traditional plankton food webs, the ecological and biogeochemical role of pelagic tunicates may increase; yet, pelagic tunicates were not resolved in the previous generation of global earth system climate projections. Here we present a global modeling study using a coupled physical-biogeochemical model, run from 1948-2007, to assess the impact of pelagic tunicates to the pelagic food web and biogeochemical cycling. We added two tunicate groups, a large salp/doliolid and a small appendicularian to the NOAA-GFDL Carbon, Ocean Biogeochemistry, and Lower Trophics (COBALT) model, which was originally formulated to represent carbon flows to crustacean zooplankton. The new GZ-COBALT simulation was able to simultaneously satisfy new pelagic tunicate biomass constraints and existing ecosystem constraints, including crustacean zooplankton observations. We simulated a global tunicate biomass of 0.22 Pg C, and annual production of 1.87 Pg C y^-1 in the top 100 m. Pelagic tunicates produced 2.68 Pg C y^-1 of detritus, of which 84% sank past 100 m. Overall export from the euphotic zone remained largely constant, though the GZ-COBALT pe-ratio increased 20% compared to the COBALT control. This indicates that while the bulk of the tunicate-mediated export production came as a result of the rerouting of phytoplankton- and mesozooplankton-mediated export, tunicates did shift the overall balance of the upper oceans away from recycling and towards export. Our results suggest that pelagic tunicates represent a significant source of competition against microzooplankton, and may serve as a trophic and carbon export shunt away from the microbial loop.