Global ecological and biogeochemical impacts of pelagic tunicates: a modeling study

Pelagic tunicates such as salps, doliolids, and appendicularians are ubiquitous throughout the world’s oceans, and are ecologically important grazers of primary producers. Additionally, their fast sinking fecal pellets, carcasses and/or discarded houses may represent a globally significant source of particulate organic carbon (POC) to the deep oceans. As climate change is projected to decrease both the average size of plankton as well as POC export from traditional plankton food webs, the ecological and biogeochemical role of small filter feeders may also be affected. However, this dynamic has largely been overlooked in marine biogeochemical models. Here we present a modeling study using a coupled physical-biogeochemical model, run over the period of 1948-2007, to assess the impact of pelagic tunicates to the upper ocean food web and to biogeochemical cycling. We added two tunicate types, a large salp/doliolid and a small appendicularian to the 6-plankton NOAA-GFDL Carbon, Ocean Biogeochemistry, and Lower Trophics (COBALT) model, and validated against appendicularian data in the COPEPOD database as well as salps in the Jellyfish Database Initiative (JeDI) database. Model results suggest that pelagic tunicates, particularly the appendicularians, represent a significant source of competition against microzooplankton and small mesozooplankton. Pelagic tunicates occupy a distinct biogeographical niche from crustacean zooplankton, showing later bloom phenology at higher latitudes than non-gelatinous plankton, which has implications for the timing of high latitude POC transfer efficiency to depth. This work represents an initial attempt at including pelagic tunicates in a global biogeochemical model, and highlights their unique niche, compared to crustacean mesozooplankton, in marine food webs and biogeochemical cycling.