S3-2.3 Jan. 4 Pelagic Tunicates Pack and Ship the Carbon MADIN, Larry; Woods Hole Oceanographic Institution email@example.com
The biological carbon cycle in the ocean begins in the surface waters with photosynthetic fixation of carbon dioxide into phytoplankton cells, which are consumed by a variety of grazers. Some respire and recycle the carbon near the surface, while others transport significant portions of ingested C to deeper ocean strata as rapidly sinking fecal pellets. An ideal grazer for this purpose would feed rapidly and indiscriminately on a wide range of particles, compact them into much larger and more rapidly sinking particles, and sometimes accelerate the vertical transport by diel migration to greater depths. In the midwater zone, an ideal particle-feeder would have an efficient mechanism to collect and ingest detrital particles, and would also produce large, sinking excreta. Pelagic tunicates appear to meet these specifications better than any other zooplankters. Epipelagic salps can occur in huge populations over large areas, consuming and sedimenting tons of C daily from the surface to deeper waters. In midwater, salps are rare, but doliolids and appendicularians are often abundant, collecting food with internal or external filters and producing both fecal pellets and discarded mucous houses. These species repackage small detrital material and accelerate its transport out of mesopelagic depths. The activity of pelagic tunicates has consequences both for the food supply to the deep sea and benthos, and for the removal of C from the ocean-atmosphere equilibrium. This natural sequestration of C has been occurring for millions of years. If currently rising carbon dioxide levels in the atmosphere and upper ocean lead to increased primary production, it is conceivable that pelagic tunicate populations will increase to remove larger fractions of organic carbon to the deep ocean, where it may be respired to carbon dioxide by deep-living biota, but will be isolated from the atmosphere.