Carbon stored in living organisms is called Biomass Carbon, and is found in all marine vertebrates. Large animals such as whales, which may weigh up to 50 tons and live for over 200 years, can store large quantities of carbon for long periods of time.
When they die, their carcasses sink to the seafloor, bringing a lifetime of trapped carbon with them. This is called Deadfall Carbon. On the deep seafloor, it can be eventually buried in sediments and potentially locked away from the atmosphere for millions of years.
Whales can also help to trap carbon by stimulating production of tiny marine plants called phytoplankton, which use sunlight and carbon dioxide to make plant tissue just like plants on land. The whales feed at depth, then release buoyant, nutrient-rich fecal plumes while resting at the surface, which can fertilize phytoplankton in a process that marine scientists call the Whale Pump.
And whales redistribute nutrients geographically, in a sequence we refer to as the Great Whale Conveyor Belt. They take in nutrients while feeding at high latitudes then release these nutrients while fasting on low-latitude breeding grounds, which are typically nutrient-poor. Influxes of nutrients from whale waste products such as urea can help to stimulate phytoplankton growth.
Finally, whales can bring nutrients to phytoplankton simply by swimming throughout the water column and mixing nutrients towards the surface, an effect researchers term Biomixing Carbon.
Fish poo also plays a role in trapping carbon. Some fish migrate up and down through the water column each day, swimming toward the surface to feed at night and descending to deeper waters by day. Here they release carbon-rich fecal pellets that can sink rapidly. This is called Twilight Zone Carbon.
These fish may descend to depths of 1,000 feet or more, and their fecal pellets can sink even farther. Twilight Zone Carbon can potentially be locked away for tens to hundreds of years because it takes a long time for water at these depths to recirculate back towards the surface.