Whales have been viewed as a source of CO2 because they respire tons of CO2 annually. However, their feces could possibly offset this impact, as they may be a great contributor to carbon export (removal from the atmosphere) to the depths of the ocean. Iron-rich whale feces stimulate the growth of phytoplankton, which leads to more CO2 drawn into the ocean through photosynthesis.
Lavery et al. conducted this study to find out whether the 12,000 sperm whales in the Southern Ocean are acting as a carbon sink. The authors wondered whether the whales help the ocean absorb more carbon from the atmosphere than the whales themselves release through respiration. They note that these animals consume prey outside of but defecate within the photic zone (the layer nearest to the ocean surface), raising nutrient availability in the layer of ocean where photosynthesis is possible. Whale feces are also in liquid form, which disperses and persists within this area.
Using existing data on whale populations, consumption patterns, and average rates of iron retention compared to what is expelled, the authors estimate that the South Ocean sperm whales contribute 36 tons of iron per year to the photic zone. After accounting for respiration rates, the authors conclude that whales do act as a net carbon sink by removing 2.4 X 105 metric tons of carbon from the atmosphere annually. Even under conservative scenarios (consumption of prey with lower iron concentrations), whales still help sequester more carbon than they respire.
These animals’ contribution to nutrient and carbon cycling in the ocean has previously been overlooked. Their feces not only enhance carbon sink in the ocean but also contribute to increasing numbers of prey. However, the reduction of sperm whales by commercial whaling has reduced krill populations and decreased allochthonous (originating externally) iron inputs to the Southern Ocean by 450 tons annually.
The reduction in sperm whale numbers owing to whaling has resulted in an extra 2 X 106 tonnes of carbon remaining in the atmosphere annually [Lavery 2010: 3].
In addition to sperm whales, there could be more organisms acting as carbon sinks in the ocean:
We have restricted our analysis to sperm whales; however, any organism that consumes prey outside the photic zone and defecates nutrient-rich waste that persists in the photic zone would stimulate new production and carbon export. Pygmy and dwarf sperm whales (Kogia spp.) and beaked whales (Family Ziphiidae) fulﬁll these criteria. The proportion of time baleen whales consume prey at depth is currently unknown, but ﬁn whales (Balaenoptera physalus) dive to at least 470 m while feeding. Seals and sealions often consume prey at depth, but whether the[ir] waste is liquid (and buoyant) requires further investigation. [Lavery 2010: 4]
Lavery, Trish J., et al., 2010, Iron defecation by sperm whales stimulates carbon export in the Southern Ocean, Proceedings of the Royal Society B, http://dx.doi.org/10.1098/rspb.2010.0863.