Whale poop can enhance ocean productivity

The biological pump, a way of moving nutrients and carbon down the water column, in the ocean is fairly straight-forward.  Phytoplankton–tiny, photosynthesizing critters bobbing around in the surface ocean–are eaten by larger organisms, like zooplankton and fish.  When the zooplankton and their brethren die (or are eaten and excreted by fishes), they sink towards the seafloor.  When these organisms produce waste, either the zooplankters themselves or things that ate them, that sinks in the water column as well–something that a former professor of mine lovingly deemed the ‘fecal pellet express’.  This entire process exports nutrients, like nitrogen, phosphorous, and iron, as well as carbon, to the deep ocean.  However, researchers from the University of Vermont and Harvard recently found another component to this long held oceanographic concept:  the whale pump.

Marine mammals breath air.  When these creatures dive to feed, they must surface at some point, defecating along the way.  Even if they can cross their fins and hold it, the excrement on average would occur higher in the water column than where these animals feed, as their dive depths are related to where the food is.  Water samples taken near humpback whale fecal plumes in the Gulf of Maine showed hugely elevated nitrogen levels compared to other locations.  Applying these findings to the entire population of marine mammals in the Gulf, 23 thousand metric tons of recycled nitrogen year is released to the surface annually, more than all riverine input for this area.  That’s slightly less than adding the mass of the Titanic in nitrogen yearly.

Whale (and other marine mammals’) poop can enhance primary productivity by concentrating nitrogen in surface waters, essentially adding an upward component to the known mechanisms of nutrient recycling in the ocean.  This additional nitrogen, which can be the limiting factor in phytoplankton growth, can boost primary productivity, increasing the base of the food web and thus supporting more consumers, like zooplankton, fish, and potentially the whales themselves–a very neat positive feedback.

Marine mammal populations have been drastically reduced by humans; one consequence of this could be that the carrying capacity (the sustainable population size) of coastal ecosystems could be reduced due to the lessening of this nitrogen recycling loop.  In the Gulf of Maine, the amount of nitrogen contributed by whales was much higher before commercial hunting.  The recovery of whale populations has other ecosystem service benefits as well.  As the planets heats up, phytoplankton abundances have been seen to decrease due declining nutrients.  Robust whale populations could help counter this negative effect on marine primary producers.

Images 1:  Humpback whale breaching, Whit Welles, Wikipedia  2:  A simplifed model of the biological and whale pump.  Roman and McCarthy 2010.

Roman J, & McCarthy JJ (2010). The whale pump: marine mammals enhance primary productivity in a coastal basin. PloS one, 5 (10) PMID: 20949007

This article is cross-posted at The Urban Times.


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