The whale pump: marine mammals enhance primary productivity in a coastal basin

Abstract

It is well known that microbes, zooplankton, and fish are important sources of recycled nitrogen in coastal waters, yet marine mammals have largely been ignored or dismissed in this cycle. Using field measurements and population data, we find that marine mammals can enhance primary productivity in their feeding areas by concentrating nitrogen near the surface through the release of flocculent fecal plumes. Whales and seals may be responsible for replenishing 2.3×10(4) metric tons of N per year in the Gulf of Maine's euphotic zone, more than the input of all rivers combined. This upward "whale pump" played a much larger role before commercial harvest, when marine mammal recycling of nitrogen was likely more than three times atmospheric N input. Even with reduced populations, marine mammals provide an important ecosystem service by sustaining productivity in regions where they occur in high densities.

Figure 1. A conceptual model of the whale pump.

In the common concept of the biological pump, zooplankton feed in the euphotic zone and export nutrients via sinking fecal pellets, and vertical migration. Fish typically release nutrients at the same depth at which they feed. Excretion for marine mammals, tethered to the surface for respiration, is expected to be shallower in the water column than where they feed.

Figure 2. Shipboard incubation time-course experiments on Humpback whale samples collected on Stellwagen Bank, Gulf of Maine.

(a) Net NH₄ ⁺ production vs. fecal PON concentration in time course incubations of material collected in whale fecal plumes. Samples 1 and 2 had the highest initial NH₄ ⁺ concentrations, yet their rates of NH₄ ⁺ production ranged from the second lowest to the highest in the entire data set. (b) NH₄ ⁺ concentration vs. incubation time.

Figure 3. The flux of nitrogen in the Gulf of Maine (a) at present and (b) before commercial hunting.

Point-source pollution, industrial emissions of nitrogen, and allochthonous sources from Townsend . The range of historical estimates are adapted from Lotze . Sources that are not expected to be influenced by anthropogenic change, such as offshore transport from Scotian Shelf water, are not included in this graph.

Figure 4. The role of cetaceans in the nitrogen cycle by season.

Seasonal estimates based on the percentage of total consumption in the Gulf of Maine .