Can whales mix the ocean?

Abstract

Ocean mixing influences global climate and enhances primary productivity by transporting nutrient rich water into the euphotic zone. The contribution of the swimming biosphere to diapycnal mixing in the ocean has been hypothesised to occur on scales similar to that of tides or winds, however, the extent to which this contributes to nutrient transport and stimulates primary productivity has not been explored. Here, we introduce a novel method to estimate the diapycnal diffusivity that occurs as a result of a sperm whale swimming through a pycnocline. Nutrient profiles from the Hawaiian Ocean are used to further estimate the amount of nitrogen transported into the euphotic zone and the primary productivity stimulated as a result. We estimate that the 80 sperm whales that travel through an area of 10<sup>4</sup> km<sup>2</sup> surrounding Hawaii increase diapycnal diffusivity by 10<sup>–6</sup> m<sup>2</sup> s<sup>−1</sup> which results in the flux of 10<sup>5</sup> kg of nitrogen into the euphotic zone each year. This nitrogen input subsequently stimulates 6 × 10<sup>5</sup> kg of carbon per year. The nutrient input of swimming sperm whales is modest compared to dominant modes of nutrient transport such as nitrogen fixation but occurs more consistently and thus may provide the nutrients necessary to enable phytoplankton growth and survival in the absence of other seasonal and daily nutrient inputs

Biologically Generated Mixing in the Ocean

“…While not in the ocean, measurements in lakes have reported turbulence levels associated with shoaling perch that, although elevated, fall two orders of magnitude below predictions (Lorke & Probst 2010). There have been published claims of elevated turbulent mixing associated with jellyfish in unstratified water (Katija & Dabiri 2009) and cetaceans (Lavery et al 2012), but these were not accompanied by microstructure measurements or credible quantitative estimates of mixing. Dewar et al (2006) calculated that sperm whales could produce at most 10 −3 TW of turbulence globally.…”