A new study published in Nature sheds light on the roles whales play in marine ecosystems. Baleen whales are the largest carnivorous marine mammals, so naturally, they feed on tremendous amounts of krill, zooplankton, and other prey. Krill is turned over in the stomachs of whales (Mysticeti). Once krill have been digested, their iron contents are released back out into ocean ecosystems, where it floats toward the water’s surface due to water pressure. Iron-rich excrement yields nutrients for phytoplankton, which are microscopic plants that use photosynthesis to make energy.
Phytoplankton are then consumed by other creatures in the environment, including krill! Krill feed on the phytoplankton that grow using the nutrients from recycled metabolized – recycled – krill. In other words, baleen whales populations perpetuate nutrient cycling. At one level, krill are consumed by whales. Subsequently, whale waste supplements phytoplankton growth, which helps sustains krill populations.
By comparing the prey consumption more than 300 tracked whales in this new study to per-capita consumption estimates from the early 20th century, researchers were able to reason that southern krill populations has to be considerably higher than they are today. Whales were found to eat up to three times more krill and other prey than previous assessments have supposed.
Research Method and Design
Researchers used metabolic models to estimate whale feeding volumes. Whale tagging and acoustic acoustic measurements were used to calculate whale prey densities in the Atlantic, Pacific, and Southern Oceans. Their results suggest that previous assessments greatly underestimated baleen whale prey consumption. Further, researchers reason that larger whale populations would add to the “productivity” of marine ecosystems by perpetuating iron recycling.
Researchers were able to determine how much whales eat by tagging individual whales by attaching electronic devices on their backs. These electronic devices carry cameras, microphones and of course, GPS locators. These electronic tags, in conjunction with acoustic measurements of prey biomass, informed researchers on whale eating cycles and intake volume. Of course, prey intake varies between different species of whale.
The Krill Paradox
The famous krill paradox refers to the mystery in marine ecosystems regarding the removal of large predators, like whales. When whales are hunted, and their populations consequently decrease, so do the population sizes of krill. This perplexes researchers because they intuitively expect krill populations to grow wildly in the absence of whales which eats thousands of tons of krill daily. Instead, the opposite is true: as whales are removed from the ecological system, krill populations shrink. The new study illuminates exactly why this phenomenon occurs. Krill depend on whales to produce nutrients for the microscopic plants that they eat. Declines in whale species members leads to fewer iron being sent toward the water’s surface in the form of whale excrement. Which ultimately contributes to less plentiful meals available for krill populations.
The conclusions of this study may have potential for marine ecosystem restoration efforts. Species like whales are evidently essential for the continued functionality of marine ecosystems, and should therefore be protected.