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Scientists have been aware for some time that whales contribute to marine ecosystems by "recycling" nutrients. However, the exact impact on ocean productivity was unclear and had not been studied quantitatively. A new study, in the journal Proceedings of the National Academy of Sciences, has tackled this problem by making direct measurements of nutrients excreted by baleen whales. The results show that these whales are boosting ocean primary productivity significantly.

Primary productivity and the ocean food web

Primary productivity is a measure of how some organisms—in this case, phytoplankton—convert energy into organic compounds through processes like photosynthesis or chemosynthesis, which ultimately act as the base of the food web. When primary productivity is higher, organisms up through the food chain have more sustainable food sources.

In order for phytoplankton to make these organic compounds, they must be supplied with a healthy dose of nutrients, like nitrogen, phosphorus, and iron, which are required for photosynthesis and other processes. Unfortunately, many parts of the upper layer of ocean water are depleted of such nutrients. Some of these nutrients get replenished by terrestrial runoff, mixing with deeper ocean water, and nutrient recycling by zooplankton, bacteria and larger marine life.

Waste not, want not

According to the new study, baleen whales—a group which includes minke, fin, blue, sei, humpback, and bowhead whales—contribute a significant amount of these nutrients through their waste. To determine the actual contribution, the research team collected and analyzed feces and urine from minke and fin whales, testing for the nutrient content of nitrogen, phosphorus, iron and 14 other trace elements.

Then, they estimated the daily and annual nutrient release for six other baleen whale species and used an ecosystem model, called NORWECOM.E2E to simulate impacts of whale nutrient release on primary production and zooplankton. The study focused on the Greenland and Norwegian Seas and the Iceland Seas, where baleen whales migrate to in the summer, before returning to their winter breeding grounds at lower latitudes.

The results showed that baleen whales release significant amounts of nitrogen, mainly via urine, and phosphorus and trace elements, mainly via feces. In total, these whales recycle over 815 tons of nitrogen and 325 tons of phosphorus daily in the Nordic and Barents Seas during feeding season. The contribution varies by specific species.

"The amount of nutrients released by whale populations is expected to depend on prey consumption rates and population abundance. Given their large body size, blue, bowhead, and fin whales are the species contributing the highest nutrient loads per individual. However, when population abundance is considered, minke whales are the species that contribute the most to nutrient pools in most regions, including the Barents Sea, Norwegian Sea, and Greenland Sea," the study authors explain.

Seasonal and spatial fluctuations

As might be expected, the nutrient content varies over time, with higher amounts found in late summer, increasing from May, when the whales migrate to these areas. In some areas, primary production was found to be 10% higher in August. This effect was seen more in offshore, nutrient-poor regions.

"The change in annual net primary production was relatively small in most areas, with a spatial mean of 0.63%, but reached 2 to 4.5% within the Norwegian Sea basin and Northern Barents Sea, east of Svalbard. In the Barents Sea, phytoplankton bloom development during spring and summer is primarily governed by available light and the progression of the sea ice melt.

"In general, the Arctic-influenced northern Barents Sea is less productive due to lower light availability and nutrient concentrations compared to the southern Barents Sea. Our models indicate that whale-derived nutrients have a more pronounced impact in such remote regions distant from alternative nutrient sources," the study authors write.

Cascading effects of increased primary productivity

The increase in primary production cascades up the food web, benefiting many organisms. Mesozooplankton—organisms that forage on phytoplankton and microzooplankton—were found to increase in biomass by up to 10%. The team points out drops in smolt survival and growth of Norwegian salmon were noted in other studies, as well as changes in herring feeding migration patterns. These have been attributed to changes in zooplankton biomass in the Nordic Seas.

"Thus, mesozooplankton appears to establish an important link between the primary producers and higher trophic levels. While not all nutrients released by whales may be readily utilized by phytoplankton, our study highlights their potential to increase both primary production and secondary growth," the authors say.

In addition to boosting the food web, increased primary productivity contributes to carbon sequestration. This is due to the inherent processes involved, which utilize carbon dioxide from the atmosphere by turning it into organic compounds.

The future of marine ecosystems

It is clear that the ocean ecosystem has complex processes that sustain and nourish its inhabitants. The study shows that protecting whale populations benefits not just whales, but entire ocean ecosystems and the global climate.

Future research may help refine these models with improved data on whale movement, excretion depth, and nutrient dissolution and by incorporating other marine vertebrates and more trace elements into models.

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