Popular Science: The southern giant is heading north
With global climate change and human activities in general, the water phase, water chemistry and water cycle on Earth are fundamentally changing. The oceans, including the Southern Ocean, are no exception. The most pronounced changes are increasing ocean temperatures, a shift of atmospheric fronts to the south, ocean acidification (lowering water pH) and changes in the seasonal sea-ice extent. These changes also have a significant impact on overall ocean life, including the density of krill (a group of small crustaceans living in the world’s oceans, especially in the Arctic Belt), one of the most important components of the marine food chain. It is estimated that a warming of 1 °C could cause a decrease in the amount of Antarctic krill biomass in the southwest Atlantic by up to 95%. Changes in krill availability are also a concern for large mammals in the ocean, including the southern right whale (Eubalaena australis).
Experts from several countries, including the Czech Republic, have focused on this gigantic beauty, which is an important part of the ecosystem. They aimed to find out what changes or shifts have taken place in recent decades and what could influence the whale’s significant reproductive decline in recent times. They used skin biopsy samples from the 1990s, as well as from 2015, 2016 and newly collected samples in 2019. They compared a total of 122 skin biopsy samples, in which they determined the values of the content of stable isotopes of carbon (δ13C) and nitrogen (δ15N). Based on the stable isotopes, it was possible to determine the area of the isotope niche, which reflects the food composition of the individuals and populations. These values were then compared with zooplankton isotopic data from areas representing the southern right whale range.
Obvious differences were observed in the isotopic composition of skin biopsy samples from the 1990s and from the beginning of the 21st century. Interestingly, overall, there were significant changes in the δ13C carbon isotope ratios, which increased, but the δ15N nitrogen isotopes changes were less pronounced. Nitrogen isotopes showed considerable seasonal differences in the 1990s, when values in September were lower than in July and August. Seasonality was not evident in early 21st century samples or in all carbon isotopes. Comparing the two decades, the isotopic niche space of the southern right whale indicated a substantial expansion and northward shift. This is probably a reflection of the decrease in abundance and southward range contraction of Antarctic krill due to global warming, which is why whales have had to reorient themselves to another type of food they are looking for in other areas.
Global climate change is affecting the entire Earth, including the ocean life. The results of the current study show a significant change in the isotopic niche space and diet composition of the southern right whale in recent decades, which also suggests an extensive transformation of the ecosystem in the Southern Ocean. It can be considered as a reason for being slightly optimistic that the southern right whale could potentially adapt to changing resources. However, this strategy still appears to be insufficient, as there has been a significant decline in reproductive success in recent times, which is very worrying. Besides the fact that southern right whales are one of the most sublime giants on the planet, they are also an important stabiliser of the Earth’s climate. Their existence supports the growth of phytoplankton which absorbs up to 40% of all carbon on Earth. The gradual decline of the southern right whale population could thus contribute to further warming of the Earth.
van den Berg, GL, Vermeulen, E, Valenzuela, LO, Bérubé, M., Ganswindt, A, Gröcke, DR, Hall, G, Hulva, P, Neveceralova, P, Palsbøll, PJ, Carroll, EL, 2020. Decadal shift in foraging strategy of a migratory southern ocean predator. Global Change Biology 1–16. https://doi.org/10.1111/gcb.15465.