Recovery of carbon benefits by overharvested baleen whale populations is threatened by climate change

Abstract

Despite the importance of marine megafauna on ecosystem functioning, their contribution to the oceanic carbon cycle is still poorly known. Here, we explored the role of baleen whales in the biological carbon pump across the southern hemisphere based on the historical and forecasted abundance of five baleen whale species. We modelled whale-mediated carbon sequestration through the sinking of their carcasses after natural death. We provide the first temporal dynamics of this carbon pump from 1890 to 2100, considering both the effects of exploitation and climate change on whale populations. We reveal that at their pre-exploitation abundance, the five species of southern whales could sequester 4.0 × 10⁵ tonnes of carbon per year (tC yr^-1). This estimate dropped to 0.6 × 10⁵ tC yr^-1 by 1972 following commercial whaling. However, with the projected restoration of whale populations under a RCP8.5 climate scenario, the sequestration would reach 1.7 × 10⁵ tC yr^-1 by 2100, while without climate change, recovered whale populations could sequester nearly twice as much (3.2 × 10⁵ tC yr^-1) by 2100. This highlights the persistence of whaling damages on whale populations and associated services as well as the predicted harmful impacts of climate change on whale ecosystem services.

Keywords: blue carbon; climate change scenarios; deadfall carbon; modelling; whales fall; whaling.

Figure 1.

Conceptual diagram of the carbon sequestration mediated by baleen whales. (1) After natural death, the carcasses start sinking. (2) They can be partially consumed by scavengers like killer whales and sharks as well as degraded by microorganisms. Once a carcass has reached the seafloor, the organic carbon in tissues can be (3a) consumed and respired at depth by local abyssal fauna (like Zoarcidae) and microorganisms. Once respired, the inorganic carbon can be brought back to the surface water and outgassed into the atmosphere on a timescale from centuries to thousands of years depending on the depth and the water circulation [5]. Or (3b), it can be buried in the sediments where it can be trapped on longer timescales, up to millennia [12]. (Online version in colour.)

Figure 2.

Amount of carbon sequestered annually by each whale species and for all the five species together (total) at their pre-exploitation levels through the sinking of the carcasses. On the top right, the relative contribution of each species. Errors bars represent high and low estimations for carbon sequestration given parameter uncertainties. (Online version in colour.)

Figure 3.

Dynamics of carbon sequestration mediated by the five baleen whale species between 1890 and 2100 without climate change (a,c) and with climate change (b,d); (a,b) represent the total sequestration by the five whale species; (c,d) represent the sequestration dynamic for each species. Shaded areas represent the high and low estimations of carbon sequestration given parameter uncertainties. (Online version in colour.)

Figure 4.

Cumulative carbon sequestration deficit from 1890 to 2100 without climate change (a) and with climate change (b). For each year, the total amount of non-sequestered carbon is compared to that corresponding to the pre-exploitation levels of whale populations. Shaded areas represent the high and low estimations for carbon deficit given parameter uncertainties. (Online version in colour.)