I’ve written in extent on how much I do enjoy my interactions with SPC. There are not as many as I would like… the main focus of my work in compliance these days, while they do mostly science and data… Yet as a scientist (that does not do much science anymore), I do enjoy keeping up to date their work, have an enourmous respect for my friends working there (and they use many of my pictures!)
Among the top scientists that work there, I have a lot of respect and admiration for Valerie Allain, I blogged on her work before in climate issues and mercury, now here she is co-authoring (with Anne Lorrain as lead) a really interesting article and very nicely illustrated (as usual for SPC)
Their work tackles tuna from an interesting angle as climate sentinels, by measuring relative abundances of carbon isotopes (also referred to as measuring isotopic ratios) in tuna muscle, to trace the proportion of CO2 emitted by humans and absorbed by the ocean.
The article was published today in the SPC fisheries newsletter (you should subscribe!) and I just refer to the parts and illustrations I enjoy the most, but as usual, read the original!
Intro
Research on the carbon composition of tuna flesh has revealed that, over the past 15 years, deep changes have occurred in the carbon cycle and the phytoplankton underpinning ocean food webs. A multidisciplinary study published in November 2019 (Lorrain et al. 2019) is based on a broad network of international cooperation making it possible to collectively assess 4500 muscle samples from three tuna species caught in the Pacific, Indian and Atlantic oceans between 2000 and 2016. Biological observations on such an extensive spatial and temporal scale are unusual and of prime importance for the validation of climate forecasts and their consequences for food webs.
INSERT 2 a, b, c, d (from left to right and top to bottom)
Tracing the carbon cycle through isotopes
Carbon is a fundamental element that can be inorganic, like that contained in atmospheric carbon dioxide (CO2), or organic. The human body contains 18% carbon in terms of weight, making it the second biggest component after oxygen, and this carbon can be found throughout the body, e.g. in muscle proteins, fats and DNA. It is therefore present in living beings, the air, the Earth’s crust and the oceans. The ocean absorbs more than 90% of the heat associated with climate warming and over 30% of the carbon emissions from fossil fuel burning. The consequences of this on the functioning of the ecosystem and marine organisms through, for example, ocean acidification are not yet fully known. Until now, only some localised observations from certain oceanic regions have provided fragmented information on this topic. This new study, carried out by some 20 international researchers, for the first time provides some elements of overall understanding through analysis of the stable isotopes in the carbon present in 4500 specimens of tuna harvested from the Pacific, Indian and Atlantic oceans between 2000 and 2016.
Carbon exists in various forms, called stable isotopes, with special reference to 12C and 13C (articulated as Carbon 12 and Carbon 13, please see Insert 2a). These isotopes do not have the same mass, with 12C being lighter than 13C. Because of this difference in mass, 12C and 13C react differently during chemical, physical or biological change processes. For example, when a process of water evaporation involving dissolved carbon occurs, the light carbon (12C) tends to evaporate more readily and the water vapour contains more 12C than the residual unevaporated water. The distribution of 12C and 13C is not uniform throughout the world, in the atmosphere or in living organisms with a majority carbon content. Measuring their respective abundance levels makes it possible to shed light on these various processes and understand the carbon cycle. For example, it makes it possible to trace atmospheric CO2 emissions due to human activity.
Fossil fuels at the dinner table
Since the end of the 19th century, the burning of fossil fuels (oil, coal) has released into the atmosphere light carbon enriched with 12C, (or depleted in 13C): this is what is commonly referred to as the Suess effect (Insert 2B). The heavy isotope content reduction in the atmosphere moves by diffusion into the ocean and then travels up the food web to the tunas (Insert 2D). Measuring relative abundances of carbon isotopes (also referred to as measuring isotopic ratios) in tuna muscle makes it possible to trace the proportion of CO2 emitted by humans and absorbed by the ocean. The reduction in 13C in tuna muscle is five times higher than that expected if it was solely due to the Suess effect. Increasing use of fossil energies is therefore not sufficient to explain the low 13C value observed in tunas.
Insert 3
But what causes tuna’s isotopic composition to fall?
In our study, we sought to determine what other factors could explain the steep decline in 13C in tuna by examining every stage in carbon conversion through the marine cycle, from water composition to tuna. The carbon composition of tunas is governed by a number of factors, acting synergistically, i.e. (Insert 3): the quantity of CO2 present in the oceans, a majority of which is due to the CO2 emissions associated with human activities; the types of phytoplankton present in the oceans and their growth rates; and the various trophic relationships at play and culminating at the tuna level.
Atmospheric carbon enters the oceans through diffusion and is absorbed by phytoplankton, which needs it in order to develop. The proportion of 12C and 13C absorbed is variable depending on the kind of phytoplankton and their growth rate (Insert 2C). Phytoplankton is the foundation of the food web and is consumed by larger organisms, which themselves are in turn consumed by bigger and bigger organisms up to the top predators like tunas. The proportion of 12C/13C in phytoplankton is then propagated throughout all levels of the food web and can be changed at each level depending on the organisms concerned. In this way, the changes in 12C/13C proportions in the phytoplankton populations find their way through the food webs to the apex tunas (Insert 2D). Changes in the type of trophic relationship (changes in the type of prey or the number of different steps in the food web) can also influence the proportions of 12C/13C observed in tuna muscle.
Tuna as climate change sentinels?
Through a modelling approach taking into consideration all the processes set out above and known to have an influence on isotopic values (summarised in Insert 3), we demonstrate that, while all the factors at work can influence the isotopic composition of tuna muscle, the one with the most impact is linked to the kind of phytoplankton occurring in the oceans. These results suggest that deep changes in the phytoplankton population structures at the foundation of the food webs that culminate in tuna have been taking place for the past 15 years. These data are of inestimable value for the calibration and validation of climate models and for projecting the effects of climate change onto ocean productivity. Few biological datasets are in fact available at such spatial and temporal scales.
We also suggest that the phytoplankton communities are constantly shrinking because the smaller species contain more 12C than the larger species such as the diatoms. These changes in populations are not improbable because, with climate warming, changes are being forecast in the way water masses are structured (ocean stratification, in other words, less mixing between surface water and deep water), with a reduction in the quantity of nutrients present in surface waters. Faced with the available nutrient quantities, not all phytoplankton species adapt in the same way and, for example, smaller-sized species show higher suitability when the waters are nutrient-poor, which could explain a change in population structure.
Consequences on energy transfers and health?
A change in the phytoplankton communities could have extensive repercussions on trophic webs, for example by reducing the amount of energy and nutrients available for fish. Research, in fact, suggests that the smallest phytoplankton species synthesise less of the omega-3 polyunsaturated fatty acids essential for the growth of many species of fish and beneficial for human health. This opens promising research avenues for further exploration, as tunas are a source of the fatty acids essential for human health.