Carbon and the Deep Blue Sea: Why Recovering Fish Stocks Hold the Key to Sequestering Carbon at the Bottom of the Sea

Important progress is underway around the world to emplace sustainability in wild ocean fisheries. The big surprise, however, is that getting fisheries right at the global scale may also make an unexpected and potentially very significant down payment on helping fight global warming.

We have known for some time that fixing fisheries management is the right answer for economic and ecosystem well-being under today’s conditions. That outcome is moving from the  theoretical to the realm of the possible. For example, the past 12 months have seen the profound transformation of fisheries framework laws in Japan and Cuba, and Belize is about to follow suit. Even China has joined the hunt, both for reforming its own domestic fisheries — the biggest in the world — and also in the way the country imagines “environmental civilization” and the future of the world ocean.

But can this bounty be attained when the world ocean is changing so profoundly as a result of climate change? To find out, we re-ran the models noted above to test what would happen under different emissions scenarios. That analysis shows that, as long as the most egregious emissions scenarios are avoided, good management today of changing fish stocks can offset nearly all of the catch losses of the ocean as a whole. True, there are shifts in production patterns that need to be addressed, but it turns out that good strategies exist today to begin building climate-ready fisheries for tomorrow. My colleagues are publishing a series of blogs that will chart that course. Stay tuned for this series, launching Sept. 26.

Perhaps most exciting, even that good news is not the whole story. The ocean, of course, is the world’s biggest carbon sink — the so-called “biological pump,” where surface photosynthesis by phytoplankton drives a series of transformations that end up with carbon in various fixed forms raining into the deep sea, where it is sequestered for the long term.

Scientists are just beginning to ask the right question: what might happen to ocean ecosystems and global carbon cycles when widespread ocean restoration occurs? What if we can manage ocean ecosystems with a specific eye toward not only increasing living carbon compartments (e.g., populations of fish and plants), but also with an eye toward maximizing the carbon fixed from the atmosphere in the deep sea — taking carbon out of circulation for millennia? What if we can do that naturally, not by artificial additions of iron or other growth-stimulating nutrients, but by “re-priming the biological pump” with growth-stimulating nutrients that are brought back to the surface by large marine animals who feed in the deep sea, like whales and large fish?

Recent studies show clearly that great whales act as ecosystem engineers — with a cascading and disproportionate impact on the structure and function of ocean ecosystems —through the so-called “poop pump” or “whale pump.” Though dead whale bodies falling to the seafloor provide nutrients that create fascinating oases of life on the bottom, live whales poop near the surface, bringing nutrients from their deepwater prey back where they then fuel primary production. This in turn drives secondary (animal) production, which result together in “marine snow,” the rain of organic matter into the abyss. Severe reductions in whale abundance have greatly depressed surface productivity.

The same relationship seems likely to emerge as we begin to understand parallel patterns related to fish feeding, especially those of large fish that feed on prey in the deep sea. Although knowledge is accumulating rapidly about how actions at the surface affect the deep and vice versa — together sometimes called “benthopelagic coupling” — it is time we invest in better understanding of these interactions, including the “trickle down” effects of rebuilding large animal populations for the biological pump.

Another exciting opportunity lies in an expanded view of “blue carbon,” a term generally used to describe the fixation and storage of atmospheric carbon in the coastal zone in the form of living biomass of wetlands — mangroves, sea grasses, marshes, etc. — and organic soils associated with those. Protecting blue carbon assets is well-known to be critically important, not only for carbon storage, but also in terms of habitat protection for all kinds of terrestrial animals and for many marine and estuarine animals that depend on those habitats. Properly managed marshes and wet forests will also help protect shorelines against the rising seas and intensifying storms of the future.

But what if even that version of “blue carbon” also falls short of its true potential for fighting global warming? What if blue carbon is not just about stopping losses of today’s coastal wetland inventory, but also about finding and exploiting big opportunities for reforesting the changing sea? Scientists may argue about whether kelps and other macroalgae really sequester carbon — but new, large, living kelp forests could inarguably strike many tons of carbon from the ledger. And there could be places to build those underwater forests: China, for example, has begun talking with Australia and others about possible massive reforestation of their shallow waters using artificial substrates to jumpstart the process. One can imagine building large areas of new habitats that could also be a locus for restoring populations of larger finfish. These rebuilt finfish populations could be the focus of a new recreational fishery while also beginning to restore ecosystem structure and function in areas long depleted of long-lived species.

It’s important to note that “blue carbon” as typically defined includes mangroves but not seagrasses or corals. Corals are usually omitted from the blue carbon equation because a narrow view of the complicated chemistry that results in calcium carbonate deposition in coral skeletons also releases carbon dioxide as a byproduct. Solid carbonate — from corals, but also from calcareous algae that occur extensively on reefs — not only is used to form the reefs themselves, but also leads to long-term sequestration when it is deposited in sediments that are buried, contributing to the formation of carbonate oozes, reef rock and ultimately limestone and marble. Oddly, fish poop again plays a role, where parrotfish and other grazers on corals create a steady rain of calcium carbonate fragments to the bottom.

However, it may well be that viewing living corals only as carbon dioxide sources shortchanges the value of carbon investments that would rebuild integrated and tightly-linked mangrove, seagrass and coral ecosystems. These ecosystems together have tremendous potential for delivering a wide variety of natural and human services, which can be attained by linking investments in fisheries, carbon sequestration and shoreline protection. Actually, there is emerging science that keeping fish populations high may be an essential approach for maintaining coral reef ecosystem health in the face of climate change.

I am not alleging that these outcomes are precisely correct. I am arguing that it is past time for all of us to think outside the box — to seize every chance to help decarbonize our reeling planet — as long as we can do that with clear eyes as to the consequences and double-dipping where possible toward other important outcomes.

Thus, I am really saying it is time to invest in building the science and doing the field experiments needed to bring ocean interventions fully into play for actively fighting global warming. The impacts need to be deeply and systematically explored by scientists so we can understand more fully the nature of this opportunity.

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