Forests of the sea

Where is carbon stored in the ocean? (10 minute read)

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When I think of the sea, I think of the colour blue. Sometimes I think of the brilliant turquoise I’ve seen on surrounding coral reefs in the tropics. Sometimes I think of the inky blue I saw when I was lucky enough to visit Antarctica (it’s many years ago but remains a vivid memory). Most often, though, I think of the blue harbour I see from Wellington’s high hills, ever-changing as it reflects the city’s skies. Today, it’s a mottled blue, dark where there are clouds, lighter where the sky is clear, the patches shifting constantly in the wind.

Any excuse to remember Antarctica (image credit: Melanie Newfield)

Lately, though, I’ve been thinking about a different colour in the ocean – green. Beneath the surface, there is a wondrous diversity of plant life, crucial to our survival, and that of all life on earth. But most of us seldom give the plant life in the ocean a thought.

The plants that live in the sea are very different from the plants that we see on land. On land, the most obvious plants are usually the flowering plants. Even plants that we might not think about as having flowers, such as grasses, are classed as flowering plants. It’s just that their flowers aren’t very showy. The other plants which dominate some places are the conifers. These are the cone-bearing plants, like pine trees or New Zealand’s mighty kauri. There are some other groups, such as mosses, ferns and cycads, but it’s the conifers and flowering plants which dominate the land. And when it comes to the human diet, almost every plant we eat is a flowering plant.

But when we dip below the surface of the sea, the plants are different. There are very few flowering plants which grow truly submerged in the sea, and no conifers. Instead, the seas are dominated by an extraordinarily diverse group which are sometimes called “algae”. However, “algae” is a misleading term, because some types of algae are less closely related to each other than humans are to houseflies. Many types of “algae” aren’t technically even plants, even though they perform the same kinds of functions as plants (and I will refer to them as plants sometimes here, for convenience).

I admit, I’ve just been down something of a research rabbit hole trying to understand exactly how all these groups of marine species are related. I’ve concluded I don’t need to explain it here. The short version is that there are some you can see easily – these are often called seaweeds. And there are some you can’t see unless you have a microscope, mostly known as phytoplankton. There are also a couple of other groups, including seagrass, which is unusual in that it’s a flowering plant, closely related to species which live on land and not at all related to other species in the sea. What’s important is that all these groups, including seagrass, seaweed and phytoplankton, absorb carbon dioxide from the water and, using sunlight, turn it into living tissue, through the process of photosynthesis.

In the ocean, the process is slightly different from what happens on land. On land, plants absorb carbon dioxide, in gas form, directly from the atmosphere. In the ocean, plants absorb carbon dioxide which has dissolved in the water. However, since the carbon dioxide dissolved in the ocean’s water has come from the atmosphere, seaweed, seagrass and phytoplankton all play a role in absorbing the extra carbon dioxide we have been pumping into the atmosphere over the last couple of hundred years.

How much carbon do marine plants absorb? One measure of this is called Net Primary Productivity, measured in tonnes of carbon. Although the estimates vary, in total marine species absorb a similar amount to terrestrial plants, or perhaps a little less. However, here is where the oceans and the land begin to differ.

On land, the ecosystems which absorb the most carbon are tropical forests, followed by tropical savannahs. Productivity is much lower in temperate areas. But in the ocean, it’s different, for a whole range of reasons. While there is an area of high productivity on the equator, areas near the poles are highly productive too. If there is a general rule, it is that coastlines are generally more productive than the open ocean (there’s a great graphic showing seasonal changes here).

Most of the carbon absorbed from the ocean is absorbed by phytoplankton, microscopic organisms floating in the sea. Those species we can actually see – the seaweeds and seagrasses – make up only a small proportion of the carbon absorbed. However, that doesn’t mean they aren’t important. Recent research has shown that seaweed ecosystems such as kelp forests are extremely productive – that is, they absorb a lot of carbon in relation to their total area. If all the areas of seaweed, which are distributed in a thin strip around the world’s coastlines, are added together, they would make up about the same area, and biological productivity, as the Amazon rainforest.

But marine species don’t just absorb carbon from the ocean during photosynthesis. Many marine species have shells made from calcium carbonate, which, as its name suggests, contains carbon. This carbon also comes from carbon dioxide dissolved in the water of the ocean.

So, species in the ocean absorb a lot of carbon, but does that mean they can act as carbon sinks and help us with climate change? Unfortunately, it’s not quite that simple. There’s an important difference between what I’ve been talking about – absorbing carbon during photosynthesis and shell formation – and carbon sequestration, which is the process of locking carbon away somewhere it can’t contribute to climate change.

Marine plants have no equivalent of the mighty trees we find on land, but they do sequester carbon. One way this happens is through “marine snow”, which is a beautifully evocative name for the remains of dead marine creatures falling through the water column and onto the sea floor. Some of the marine snow is eaten by other creatures, but some of it, especially the shells made of calcium carbonate, falls deep into the ocean, forming a layer of sediment which holds a massive quantity of carbon. In fact, there is vastly more carbon stored in the ocean than all living plant matter and soils on land combined. Most of that carbon is in ocean sediments. Those sediments don’t necessarily remain in the ocean forever, for example some are brought to the surface by the Humboldt current off the coast of South America. However, much of the sediment can remain for thousands or even millions of years. Some will eventually compact and become rock, which means the carbon held there is not going to get into the atmosphere any time soon.

Are we at risk of releasing the sequestered carbon from these sediments, as we are releasing carbon sequestered in forests and wetlands? And is there the potential for us to enhance it in some way, and perhaps draw more carbon dioxide out of the atmosphere?

The risk of somehow disturbing these sediments and releasing the carbon is so far largely hypothetical. Nonetheless, because there’s such a massive amount of carbon stored there, releasing only a small percentage could be a big problem. Scientists have begun to consider which areas of sediment might be most at risk, and most of a risk to us.

I haven’t found studies which look at enhancing the natural process of sediments falling to the ocean floor as a solution to climate change (let me know if I’ve missed something). However, the deep ocean is being seriously considered as a storage option for carbon dioxide which has been extracted from the atmosphere in other ways. For example, one recent study investigated a product called “black pellets”, a type of artificial coal originally developed as a potential fuel. Because these black pellets are heavier than seawater and resistant to being broken down by microbes, it may be possible to store them deep in the ocean as a way of sequestering carbon. I will return to this topic when I look at carbon capture technology, in the next couple of months.

But there is one part of the ocean where carbon is stored in sediments and which is vulnerable to human interference. This is our coastline. On the boundary between land and sea, and in the shallow waters closest to the coast, are some important ecosystems which get relatively little attention. These are mangrove swamps, salt marshes and seagrass meadows.

Of the three groups, it is mangrove swamps which are the best-known, perhaps because they are the most visible. The term “mangrove” refers to any tree which grows in brackish or sea water – New Zealand has only a single species but there are about 80 species, mostly in the tropics. Over the last 50 years, the world has lost at least 20% of its mangrove forest, maybe up to 35%.

Salt marshes are areas of low-growing vegetation which are inundated by sea water at high tide. However, even at high tide, some vegetation grows above the water surface. Salt marshes occupy similar types of environment to mangroves, but are more common in cooler areas. Like mangroves, they are also being lost, but at a much slower rate than mangroves, less than 0.5% per year.

Saltmarsh near Waikanae (image credit: Melanie Newfield)

Seagrass meadows are different. While some parts of seagrass meadows may be exposed at low tide, at high tide they are completely submerged. Some seagrass meadows remain submerged even at the lowest of tides. Although they aren’t true grasses (according to botanists), seagrasses do have a way of growing that’s like grass, with a dense underground network of stems and roots. Studies have suggested that seagrass meadows have been lost at a rate of at least 1.5% per year since around 1880. But the rate of loss is increasing, and since 1980, 5% of seagrass meadows have been lost every year (some sources give even higher figures).

All three groups of coastal plants contribute to the buildup of deep soils where decomposition is very slow, because the soil has very little oxygen. Although the amount of carbon stored in the living tissue of mangroves, salt marsh plants and seagrass isn’t particularly large, there is much more carbon stored in the deep, muddy soil.

When mangrove forests, salt marshes and seagrass meadows are destroyed, either directly by coastal development, or indirectly as a result of pollution, carbon is released from the soil. In one frightening example, scientists found that converting one hectare of mangrove forests to shrimp farm was the equivalent, in terms of carbon emissions, to converting five hectares of tropical rainforest to pasture.

There is now increasing awareness of the importance of these three coastal ecosystems, and in some cases they are being restored and replanted. But how fast are they at sequestering carbon? Are they fast enough to make a difference to emissions? There is some evidence that replanting mangrove forests is a particularly good way to suck carbon out of the atmosphere, comparable to replanting other tropical forest. But these coastal areas, most of the carbon is in the soil, not the vegetation, and that soil can take thousands of years to build up. So, it is crucially important to prioritise the protection of what we already have over attempts to restore what has been lost.

But that doesn’t mean that restoration of mangroves, salt marsh and seagrass is pointless. In fact, we need them more than ever now. Because all three environments protect the coast from erosion. And erosion is something that will only get worse as we face rising seas and more intense storms, so we need all the help we can get.

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