“There hasn’t really been much interest in the dead”

The oceans are by far the largest habitat on Earth and home for an abundance of spectacular animals, large and small, predators and prey, living on all depths from the warm sunlit surface waters to the kilometers deep dark and cold trenches.

Science has always been fascinated with and studied the diversity of life in the oceans, and today we know that all living organisms in the oceans play a key role in maintaining the ecology of Earth.

- But there hasn’t really been much interest in all the tiny, dead animals in the sea, says Belén Franco-Cisterna, a post doc at the Danish Center for Hadal Research (HADAL), and the Department of Biology, University of Southern Denmark.

The ones that got away

Most scientists have assumed that there are not very many dead animals in the sea; Either they get eaten alive or the carcasses get eaten or degraded by microorganisms soon after an animal dies.

- But we now know that this is far from the case, says Belén Franco-Cisterna:

- There is an abundance of small, dead animals in the oceans, and all these carcasses seem to play an important role in the transportation and recycling of carbon and nitrogen on our planet.

Together with colleagues at the earth evolution, biogeochemistry, geobiology focused research center Nordcee and HADAL, she is now trying to unravel the role of all these dead ocean animals. The researchers are particularly interested in studying zooplankton; an extremely abundant group of small animals that float with the current and can be found in all ocean regions.

Examples are copepods and krill, but the group also includes a number of jellyfish species. Zooplankton feeds on their plant counterpart, phytoplankton, or preys on zooplankton smaller than themselves. So far, their own fate has seemed to be eaten by larger animals.

This assumption has been so strong that scientists traditionally have not distinguished between alive and dead zooplankton when analysing collected samples from the oceans; all collected copepods, krill, etc. have been assumed to be alive at the time of collection. A factor also has been that it is time-consuming and difficult to tell the difference between a living and dead copepod just by looking at it.

But science now has new tools to tell the difference; for instance, you can stain the samples in a way that will make the living ones light up in bright colors, while the dead ones remain unstained.

Copepods can die of old age

- We are discovering that not all are alive. We now estimate that maybe 12-60% of zooplankton are already dead when they are collected in samples, says Belén Franco-Cisterna.

If a copepod does not get eaten, there are a number of other ways to die: it may die of starvation, infections, exposure to contaminants, diseases or simply of old age.

Up to 60% of all zooplankton is a huge amount of biomass, so the big question is: What happens to all these carcasses and what role do they play in the cycling of organic matter?

There are of course many factors determining what happens to a dead copepod: temperature, pressure, scavengers, etc, but one thing will always happen: It will start sinking. And if it is not eaten or degraded on its way down through the water, it will eventually reach the bottom.

- Roughly speaking and depending on temperature and animal size, a dead copepod will sink 100 metres a day, and at 4-5000 metres depth, I would expect to find more dead than live copepods – so carcasses can be abundant in deep waters, says Belén Franco-Cisterna.

As all other lifeforms on Earth, zooplankton is made of carbon. This element, in the form of CO2 has steadily increased in the atmosphere over the last decades, leading to climate change. The oceans act as a sink of anthropogenic CO2, sequestering the carbon through the biological carbon pump, which is a series of processes that transfer the atmospheric CO2 to the ocean interior and marine sediments as organic carbon.

So, sinking carcasses, as part of the biological carbon pump, is a transport of carbon to the ocean bottom, where it may be buried in the sediments.

Future climate changes

Sinking carcasses remove carbon from the upper water layers and indirectly also from the atmosphere, and this makes them acutely relevant to study, she points out, adding:

- We need to learn more about these processes to understand zooplankton carcasses’ role in the global carbon and nitrogen cycles. Then we can maybe predict how the oceans will respond to future climate change more comprehensively.

There are surprisingly few studies of zooplankton carcasses for Belén Franco-Cisterna and her colleagues to draw on:

In the deep

- One study has explored the prevalence of krill carcasses, but that is only for one specific area in one ocean and not enough for global extrapolation. We need more information to be able to estimate the global impact of zooplankton carcasses.

In their pursuit of more information, Belén Franco-Cisterna and her colleagues Peter Stief and Ronnie N. Glud have recently conducted a study of the fate of sinking copepod carcasses. Copepods account for the most abundant metazoans in the oceans, representing between 70 and 90 % of the zooplankton communities.

The team wanted to test the effect of temperature on copepod carcasses´ ability to reach the bottom - and thus transport carbon to the sediments.

- We found that it takes longer time for a carcass to degrade in cold water, which means that a larger portion of a carcass will evade degradation and can reach the bottom in cold environments, says Belén Franco-Cisterna.

In their experiments, on average 50% of the total carbon in a carcass degraded within 6-12 days at 20 degrees Celsius. When they lowered the water temperature to 4 degrees Celsius, it took more than 60 days – allowing more time to reach the bottom.

This indicates, that in low temperature areas, copepod carcasses may represent an important agent for carbon export, the authors conclude.

Suddenly there are piles of carcasses

Maybe all populations of zooplankton generate a stable proportion of carcasses, but some of these animal populations are known to suddenly increase in numbers and just as suddenly diminish in massive die offs – so at least sometimes their transportation of carbon may occur in bursts.

Blooms of jellyfish occur periodically in the oceans (both on the surface and kilometres deep down) and can extend for thousands of square kilometre. Obviously, such massive amounts of biomass will not just disappear when the blooms collapse, and the jellyfish die. One fate is that they end up in piles, sometimes called jelly-falls, on the ocean bottom, and even though a jellyfish mostly consists of water and very little carbon, the sheer amount of them turns these jelly-falls into an important carbon-transporter.

Large stranding events have been reported to push enormous amounts of krill (around 1500 carcasses per square meter) onto the beaches of Antarctica, after which they are assumed to wash back into the ocean and sink to the ocean bottom.

Should we initiate massive die offs?

Could such naturally occurring blooms inspire us to create artificial blooms of zooplankton that will die off and transport carbon to the ocean bottom – could this be a tool to lower CO2 levels in the atmosphere?

In theory a great idea, but nature rarely behaves according to theory. So many things could happen if we tried to do this. In nature, things are not so straightforward. Zooplanktonic organisms generally have complex life cycles with different stages from eggs to adults.

Therefore, the success of their populations relies on the convergence of various factors in each developmental stage, mainly food supply and favorable environmental conditions.

- Maybe, one can succeed in increasing their populations, but the occurrence of “zooplankton blooms” may not necessarily lead to massive die-off and carbon sequestration. Other processes may occur instead, like more predation by fish. Nowadays it is crucial to look for solutions to mitigate climate change, but we need to be cautious with the uncertainties and limitations that we can face, and the complexity of the natural systems must always be considered, says Belén Franco-Cisterna.

Laughing gas at sea

Carbon is not the only element of interest, when studying zooplankton carcasses. They also play a key role in the global nitrogen cycle. This is more complex than the carbon cycle, but examples are that they remove nitrogen from the surrounding seawater and that they produce laughing gas (nitrous oxide), which is a potent greenhouse gas.

Intriguingly, recent studies have documented that zooplankton is surprisingly abundant in and around Oxygen Minimum Zones in the oceans; large ocean patches at 200 – 1500 m depth, where there is no or only very little oxygen in the water.

Peter Stief and Ronnie N. Glud have looked further into this phenomenon and they have discovered, that the guts of copepods in Oxygen Minimum Zones are depleted of oxygen, which is remarkable considering their small size; anoxic guts have otherwise been reported for much larger animals.

No oxygen inside the body

- Copepods and other zooplankton represent vastly abundant microbial hotspots with unique biogeochemical features. Because they are anoxic inside, they can host microbial processes, that allows the production of nitrous oxide, says Peter Stief, an Associate Professor at Department of Biology.

Oxygen Minimum Zones are also responsible for up to half of the oceanic nitrogen loss, which is also, in part, taken by zooplankton carcasses.

- What we don’t know is how much carcasses contribute to the removal of nitrogen and production of nitrous oxide on a global scale. Their contribution is not included in the current ocean models, so we might have to integrate the rates of nitrogen cycling associated with zooplankton more comprehensively, we might have to change our models of nitrogen cycling associated with zooplankton more comprehensively, says Peter Stief.