Illustration:© Lars-Erik Håkansson

Global warming alters plankton communities

Plankton are cornerstones in marine food webs. Now research in the Mediterranean Sea shows that even short-term heat waves in the oceans can cause great damage to plankton communities.

Climate change could in many ways alter the functioning of marine plankton communities. Yet descriptions of the effects of climate change on plankton are rarely found outside purely scientific publications, at least in comparison to reports of the effects on more conspicuous organisms such as fish or corals.

“Plankton are the base of the functioning in marine waters as they are the first link of the marine food chain. Eventually, they nourish fish, marine mammals and birds,” says Dr Francesca Vidussi, researcher at the MARBEC laboratory in Montpellier, a part of the French National Centre for Scientific Research (CNRS), where she co-leads a research group on climate change effects on plankton. “They also contribute to oxygen production and CO2 sequestration, similarly to forests on land. The ocean is a sink of CO2 – it absorbs CO2 and traps it in deep waters. Part of this process is related to plankton. It is important to know the response of plankton to climate change, which can modify the food web structure and diversity. In addition, changes in planktonic communities can compromise the mitigation role exerted by the oceans and impact important element cycles”.

Trying to establish climate change effects on plankton communities is not easy. Effects of warming can be studied in laboratory experiments by exposing plankton species to different temperatures. Such experiments are valuable in identifying, for instance, temperature optima and tolerance levels, but they do not consider interactions within the community. At another extreme, warming effects can be assessed through long-term field observations on the effects of temperature on plankton. Such studies are highly valuable, but due to natural variations in factors other than temperature, it can be difficult to distinguish between the effects of warming and those due to other parameters.

The researchers at MARBEC chose a method that is in between laboratory experiments and complex field observations. They performed experiments in field enclosures called mesocosms, which are enclosed experimental units that replicate some of the conditions of the natural environment1. In this case, the mesocosms consisted of transparent bags that were filled with water from the Mediterranean Thau lagoon, in which the bags were also immersed. Prior to the filling of the bags, the water was passed through a 1 mm mesh to remove larger particles and organisms, and the community studied hence consisted of plankton smaller than 1 mm in size, representing almost the whole plankton community in the lagoon.

“The water mass in a natural environment is usually submitted to horizontal and vertical movements. It is hard to study the same plankton communities in natural waters, since the water mass moves, and the populations change”, explains Dr Behzad Mostajir, the other co-leader of the research group. “To study the warming effects on the dynamics of the same plankton communities we need to confine several cubic metres of natural lagoon water, which can be heated in a controlled manner, according to the future scenario of global warming. Mesocosms provide this confinement of the natural water mass and the possibility to heat the water mass and follow associated plankton communities over a period of several weeks”.

Two of the experiments – conducted for slightly less than three weeks in spring and autumn, respectively – investigated the effects of a 3°C temperature increase in relation to controls. All the bags reflected the natural conditions in the lagoon with its day/night temperature variations etc. The treatment process was intricate, as the temperature was not static. The water temperature in the heated bags was constantly adjusted to follow the natural water temperature variations in the lagoon, but was kept 3°C warmer.

In a third experiment, the researchers mimicked a marine heatwave (a major threat to oceans; see Editorial). As in the studies described above, a 3°C water temperature increase was used. This time the warming was halted halfway (10 days) through the experiment, after which the recovery of the plankton community was investigated for another 10 days.

In all three experiments, the composition of the plankton community was monitored. Several parameters that indicate the overall performance or condition of the plankton community were also measured. These were related to biomass, growth, losses, metabolism etc. of the plankton.

One of the main results of the two warming experiments measuring responses in spring and autumn was that a 3°C warming was enough to depress whichever phytoplanktonic groups dominated the community at the time, and hence changed the entire composition of the plankton community in the two most productive seasons.

Comparing these two experiments showed that warming shifted the production balance in the lagoon from spring to autumn. This resulted from a strong depression of phytoplankton biomass and oxygen production in spring, and an enhancement of the biomass in autumn. Contrary to the natural dynamics, the autumn community was as productive as the spring community. Taken together, the results show that the strong negative effects in spring were only partially compensated by the increases in autumn, and the overall effects of warming were negative for oxygen production, respiration, and phytoplankton biomass.

“It is clear that profound changes are expected in marine coastal waters, notably during the spring bloom, which is an important event in temperate waters, just like flowering in spring on land,” says Dr Vidussi. “This is especially important in productive coastal waters like the Thau lagoon, where plankton productivity is not only important for natural species, but also sustains aquaculture. This lagoon is one of the most important aquaculture sites in France – there are a lot of oyster farms. Oysters feed on plankton and the diminishing of plankton during the spring bloom could compromise aquaculture”.

The results from the heatwave experiment2 showed that many of the functional processes related to phytoplankton biomass, growth, losses and metabolism were amplified by the heatwave. The effects persisted for several days, and in conclusion the resilience of the community was low. Not all effects were reversible, even if some functions recovered.

“This means that even during a short and not very intense heatwave, persistent effects are expected to occur. These lasted longer than expected, so the changes induced for some important functions in the community are profound”, says Dr Mostajir.  And goes on to consider all three experiments: “It seems that with an increase of 3°C the functioning and structure of the plankton community would be deeply modified. Thus, what we know about its functioning and its related ecosystem services today would not apply in the future. As one example, it is not certain that it would be possible to culture oysters in the Thau lagoon, and this could potentially also apply to other aquaculture and fishing activities in similar productive coastal waters”.

Dr Vidussi sets their results in a global context: “Plankton provide us with half of the oxygen we respire. It contributes to climate regulation and to reducing carbon dioxide in the atmosphere by partly assimilating emissions. All of this is cost-free for us! At the same time, climate change has an impact on plankton. Plankton and oceans are our allies – the best that we can do is of course to reduce our carbon emissions. But we also need to reduce additional stressors, such as pollution and overfishing.”

Marko Reinikainen

1 Mesocosms also have their own challenges, such as that they are closed, all levels of food chains are hard to include etc. Commonly, it is recommended to approach research topics through a combination of methods.
2 Note that this experiment was conducted in a different year and season (May/June) than the two other experiments, so the results are not directly comparable.
This study has received funding from the AQUACOSM-plus project of the European Union's Horizon 2020 research and innovation programme under grant agreement No. 871081.

Courboulès J, Mostajir B, Trombetta T, Mas S, Vidussi F. 2022. Warming Disadvantages Phytoplankton and Benefits Bacteria During a Spring Bloom in the Mediterranean Thau Lagoon. Frontiers in Marine Science 9.
Courboulès J, Vidussi F, Soulie ́ T, Mas S, Pecqueur D, Mostajir B. 2021. Effects of experimental warming on small phytoplankton, bacteria and viruses in autumn in the Mediterranean coastal Thau Lagoon. Aquatic Ecology 55.
Soulié T, Vidussi, F, Courboulès J. Mas S, Mostajir B. 2022. Metabolic responses of plankton to warming during different productive seasons in coastal Mediterranean waters revealed by in situ mesocosm experiments. Scientific Reports 12.
Soulié T, Vidussi F, Mas S, Mostajir B. 2022. Functional Stability of a Coastal Mediterranean Plankton Community During an Experimental Marine Heatwave. Frontiers in Marine Science 9.

Exeprimental setup in  the Mediterranean Thau lagoon.Photo: Behzad Mostajir



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