The ocean is teeming with microscopic plants and animals known collectively as plankton. Each individual organism is tiny, yet taken as a whole this floating community provides important ecosystem services. For example, plant plankton, or phytoplankton, uses photosynthesis to fix carbon from carbon dioxide, making it a key part of the ocean carbon cycle. Phytoplankton are also a food source for zooplankton, which in turn feeds fish and sea creatures up to the blue whale included.
As the climate warms and ocean temperatures rise, researchers expect to see significant changes in the distribution of plankton. Yet, there are hardly any studies on where different species of plankton might thrive in the future.
Part of this knowledge gap has now been filled by a research team led by Fabio Benedetti and Meike Vogt, the first postdoctoral researcher and the second principal investigator in Nicolas Gruber’s group at ETH Zurich, together with colleagues of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).
The project team assembled a new global dataset to create distribution maps for more than 860 species of phytoplankton and zooplankton based on various statistical algorithms and climate models. They then overlaid these maps to determine what plankton communities might look like in the future and where they might occur. The results of their work were recently published in the journal Nature Communication.
Global warming leads to diversity
Benedetti and his colleagues have shown that the diversity of phytoplankton and zooplankton can be expected to increase in the future in many regions, as warmer water generally tends to promote greater diversity.
However, at very high temperatures, i.e. above 25 degrees Celsius, phytoplankton and zooplankton respond to warming differently: phytoplankton diversity continues to increase, while zooplankton diversity decreases. This will lead to a reduction in the diversity of zooplankton in the tropics.
Emergence of new communities
Plankton species from the tropics and subtropics will move to the poles and replace species adapted to colder waters. This will give rise to many new communities that have never existed in these combinations before, a convergence of species that do not currently occupy the same habitat and whose interrelationships are not clearly aligned.
Researchers expect the greatest changes to occur in the oceans at high and temperate latitudes, precisely in regions critical for CO.2 fixing and fishing.
“In some areas of the ocean we will see an increase in the number of species which may at first glance seem positive. But this increase in diversity could actually pose a serious threat to the existence and functioning of ‘well-established marine ecosystems at higher latitudes,’ says lead author Benedetti.
The marine ecosystems of high and middle latitudes currently depend on planktonic communities poor in species. The size distribution of planktonic organisms also has an important influence on the quality of ecosystem service.
To determine if these factors change when planktonic communities and therefore their size distributions change, the researchers simulated the effects of climate change on the size structure of two important plankton groups, diatoms and copepods. Data on the size of individual species are available for these organisms.
Small organisms replace larger ones
Using simulations, scientists have shown that the quality of habitat increases for small organisms, while it decreases for larger ones. As a result, planktonic communities may change, as may the relative proportions of small and large species: smaller organisms become more abundant and more numerous, especially at high and temperate latitudes, while larger organisms decrease in number. .
According to the researchers, this will affect the ecosystem services provided by the plankton: if changes occur in the species composition and size structure of the plankton, it could have a negative impact on the ecological pyramid and therefore on fish yields.
Plankton also play an important role in fixing oceanic carbon. Some of the carbon fixed by phytoplankton sinks into the depths of the ocean and is effectively removed from exchanges with the atmosphere.
For example, the Arctic Ocean is currently home to larger phytoplankton than tropical seas. Many of them have shells, and their excretions are also larger and heavier. As a result, dead organisms and their droppings sink faster and to greater depths before the carbon they contain is broken down into CO.2. Dissolved in deeper water, this CO2 remains trapped in the depths for long periods of time due to density stratification and slower circulation of the deep ocean.
If smaller species replace larger ones, this transfer of carbon to the deep ocean will decrease.
However, scientists cannot say exactly how important these effects will be. “The only thing we can determine at the moment is the current importance of certain areas of the ocean in terms of different ecosystem services and whether this service provision will change in the future,” said Benedetti.
The shift in distribution well underway
Scientists have observed changes in the distribution of plankton since researchers observed that the distribution of plankton has changed for several decades. The first systematic monitoring program, called a continuous plankton recorder (CPR), began in the North Atlantic in the 1930s. Using data from the CPR, other researchers were recently able to show that smaller copepods have displaced the largest species in the North Atlantic since the 1950s due to global warming. It also reduced atmospheric CO sequestration2 in the deep sea.
Jellyfish, another type of zooplankton, also migrate north. In 2005, Ireland detected a huge influx of tropical jellyfish, which devastated salmon farms along the coast. “Events like this show that changes in the distribution of plankton are already well advanced,” said co-author Meike Vogt.