Source: Basque Research Newsroom
Posted by Jenny Griffin
Sea surface temperature is expected to increase 2ºC on average globally by 2080-2100. Some of the consequences of this increase include changes in ocean circulation and higher water column stratification, thus affecting the nutrient availability for the growth of marine phytoplankton.
The research team led by Azti-Tecnalia points out the effects to primary production (phytoplankton mass produced annually by photosynthetic single-celled organisms that are suspended in the ocean), and to secondary production (zooplankton biomass, made up of small animal organisms that feed mainly on phytoplankton and which fish feed on).
Globally, it is estimated that the sea temperature rise will cause phytoplankton and zooplankton biomass to decrease by 6% and 11% respectively. This suggests that there will be a negative amplification of climate change, which will spread through the marine food web, i.e. zooplankton biomass will decrease more than phytoplankton. This process will take place mainly in tropical oceans, which cover 47% of the global ocean surface.
Differences by region
Phytoplankton and zooplankton reduction, however, will affect different regions in different ways. In the seas in Central and Southern Europe (North Sea and temperate Northeast Atlantic), higher thermal stratification of the ocean water layers and, consequently, a lower presence of nutrients for phytoplankton to grow, will reduce primary production; and in the Baltic, Barents and Black Sea phytoplankton production is expected to increase.
Azti-Tecnalia researcher Guillem Chust, leader of the scientific work and main author of the paper, says that "in the ocean regions that lose more phytoplankton and zooplankton biomass, that is, with a negative amplification, fish biomass may also decrease dramatically, especially pelagic species (i.e. those living the water column, excluding the seabed)".
"Climate regulation will also be affected negatively by the primary and secondary production decrease globally," Chust explains, "because, as there will be less phytoplankton, absorption of CO2 from the atmosphere by the oceans will be lower, as plankton is responsible for half of the planet's photosynthetic activity. This in turn will reduce the ocean's capacity to regulate the climate".
Guillem Chust, J. Icarus Allen, Laurent Bopp, Corinna Schrum, Jason Holt, Kostas Tsiaras, Marco Zavatarelli, Marina Chifflet, Heather Cannaby, Isabelle Dadou, Ute Daewel, Sarah L. Wakelin, Eric Machu, Dhanya Pushpadas, Momme Butenschon, Yuri Artioli, George Petihakis, Chris Smith, Veronique Garçon, Katerina Goubanova, Briac Le Vu, Bettina A. Fach, Baris Salihoglu, Emanuela Clementi, Xabier Irigoien. Biomass changes and trophic amplification of plankton in a warmer ocean. Global Change Biology, 2014; DOI: 10.1111/gcb.12562
By Jenny Griffin
In the Southern Ocean far-ranging seabirds are apex predators at the top of the food chain. Yet surprisingly, through a mutualistic relationship they have developed with primary producers at the bottom of the food chain – phytoplankton, they play an essential role in keeping the ocean healthy while at the same time regulating global climate.
According to a new study, which was recently published online in the Proceedings of the National Academy of Sciences, when phytoplankton are consumed by herbivorous grazers known as krill, they give off a chemical signal that attracts krill-eating seabirds.
These chemical cues also have another important function; when dimethyl sulfide (DMS) – the chemical attractant released by the phytoplankton – evaporates, it forms sulfur dioxide – a compound that is known to promote the formation of clouds. Because clouds increase the Earth's albedo affect by reflecting a great deal of solar radiation back into space, by promoting cloud formation, plankton play an important role in climate regulation. This theory, first put forward by Charlson et al, is known as the CLAW hypothesis, an acronym for each of the authors of the paper where this theory was originally proposed. Seabirds benefit the phytoplankton in two ways: firstly they reduce predation pressure on the primary producers by consuming the grazing zooplankton that are feeding on them; and secondly, they fertilize the producers by providing a rich source of iron, which is necessary for primary production, and is in relatively short supply in the Southern Ocean.
“The data are really striking,” said Gabrielle Nevitt, professor of neurobiology, physiology and behavior at the University of California, Davis and co-author on the paper with graduate student Matthew Savoca. “This suggests that marine top predators are important in climate regulation, although they are mostly left out of climate models.”
“In addition to studying how these marine top predators are responding to climate change, our data suggest that more attention should be focused on how ecological systems, themselves, impact climate," said Nevitt. “Studying DMS as a signal molecule makes the connection.”
Nevitt, who has been researching the sense of smell in ocean-faring seabirds for around 25 years, was the first to show that top predators in marine ecosystems make use of climate-regulating chemicals to forage and navigate across the featureless expanse of the ocean.
“DMS is now known to be an important signal for petrels and albatrosses, and the idea has been extended to various species of penguins, seals, sharks, sea turtles, coral reef fishes and possibly baleen whales,” she said.
Phytoplankton are the primary producers or plants of the open ocean. They absorb carbon dioxide and energy from the sun for growth, and when they die they produce an enzyme that releases DMS.
According to the CLAW hypothesis, ocean warming promotes growth of phytoplankton, which upon their death then produce an enzyme that stimulates DMS release. As atmospheric DMS levels rise, more clouds are formed, which reflect sunlight and help keep the planet cool, effectively serving as a negative feedback loop that helps control the Earth's temperature.
The scientists analyzed 50 years of data on seabird stomach contents, combined with results of Nevitt's previous studies that looked at which seabird species used DMS as an olfactory cue for foraging. They discovered that seabirds that are attracted to DMS prey primarily on herbivorous crustaceans, such as krill, that feed on phytoplankton.
This relationship has previously been shown on land, where there are several examples of plant species that when under attack from insects, produce chemicals that attract predators who feed on those insects. The authors propose that a similar scenario takes place in the open ocean: this same mechanism is used by phytoplankton when they are attacked by krill; the DMS that they release from their cells as they die attracts avian predators that feed on krill.
Besides preying on their predators, the seabirds have a further benefit to offer phytoplankton. Because the Southern Ocean is devoid of large land masses – a natural source of iron, which washes into the sea from rivers – it is also limited in iron, which in turn limits primary production. Krill are a rich source of iron, yet this is toxic to vertebrate predators in high doses. Consequently, seabirds excrete much of the iron that they ingest from the krill, returning this to the ocean water where it is available to phytoplankton for growth.
According to Nevitt, the work suggests that by linking predatory seabirds and phytoplankton — the top and bottom levels of the food chain — DMS plays an important role in the ocean ecosystem, which affects climate by taking up carbon, as well as a physical role in generating clouds.
“Studying how seabirds use scent cues to forage has shown us a mechanism by which the seabirds themselves contribute to climate regulation. That's not what we expected, but I really think our results will have global significance,” she said.
It is concerning that seabird numbers are declining – nearly half of the species are listed as threatened, some of them critically. This newly discovered link between top predators and phytoplankton at the base of marine food webs means that a decline in the number of seabirds could have a significant impact on marine productivity, and ultimately on climate regulation.
Charlson RJ, Lovelock JE, Andreae MO, Warren SG. (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655-661; doi:10.1038/326655a0
Nevitt GA, Veit RR, Kareiva P. (1995) Dimethyl sulphide as a foraging cue for Antarctic Procellariiform seabirds. Nature 376, 680-682; doi:10.1038/376680ao
Savoca MS, Nevitt GA. (2014) Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators. PNAS, doi:10.1073/pnas.1317120111
Wright KLB, Pichegru L, Ryan PG. (2011) Penguins are attracted to dimethyl sulphide at sea. J Exp Biol 214, 2509-2511; doi: 10.1242/jeb.058230
A species of one of the world's tiniest creatures, ocean plankton, is heading for extinction as it struggles to adapt to changes in sea temperature. And it may take local fisheries with it.
Research led by Deakin University (Warrnambool, Australia) and Swansea University (UK) has found that a species of cold water plankton in the North Atlantic, that is a vital food source for fish such as cod and hake, is in decline as the oceans warm. This will put pressure on the fisheries that rely on abundant supplies of these fish.
"There is overwhelming evidence that the oceans are warming and it will be the response of animals and plants to this warming that will shape how the oceans look in future years and the nature of global fisheries," explained Deakin's professor of marine science, Graeme Hays.
"We know that warm water species are expanding their ranges as warming occurs, and vice versa. What is not known is whether species are able to adapt to new temperatures. Will, for example, cold water species gradually adapt so they can withstand warming seas and not continually contract their ranges. From the results of our study, it is looking like the answer is no."
Answering the question of adaptation is not easy as it requires long-term observations spanning multiple generations. For this study, the research team examined a 50-year time series from the North Atlantic on the distribution and abundance of two very common but contrasting species of ocean plankton, Calanus helgolandicus that lives in warmer water and Calanus finmarchicus that lives in cold water. These crustaceans are vital food for fish and underpin many commercial fisheries in the North Atlantic region.
The researchers were surprised to find that the cold water C. finmarchicus has continued to contract its range over 50 years of warming.
"In other words, even over 50 generations (each plankton lives for one year or less) there is no evidence of adaptation to the warmer water," Professor Hays said.
"The consequences of this study are profound. It suggests that cold water plankton will continue to become scarcer as their ranges contract to the poles, and ultimately disappear. So certainly for these animals, thermal adaptation appears unlikely to limit the impact of climate change.
"C. finmarchicus is a key food source for fish such as cod and hake. So continued declines in abundance will have a negative impact on the long-term viability of cold water fisheries in the North Sea and other areas in the southern part of their range. At the same time the continued increase in abundance of the warm water plankton, C. helgolandicus, will likely play a role in the emergence of new fisheries for warm water species."
Professor Hays said that the impact of ocean warming was not confined to the North Atlantic region.
"Ocean warming is occurring globally and so these findings are likely to apply to other areas around the world including southern hemisphere locations such as Australia, South Africa and South America that support important fisheries dependant on plankton," Professor Hays said.
"Plankton recorders deployed in the southern hemisphere, for example as part of the Australian Continuous Plankton Recorder Project (a joint project of CSIRO Marine and Atmospheric Research and the Australian Antarctic Division), will continue to document these changes."
Hinder, S. L., Gravenor, M. B., Edwards, M., Ostle, C., Bodger, O. G., Lee, P. L. M., Walne, A. W. and Hays, G. C. (2014), Multi-decadal range changes vs. thermal adaptation for north east Atlantic oceanic copepods in the face of climate change. Global Change Biology, 20: 140–146. doi: 10.1111/gcb.12387