Fall arrives later than before, below the surface like above. Summers become longer, winters shorter. The sea is warmer, and hearing that mackerel have reached Svalbard, one might think that all living creatures will be displaced northward, that all fishermen need to do, is follow them.
But life is more than temperature.
"We cannot move the light," says Mari Myksvoll.
The oceanographer, from the Norwegian Instititue of Marine Research and the Bjerknes Centre for Climate Research, has coordinated a study of future primary production.
Behind the term primary production lie algae and other phytoplankton at the bottom of the food chain – microscopic plants that are eaten by zooplankton that are eaten by capelin that are eaten by cod that are eaten from white dinner plates, unless something in the chain has already become food for auks, seals or starfish.
Fish need light at the right time
Like all plants, phytoplankton requires light for growth. The light follows neither ocean currents nor the temperature of the water, but the course of the sun. At high latitudes, plankton blooms in spring, regardless of where the fish might be.
Algae and fish may not necessarily find new habitats if climate change makes their old regions too warm.
"Everything must match in time," says Mari Myksvoll.
"If the algal bloom comes at the wrong time, the fish may not spawn simultaneously. Then the fish larvae will not get food when they need it."
More food where the ice disappears
Areas that are covered with ice during the growth season, can be livelier in an ice-free future.
Mari Myksvoll and her colleagues have used a climate model to study the living conditions of phytoplankton in the Barents Sea and the Nordic Seas till the end of the century. Considering these plants at the bottom of the food chain is a necessary, first step.
"To be able to predict future fisheries, we need to know how much food will be available for the fish," she says.
Their results show that regions that are totally or partly covered by sea ice today, will have more phytoplankton. With the ice gone, sunlight can penetrate farther into the sea, opening up new space for growth.
Less clear in regions with no ice
The development in regions that are not covered by ice today, is less certain. Not everything can be explained by sunlight and temperature. Whether the water column is unstable or stable, determines if nutrients remain in the deep ocean or are mixed up to levels where plants may make use of them.
"Changes in Norwegian waters may not be that substantial," says Myksvoll, though emphasizing an important exception.
Spring blooms in Norwegian fjords do not play second fiddle to bursting buds on land. In May, from one day to the following, the sea can go from translucent to opaque turquoise. Light has replaced the darkness of winter, and when nutrients suddenly well up from the depths, Emiliana huxleyi and other algae reproduce explosively.
In regions with such spring blooms, model simulations suggest that the growth conditions for algae will improve even further in the coming decades.
Oceans of desert
Half of the oxygen in the atmosphere comes from plants in the ocean. Still, the vast oceans are as barren as deserts. As little as twenty percent of the global ocean provides eighty percent of the fish catch.
One reason behind the wealth of Norwegian fisheries, is that algae bloom in the right place at the right time. Herring and mackerel shoal through the Norwegian Sea, and on the bottom of the Barents Sea, cod and haddock feed on shrimp and small fish. Food is available for hungry fish and fish larvae.
The really great oases are otherwise found in concentrated regions near land, like off the coast of Peru and the southwestern coast of Africa, where cold and nutrient-rich water wells up from the depths.
In such upwelling regions, conditions are good for phytoplankton – so good that the larger organisms present are too few to digest all the food. Resources for fisheries are abundant, but if the production of phytoplankton should increase, the resulting growth would probably be limited to jellyfish.
Fish swim their own seas
It is November. Mackerel swimming south meet fish heading north, for not all species are as simple as migratory birds. Fish find their own routes.
What the future will bring at higher levels of the food chain, for those eating those that eat phytoplankton, is complicated to predict, in areas with, as well as without, sea ice.
"Finding out what will happen to future fisheries, requires very detailed climate models," says Mari Myksvoll.
Global fisheries depend on the cumulative development in various ocean basins. One needs to know how nutrients are mixed and distributed, in seas where the sun always shines, and in those lying in darkness for half of the year.
The ocean's color of fall is black. Soon winter storms will stir the sea, so that plankton could hardly grow, not even with light and warmer water. But below the surface dead organisms and algae hit the seafloor, where they will decompose into nutrients for new life.
The ocean will soon be ready for spring.
Reference
Mari S. Myksvoll, Anne Britt Sandø, Jerry Tjiputra, Annette Samuelsen, Veli Çağlar Yumruktepe, Camille Li, Erik A. Mousing, Joao P.H. Bettencourt, Geir Ottersen (2023): Key physical processes and their model representation for projecting climate impacts on subarctic Atlantic net primary production: A synthesis. Progress in Oceanography, 217, 2023, 103084. ISSN 0079-6611, https://doi.org/10.1016/j.pocean.2023.103084.
Scientists from all the Bjerknes Centre partner institutions – the Norwegian Institute of Marine Research, NORCE, the Nansen Environmental and Remote Sensing Center, and the University of Bergen – collaborated in this study.