NEWS Many current climate studies focus solely on increases in temperature, and in doing so, the ecosystem effects caused by climate change may be misinterpreted. In a new study, Umeå researchers are emphasising that shortage of nitrogen can limit how arctic streams respond to global warming. The results are published in the journal Global Change Biology.
Field study with "cups" filled with different nutritional solutions. They are used to test which nutritional solution gives the greatest response and provides information about which nutrients stimulate growth of algae and bacteria, which then grow on top of the cup. The cups are placed in the stream for 3 weeks. Photo: Maria Myrstener
As temperatures continue to increase globally, we expect to see inevitable and irreversible effects on our nature. The signs are particularly noticeable in the Arctic where permafrost thaws, tree lines move further up the mountains, and shrubs expand over otherwise open tundra. Doctoral student Maria Myrstener and her research colleagues at Umeå University wanted to learn how these changes might influence streams and rivers draining those landscapes, which may be influenced directly by temperature, but also by other factors, like light and water chemistry. So they conducted a study in streams near Abisko, in northern Sweden, to try and find out.
They took advantage of the natural climate and vegetation changes in the northern Swedish landscape. If you have travelled west from Kiruna towards Norway, you have seen how this landscape changes dramatically. It begins with coniferous forests, such as pine and spruce, but as you head west, vegetation becomes sparser, and coniferous trees give way to deciduous birch forest. Eventually you find yourself in in open areas dominated by birch forests in the valleys and if you lift your eyes beyond those trees, you see a vast, treeless tundra. The researchers used this change in the landscape, going from a warmer coniferous forest to a colder tundra, to test how climate factors influence the small streams within those landscapes.
Their field study in the Abisko area shows that arctic streams do not necessarily respond to climate warming in the same way as terrestrial ecosystems, i.e. by increased productivity.
“Our results indicated that the productivity in all of our streams, independent of their landscape, was strongly and primarily limited by the nutrient element nitrogen. This means that algae and bacteria, also called biofilms, living on the bottoms of the streams cannot grow more without increased nitrogen concentrations,” says Maria Myrstener, doctoral student at the Climate Impacts Research Centre (CIRC) at Umeå University.
Only after the researchers added nitrogen did the growth of biofilms relate to any climate variables such as temperature increase and light, along the landscape gradient. However, for northern Sweden, the current evidence suggests that nitrogen concentrations have been declining in streams and rivers over the last decades. This decline has been explained both by decreases in nitrogen deposition from the air and by an increase in plant growth due to climate change – because plants require more of these nutrients when they grow.
“If climate warming continues to increase the ‘greenness’ of the northern Sweden, the supply of these important nutrients to streams and rivers could also continue to decline. Such a decline could reduce the productivity of aquatic ecosystems, with has important implications for the food webs that they support.”
Maria Myrstener, Gerard Rocher-Ros, Ryan Burrows, Ann-Kristin Bergström, Reiner Giesler and Ryan Sponseller: Persistent nitrogen limitation of stream biofilm communities along climate gradients in the Arctic. Global Change Biology. 2018. Doi: 10.1111/gcb.14117
Maria Myrstener, Department of Ecology and Environmental Science, CIRCPhone: +46 73-847 76 83
Editor: Ingrid Söderbergh