Warming and coastal darkening alter nitrogen cycling by primary producersBiogeochemical Cycling

Grace Edwards¹, Hannah S. Earp², Antoine J. P. Minne¹, Catriona L. Hurd³, Karen Filbee-Dexter^1,2, Thomas Wernberg^1,2

¹University of Western Australia, Australia - ²Institute of Marine Research, Norway - ³University of Tasmania, Australia

Marine primary producers ˗ macroalgae and phytoplankton ˗ play a critical role in the biogeochemical cycling of Earth’s elements. The activity of these organisms is often controlled by nitrogen (N), which is considered the most limiting bio-essential nutrient for marine metabolic and photosynthetic processes, but is also being added to coastal waters globally as run-off, sewage and other pollution. Increasing our understanding of the ability of coastal ecosystems to transform and process N will reduce uncertainty surrounding the functioning of ocean nutrient cycles, which in turn influences biodiversity and climate regulation. However, key questions remain on how competition for N among ecosystem constituents influence nutrient cycles in marine temperate zones, and how these interactions might change under human stressors. This talk will share findings of an ongoing project that assesses the co-occurring impacts of warming and coastal darkening on the N cycling potential and photosynthetic capacity of phytoplankton and two ecologically significant kelps (Laminaria hyperborea and Saccharina latissima) using laboratory experiments in the Skagerrak region of southern Norway. We expect that these producers will compete for nutrients, especially under stressful (limiting) conditions. The combined effects of warming and darkening appear to have caused a net decrease in nutrient uptake potential for all primary producers, with elevated temperature offsetting any positive effects of light, particularly for phytoplankton. We also observed strong seasonal characteristics in the nutrient uptake dynamics by kelps and plankton, with slower rates of uptake during the warm autumn storage period. Our study supports predictions that warming and coastal darkening will alter physiological and biogeochemical processes in our oceans.