title: Long photoperiods sustain high pH in Arctic kelp forests.
abstract: Concern on the impacts of ocean acidification on calcifiers, such as bivalves, sea urchins, and foraminifers, has led to efforts to understand the controls on pH in their habitats, which include kelp forests and seagrass meadows. The metabolism of these habitats can lead to diel fluctuation in pH with increases during the day and declines at night, suggesting no net effect on pH at time scales longer than daily. We examined the capacity of subarctic and Arctic kelps to up-regulate pH in situ and experimentally tested the role of photoperiod in determining the capacity of Arctic macrophytes to up-regulate pH. Field observations at photoperiods of 15 and 24 hours in Greenland combined with experimental manipulations of photoperiod show that photoperiods longer than 21 hours, characteristic of Arctic summers, are conducive to sustained up-regulation of pH by kelp photosynthesis. We report a gradual increase in pH of 0.15 units and a parallel decline in pCO2of 100 parts per million over a 10-day period in an Arctic kelp forest over midsummer, with ample scope for continued pH increase during the months of continuous daylight. Experimental increase in CO2concentration further stimulated the capacity of macrophytes to deplete CO2and increase pH. We conclude that long photoperiods in Arctic summers support sustained up-regulation of pH in kelp forests, with potential benefits for calcifiers, and propose that this mechanism may increase with the projected expansion of Arctic vegetation in response to warming and loss of sea ice.
authors.affiliation: Arctic Research Centre, Aarhus University, Ny Munkegade 114, Building 1540, 8000 Århus C, Denmark.; Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark.
authors.forename: Dorte
authors.identifier: 0000-0001-9792-256X
authors.initials: D
authors.lastname: Krause-Jensen
authors.affiliation: Department of Global Change Research, Institut Mediterrani d'Estudis Avançats (Consejo Superior de Investigaciones Científicas-Universidad de las Islas Baleares), Miquel Marquès 21, 07190 Esporles, Spain.
authors.forename: Núria
authors.identifier: 0000-0002-8048-6789
authors.initials: N
authors.lastname: Marbà
authors.affiliation: Department of Global Change Research, Institut Mediterrani d'Estudis Avançats (Consejo Superior de Investigaciones Científicas-Universidad de las Islas Baleares), Miquel Marquès 21, 07190 Esporles, Spain.; Facultat de Geologia, Universitat de Barcelona, 08028 Barcelona, Spain.
authors.forename: Marina
authors.identifier: 0000-0002-9777-5894
authors.initials: M
authors.lastname: Sanz-Martin
authors.affiliation: Department of Global Change Research, Institut Mediterrani d'Estudis Avançats (Consejo Superior de Investigaciones Científicas-Universidad de las Islas Baleares), Miquel Marquès 21, 07190 Esporles, Spain.
authors.forename: Iris E
authors.identifier: 0000-0002-2238-6018
authors.initials: IE
authors.lastname: Hendriks
authors.affiliation: Arctic Research Centre, Aarhus University, Ny Munkegade 114, Building 1540, 8000 Århus C, Denmark.; Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark.
authors.forename: Jakob
authors.identifier: 0000-0002-1029-3105
authors.initials: J
authors.lastname: Thyrring
authors.affiliation: Department of Bioscience, Aarhus University, Frederiksborgvej 399, DK-4000 Roskilde, Denmark.
authors.forename: Jacob
authors.identifier: 0000-0003-0016-6118
authors.initials: J
authors.lastname: Carstensen
authors.affiliation: Arctic Research Centre, Aarhus University, Ny Munkegade 114, Building 1540, 8000 Århus C, Denmark.; Department of Bioscience, Aarhus University, Vejlsøvej 25, DK-8600 Silkeborg, Denmark.
authors.forename: Mikael Kristian
authors.initials: MK
authors.lastname: Sejr
authors.affiliation: King Abdullah University of Science and Technology, Red Sea Research Center, Thuwal 23955-6900, Kingdom of Saudi Arabia.; Faculty of Biosciences, Fisheries and Economics, University of Tromsø, Tromsø, Norway.
authors.forename: Carlos M
authors.identifier: 0000-0002-1213-1361
authors.initials: CM
authors.lastname: Duarte
mesh_terms: D001110:Arctic Regions; D006116:Greenland; D006863:Hydrogen-Ion Concentration; D044423:Kelp / Q000502:physiology*; D017440:Photoperiod; D010788:Photosynthesis; D012623:Seawater / Q000737:chemistry*
keywords: Arctic kelp; Photosynthesis; acidification; macrophytes; metabolism; pH; photoperiod; plant-climate interactions; respiration