Dag Olav Hessen

• Baho, Didier Ludovic; Pomati, Francesco; Leu, Eva; Hessen, Dag Olav; Moe, S. Jannicke; Norberg, Jon & Nizzetto, Luca (2019). A single pulse of diffuse contaminants alters the size distribution of natural phytoplankton communities. Science of the Total Environment.  ISSN 0048-9697.  683, s 578- 588 . doi: 10.1016/j.scitotenv.2019.05.229
• Hessen, Dag Olav; Jensen, Thomas Correll & Walseng, Bjørn (2019). Zooplankton diversity and dispersal by birds; Insights from different geographical scales. Frontiers in Ecology and Evolution.  ISSN 2296-701X.  7:74, s 1- 11 . doi: 10.3389/fevo.2019.00074 Show summary

  Given the major ecological and evolutionary role of dispersal abilities for organisms, as well as the current interest in species' potential for further migration and colonization as a result of climatic changes or human-mediated invasions, our knowledge about dispersal abilities on spatial and temporal scales in many taxa is surprisingly limited. Zooplankton inhabit lakes and ponds that functionally are “aquatic islands” in the landscape, and both community composition and richness depend on their ability to disperse, and their post-dispersal colonization abilities. We here assess the diversity and dispersal of freshwater microcrustaceans based on three types of data; (1) > 2000 lakes on mainland Norway spanning a wide range in longitude, latitude and altitude, (2) a more limited number of ponds at Svalbard that are differently affected by migrating birds, and (3) immigration and colonization of recently constructed wetlands and man-made ponds. At all scales we discuss whether observed patterns in diversity can be explicitly linked to birds as vectors, or if confounding factors such as climate, productivity, age of locality—or other means of immigration, precludes conclusive evidence. The spatial patterns of zooplankton distribution strongly suggest that local sorting is a major determinant of richness and community composition. This sorting may not necessarily lead to similar community composition (the “quorum effect”) however. Despite the fact that rapid colonization occurs at local scales, and that birds undoubtedly can transmit animals or resting stages, their role in modulating community structure and richness is still an unsettled issue due to the many confounding parameters. The fact that birds often play a dual role in shaping diversity and community composition, first by direct dispersal, and secondly via affecting post-dispersal species sorting by changing water quality and productivity, is an important aspect of zoochory. Direct experimental evidence (colonization with and without bird exclusion), or genetic analysis of zooplankton species along migration routes, would however be the only ways to establish firm evidence for this case of zoochory.

• Jensen, Thomas Correll; Walseng, Bjørn; Hessen, Dag Olav; Dimante-Deimantovica, Inta; Novichkova, Anna A.; Chertoprud, Elena S.; Chertoprud, Mikhail V.; Sakharova, Ekaterina G.; Krylov, Aleksandr V.; Frisch, Dagmar & Christoffersen, Kirsten Seestern (2019). Changes in trophic state and aquatic communities in high Arctic ponds in response to increasing goose populations. Freshwater Biology.  ISSN 0046-5070.  64(7), s 1241- 1254 . doi: 10.1111/fwb.13299 Show summary

  1. The high Arctic, including the Svalbard archipelago in the North Atlantic, has been exposed to direct and indirect drivers of climatic change such as rising temperatures and associated changes in hydrology and nutrient fluxes. In addition, the number of migrating birds, particularly geese, increased remarkably in the Svalbard archipelago during the second half of the last century. The higher number of breeding birds potentially affects water quality and the biota in ponds and lakes. 2. We aimed to investigate the potential influence of increasing goose abundance on trophic state, taxon richness, and species composition of freshwater communities in the high Arctic. We hypothesised that higher goose abundance affects the trophic state of shallow lakes and ponds and their taxon richness and species composition. We conducted a survey of selected ponds at Svalbard along a goose abundance gradient. We used the number of area-specific goose droppings (range of 0–94 droppings m2) as a proxy of goose presence and measured proxies for productivity as well as taxon richness and composition of phytoplankton and invertebrate communities. 3. Presence and abundance of geese were associated with higher productivity of ponds. Invertebrate and phytoplankton taxon richness correlated (positively) with goose abundance. Both phytoplankton and invertebrate taxon richness increased with increasing nitrogen (N) concentrations. Goose abundance significantly affected phytoplankton species composition, while concentrations of total-N and total phosphorus (P) did not. Species composition of aquatic invertebrates was most strongly affected by goose abundance, but the effect of total-N concentration was also significant. 4. Increased goose abundance was associated with bird driven nutrient enrichment, increased phytoplankton and invertebrate taxon richness and changes of these biological communities. Thus, in addition to climate change, the higher abundances of large migratory water fowl in many polar areas may pose a major additional stress to arctic lakes and ponds. In fact, climate change and bird impact may interact, accelerating ongoing environmental change of arctic freshwater ecosystems. climate change, geese, invertebrates, phytoplankton, Svalbard

• Moe, Therese Fosholt; Hessen, Dag Olav & Demars, Benoît Olivier Laurent (2019). Functional biogeography: Stoichiometry and thresholds for interpreting nutrient limitation in aquatic plants. Science of the Total Environment.  ISSN 0048-9697.  677, s 447- 455 . doi: 10.1016/j.scitotenv.2019.04.366
• Simon, Carsta & Hessen, Dag Olav (2019). Selection as a domain-general evolutionary process. Behavioural Processes.  ISSN 0376-6357.  s 1- 14 . doi: 10.1016/j.beproc.2017.12.020
• Yu, ChaoQing; Huang, Xiao; Chen, Han; Godfray, H. Charles J.; Wright, Jonathon S.; Hall, Jim W.; Gong, Peng; Ni, ShaoQiang; Qiao, ShengChao; Huang, GuoRui; Xiao, YuChen; Zhang, Jie; Feng, Zhao; Ju, XiaoTang; Ciais, Philippe; Stenseth, Nils Christian; Hessen, Dag Olav; Sun, ZhanLi; Yu, Le; Cai, WenJia; Fu, HaoHuan; Huang, XiaoMeng; Zhang, Chi; Liu, HongBin & Taylor, James (2019). Managing nitrogen to restore water quality in China. Nature.  ISSN 0028-0836.  567, s 516- 520 . doi: 10.1038/s41586-019-1001-1 Show summary

  The nitrogen cycle has been radically changed by human activities1 . China consumes nearly one third of the world’s nitrogen fertilizers. The excessive application of fertilizers2,3 and increased nitrogen discharge from livestock, domestic and industrial sources have resulted in pervasive water pollution. Quantifying a nitrogen ‘boundary’4 in heterogeneous environments is important for the effective management of local water quality. Here we use a combination of water-quality observations and simulated nitrogen discharge from agricultural and other sources to estimate spatial patterns of nitrogen discharge into water bodies across China from 1955 to 2014. We find that the critical surface-water quality standard (1.0 milligrams of nitrogen per litre) was being exceeded in most provinces by the mid-1980s, and that current rates of anthropogenic nitrogen discharge (14.5 ± 3.1 megatonnes of nitrogen per year) to fresh water are about 2.7 times the estimated ‘safe’ nitrogen discharge threshold (5.2 ± 0.7 megatonnes of nitrogen per year). Current efforts to reduce pollution through wastewater treatment and by improving cropland nitrogen management can partially remedy this situation. Domestic wastewater treatment has helped to reduce net discharge by 0.7 ± 0.1 megatonnes in 2014, but at high monetary and energy costs. Improved cropland nitrogen management could remove another 2.3 ± 0.3 megatonnes of nitrogen per year—about 25 per cent of the excess discharge to fresh water. Successfully restoring a clean water environment in China will further require transformational changes to boost the national nutrient recycling rate from its current average of 36 per cent to about 87 per cent, which is a level typical of traditional Chinese agriculture. Although ambitious, such a high level of nitrogen recycling is technologically achievable at an estimated capital cost of approximately 100 billion US dollars and operating costs of 18–29 billion US dollars per year, and could provide co-benefits such as recycled wastewater for crop irrigation and improved environmental quality and ecosystem services.

• Creed, Irena F.; Bergström, Ann-Kristin; Trick, Charles G.; Grimm, Nancy B.; Hessen, Dag Olav; Karlsson, Jan; Kidd, Karen A.; Kritzberg, Emma; McKnight, Diane M.; Freeman, Erika C.; Senar, Oscar E.; Andersson, Agneta; Ask, Jenny; Berggren, Martin; Cherif, Mehdi; Giesler, Reiner; Hotchkiss, Erin R.; Kortelainen, Pirkko; Palta, Monica M.; Vrede, Tobias & Weyhenmeyer, Gesa A. (2018). Global change-driven effects on dissolved organic matter composition: Implications for food webs of northern lakes. Global Change Biology.  ISSN 1354-1013.  24(8), s 3692- 3714 . doi: 10.1111/gcb.14129
• Lindholm, Markus; Eie, Martin; Hessen, Dag Olav; Johansen, Joachim Tørum; Weiby, Kristoffer & Thaulow, Jens (2018). Effects of water browning on freshwater biodiversity: the case of the predatory phantom midge Chaoborus nyblaei. Hydrobiologia.  ISSN 0018-8158.  813(1), s 33- 40 . doi: 10.1007/s10750-017-3503-x Full text in Research Archive. Show summary

  Water browning, due to increased runoff of terrestrial dissolved organic carbon (DOC), has recently gained considerable attention. While it is well settled how browning affects light regime and thereby aquatic primary production, other impacts on the aquatic biota is less explored. Water browning shelters against UV radiation, and may thus benefit range expansion of UV sensitive organisms, such as midges. We mapped occurrence of Chaoborids in 148 subalpine and alpine ponds in Norway, and identified an apparent threshold for their presence around 3 mg total organic carbon (TOC) l−1. The field study was complemented with laboratory experiments on Chaoborus nyblaei (Zetterstedt, 1838), to test if this species is able to identify and select water colour (concentrations of DOC) for oviposition. Number of egg rafts on brown water tanks was significantly higher than in clear water tanks, indicating that C. nyblaei performs oviposition habitat selection. Chaoborids are effective predators in planktonic habitats, and our findings support the hypothesis that climate change may cascade through the ecosystem and promote range shifts of species due to alternated habitat frame conditions.

• Walseng, Bjørn; Jensen, Thomas Correll; Dimante-Deimantovica, Inta; Christoffersen, Kirsten Seestern; Chertoprud, Mikhail; Chertoprud, Elena; Novichkova, Anna & Hessen, Dag Olav (2018). Freshwater diversity in Svalbard: providing baseline data for ecosystems in change. Polar Biology.  ISSN 0722-4060.  41, s 1995- 2005 . doi: 10.1007/s00300-018-2340-3 Full text in Research Archive.
• Wolf, Raoul; Thrane, Jan-Erik; Hessen, Dag Olav & Andersen, Tom (2018). Modelling ROS formation in boreal lakes from interactions between dissolved organic matter and absorbed solar photon flux. Water Research.  ISSN 0043-1354.  132, s 331- 339 . doi: 10.1016/j.watres.2018.01.025 Full text in Research Archive. Show summary

  Concentrations of dissolved organic matter (DOM) are increasing in a large number of lakes across the Northern hemisphere. This browning serves a dual role for biota by protecting against harmful ultraviolet radiation, while also absorbing photosynthetically active radiation. The photochemical activation of DOM and subsequent formation of reactive oxygen species (ROS) is a potentially harmful side effect, but can be difficult to measure directly in situ. In this study, we combine a data set of physico-chemical properties from 71 Nordic lakes with in vitro ROS formation quantum yields to predict ROS formations across a representative boreal ecosystem gradient. For the upper centimeter of the water column, we calculate ROS formations in the range of 7.93–12.56 μmol L−1 h−1. In the first meter, they range between 1.69 and 6.69 μmol L−1 h−1 and in the remaining depth the range is 0.01–0.46 μmol L−1 h−1. These ROS formations are comparable with previously field-measured hydrogen peroxide formation rates and likely affect both phyto- and zooplankton, as well as lake chemistry. Interestingly, wavelengths of the visible spectrum (>400 nm) contribute more than half of the overall ROS formation in surface-near water layers. The association between DOM and ROS formation was found to be two-fold. While DOM promotes ROS formation in the first centimeters of the water column, the shading effect of light attenuation overpowers this with increasing depth. In the context of water browning, our results indicate the emergence of an underestimated oxidative stress environment for lake biota in the upper centimeters of the water column.

• Alfsnes, Kristian; Leinaas, Hans Petter & Hessen, Dag Olav (2017). Genome size in arthropods; different roles of phylogeny, habitat and life history in insects and crustaceans. Ecology and Evolution.  ISSN 2045-7758.  7(15), s 5939- 5947 . doi: 10.1002/ece3.3163 Full text in Research Archive.
• Anderson, Thomas R.; Hessen, Dag Olav; Boersma, Maarten; Urabe, Jotaro & Mayor, Daniel J. (2017). Will invertebrates require increasingly carbon-rich food in a warming world?. American Naturalist.  ISSN 0003-0147.  190(6), s 725- 742 . doi: 10.1086/694122 Full text in Research Archive.
• Guignard, Maïté S.; Leitch, Andrew R.; Aquisti, Claudia; Eitzaguirre, Christophe; Elser, James J.; Hessen, Dag Olav; Jeyasingh, Punidan D.; Neiman, Maurine; Richardson, Alan E.; Soltis, Pamela S.; Soltis, Douglas E.; Stevens, Carly J.; Trimmer, Mark; Weider, Lawrence J.; Woodward, Guy & Leitch, Ilia J. (2017). Impacts of nitrogen and phosphorus: From genomes to natural ecosystems and agriculture. Frontiers in Ecology and Evolution.  ISSN 2296-701X.  5, s 1- 9 . doi: 10.3389/fevo.2017.00070 Full text in Research Archive.
• Hessen, Dag Olav; Andersen, Tom; Tominaga, Koji & Finstad, Anders Gravbrøt (2017). When soft waters becomes softer; drivers of critically low levels of Ca in Norwegian lakes. Limnology and Oceanography.  ISSN 0024-3590.  62(1), s 289- 298 . doi: 10.1002/lno.10394 Full text in Research Archive.
• Hessen, Dag Olav; Hafslund, Ola Tobias; Andersen, Tom; Broch, Catharina; Shala, Nita Kaupang & Wojewodzic, Marcin (2017). Changes in stoichiometry, cellular RNA, and alkaline phosphatase activity of chlamydomonas in response to temperature and nutrients. Frontiers in Microbiology.  ISSN 1664-302X.  8(18), s 1- 8 . doi: 10.3389/fmicb.2017.00018 Full text in Research Archive.
• Hessen, Dag Olav; Håll, Johnny Peter; Thrane, Jan-Erik & Andersen, Tom (2017). Coupling dissolved organic carbon, CO2 and productivity in boreal lakes. Freshwater Biology.  ISSN 0046-5070.  62(5), s 945- 953 . doi: 10.1111/fwb.12914 Full text in Research Archive.
• Hessen, Dag Olav; Tombre, Ingunn; van Geest, Gerben & Alfsnes, Kristian (2017). Global change and ecosystem connectivity: how geese link fields of central Europe to eutrophication of Arctic freshwaters. Ambio.  ISSN 0044-7447.  46(1), s 40- 47 . doi: 10.1007/s13280-016-0802-9 Full text in Research Archive. Show summary

  arctic connectivity eutrophication migration

• Neiman, Maurine; Beaton, Margaret J.; Hessen, Dag Olav; Jeyasingh, Punidan D. & Weider, Lawrence J. (2017). Endopolyploidy as a potential driver of animal ecology and evolution. Biological Reviews.  ISSN 1464-7931.  92(1), s 234- 247 . doi: 10.1111/brv.12226 Full text in Research Archive.
• Skau, Lars Fredrik; Andersen, Tom; Thrane, Jan-Erik & Hessen, Dag Olav (2017). Growth, stoichiometry and cell size; temperature and nutrient responses in haptophytes. PeerJ.  ISSN 2167-8359.  5 . doi: 10.7717/peerj.3743 Full text in Research Archive.
• Spears, Bryan; Futter, Martyn N.; Jeppesen, Erik; Huser, Brian J; Ives, Stephen; Davidson, Thomas A; Adrian, Rita; Angeler, David G.; Burthe, Sarah J; Carvalho, Laurence; Daunt, Francis; Gsell, Alena S; Hessen, Dag Olav; Janssen, Annette B G; MacKay, Eleanor B; May, Linda; Moorhouse, Heather; Olsen, Saara; Søndergaard, Martin; Woods, Helen & Thackeray, Stephen J. (2017). Ecological resilience in lakes and the conjunction fallacy. Nature Ecology and Evolution.  ISSN 2397-334X.  1(11), s 1616- 1624 . doi: 10.1038/s41559-017-0333-1 Full text in Research Archive.
• Thrane, Jan-Erik; Hessen, Dag Olav & Andersen, Tom (2017). Plasticity in algal stoichiometry: Experimental evidence of a temperature-induced shift in optimal supply N:P ratio. Limnology and Oceanography.  ISSN 0024-3590.  62(4), s 1346- 1354 . doi: 10.1002/lno.10500 Full text in Research Archive.
• Wolf, Raoul; Andersen, Tom; Hessen, Dag Olav & Hylland, Ketil (2017). The influence of dissolved organic carbon and ultraviolet radiation on the genomic integrity of Daphnia magna. Functional Ecology.  ISSN 0269-8463.  31(4), s 848- 855 . doi: 10.1111/1365-2435.12730 Full text in Research Archive.
• Adrian, Rita & Hessen, Dag Olav (2016). Environmental Impacts—Lake Ecosystems, In Markus Quante & Franciscus Colijn (ed.),  North Sea Region Climate Change Assessment.  Springer Publishing Company.  ISBN 978-3-319-39743-6.  Kapittel 10.  s 315 - 340
• Alfsnes, Kristian; Hobæk, Anders; Weider, Lawrence J. & Hessen, Dag O. (2016). Birds, nutrients, and climate change: mtDNA haplotype diversity of Arctic Daphnia on Svalbard revisited. Polar Biology.  ISSN 0722-4060.  39(8), s 1425- 1437 . doi: 10.1007/s00300-015-1868-8
• de Wit, Heleen A.; Valinia, Salar; Weyhenmeyer, Gesa A; Futter, Martyn N.; Kortelainen, Pirkko; Austnes, Kari; Hessen, Dag O; Raike, Antti; Laudon, Hjalmar & Vournenmaa, Jussi (2016). Current Browning of Surface Waters Will Be Further Promoted by Wetter Climate. Environmental Science and Technology Letters.  ISSN 2328-8930.  3(12), s 430- 435 . doi: 10.1021/acs.estlett.6b00396 Full text in Research Archive.
• Finstad, Anders Gravbrøt; Andersen, Tom; Larsen, Søren; Tominaga, Koji; Blumentrath, Stefan; de Wit, Heleen A.; Tømmervik, Hans & Hessen, Dag Olav (2016). From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes. Scientific Reports.  ISSN 2045-2322.  6 . doi: 10.1038/srep31944 Full text in Research Archive. Show summary

  Increased concentrations of dissolved organic carbon (DOC), often labelled “browning”, is a current trend in northern, particularly boreal, freshwaters. The browning has been attributed to the recent reduction in sulphate (S) deposition during the last 2 to 3 decades. Over the last century, climate and land use change have also caused an increasing trend in vegetation cover (“greening”), and this terrestrially fixed carbon represents another potential source for export of organic carbon to lakes and rivers. The impact of this greening on the observed browning of lakes and rivers on decadal time scales remains poorly investigated, however. Here, we explore time-series both on water chemistry and catchment vegetation cover (using NDVI as proxy) from 70 Norwegian lakes and catchments over a 30-year period. We show that the increase in terrestrial vegetation as well as temperature and runoff significantly adds to the reduced SO4-deposition as a driver of freshwater DOC concentration. Over extended periods (centuries), climate mediated changes in vegetation cover may cause major browning of northern surface waters, with severe impact on ecosystem productivity and functioning.

• Hanevik, Hans Ivar; Hessen, Dag Olav; Sunde, Arne & Breivik, Jarle (2016). Can IVF influence human evolution?. Human Reproduction.  ISSN 0268-1161.  31(7), s 1397- 1402 . doi: 10.1093/humrep/dew089
• Hessen, Dag Olav (2016). Seleksjon ved konsekvenser – og konsekvensene av seleksjon. Norsk Tidsskrift for Atferdsanalyse.  ISSN 0809-781X.  43, s 51- 55
• Leinaas, Hans Petter; Jalal, Marwa; Gabrielsen, Tove M. & Hessen, Dag Olav (2016). Inter- and intraspecific variation in body- and genome size in calanoid copepods from temperate and arctic waters. Ecology and Evolution.  ISSN 2045-7758.  6(16), s 5585- 5595 . doi: 10.1002/ece3.2302 Full text in Research Archive.
• Lindholm, Markus; Wolf, Raoul; Finstad, Anders & Hessen, Dag O (2016). Water browning mediates predatory decimation of the Arctic fairy shrimp Branchinecta paludosa. Freshwater Biology.  ISSN 0046-5070.  61(3), s 340- 347 . doi: 10.1111/fwb.12712
• Thrane, Jan-Erik; Hessen, Dag Olav & Andersen, Tom (2016). The impact of irradiance on optimal and cellular nitrogen to phosphorus ratios in phytoplankton. Ecology Letters.  ISSN 1461-023X.  19(8), s 880- 888 . doi: 10.1111/ele.12623
• Feng, Jianfeng; Durant, Joel Marcel; Stige, Leif Christian; Hessen, Dag Olav; Hjermann, Dag Øystein; Zhu, Lin; Llope, Marcos & Stenseth, Nils Christian (2015). Contrasting correlation patterns between environmental factors and chlorophyll levels in the global ocean. Global Biogeochemical Cycles.  ISSN 0886-6236.  29(12), s 2095- 2107 . doi: 10.1002/2015GB005216
• Hessen, Dag Olav (2015). Noncoding DNA as a Phenotypic Driver. Evolutionary biology.  ISSN 0071-3260.  42, s 427- 431 . doi: 10.1007/s11692-015-9335-6
• Jalal, Marwa; Andersen, Tom & Hessen, Dag Olav (2015). Temperature and developmental responses of body and cell size in Drosophila; effects of polyploidy and genome configuration. Journal of Thermal Biology.  ISSN 0306-4565.  51, s 1- 14 . doi: 10.1016/j.jtherbio.2015.02.011
• Jeyansingh, Pinidian; Chowdhury, Priyanka; Wojewodzic, Marcin; Frisch, Dagmar; Hessen, Dag Olav & Weider, Lawrence J. (2015). Phosphorus use and excretion varies with ploidy level in Daphnia. Journal of Plankton Research.  ISSN 0142-7873.  37, s 1210- 1217 . doi: 10.1093/plankt/fbv095
• Lindholm, Markus; Hessen, Dag Olav; Færøvig, Per-Johan; Rognerud, Bjørg; Andersen, Tom & Stordal, Frode (2015). Is distribution of cold stenotherms constrained by temperature? The case of the Arctic fairy shrimp (Branchinecta paludosa O.F. Müller 1788). Journal of Thermal Biology.  ISSN 0306-4565.  53, s 46- 52 . doi: 10.1016/j.jtherbio.2015.08.005
• Walseng, Bjørn; Andersen, Tom & Hessen, Dag Olav (2015). Higher zooplankton species richness associated with an invertebrate top predator. Freshwater Biology.  ISSN 0046-5070.  60(5), s 903- 910 . doi: 10.1111/fwb.12543 Show summary

  1. A dataset of 1735 Norwegian lakes, spanning a wide range of geographical, physical, chemical and biological properties, was analysed to assess the role of the top invertebrate predator, Bythotrephes longimanus, on the crustacean species richness and community structure. 2. Bythotrephes was associated with a 25.5% increase in mean richness of other zooplankton species, and this effect could not be related to confounding factors. Based on presence–absence data, community composition did not differ significantly between lakes with and without this top predator. 3. Neither fish predation nor lake area, altitude, and geographical or physicochemical parameters offer any obvious impact on the observed diversity patterns. We suggest that Bythotrephes facilitates species richness by reducing the abundance of species controlling subdominant species. 4. This pattern of increased diversity offers an interesting contrast to North American lakes recently invaded by Bythotrephes, where species richness typically has decreased, suggesting different shortterm and long-term effects of predatory invaders. Keywords: diversity, lakes, predator, richness, zooplankton

• Yang, Hong; Andersen, Tom; Dörsch, Peter; Tominaga, Koji; Thrane, Jan-Erik & Hessen, Dag Olav (2015). Greenhouse gas metabolism in Nordic boreal lakes. Biogeochemistry.  ISSN 0168-2563.  126(1-2), s 211- 225 . doi: 10.1007/s10533-015-0154-8 Full text in Research Archive. Show summary

  Boreal lakes are important net sources of greenhouse gases (GHGs). In this study we analyzed concentrations of CO2, CH4, N2O as well as O2, N2 and argon (Ar) from the epilimnion of 75 boreal lakes covering gradients in total organic carbon (TOC), phosphorus (P) and nitrogen (N) deposition. The Ar-corrected gas saturation deficit was used as a proxy of net metabolic changes from spring overturn to mid-summer sampling (all lakes were dimictic). Emission fluxes were calculated for CO2, CH4 and N2O based on partial pressure, water temperature and wind speed. Gas concentrations, actual and Ar-corrected, were related to lake specific properties. TOC was the main predictor of CO2 concentrations and fluxes, followed by total P, while total P and chlorophyll a governed CH4 concentrations and fluxes. Nitrogen (NO3 - or total N) were key predictors of N2O concentrations and fluxes, followed by total P. Altitude, area and depth were not strong predictors of CO2, CH4 and N2O concentrations and fluxes, likely because only lakes with an area of [1 km2 were included. CO2 molar concentrations were negatively correlated with O2 concentrations, while the slope of CO2 concentration to Ar corrected O2 deficit was 1.039. Together with the poor correlation between area-specific primary production and CO2 as well as O2, this suggests that these gases are mostly affected by catabolic processes and probably photo-oxidation in these nutrient-poor, boreal lakes investigated in this study. Increasing inputs of TOC (i.e. lake ‘‘browning’’) is likely to promote the net heterotrophy and hence emissions of all GHGs, while elevated N deposition in particular may cause elevated emissions of N2O.

• Feng, Jianfeng; Stige, Leif Christian; Durant, Joel Marcel; Hessen, Dag Olav; Zhu, Lin; Hjermann, Dag Øystein; Llope, Marcos & Stenseth, Nils Christian (2014). Large-scale season-dependent effects of temperature and zooplankton on phytoplankton in the North Atlantic. Marine Ecology Progress Series.  ISSN 0171-8630.  502, s 25- 37 . doi: 10.3354/meps10724
• Finstad, Anders Gravbrøt; Helland, Ingeborg Palm; Ugedal, Ola; Hesthagen, Trygve H. & Hessen, Dag Olav (2014). Unimodal response of fish yield to dissolved organic carbon. Ecology Letters.  ISSN 1461-023X.  17(1), s 36- 43 . doi: 10.1111/ele.12201 Full text in Research Archive.
• Frigstad, Helene; Andersen, Tom; Bellerby, Richard; Silyakova, Anna & Hessen, Dag Olav (2014). Variation in the seston C:N ratio of the Arctic Ocean and pan-Arctic shelves. Journal of Marine Systems.  ISSN 0924-7963.  129, s 214- 223 . doi: 10.1016/j.jmarsys.2013.06.004
• Hessen, Dag Olav & Kaartvedt, Stein (2014). Top-down cascades in lakes and oceans: Different perspectives but same story?. Journal of Plankton Research.  ISSN 0142-7873.  36(4), s 914- 924 . doi: 10.1093/plankt/fbu040
• Jalal, Marwa; Shala, Nita Kaupang; Wojewodzic, Marcin; Andersen, Tom; Hessen, Dag Olav & Schwarzacher, T (2014). Multigenerational genomic responses to dietary phosphorus and temperature in Daphnia. Genome.  ISSN 0831-2796.  57(8), s 439- 448 . doi: 10.1139/gen-2014-0047
• Reusch, Marianne & Hessen, Dag Olav (2014). Friluftslivets høstingsrett – I biologisk og juridisk perspektiv. Nordisk miljörättslig tidskrift.  ISSN 2000-4273.  2013(2), s 67- 84
• Skurdal, Jostein; Johnsen, Stein Ivar; Garnås, Erik; Hessen, Dag Olav & Qvenild, Tore (2014). Hva har vi lært etter 35 års overvåking av ferskvannskreps i Steinsfjorden?. Vann.  ISSN 0042-2592.  (1), s 6- 16
• Thrane, Jan-Erik; Hessen, Dag Olav & Andersen, Tom (2014). The Absorption of Light in Lakes: Negative Impact of Dissolved Organic Carbon on Primary Productivity. Ecosystems.  ISSN 1432-9840.  17(6), s 1040- 1052 . doi: 10.1007/s10021-014-9776-2
• Wilson, David S. & Hessen, Dag Olav (2014). Blueprint for the Global Village. Cliodynamics.  ISSN 2373-7530.  5, s 123- 157
• Anderson, Thomas R.; Hessen, Dag Olav; Mitra, Aditee; Mayor, Daniel J. & Yool, Andrew (2013). Sensitivity of secondary production and export flux to choice of trophic transfer formulation in marine ecosystem models. Journal of Marine Systems.  ISSN 0924-7963.  125, s 41- 53 . doi: 10.1016/j.jmarsys.2012.09.008
• Aubert, Anais; Svensen, Camilla; Hessen, Dag Olav & Tamelander, Tobias (2013). CNP stoichiometry of a lipid-synthesising zooplankton, Calanus finmarchicus, from winter to spring bloom in a sub-Arctic sound. Journal of Marine Systems.  ISSN 0924-7963.  111, s 19- 28 . doi: 10.1016/j.jmarsys.2012.09.004
• Frigstad, Helene; Andersen, Tom; Hessen, Dag Olav; Jeansson, Emil; Skogen, Morten D.; Naustvoll, Lars Johan; Miles, Martin W.; Johannessen, Truls & Bellerby, Richard (2013). Long-term trends in carbon, nutrients and stoichiometry in Norwegian coastal waters: Evidence of a regime shift. Progress in Oceanography.  ISSN 0079-6611.  111, s 113- 124 . doi: 10.1016/j.pocean.2013.01.006 Show summary

  A 20-year time series from the Norwegian Coastal Current was explored to examine the effects of advected nutrient supply from the southern North Sea and of large-scale climate variability on hydrography, nutrients and particulate organic matter (seston), focusing on trends in the January to April period in the upper layers (0–30 m). The interannual variability in hydrography, nutrients and seston was correlated with the NAO index, mostly through the inflow of nutrient-rich waters from the southern North Sea. There was a long-term decrease in nutrient concentrations, which according to a water mass analysis followed a reduction in nutrients advected from the German Bight and southern North Sea. The concentrations of carbon and nitrogen in seston, dissolved organic nitrogen and the estimated fraction of non-autotrophic material increased significantly and non-linearly through a sharp transition between 1998 and 2000, and have remained at this level since. Humic coagulation was suggested as the mechanism behind the increase in the non-autotrophic fraction of seston, which could be connected with the reported “darkening” of the coastal Skagerrak and Baltic Sea. Concurrent with the thresholds in suspended material, a decimation of the sugar kelp forest and recruitment failure of key carnivorous fish was reported for the same region, suggesting that a regime shift took place in the early 2000s in the coastal waters of the Norwegian Skagerrak. Our data suggests that the effects of increased freshwater runoff, especially the increased inputs of terrestrial-derived, humic material, could play an important role in the observed, coastal responses.

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• Hessen, Dag Olav (2017). The many lives of carbon. Reaktion Books.  ISBN 9781780238517.  272 s.
• Hessen, Dag Olav (2017). Vi. Samarbeid fra celle til samfunn. Cappelen Damm AS.  ISBN 978-82-02-52606-1.  262 s.
• Weslawski, Jan Marcin & Hessen, Dag Olav (2017). Size: Why are some animals small and other large?. Polish Institute of Oceanography.  ISBN 978-83-949491-0-5.  25 s.
• Hessen, Dag Olav (2016). Landskap i endring. Pax Forlag.  ISBN 978-82-530-3899-5.  159 s.
• Hessen, Dag Olav (2015). C. Karbon - en uautorisert biografi. Cappelen Damm AS.  ISBN 978-82-02-45867-6.  260 s.
• Hessen, Dag Olav (2015). Livet fra A til Å - en bok om deg. Cappelen Damm AS.  ISBN 978-82-02-47688-5.  91 s.
• Hessen, Dag Olav; Lie, Thore & Stenseth, Nils Christian (red.) (2015). Mendels arv - Genetikkens æra. Gyldendal Norsk Forlag A/S.  ISBN 978-82-05-45818-5.  440 s.
• Kleiven, Helga Flesche; Hessen, Dag Olav; Svensen, Henrik; Samset, Bjørn Hallvard; Østli, Kjetil & Kristiansen, Nina (2014). Isfritt. Spartacus.  ISBN 9788243008342.  160 s.
• Alfsen, Knut H.; Hessen, Dag Olav & Jansen, Eystein (2013). Klimaendringer i Norge. Universitetsforlaget.  ISBN 978-82-15-02158-4.  248 s.
• Alfsen, Knut H.; Hessen, Dag Olav & Jansen, Eystein (2013). Klimaendringer i Norge - forskernes forklaringer. Universitetsforlaget.  ISBN 9788215021584.  256 s.

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• Jensen, Thomas Correll; Walseng, Bjørn; Hessen, Dag Olav; Dimante-Deimantovica, Inta; Novichkova, Anna; Chertoprud, Elena S.; Chertoprud, Mikhail V.; Sakharova, E. G.; Krylov, A. V.; Frisch, Dagmar & Christoffersen, Kirsten Seestern (2019). Konsekvenser av økte gåsebestander i Arktis; næringsanriking og endret biomangfold i ferskvann..
• Källqvist, Torsten; Eklo, Ole Martin; Låg, Marit; Rakkestad, Kirsten Eline; Sipinen, Ville Erling; Steffensen, Inger-Lise; Agdestein, Angelika; Bodin, Johanna Eva; Elvevoll, Edel O.; Hemre, Gro Ingunn; Hessen, Dag Olav; Hofshagen, Merete; Husøy, Trine; Knutsen, Helle Katrine; Krogdahl, Åshild; Nilsen, Asbjørn Magne; Rafoss, Trond; Skjerdal, Olaug Taran; Strand, Tor A; Vandvik, Vigdis; Velle, Gaute; Wasteson, Yngvild & Alexander, Jan (2019). Genetically modified glyphosate tolerant maize, soybean and oilseed rape versus conventionally grown varieties – agricultural practices, residues of glyphosate, other pesticides and metabolites, and implications for toxicity. Opinion of the Scientific Steering Committee of the Norwegian Scientific Committee for Food and Environment (VKM). VKM Report. 2019:06.
• Bøckman, Petter & Hessen, Dag Olav (2018, 27. januar). Evig din ... ... og din.  VG.
• Haugnes, Gunhild M.; Tallaksen, Lena M.; Vogt, Rolf David; Storelvmo, Trude; Hessen, Dag Olav; Stordal, Frode; Lefevre, Sjannie; Kääb, Andreas & Uggerud, Einar (2018, 27. august). Klimaforskerne tar temperaturen på tropesommeren. [Tidsskrift].  Titan.uio.no.
• Hessen, Dag Olav; Stordal, Frode & Vogt, Rolf David (2018). Vi må forstå hvorfor to graders temperaturøkning er farlig. Aftenposten Viten.
• Smedsrud, Morten Skipenes; Hessen, Dag Olav & Vogt, Rolf David (2018, 30. mai). Svekket livsgrunnlag for fossekall og røye. [Fagblad].  Apollon.
• Hessen, Dag Olav (2017). Impact of Noncoding DNA on Phenotype Evolution. Encyclopedia of Life Sciences.  ISSN 1476-9506. . doi: 10.1002/9780470015902.a0027507
• Jensen, Nina; Martinsen, Siri; Hessen, Dag Olav; Bøckman, Petter & Moen, Ole Martin (2017). Dyrene dør. Du lever som før?.
• Jensen, Thomas Correll; Walseng, Bjørn; Christoffersen, Kirsten Seestern; Novichkova, Anna; Chertoprud, Elena S.; Chertoprud, Mikhail V.; Makarova, O. L.; Dimante-Deimantovica, Inta; Sakharova, E. G.; Krylov, A. V.; Frisch, Dagmar & Hessen, Dag Olav (2017). The impact of geese on aquatic biodiversity in the high Arctic.
• Schultze, Sabrina; Poste, Amanda; Ruus, Anders; Hessen, Dag Olav; Andersen, Tom & Borgå, Katrine (2017). Do high levels of terrestrial DOM uptake lead to higher contaminant concentration and changes in energy allocation and growth? A study on three marine lower food web taxa.
• Wolf, Raoul; Andersen, Tom; Hessen, Dag Olav & Hylland, Ketil (2017). From genetic damage to population effects: How interactions of dissolved organic carbon and ultraviolet radiation affect Daphnia magna.
• Benestad, Esben Esther Pirell; Hessen, Dag Olav & Mühleisen, Wencke (2016). Til bunns i seksualiteten - en mailsamtale. Fanfare.  ISSN 1893-8698.  (7), s 14- 23
• Bernhoft, Aksel; Albihn, Ann; Hessen, Dag Olav; Holmboe-Ottesen, Gerd; Keilman, Nico; Børresen, Trond; Haug, Ruth; Nicolaysen, Anna Marie & Andersen, Regine (2016). Hvordan skal vi sikre et landbruk som kan fø oss i framtiden?. Show summary

  Menneskenes endring av naturen og jordas ressurser truer framtidens landbruk. Så hvordan skal den sårbare matproduksjonen best foregå på lang sikt når jordas befolkning øker?

• Eriksen, Thomas Hylland; Hessen, Dag Olav & Svensen, Henrik (2016). En røff guide til antropocen. Nytt Norsk Tidsskrift.  ISSN 0800-336X.  33(1-2), s 71- 83
• Grinde, Bjørn; Wilson, David Sloan; Lieberman, Jerry; Hessen, Dag Olav; Gule, Lars; Lieberman, Gin Kohl; Moene, Kalle; Gowdy, John & Krall, Lisi (2016). The Norway Project: A deep look at its lessons to help us to improve Our society. Full text in Research Archive.
• Hessen, Dag Olav (2016). Banch/Stim, I: Knut Stene-Johansen; Christian Refsum & Johan Henrik Schimanski (red.),  Å leve sammen. Roland Barthes, individet og fellesskapet.  Spartacus.  ISBN 978-82-304-01781.  Kapittel.
• Svensen, Henrik; Eriksen, Thomas Hylland & Hessen, Dag Olav (2016). Menneskeheten er inne i en ny geologisk epoke - antropocen. Aftenposten (morgenutg. : trykt utg.).  ISSN 0804-3116.
• Vogt, Rolf David; Gao, Pingchuan; Hessen, Dag Olav & Haaland, Ståle (2016). Effect of solar radiation on Dissolved Natural Organic Matter. Show summary

  Lakes and rivers are the main sources of drinking water in Nordic countries. For aesthetic and hygienic reasons high concentration levels of dissolved natural organic matter (DNOM) are not acceptable in drinking water. An increasing amount of DNOM in raw water sources for drinking water in Fennoscandia is thus raising our concerns. Moreover, the DNOM constitute an important transport mechanism for the flux of allochthonous nutrients to surface water. Once in the lake the DNOM photo-degrades due to adsorption of energy from sunlight. This causes its larger molecular weight (MW) compound to decompose into a variety of smaller organic photoproducts. Moreover, some of the DNOM is mineralized completely to inorganic compounds. Mineralization of DNOM causes a release of aluminum (Al) and iron (Fe) ions along with the release of orthophosphate (PO43-). The metal ions rapidly hydrolyze and form oxy-hydroxides which co-sorb phosphate and precipitate. Due to the removal of mineralized PO43- by subsequent precipitation with Fe- and Al-oxy hydroxides it is difficult to detect how much orthophosphate becomes mineralized by solar radiation. This is needed in order to assess the amount of this limiting nutrient becoming available for autotrophs. Thus the essence of this study is to test the option of using algae growth response as an indication of phosphorus release. Considerable research has been conducted comparing the changes in characteristics of the matrix that DNOM is in and structural characterization of DNOM, before and after solar exposure. A continuous increase in the relative contribution of lower to higher MW DNOM compounds along with a decrease in DOC concentration has been found with the extension of exposure time. This implies that the photochemical mineralization of DNOM by the photo-oxidation transforms the higher MW DNOM compounds into lower MW compounds, and that some of the DNOM is completely mineralized. This is further corroborated with fluorescence spectroscopy, indicating a decreased ratio of Humic to Fulvic acids in the DNOM as a response to sunlight exposure. This would imply that when sunlight is absorbed by DNOM, the average molecular weight is reduced. Results from assays showing increased algal growth imply that PO43- has been mineralized from DNOM due to photochemical reaction. On the other hand the growth response is not always significant. This may partly be due to concurrent heterotopic mineralization by bacteria.

• Edvardsen, Bente & Hessen, Dag Olav (2015). Genome size variation in microalgae”. Show summary

  Genome size among eukaryotic microalgae vary with a factor of more than 17 000. The rRNA gene copy number similarly vary more than 12 000 times. The lowest genome size and rRNA gene copy number has been found in picoplanktonic chlorophytes (Ostreococcus, 12 Mb) and ochrophytes (Nannochloropsis, 25 Mb) and the highest in large dinoflagellate cells (e.g. c. 450 Gb in Prorocentrum micans and 12000 rDNA copies in Akashiwo sanguinea). For taxa analysed so far, the genome size is correlated with cell size and carbon-content both between and within species. Cell size is also generally positively correlated with growth rate within a taxonomic group, and smaller genome size thus corresponds to higher growth rate. A great diversity of life cycles is found among microalgae including diploid (diatoms), haplo-diploid (haptophytes), or haploid (dinoflagellates, many chlorophytes). The nutrient-sparing hypothesis implies selection for small cells and genomes under nutrient limited conditions, and may explain why some groups have retained a haploid or a haplo-diploid life cycle. A biphasic life cycle may be favorable under variable environmental conditions through niche partitioning or provide escape from biotic pressures such as pathogens and grazers, but may get lost under stable conditions. Nutrient scarcity may however also promote smaller cells due to their larger surface : volume ratio. Here we have compiled published data on eukaryotic microalgal genome size, cell size and growth rate and added own data for haptophytes to compare patterns among taxonomic groups, reveal driving forces and relate this to life cycle strategies.

• Hessen, Dag Olav (2015). Evolusjon, naturvitenskap og mening : tilsvar å Geir Gellemos artikkel i forrige nummer. Kirke og kultur.  ISSN 0023-186X.  120(4), s 340- 341
• Wolf, Raoul; Andersen, Tom; Hessen, Dag Olav & Hylland, Ketil (2015). The influence of UVR and DOC on the genomic integrity of Daphnia magna.
• Wolf, Raoul; Varga, Lilia; Andersen, Tom; Hessen, Dag Olav & Hylland, Ketil (2015). Effects of Environmental Stress on the Genomic Integrity of Daphnia magna STRAUS, 1820.
• Bøckman, Petter; Hessen, Dag Olav & Sætre, Glenn-Peter (2014, 08. august). Homo futurus. [Tidsskrift].  Aftenposten Ammagasinet.
• Hessen, Dag Olav (2014). Albedo - og et grønnere Artktis, I: Henrik Svensen (red.),  Isfritt. Populærvitenskap som angår deg.  Spartacus.  ISBN 9788243008342.  Kapittel.  s 93 - 95
• Hessen, Dag Olav (2014). Den tause våren. Harvest : Mennesket & naturen.
• Hessen, Dag Olav (2014). En utrydningstruet art?. Klassekampen.  ISSN 0805-3839.
• Hessen, Dag Olav (2014). Fri vilje - biologisk umulig, men moralsk nødvendig?, I: Svein Harald Gullbekk (red.),  Ja, vi elsker frihet.  Dreyer Forlag A/S.  ISBN 978-82-8265-093-9.  Kapittel.  s 32 - 36
• Hessen, Dag Olav (2014). Høyere, tettere, gråere. Aftenposten (morgenutg. : trykt utg.).  ISSN 0804-3116.
• Hessen, Dag Olav (2014). Mer av alt. NRK - www.nrk.no.
• Hessen, Dag Olav (2014). Motorferdsel og demogratiske fikenblad. VG : Verdens gang.  ISSN 0805-5203.
• Hessen, Dag Olav (2014). Oljefondet som mulighet. Klassekampen.  ISSN 0805-3839.
• Hessen, Dag Olav (2014). Oljefondets etikk; både råd og forvaltning. Dagens næringsliv.  ISSN 0803-9372.
• Hessen, Dag Olav (2014). Oljefondets etiske mandat. Dagbladet.  ISSN 0805-3766.
• Hessen, Dag Olav (2014). Pose og sekk-nasjonen. Klassekampen.  ISSN 0805-3839.
• Hessen, Dag Olav (2014). På skattejakt i "søppel-DNA". Aftenposten (morgenutg. : trykt utg.).  ISSN 0804-3116.
• Hessen, Dag Olav (2014). Verdien av det ubetalelige, I: Henrik Svensen (red.),  Isfritt. Populærvitenskap som angår deg.  Spartacus.  ISBN 9788243008342.  Kapittel.  s 64 - 65
• Reusch, Marianne & Hessen, Dag Olav (2014). Kan forbud fremme frihet?. Dagbladet.  ISSN 0805-3766.
• Svensen, Henrik & Hessen, Dag Olav (2014). Eksperimentet, I: Henrik Svensen (red.),  Isfritt. Populærvitenskap som angår deg.  Spartacus.  ISBN 9788243008342.  Forord.  s 9 - 11
• Tønnessen, Morten; Tønnessen, Morten & Hessen, Dag Olav (2014, 12. november). – Et godt liv kan ikke realiseres i pelsdyrindustrien. [Fagblad].
• Aall, Carlo; Grønås, Sigbjørn; Hessen, Dag Olav; Hjertager, Audun; Hylland-Eriksen, Thomas; Innbjør, Linda; Jakobsen, Ove D; Kroglund, Andrew P.; Kvåle, Gunnar; Fossgard Sandøy, Ingvild; Svae, Per Hjalmar & Vetlesen, Arne Johan (2013). Opprop for De Grønne. Klassekampen.  ISSN 0805-3839.

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