Dag Olav Hessen

• Allesson, Lina; Andersen, Tom; Dörsch, Peter; Eiler, Alexander; Wei, Jing & Hessen, Dag Olav (2020). Phosphorus availability promotes bacterial DOC-mineralization, but not cumulative CO2-production. Frontiers in Microbiology.  ISSN 1664-302X.  11:569879, s 1- 12 . doi: 10.3389/fmicb.2020.569879
• Allesson, Lina; Koehler, Birgit; Thrane, Jan-Erik; Andersen, Tom & Hessen, Dag O (2020). The role of photomineralization for CO2 emissions in boreal lakes along a gradient of dissolved organic matter. Limnology and Oceanography.  ISSN 0024-3590. . doi: 10.1002/lno.11594 Show summary

  Many boreal lakes are experiencing an increase in concentrations of terrestrially derived dissolved organic matter (DOM)—a process commonly labeled “browning.” Browning affects microbial and photochemical mineralization of DOM, and causes increased light attenuation and hence reduced photosynthesis. Consequently, browning regulates lake heterotrophy and net CO2‐efflux to the atmosphere. Climate and environmental change makes ecological forecasting and global carbon cycle modeling increasingly important. A proper understanding of the magnitude and relative contribution from CO2‐generating processes for lakes ranging in dissolve organic carbon (DOC) concentrations is therefore crucial for constraining models and forecasts. Here, we aim to study the relative contribution of photomineralization to total CO2 production in 70 Scandinavian lakes along an ecosystem gradient of DOC concentration. We combined spectral data from the lakes with regression estimates between optical parameters and wavelength specific photochemical reactivity to estimate rates of photochemical DOC mineralization. Further, we estimated total in‐lake CO2‐production and efflux from lake chemical and physical data. Photochemical mineralization corresponded on average to 9% ± 1% of the total CO2‐evasion, with the highest contribution in clear lakes. The calculated relative contribution of photochemical mineralization to total in‐lake CO2‐production was about 3% ± 0.2% in all lakes. Although lakes differed substantially in color, depth‐integrated photomineralization estimates were similar in all lakes, regardless of DOC concentrations. DOC concentrations were positively related to CO2‐efflux and total in‐lake CO2‐production but negatively related to primary production. We conclude that enhanced rates of photochemical mineralization will be a minor contributor to increased heterotrophy under increased browning.

• Andersen, Tom; Hessen, Dag Olav; Håll, Johnny Peter; Khomich, Maryia; Kyle, Marcia; Lindholm, Markus; Rasconi, Serena; Skjelbred, Birger; Thrane, Jan-Erik & Walseng, Bjørn (2020). Congruence, but no cascade—Pelagic biodiversity across three trophic levels in Nordic lakes. Ecology and Evolution.  ISSN 2045-7758.  10(15), s 8153- 8165 . doi: 10.1002/ece3.6514 Full text in Research Archive. Show summary

  Covariation in species richness and community structure across taxonomical groups (cross‐taxon congruence) has practical consequences for the identification of biodiversity surrogates and proxies, as well as theoretical ramifications for understanding the mechanisms maintaining and sustaining biodiversity. We found there to exist a high cross‐taxon congruence between phytoplankton, zooplankton, and fish in 73 large Scandinavian lakes across a 750 km longitudinal transect. The fraction of the total diversity variation explained by local environment alone was small for all trophic levels while a substantial fraction could be explained by spatial gradient variables. Almost half of the explained variation could not be resolved between local and spatial factors, possibly due to confounding issues between longitude and landscape productivity. There is strong consensus that the longitudinal gradient found in the regional fish community results from postglacial dispersal limitations, while there is much less evidence for the species richness and community structure gradients at lower trophic levels being directly affected by dispersal limitation over the same time scale. We found strong support for bidirectional interactions between fish and zooplankton species richness, while corresponding interactions between phytoplankton and zooplankton richness were much weaker. Both the weakening of the linkage at lower trophic levels and the bidirectional nature of the interaction indicates that the underlying mechanism must be qualitatively different from a trophic cascade.

• Rizzuto, Simone; Thrane, Jan-Erik; Baho, Didier Ludovic; Jones, Kevin C.; Zhang, Hao; Hessen, Dag Olav; Nizzetto, Luca & Leu, Eva Susanne (2020). Water browning controls adaptation and associated trade-offs in phytoplankton stressed by chemical pollution. Environmental Science and Technology.  ISSN 0013-936X.  54(9), s 5569- 5579 . doi: 10.1021/acs.est.0c00548 Full text in Research Archive. Show summary

  The acquisition of tolerance to an environmental stressor can result in organisms displaying slower growth after stress release. While well-grounded in the theory, empirical evidence of the trade-off between stress tolerance and organism fitness is scarce and blurred by the interaction with different environmental factors. Here, we report the effects of water browning on the responses, tolerance acquisition, and associated trade-offs in a population of microalgae exposed to sublethal concentrations of organic micropollutants over multiple generations. Our results show that dissolved organic matter (DOM) reduces toxic responses and modulates tolerance acquisition by the algae, possibly by complexing micropollutants. Microalgae that acquire tolerance allocate resources to fitness at the cost of reduced cell size. They yield higher productivity than nonadapted ones when grown in the presence of micropollutants but lower in their absence. The net trade-off was positive, indicating that adaptation can result in a higher productivity and fitness in tolerant species in recurrently stressed environments.

• Sperfeld, Erik; Nilssen, Jens Petter; Rinehart, Shelby Anne; Klaus, Schwenk & Hessen, Dag Olav (2020). Ecology of predator-induced morphological defense traits in Daphnia longispina (Cladocera, Arthropoda). Oecologia.  ISSN 0029-8549.  192, s 687- 698 . doi: 10.1007/s00442-019-04588-6
• Baho, Didier L; Leu, Eva; Pomati, Francesco; Hessen, Dag O; Nordberg, Jon; Moe, S. Jannicke; Skjelbred, Birger & Nizzetto, Luca (2019). Resilience of Natural Phytoplankton Communities to Pulse Disturbances from Micropollutant Exposure and Vertical Mixing. Environmental Toxicology and Chemistry.  ISSN 0730-7268.  38(10), s 2197- 2208 . doi: 10.1002/etc.4536 Full text in Research Archive. Show summary

  Freshwaters are increasingly exposed to complex mixtures of pharmaceutical and personal care products (PPCPs) from municipal wastewater, which are known to alter freshwater communities’ structure and functioning. However, their interaction with other disturbances and whether their combined effects can impact ecological resilience (i.e., the ability of a system to tolerate disturbances without altering the system's original structure and processes) remain unexplored. Using in situ mesocosms in 2 lakes with different nutrient levels (mesotrophic and eutrophic), we assessed whether a pulse exposure to sublethal concentrations of 12 PPCPs affects the ecological resilience of natural phytoplankton communities that experienced an abrupt environmental change involving the destabilization of the water column through mixing. Such mixing events are predicted to increase as the effects of climate change unfold, leading to more frequent storms, which disrupt stratification in lakes and force communities to restructure. We assessed their combined effects on community metrics (biomass, species richness, and composition) and their relative resilience using 4 indicators (cross‐scale, within‐scale, aggregation length, and gap length), inferred from phytoplankton communities by discontinuity analysis. The mixing disturbance alone had negligible effects on the community metrics, but when combined with chemical contaminants significant changes were measured: reducing total biomass, species richness, and altered community composition of phytoplankton. Once these changes occurred, they persisted until the end of the experiment (day 20), when the communities’ structures from the 2 highest exposure levels diverged from the controls. The resilience indicators were not affected by PPCPs but differed significantly between lakes, with lower resilience found in the eutrophic lake. Thus, PPCPs can significantly alter community structures and reinforce mechanisms that maintain ecosystems in a “degraded state.”

• Baho, Didier L; Pomati, Francesco; Leu, Eva; Hessen, Dag O; 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 Full text in Research Archive. Show summary

  The presence of a multitude of bioactive organic pollutants collectively classified as pharmaceuticals and personal care products (PPCPs) in freshwaters is of concern, considering that ecological assessments of their potential impacts on natural systems are still scarce. In this field experiment we tested whether a single pulse exposure to a mixture of 12 pharmaceuticals and personal care products, which are commonly found in European inland waters, can influence the size distributions of natural lake phytoplankton communities. Size is one of the most influential determinants of community structure and functioning, particularly in planktonic communities and food webs. Using an in-situ microcosm approach, phytoplankton communities in two lakes with different nutrient levels (mesotrophic and eutrophic) were exposed to a concentration gradient of the PPCPs mixture at five levels. We tested whether sub-lethal PPCPs doses affect the scaling of organisms' abundances with their size, and the slope of these size spectra, which describe changes in the abundances of small relative to large phytoplankton. Our results showed that a large proportion (approximately 80%) of the dataset followed a power-law distribution, thus suggesting evidence of scale invariance of abundances, as expected in steady state ecosystems. PPCPs were however found to induce significant changes in the size spectra and community structure of natural phytoplankton assemblages. The two highest treatment levels of PPCPs were associated with decreased abundance of the most dominant size class (nano-phytoplankton: 2–5 μm), leading to a flattening of the size spectra slope. These results suggest that a pulse exposure to PPCPs induce changes that potentially lead to unsteady ecosystem states and cascading effects in the aquatic food webs, by favoring larger non-edible algae at the expense of small edible species. We propose higher susceptibility due to higher surface to volume ratio in small species as the likely cause of these structural changes.

• Bostad, Inga & Hessen, Dag Olav (2019). Learning and loving of nature in the Anthropocene. How to broaden science with curiosity and passion. Studier i Pædagogisk Filosofi.  ISSN 2244-9140.  8(1), s 28- 42 . doi: 10.7146/spf.v8i1.113924 Full text in Research Archive.
• 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, s 1- 11 . doi: 10.3389/fevo.2019.00074 Full text in Research Archive. 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 F; Hessen, Dag O & Demars, Benoît O L (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 Full text in Research Archive. Show summary

  Atmospheric N pollution may shift nutrient limitations in aquatic autotrophs from N to P or cause an intensification of P limitation in formerly pristine areas. Small changes in nutrient supply in oligotrophic lakes and rivers could lead to large changes in relative plant growth and yield with possible knock on effects on ecosystem carbon cycling through changes in the decomposition rate of their tissue. Previous biogeographical studies have shown inconsistent responses of plant nutrient tissue content and stoichiometry (functional traits) to external nutrient availability. Here we used a single species, Juncus bulbosus, to test the interplay between plant tissue nutrient (content and stoichiometry) and external environmental factors (local and catchment scale). We developed a comparative approach applicable globally to assess the thresholds for nutrient limitation in aquatic plants in the wild. Phosphorus in Juncus bulbosus tissue was negatively related to sediment organic matter (Fe root plaque limiting P uptake) and catchment vegetation cover (less P leaching to lakes). Our comparative approach revealed that the lack of increase in N plant tissue along the strong gradient in external N concentration may be explained by P limitation and strict plant tissue N:P ratio. Our comparative approach further showed that the nutrient content and stoichiometry of Juncus bulbosus was similar to other submerged aquatic plants growing in nutrient poor aquatic ecosystems. In southern Norway, mass development of Juncus bulbosus may be primarily triggered by changes in P availability, rather than CO2 or inorganic N, as previously thought, although co-limitations are also possible. If so, the mass development of Juncus bulbosus in oligotrophic aquatic ecosystems could be an early indicator of increasing P fluxes through these ecosystems which are less limited by N due to high atmospheric N deposition.

• 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 Full text in Research Archive.
• Weyhenmeyer, Gesa A; Hartmann, Jens; Hessen, Dag O; Kopáček, Jiří; Hejzlar, Josef; Jacquet, Stéphan; Hamilton, Stephen K; Verburg, Piet; Leach, Taylor H; Schmid, Martin; Flaim, Giovanna; Nõges, Tiina; Nõges, Peeter; Wentzky, Valerie C; Rogora, Michela; Rusak, James A; Kosten, Sarian; Paterson, Andrew M; Teubner, Katrin; Higgins, Scott N; Lawrence, Gregory; Kangur, Külli; Kokorite, Ilga; Cerasino, Leonardo; Funk, Clara; Harvey, Rebecca; Moatar, Florentina; de Wit, Heleen A & Zechmeister, Thomas (2019). Widespread diminishing anthropogenic effects on calcium in freshwaters. Scientific Reports.  ISSN 2045-2322.  9 . doi: 10.1038/s41598-019-46838-w Full text in Research Archive. Show summary

  Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L−1 with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L−1, a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters carbonate alkalinity concentrations have increased or remained constant, while Ca concentrations have rapidly declined towards or even below pre-industrial conditions as a consequence of recovery from anthropogenic acidification. Thus, a paradoxical outcome of the successful remediation of acid deposition is a globally widespread freshwater Ca concentration decline towards critically low levels for many aquatic organisms.

• 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.

• Zheng, Xiaoqi; Lu, Yonglong; Yuan, Jingjing; Baninla, Yvette; Zhang, Yvette; Stenseth, Nils Christian; Hessen, Dag Olav; Tian, Hanqin; Obersteiner, Michael & Chen, Deliang (2019). Drivers of change in China’s energy-related CO2 emissions. Proceedings of the National Academy of Sciences of the United States of America.  ISSN 0027-8424.  117(1), s 29- 36 . doi: 10.1073/pnas.1908513117 Full text in Research Archive.
• 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 (ES&T 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 (NTA).  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

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• Slettebakk, Marianne; Håpenes, Arnodd; Hessen, Dag Olav; Eskeland, Ragnhild; Gjærevoll, Inger & Borge, Ole Johan (2019). BIOS 2. Cappelen Damm AS.  ISBN 9788202604851.
• 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.

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• Bryn, Anders; Hessen, Dag Olav & Stordal, Frode (2020). Før det bikker over. Klassekampen.  ISSN 0805-3839.  s 18- 19
• Bøckman, Petter & Hessen, Dag Olav (2019, 31. oktober). Padde unnslipper rovdyrene ved å etterligne en huggorm. [Internett].  Forskning.no.
• Hessen, Dag Olav (2019). Publiseringspirater og akademiske kappløp, I: Aksel Tjora (red.),  Universitetskamp.  Scandinavian Academic Press.  ISBN 978-82-304-0247-4.  Kapittel 2.  s 43 - 56
• 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.
• McGovern, Maeve; Evenset, Anita; Borgå, Katrine; Wit, Heleen de; Braaten, Hans Fredrik Veiteberg; Hessen, Dag Olav; Schultze, Sabrina; Ruus, Anders & Poste, Amanda (2019). Implications of Coastal Darkening for Contaminant Transport, Bioavailability, and Trophic Transfer in Northern Coastal Waters. Environmental Science and Technology.  ISSN 0013-936X.  53(13), s 7180- 7182 . doi: 10.1021/acs.est.9b03093 Full text in Research Archive. Show summary

  The increased transport of terrestrially derived organic material (tOM) from catchments to rivers and lakes is leading to browning of northern freshwaters.(1) Although the same environmental drivers can be expected to lead to widespread increases in tOM and subsequent darkening in adjacent coastal systems,(2) this has received considerably less attention, and long-term monitoring data for detecting these changes are scarce. This is despite expectations that darkening is likely to have a range of implications for coastal biogeochemistry and food-web ecology. tOM is accompanied by other catchment-derived materials including contaminants, inorganic particles, and nutrients. Together, these terrestrial inputs can affect cycling and food-web accumulation of contaminants, in particular, methyl mercury (MeHg) and persistent organic pollutants (POPs). Because of the human and ecosystem health risks associated with these contaminants, and to ensure informed policy decisions, there is a need for a better understanding of the drivers of darkening and their direct and indirect impacts on the transport, bioavailability, and trophic transfer of contaminants in northern coastal waters.

• Skåre, Janneche Utne; Alexander, Jan; Haave, Marte; Jakubowicz, Ignacy; Knutsen, Helle Katrine; Lusher, Amy; Ogonowski, Martin; Rakkestad, Kirsten Eline; Skaar, Ida; Sverdrup, Line Emilie; Wagner, Martin; Agdestein, Angelika; Bodin, Johanna Eva; Elvevoll, Edel O.; Hemre, Gro Ingunn; Hessen, Dag Olav; Hofshagen, Merete; Husøy, Trine; Krogdahl, Åshild; Nilsen, Asbjørn Magne; Rafoss, Trond; Skjerdal, Olaug Taran; Steffensen, Inger-Lise; Strand, Tor A; Vandvik, Vigdis & Wasteson, Yngvild (2019). Microplastics; occurrence, levels and implications for environment and human health related to food. Scientific opinion of the Scientific Steering Committee of the Norwegian Scientific Committee for Food and Environment. VKM Report. 2019:16. Full text in Research Archive.
• Steffensen, Inger-Lise; Fæste, Christiane Kruse; Husøy, Trine; Knutsen, Helle Katrine; Mathisen, Gro Haarklou; Ørnsrud, Robin; Agdestein, Angelika; Bodin, Johanna Eva; Elvevoll, Edel O.; Hessen, Dag Olav; Hofshagen, Merete; Krogdahl, Åshild; Nilsen, Asbjørn Magne; Rafoss, Trond; Skjerdal, Olaug Taran; Velle, Gaute; Wasteson, Yngvild; Hemre, Gro Ingunn; Vandvik, Vigdis & Alexander, Jan (2019). Ranking of substances for monitoring in foods, drinks and dietary supplements - based on risk and knowledge gaps. Scientific Opinion of the Scientific Steering Committee of the Norwegian Scientific Committee for Food and Environment. VKM Report. 2019:13. Full text in Research Archive.
• 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 Vitenskap.  ISSN 2464-3033.
• 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

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