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Lake ecosystems and the services that they provide to people are profoundly influenced by dissolved organic matter derived from terrestrial plant tissues. These terrestrial dissolved organic matter (tDOM) inputs to lakes have changed substantially in recent decades, and will likely continue to change. In this paper, we first briefly review the substantial literature describing tDOM effects on lakes and ongoing changes in tDOM inputs. We then identify and provide examples of four major challenges which limit predictions about the implications of tDOM change for lakes, as follows: First, it is currently difficult to forecast future tDOM inputs for particular lakes or lake regions. Second, tDOM influences ecosystems via complex, interacting, physical-chemical-biological effects and our holistic understanding of those effects is still rudimentary. Third, non-linearities and thresholds in relationships between tDOM inputs and ecosystem processes have not been well described. Fourth, much understanding of tDOM effects is built on comparative studies across space that may not capture likely responses through time. We conclude by identifying research approaches that may be important for overcoming those challenges in order to provide policy- and management-relevant predictions about the implications of changing tDOM inputs for lakes.

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. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Riverine transport of organic carbon (OC) to the ocean is a significant component in the global carbon (C) cycle and the concentration of total organic carbon (TOC) in rivers and lakes is vital for ecosystem properties and water quality for human use. By use of a large dataset comprising chemical variables and detailed catchment information in ~1000 Norwegian pristine lakes covering a wide climatic range, we were able to predict TOC concentrations with high accuracy. We further predict, using a 'space-for-time' approach and a downscaled, moderate, climate change scenario, that northern, boreal regions likely will experience strong increases in OC export from catchments to surface waters. Median concentrations of OC in these lakes will increase by 65%, from the current median of 2.0-3.3 mg C L -1 . This is a long-term effect, primarily mediated by increased terrestrial vegetation cover in response to climate change. This increase OC will have severe impacts on food-webs, productivity and human use. Given the robustness of the estimates and the general applicability of the parameters, we suggest that these findings would be relevant to boreal areas in general.

The ozone winter maximum at high latitudes in the northern hemisphere is not evenly distributed along the longitudes. This is mainly due to the upper air circulation, both horizontally and vertically. In addition it is also strongly influenced by the largest mountain ranges. During the last two decades the air circulation in the North Atlantic has intensified. This has led to ascending motion in the upper troposphere and the lower stratosphere, which in turn has resulted in a reduced total ozone column in Northwest Europe. The large mounter ranges in Asia are initiating standing waves, with descending motions in the atmosphere behind the mountains. The descending motion leads to adiabatic warming of the lower stratosphere and the upper troposphere. Ozone-rich air is transported downwards to lower levels and stored there, where the ozone is less affected by heterogeneous chemical destruction.

Two winter months, February 1993 and February 1994, with quite different circulation patterns in the Atlantic area have been studied. February 1993 was dominated by a strong southwesterly upper air current with correspondingly strong ascending motion in the area. This resulted in a reduced ozone column in the lower stratosphere and also a low total ozone column. This was especially true in Scandinavia and Northwestern Europe. February 1994 had a more normal circulation in the same area with weaker horizontal and ascending motions. The ozone column was back to “normal” in Scandinavia and Northwestern Europe. Several of recent years have had an increased southwesterly upper air circulation over the Atlantic ocean with low ozone values in Northwestern Europe. This altered circulation might then add to the negative trend, which has been observed during the two last decades.

During the winter of 1991–1992, ozone values over Scandinavia and northern Europe were exceptionally low. The sea surface temperatures in the southern part of the North Atlantic were 0.5–1.0°C higher than the normal, whereas the corresponding temperatures in the Greenland waters were about normal or a little lower than normal. The normal is based on observations during the period 1957–1980 (WMO, 1992). The increased temperature gradient in the sea surface resulted in an increase in the air temperature gradient. As a consequence, strong southwesterly winds with frequent low pressure activity dominated in the North Atlantic during the winter of 1991–1992. In the southwest wind-flow, ascending motion in the troposphere and the lower stratosphere was persistent, especially during the month of January 1992. The ascending motion lifted the tropopause and the lower stratosphere to higher levels and attempted to establish a secondary tropopause due to tropopause folding. The ozone partial pressure between the original tropopause and the secondary was then strongly reduced.

The Airborne Arctic Stratospheric Expedition (AASE) carried out measurements from January 3 to February 15, 1989. Enhanced levels of chlorine compounds were found in the Arctic stratosphere, and on two single flights ozone decrease of 17% were measured, interpreted as essential features of the Arctic stratosphere, caused by a combined effect of enhanced amounts of chlorine compounds and presence of polar stratospheric clouds. Related model calculations also indicate extended ozone depletion maximizing in late March 1989 and amount to 5–8% in column at 70° N. Ground-based ozone measurements, however, show that the most characteristic features during this period are temporal variations and a strong enhancement of ozone, probably due to an extended stratospheric warming. From these measurements it is hard to see any effect of an eventual enhanced burden of stratospheric chlorine, which might show up as an extended and long-lasting decrease of stratospheric ozone, but its eventual existence is masked by the temporal variations.

[1] We combine a synoptic data set on organic carbon (C) concentrations in ∼1000 Norwegian lakes with detailed information on catchment properties derived from digital maps of elevation, climate, vegetation density, and land use. The resulting regression model explains 83% of the variance in total organic C from eight predictor variables: mean air temperature, area specific runoff, terrain slope, vegetation density, atmospheric nitrogen deposition, water residence time, and catchment area fractions of bogs and arable land. Vegetation density, expressed as the normalized difference vegetation index (NDVI), was by far the strongest predictor, while area fraction of bogs in the catchment also gave a strong positive effect on organic C concentrations and export fluxes. The unbiased sampling of lakes relative to whole population of catchments in the region and the general robustness of the model, as witnessed by imputation and resampling tests, allowed confident prediction of organic C concentrations and export fluxes for all 20,000 catchments of the Norwegian mainland. The method used in this study can, with some modifications, be extended to larger geographical regions and holds promise for predicting organic C fluxes under changed climatic conditions.

[1] During the past two decades, it has become evident that a majority of lakes are net conduits of CO2 to the atmosphere. This insight has implications both for lake metabolism per se and for assessing the role of lakes in the global C cycle. The concentration of dissolved organic carbon (DOC), which constitutes >90% of the total organic carbon (TOC), has been identified as a key driver of partial pressure of CO2 (pCO2). A crucial question is whether one may identify global relationships in the DOC-pCO2 relationship in lakes or whether this has to be determined regionally or locally. A second major aspect is how to best predict CO2 as a function of DOC. Based on a survey of pCO2 and a range of lake and catchment variables in 112 lakes, we support the view that DOC is by far the most important determinant of pCO2 while groundwater influx has a minor contribution. Contrary to expectations, total phosphorus (P) also apparently contributed positively to pCO2, owing to the fact that most P in these lakes is on the form of allochthonously organic P, and thus correlates strongly with DOC. Physical principles dictate that even a lake completely devoid of DOC should have a nonzero pCO2. This is not reflected in power models, which imply that the pCO2 approaches zero with zero DOC. Based on this study as well as published data on DOC-pCO2 relationships, we argue that identity link gamma–generalized linear models are appropriate for predicting pCO2 in lakes and that their application makes it possible to reach reasonably accurate global models for how pCO2 relates to DOC and other environmental factors.

Long term column ozone density measurements have been carried out in Norway and U.S.S.R. Data from Tromso and two meridian chains in U.S.S.R. are analysed, indicating that no significant decreasing trend in ozone has occurred during the last two decades. From these data it can be inferred that the increasing atmospheric load of CFC gases has played a negligible role in depleting the column ozone densities in the regions of Norway and U.S.S.R. where the measurements have been carried out.

During the first week of February 1990 the total ozone values over England and Scandinavia dropped to a minimum of approximately 200DU. This minimum lasted, however, only for a couple of days at each of the stations, which are measuring ozone in the area. The ozone ‘minihole’ was not stationary, but it moved from the North Atlantic towards Northern Scandinavia and the Kola peninsula during a 4day period. From the ozone observations its path was traced over England towards Norway, Sweden and Russia. In this paper we propose that the ‘minihole’ was mainly created by a strong cyclonic development in the North Atlantic, which moved towards Scandinavia. The cyclone development resulted in ascending air ahead of the cyclone in the troposphere and the lower stratosphere. The total ozone column was thereby reduced. However, this does not exclude the possibility that ozone also could have been reduced due to heterogeneous photochemical processes. The events took place during the CHEOPS III-Campaign, at Kiruna in Sweden, and gave most valuable information.

In this paper we have tried to show that the ozone variation along the polar front zone is very dependent on dynamical processes in the atmosphere. We have used the 500 hPa level to show that there is ascending motion at 100 hPa for south and southwesterly wind flows at 500 hPa, whereas west and northwesterly flows give descending motion at 100 hPa level. The vertical velocities at 100 hPa are remarkably large. We have examples of 7 hPa/h, which corresponds to approximately 500 m/h. These strong vertical velocities are usually set un by mountain ranges and strong horizontal wind flows. Since the vertical motion is transported eastwards by the strong westerlies, the effect on the ozone values will show up to the east of the strongest vertical motions. The dominating wind flow over the Atlantic in the winter is southwesterly. This results in low stratospheric temperatures and low ozone values over the Scandinavian sector. Over western Siberia and Russia a high pressure dominates the general winter circulation. To the east of the high the dominating wind flow will be west and northwesterly, and, combined with the mountain ranges east of the Baikal Sea, this leads to general descending motion over eastern Siberia and Kamchatka. This descending motion will raise the temperature in the lower stratosphere and is well correlated with the increase in ozone. The ozone increase is, however, caused by the lowering of the tropopause and, thereby, the vertical stretching of the ozone layer.

Long-term measurements of the thickness of the ozone layer in Tromso since 1935 show that the annual mean value can vary by as much as 18%. However, no decreasing trend of the ozone content and no correlation with the increasing amount of chlorine and halogen compounds of the atmosphere are found in the Scandinavian sector of the Arctic.

The effect of changes in the ozone layer on the incidence of skin cancer was explored using data for Norway. Attempts were made to arrive at a relationship between the "environmental effective UV-dose" and the skin cancer incidence. Norway is well suited for this purpose because of the large variation in the annual UV-dose from north to south. Furthermore we have a well developed cancer registry and a homogeneous population with regard to skin type. Four different regions of the country, each with a broadness of 1 degree in latitude (approximately 111 km), were selected (located around 69.5, 63.5, 60 and 58.5 degrees N). The annual effective UV-doses for these regions were calculated, assuming normal ozone conditions throughout the year and the action spectrum proposed by CIE, which extends up to 400 nm. The incidence rate (in the period 1970-1980) of malignant melanoma and non-melanoma skin cancer (mainly basal cell carcinoma) increased with the annual environmental UV-doses. For both these types of cancer a quadratic dose-effect relationship seems to be valid to a first approximation. The present data indicate that the incidence of skin cancer would increase by approximately 2% for each percent ozone reduction.

The Airborne Arctic Stratospheric Expedition (AASE) carried out measurements from Jan. 3 to Feb. 15, 1989. Enhanced levels of chlorine compounds were found in the Arctic stratosphere, and on two single flights ozone decrease of 17% were measured, interpreted as essential features of the Arctic stratosphere, caused by a combined effect of enhanced amounts of chlorine compounds and the presence of polar stratospheric clouds. Related model calculations also indicate extended ozone depletion maximizing in late March 1989 and amount to 5 - 8% in column at 70 degree(s) N. Ground-based ozone measurements, however, show that the most characteristic features during this period are temporal variations and a strong enhancement of ozone, probably due to an extended stratospheric warming. From these measurements it is hard to see any effect of an eventual enhanced burden of stratospheric chlorine, which might show up as an extended and long-lasting decrease of stratospheric ozone, but its eventual existence is masked by the temporal variations.

The Oslo ozone station on 60 degree(s) N has been in continuous operation since 1978. The measured ozone values for this station seem to be rather representative for the Arctic region. All the observed data have recently been reanalyzed and the results are given. In the period from 1978 to 1993 the ozone decrease is approximately 4.5%. The winter values (average of December, January, February, and March) decreased by approximately 9% whereas the summer values (average of May, June, July and August) decreased 2.3% in the 15 year period. The volcano eruptions from El Chichon and Mt. Pinatubo seem to influence the ozone layer.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ozone observations have been taken for many years in Tromso, 70°N. From 1979 new measurements started in Oslo, 60°N. The ozone amount changes from day to day and have a seasonal change. One sees the same type of variations in the south, but since 1978 the seasonal change has changed. An ozone depletion appears and normal condition is not established before early summer. The different behaviours are demonstrated and this may be connected to difference in dynamical processes and a relative importance of heterogeneous reactions in chemical processes.

Effective UV-doses were calculated based on the integrated product of the biological action spectrum (the one proposed by IEC, which extends to 400 nm, was adopted) and the spectral irradiance. The calculations include absorption and scattering of UV-radiation in the atmosphere, both for normal ozone conditions as well as for a depleted ozone layer. For Scandinavian latitudes the effective annual UV-dose increases by approximately 4% per degrees of latitude towards the Equator. An ozone depletion of one percent increases the annual UV-dose by approximately 1% at 60 degrees N (increases slightly at lower latitudes). A large depletion of 50% over Scandinavia (60 degrees N) would give these countries an effective UV-dose similar to that obtained, with normal ozone conditions, at a latitude of 40 degrees N (California or the Mediterranean countries). The Antarctic ozone hole increases the annual UV-dose by 20 to 25% which is a similar increase as that attained by moving 5 to 6 degrees of latitude nearer the Equator. The annual UV-dose at higher latitudes is mainly determined by the summer values of ozone. Both the ozone values and the effective UV-doses vary from one year to another (within +/- 4%). No positive or negative trend is observed for Scandinavia from 1978 to 1988.

We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.