Spatial and temporal patterns of stream nutrient limitation in an Arctic catchment

Arctic stream biofilm responses to ongoing climate-related changes in physical and chemical conditions have major implications for stream food webs and biogeochemical cycles. Yet, such effects have rarely been studied outside summer months or at sub-catchment scales in the Arctic. We used deployments of nutrient diffusing substrates (NDS) to assess the spatial (20 deployments) and seasonal patterns (10 deployments) and physical and chemical drivers of nutrient limitation within an Arctic stream catchment. Results show that nutrient limitation of autotrophic processes was common during summer, but that light inhibited biomass accrual under the ice in winter. Alongside single N, P and C responses, co-limitation dominated the overall pattern of limitation over time and across the catchment. However, the primary limiting nutrient to autotrophs changed from N to P in parts of the catchment with higher N concentrations. As Arctic studies are often conducted at individual sites during summer, these may miss shifts in the drivers of stream productivity that arise from variable nutrient, temperature, and light regimes. Our results caution against focusing on one single most important limiting nutrient, as we found that this can shift seasonally and over small spatial scales in this Arctic catchment

Experimental riparian forest gaps and increased sediment loads modify stream metabolic patterns and biofilm composition

Forest management operations greatly influence stream habitats. Canopyclearing and subsequent canopy development during succession, site prepara-tion, and ditching alter the light environment, and increase sediment inputsand nutrient exports from upland and riparian soils to streams. These physico-chemical changes affect aquatic biofilms and metabolic rates, and in thisstudy, we tested their individual and combined effects. We used 12 artificialstreamside channels, together with a field survey of nine streams in andaround clear-cuts, to assess the effects of shading, substrate composition, andnutrient addition on biofilm biomass and composition, as well as metabolicrates. We found that biofilm biomass and gross primary production (GPP)were light limited in channels under 70% canopy shading. Nitrate additions atthis shading level only marginally increased autotrophic biomass, while therates  of  respiration  increased  10-fold  when  carbon  was  added.  Open(unshaded) channels had three times higher rates of GPP compared with chan-nels with 70% shading, and autotrophic biomass was twice as high, largelycaused by the colonization of filamentous green algae. These changes to bio-film biomass, composition, and GPP were caused by differences in light alone,as temperature was not affected by the shading treatment. Notably, higherrates of GPP led to no positive effect on net ecosystem production. Further,fine-grained substrates negatively affected GPP as compared with stone sub-strates in the experimental channels. In the surveyed streams, the negativeeffects of fine-grained substrates exceeded the positive influence of light onbiofilm biomass. Altogether, our results highlight the need for riparian man-agement that protects headwaters from unwanted stressors by focusing onpreventing sediment erosion and carbon transport in clear-cuts, while provid-ing variable shade conditions in second-growth forests

Contrasting impacts of warming and browning on periphyton

We tested interactive effects of warming (+2 degrees C) and browning on periphyton accrual and pigment composition when grown on a synthetic substrate (plastic strips) in the euphotic zone of 16 experimental ponds. We found that increased colored dissolved organic matter (cDOM) and associated nutrients alone, or in combination with warming, resulted in a substantially enhanced biomass accrual of periphyton, and a comparatively smaller increase in phytoplankton. This illustrates that periphyton is capable of using nutrients associated with cDOM, and by this may affect nutrient availability for phytoplankton. However, warming weakened the positive impact of browning on periphyton accrual, possibly by thermal compensation inferred from altered pigment composition, and/or changes in community composition. Our results illustrate multiple impacts of climate change on algal growth, which could have implications for productivity and consumer resource use, especially in shallow areas in northern lakes

Resolving the Drivers of Algal Nutrient Limitation from Boreal to Arctic Lakes and Streams

Nutrient inputs to northern freshwaters are changing, potentially altering aquatic ecosystem functioning through effects on primary producers. Yet, while primary producer growth is sensitive to nutrient supply, it is also constrained by a suite of other factors, including light and temperature, which may play varying roles across stream and lake habitats. Here, we use bioassay results from 89 lakes and streams spanning northern boreal to Arctic Sweden to test for differences in nutrient limitation status of algal biomass along gradients in colored dissolved organic carbon (DOC), water temperature, and nutrient concentrations, and to ask whether there are distinct patterns and drivers between habitats. Single nitrogen (N) limitation or primary N-limitation with secondary phosphorus (P) limitation of algal biomass was the most common condition for streams and lakes. Average response to N-addition was a doubling in biomass; however, the degree of limitation was modulated by the distinct physical and chemical conditions in lakes versus streams and across boreal to Arctic regions. Overall, algal responses to N-addition were strongest at sites with low background concentrations of dissolved inorganic N. Low temperatures constrained biomass responses to added nutrients in lakes but had weaker effects on responses in streams. Further, DOC mediated the response of algal biomass to nutrient addition differently among lakes and streams. Stream responses were dampened at higher DOC, whereas lake responses to nutrient addition increased from low to moderate DOC but were depressed at high DOC. Our results suggest that future changes in nutrient availability, particularly N, will exert strong effects on the trophic state of northern freshwaters. However, we highlight important differences in the physical and chemical factors that shape algal responses to nutrient availability in different parts of aquatic networks, which will ultimately affect the integrated response of northern aquatic systems to ongoing environmental changes

Disentangling denitrification and its environmental drivers in northern boreal lakes

Dinitrous oxide (N2O) is a potent greenhouse gas some 354 times stronger than carbon dioxide (CO2) in the atmosphere. Recent studies show that lake denitrification contributes to a considerable part of the global N2O emissions. Despite this, lake-N2O emissions are not being accounted for in global greenhouse gas modeling because it has not yet been accurately understood and quantified. The aim of this study was to assess how denitrification varies between and within boreal lakes and how it is controlled by nitrate- (NO3) and carbon (C) availability and temperature. Studies on denitrification were performed using the acetylene inhibition technique on sediments from three lakes in northern Sweden (February to August, 2014). Results showed that denitrification was correlated (linear regression, r2=0.71) with NO3 concentrations in the hypolimnion water at ambient conditions and that additions of NO3 up to a concentration of 50 µg NO3-N L-1 increased denitrification. Temperature increased denitrification in all lakes, at all sites except in one lake in July, when nutrient concentrations were at its lowest. The spatial and temporal variation in denitrification was small at ambient conditions (1-3 µmol N2O m-2 h-1)but the variation in the response to nutrient additions and temperature increase was very high. This was in part attributed to differences in dissolved organic C (DOC). These findings have important implications for future denitrification research and how lake-N2O production is included in greenhouse gas modeling and contributes to our knowledge on how northern boreal lakes may respond to enhanced nutrient loadings and global warming

The role of nutrients for stream ecosystem function in Arctic landscapes : drivers of productivity under environmental change

Arctic and sub-Arctic freshwaters are currently experiencing substantial ecosystem changes due to the effects of global warming. Global warming effects on these freshwaters include increasing water temperatures, altered hydrological patterns, shifts in riparian vegetation and changes in the export of nutrients and carbon from soils. How these alterations to the physical and chemical hab-itat will affect stream ecosystem functioning largely depends on the responses by autotrophic pro-ducers and heterotrophic primary consumers. In this thesis, I explore how key stream ecosystem processes such as metabolic rates and nutrient cycling vary as a function of climate and landscape drivers, particularly light, temperature, and nutrient and carbon availability. To do this I leveraged natural gradients in vegetation, altitude, disturbance, and precipitation throughout the year in northern Sweden, as well as long- and short-term manipulations of nutrient availability. I also synthesized nutrient limitation data from lakes and streams to more holistically assess the re-sponses of boreal to Arctic freshwaters to changes in nutrients and climate variables. I found that nutrient availability, and especially nitrogen (N), is a main driver of spatial and temporal patterns of biofilm productivity, whole system metabolic rates, and short term N uptake in Arctic and sub-Arctic streams. I also show the importance of light and temperature constraints during early spring and late autumn, which set the limit for the aquatic growing season and annual productivity pat-terns. I present a first comparison of combined drivers of lake and stream responses to nutrient addition, which points to a shared importance of N and phosphorus (P) rather than light or tem-perature in driving the magnitude of nutrient limitation across these systems. Ultimately, I pro-pose that across large ranges in habitat variables, widespread nutrient limitation of Arctic fresh-waters constrain other climate change effects on ecosystem functions. The results presented in this thesis will promote better predictions of climate change effects on Boreal to Arctic stream ecosystem functioning

CONDUCTIVITY IN RUNNING WATERS AS A METHOD OF IDENTIFYING ACID SULPHATE SOILS

Increasing attention is being given to acid sulphate soils wherever they occur. The problems that leaching sulphate soils gives with significant lowerings of pH and mobilization of heavy metals influence large spectra of our society from fisheries to agriculture to construction. Mapping these soils is consequently of great importance and the methods of doing this is very much lacking in function and precision. This study was therefore carried out to investigate whether conductivity in running water can be used as a simple instrument to identify acid sulphate soils in the catchment. 31 coastal streams in the county of Västerbotten were analyzed for different catchment properties such as occurence of marine sediments and basic water chemistry including conductivity and sulphate. Sulphate proved to be the dominant factor controling conductivity in most streams, constituting up to 90 % of the anions. The results also showed that the concentrations of sulphate correlated to 67 % with marine sediments in the catchment. Where conductivity values exceeded 90 µS/cm the influence of acid sulphate soils could be determined for certain. The major conclusion drawn from this study is that high conductivity values serves as a reliable indicator of leaching acid sulphate soils whereas lower values can not exclude them.flisi

Spatial and temporal patterns of stream nutrient limitation in an Arctic catchment

Arctic stream biofilm responses to ongoing climate-related changes in physical and chemical conditions have major implications for stream food webs and biogeochemical cycles. Yet, such effects have rarely been studied outside summer months or at sub-catchment scales in the Arctic. We used deployments of nutrient diffusing substrates (NDS) to assess the spatial (20 deployments) and seasonal patterns (10 deployments) and physical and chemical drivers of nutrient limitation within an Arctic stream catchment. Results show that nutrient limitation of autotrophic processes was common during summer, but that light inhibited biomass accrual under the ice in winter. Alongside single N, P and C responses, co-limitation dominated the overall pattern of limitation over time and across the catchment. However, the primary limiting nutrient to autotrophs changed from N to P in parts of the catchment with higher N concentrations. As Arctic studies are often conducted at individual sites during summer, these may miss shifts in the drivers of stream productivity that arise from variable nutrient, temperature, and light regimes. Our results caution against focusing on one single most important limiting nutrient, as we found that this can shift seasonally and over small spatial scales in this Arctic catchment

Experimental riparian forest gaps and increased sediment loads modify stream metabolic patterns and biofilm composition

Forest management operations greatly influence stream habitats. Canopy clearing and subsequent canopy development during succession, site preparation, and ditching alter the light environment, and increase sediment inputs and nutrient exports from upland and riparian soils to streams. These physicochemical changes affect aquatic biofilms and metabolic rates, and in this study, we tested their individual and combined effects. We used 12 artificial streamside channels, together with a field survey of nine streams in and around clear-cuts, to assess the effects of shading, substrate composition, and nutrient addition on biofilm biomass and composition, as well as metabolic rates. We found that biofilm biomass and gross primary production (GPP) were light limited in channels under 70% canopy shading. Nitrate additions at this shading level only marginally increased autotrophic biomass, while the rates of respiration increased 10-fold when carbon was added. Open (unshaded) channels had three times higher rates of GPP compared with channels with 70% shading, and autotrophic biomass was twice as high, largely caused by the colonization of filamentous green algae. These changes to biofilm biomass, composition, and GPP were caused by differences in light alone, as temperature was not affected by the shading treatment. Notably, higher rates of GPP led to no positive effect on net ecosystem production. Further, fine-grained substrates negatively affected GPP as compared with stone substrates in the experimental channels. In the surveyed streams, the negative effects of fine-grained substrates exceeded the positive influence of light on biofilm biomass. Altogether, our results highlight the need for riparian management that protects headwaters from unwanted stressors by focusing on preventing sediment erosion and carbon transport in clear-cuts, while providing variable shade conditions in second-growth forests.

Stream metabolism controls diel patterns and evasion of CO2 in Arctic streams

Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO2 evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO2 production and evasion from streams remains poorly understood. In this study, we measured O-2 and CO2 continuously in streams draining tundra-dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO2 evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO2 concentrations and evasion at multiple timescales. Photosynthesis caused CO2 concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low-turbulence streams. Diel patterns in CO2 concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO2 evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO2 evasion, suggesting that some stream reaches serve as net sources of CO2, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO2 evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes.Originally included in thesis in manuscript form.</p