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

Globala förändringars påverkan på primärproduktionen i nordliga sjöar

Algae are primary producers, a major component of the aquatic foodweb, and changes in primary production affect aquatic ecology in general. Global changes such as warming, recovery of acidification and changes in land-use have caused warming and browning of northern lakes. Warming is a direct effect of increasing air temperatures, whereas browning is mainly caused by increasing amounts of terrestrially derived colored dissolved organic carbon (DOC). Altogether, such global changes impact important environmental drivers for aquatic gross primary production (GPP). Increased temperatures and nutrient supplementation by DOC at low concentrations enhance GPP, but the browning by DOC at high concentrations inhibits GPP by light reduction, resulting in contrasting controls of global changes on primary production in northern lakes. Primary producers grow in two distinct habitats; free-floating algae (phytoplankton) and stationary periphytic (attached) algae that are restricted to use the light that reaches them. Periphyton includes algae growing on submerged surfaces ranging from nutrient-poor rocks to nutrient-rich sediments (here: benthic algae), and both often exceed pelagic GPP but are overlooked and often simply excluded from algal biomass estimates. In this thesis, I investigate how global change influences key environmental drivers of GPP, and how those changes impact GPP in the benthic and pelagic habitat, and the sum and partitioning of GPP between these habitats (the autrotrophic structuring). I do this by interpreting a dataset with GPP measurements in several lakes over the Swedish Arctic, subarctic and boreal landscape that representa wide range of DOC concentrations. I also assess to what extent temperature and DOC impact periphytic algae growth on plastic strips in an experimental study where DOC and temperature are manipulated in 20 ponds. Besides assessing the direct impacts of changes in nutrients and light climate associated with changes in DOC, I assess indirect impacts of global changes on primary production, e.g., through intensified warming, CO2 supersaturation, changes in pH, and the role of landscape processes and properties. Results confirm that DOC is dominant in structuring GPP in northern lakes by light inhibition, nutrient supplementation, indirect warming of surface waters, and additionally by CO2 fertilization. In addition, warming can enhance growth rates, but thermal compensation can also lead to reduced algae growth. Moreover, periphytic GPP of algae growing on both soft nutrient-rich sediments and nutrient-poor plastic strips GPP was generally much higher than pelagic GPP, and should thus not be excluded in studies assessing global change impacts on GPP. DOC affects the total GPP, as well as the autotrophic structuring in northern lakes, and likely also higher trophic levels productivity and community composition

Globala förändringars påverkan på primärproduktionen i nordliga sjöar

Algae are primary producers, a major component of the aquatic foodweb, and changes in primary production affect aquatic ecology in general. Global changes such as warming, recovery of acidification and changes in land-use have caused warming and browning of northern lakes. Warming is a direct effect of increasing air temperatures, whereas browning is mainly caused by increasing amounts of terrestrially derived colored dissolved organic carbon (DOC). Altogether, such global changes impact important environmental drivers for aquatic gross primary production (GPP). Increased temperatures and nutrient supplementation by DOC at low concentrations enhance GPP, but the browning by DOC at high concentrations inhibits GPP by light reduction, resulting in contrasting controls of global changes on primary production in northern lakes. Primary producers grow in two distinct habitats; free-floating algae (phytoplankton) and stationary periphytic (attached) algae that are restricted to use the light that reaches them. Periphyton includes algae growing on submerged surfaces ranging from nutrient-poor rocks to nutrient-rich sediments (here: benthic algae), and both often exceed pelagic GPP but are overlooked and often simply excluded from algal biomass estimates. In this thesis, I investigate how global change influences key environmental drivers of GPP, and how those changes impact GPP in the benthic and pelagic habitat, and the sum and partitioning of GPP between these habitats (the autrotrophic structuring). I do this by interpreting a dataset with GPP measurements in several lakes over the Swedish Arctic, subarctic and boreal landscape that representa wide range of DOC concentrations. I also assess to what extent temperature and DOC impact periphytic algae growth on plastic strips in an experimental study where DOC and temperature are manipulated in 20 ponds. Besides assessing the direct impacts of changes in nutrients and light climate associated with changes in DOC, I assess indirect impacts of global changes on primary production, e.g., through intensified warming, CO2 supersaturation, changes in pH, and the role of landscape processes and properties. Results confirm that DOC is dominant in structuring GPP in northern lakes by light inhibition, nutrient supplementation, indirect warming of surface waters, and additionally by CO2 fertilization. In addition, warming can enhance growth rates, but thermal compensation can also lead to reduced algae growth. Moreover, periphytic GPP of algae growing on both soft nutrient-rich sediments and nutrient-poor plastic strips GPP was generally much higher than pelagic GPP, and should thus not be excluded in studies assessing global change impacts on GPP. DOC affects the total GPP, as well as the autotrophic structuring in northern lakes, and likely also higher trophic levels productivity and community composition

Experimental warming and browning influence autumnal pelagic and benthic invertebrate biomass and community structure

Globally, lakes are warming and browning with ongoing climate change. These changes significantly impact a lake's biogeochemical properties and all organisms, including invertebrate consumers. The effects of these changes are essential to understand, especially during critical periods after and before the growing season, that is, autumn and spring, which can determine the composition of the invertebrate consumer community. In this study, we used a large-scale experimental pond system to test the combined effect of warming (+3°C) and increased input of terrestrial and coloured dissolved organic carbon (gradient of 1.6–8.8 mg/L in the ambient and 1.6–9.3 mg/L in the warm)—which causes browning—on zooplankton and benthic macroinvertebrate biomass and composition during the autumn and the following spring. Total zooplankton biomass decreased with warming and increased with browning, while total zoobenthos did not respond to either treatment. Warming and browning throughout the autumn had no overall interactive effects on zooplankton or zoobenthos. Autumnal warming decreased total pelagic consumer biomass, caused by a decrease in both Rotifera and Copepoda. In contrast, there was no effect on overall benthic consumer biomass, with only Asellus sp. biomass showing a negative response to warming. An autumnal increase in dissolved organic carbon led to increased total pelagic consumer biomass, which was related to increases in Daphnia sp. biomass but did not affect zoobenthos biomass. While we expected zooplankton and zoobenthos biomass to follow responses in primary and bacterial production to treatments, we did not find any relationship between consumer groups and these estimates of resource production. Our results suggest that consumer responses to warming and browning during autumn may lead to less overarching general changes in consumer biomass, and responses are mostly taxon-specific. This study gives novel insights into the effects of warming and browning on consumer biomass during autumn and spring and increases the understanding of the effects of climate change on invertebrate community biomass in the different habitats

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