From clear lakes to murky waters – tracing the functional response of high‐latitude lake communities to concurrent ‘greening’ and ‘browning’

Abstract

Climate change and the intensification of land use practices are causing widespread eutrophication of subarctic lakes. The implications of this rapid change for lake ecosystem function remain poorly understood. To assess how freshwater communities respond to such profound changes in their habitat and resource availability, we conducted a space‐for‐time analysis of food‐web structure in 30 lakes situated across a temperature‐productivity gradient equivalent to the predicted future climate of subarctic Europe (temperature +3°C, precipitation +30% and nutrient +45 μg L−1 total phosphorus). Along this gradient, we observed an increase in the assimilation of pelagic‐derived carbon from 25 to 75% throughout primary, secondary and tertiary consumers. This shift was overwhelmingly driven by the consumption of pelagic detritus by benthic primary consumers and was not accompanied by increased pelagic foraging by higher trophic level consumers. Our data also revealed a convergence of the carbon isotope ratios of pelagic and benthic food web endmembers in the warmest, most productive lakes indicating that the incorporation of terrestrial derived carbon into aquatic food webs increases as land use intensifies. These results, reflecting changes along a gradient characteristic of the predicted future environment throughout the subarctic, indicate that climate and land use driven eutrophication and browning are radically altering the function and fuelling of aquatic food webs in this biome.

Hotspots of biotic compositional change in lakes along vast latitudinal transects in northern Canada

“…These physical variables exert a strong influence on oxygen availability and, therefore, on the available living space for benthos. Nevertheless, in a recent study, Engels et al (2019) linked to changes in terrestrial inputs will affect aquatic biodiversity and ecosystem functioning at various levels (Finstad et al, 2016;Hayden et al, 2019). With warming climates, these effects may include alterations in food webs and biogeochemical cycles, with loss of phytoplankton diversity (Urrutia-Cordero et al, 2017), shifts to increased mixotrophy in eukaryotic phytoplankton (Wilken et al, 2018) and decreases in fish biomass production (Tanentzap et al, 2014;van Dorst et al, 2019).…”

“…Increasing temperatures and dissolved organic matter (DOM) concentrations (and dissolved organic carbon [DOC], one of its main components) in arctic‐boreal lakes due to anthropogenic climate warming have been linked to fundamental ecosystem changes (Post et al, ). These limnological changes lead to marked biological shifts, including altered community compositions and food web interactions (e.g., Creed et al, ; Finstad et al, ; Hayden et al, ). The examination of lakes along transects that cross biomes allows not only the detection of broad‐scale trends in limnological and biotic characteristics through vast environmental gradients but also enables the detection of displacements in the position of transition zones.…”

“…(Post et al, 2009). These limnological changes lead to marked biological shifts, including altered community compositions and food web interactions (e.g., Creed et al, 2018;Finstad et al, 2016;Hayden et al, 2019). The examination of lakes along transects that cross biomes allows not only the detection of broad-scale trends in limnological and biotic characteristics through vast environmental gradients but also enables the detection of displacements in the position of transition zones.…”

Abruptly and irreversibly changing Arctic freshwaters urgently require standardized monitoring

“…With continued Arctic amplification, which is the accelerated warming of Arctic regions due to factors such as shrinking sea ice cover (Serreze, Barrett, Stroeve, Kindig, & Holland, 2009), there is heightened urgency to develop a standardized means of monitoring rapid ecological change in these freshwater environments. In high-latitude freshwater systems, recent environmental changes are mainly driven by anthropogenic activities, particularly climate warming (Heino, Virkkala, & Toivonen, 2009), but also long-range transport of pollutants (Reid et al, 2019), land-use alterations and eutrophication and brownification, which have recently intensified in the Arctic (Hayden et al, 2019). While circum-Arctic regions have some of the largest remaining undammed river basins in the world (Grill et al, 2019), they comprise fragile riverine and lacustrine ecosystems with unique flora and fauna maintained through the interplay of ecological connectivity and environmental conditions (Kärnä et al, 2015).…”

“…Brownification, caused by increasing concentrations of dissolved organic matter, is another major environmental change recently predicted to occur in Arctic freshwaters (Hayden et al, 2019). Like nutrient enrichment, it is partly a consequence of the joint effects of landscape alterations and climate warming in high latitudes.…”