Spatio-temporal perspectives on aquatic carbon dynamics and limnoecological change in climate-sensitive lakes

Northern lakes are displaying widespread ecological reorganizations in response to global change. These sensitive freshwater ecosystems are an integral component of subarctic and arctic landscapes and play a critical role in regional and global biogeochemical cycles. A myriad of direct and indirect climate drivers shape their structure and functioning, often matched by equally diverse ecosystem responses. To understand the underlying mechanism and their complex interactions, and to estimate future trajectories of environmental change, we must look for past analogies of climate-lake interaction recorded in lake sedimentary archives. The present, in turn, holds the key to understand the past. This work examines patters and drivers of limnoecological change in shallow subarctic lakes across the treeline ecotone in northern Finland. A diverse set of neolimnological and paleolimnological tools and approaches were employed to explore aquatic ecosystem responses to landscape variability and direct climate forcing through space and time. Particular emphasis was placed on aquatic organic carbon that regulates vital biogeochemical processes in lakes and couples them to the global carbon cycle. To address landscape-mediated climate impact on shallow subarctic lakes, catchment controls on pelagic (lake water) and benthic (sediment) carbon pools, nutrients and productivity, periphytic algal (diatom) communities, and carbon utilization of benthic macrofauna (Cladocera) were assessed in 31 lakes spanning the treeline. To investigate long-term ecosystem development and carbon dynamics under natural and anthropogenic climate variability, two downcore sediment sequences from shallow and oligotrophic treeline lakes, covering the postglacial period (~11 500 years) and the late neoglacial (~600 years), were examined for diverse biogeochemical and paleobiological indices. Across the treeline, lake water carbon pools were fundamentally shaped by wetland in uence, largely superimposing the e ects of the latitudinal vegetation transition. Concomitant e ects on aquatic biota were modulated by parallel allochthonous nutrient inputs and lake depth, indicating that the shading e ect of colored terrestrial organic carbon was largely diminished in the shallow basins and superseded by nutrient stimulation of primary production. Catchment and soil geomorphology and related hydrological processes were further identi ed as important controls over aquatic carbon pools, nutrient regimes, and lake water acidbase balance. Diatom community structure was relatively little in uenced by terrestrial carbon and nutrient uxes but responded sensitively to variations in lake water pH largely controlled by catchment geology, soils and vegetation cover. The late neoglacial paleolimnological record demonstrated a strong coupling between centennial temperature uctuations, the structure and productivity of aquatic communities, and carbon sequestration, likely mediated primarily through changes in the length of the ice cover period. Anthropogenic warming stimulated primary production in the lake, re ected also as increased sediment carbon accumulation. An analogy from a previous warm anomaly further Rantala M.V., 2017. Spatio-temporal perspectives on aquatic carbon dynamics and limnoecological change in climate-sensitive lakes. Unigra a. Helsinki. 44 pages and 6 gures.