Ecological Regime Shifts in Lake Kälksjön, Sweden, in Response to Abrupt Climate Change Around the 8.2 ka Cooling Event

Abstract

A detailed diatom record from Lake Kalksjon, west- central Sweden, reveals two periods of abrupt ecological change correlative with the 8.2 ka cool- ing event. Using a combination of abrupt step changes and piece-wise linear regressions, the diatom data were analyzed for change points over time, and two sudden and large events that are described as regime shifts were detected. During the first event at c. 8040 cal. y BP, a doubling in diatom biomass took place over 5-10 years. This increase in primary productivity can be connected to an erosion event in the catchment that resulted in an abrupt increase in nutrient supply to the lake. The second event was characterized by a sub- stantial shift within the planktonic diatom com- munity from taxa indicative of colder conditions to those indicating warm over 5-10 years at c. 7850 cal. y BP. This event was superimposed on a successive change from periphytic to planktonic diatom dominance over a 250-year period and a gradual diversification of the periphytic community that spanned c. 150 years. Rapid climate warming following the 8.2 ka event likely caused these changes and both regime shifts are examples of externally driven abrupt ecological change. This study demonstrates that it is possible to detect, quantify and test for regime shifts in paleoecologi- cal data, and it highlights the need for high sam- pling resolution and precise chronological control. High-resolution paleoecological reconstructions of ecological regime shifts in response to climate change can provide useful analogues of future changes in ecosystem structure and functioning.