Fire activity has increased in recent decades due to historical landscape management and climate change, with consequences for lake and reservoir (hereafter lakes) ecology and biogeochemistry. Fires can burn thousands of hectares adjacent to or around lake ecosystems, but the larger-scale effects of fires on lake ecosystem properties have not been quantified. Particularly in the semi-arid western USA, lakes represent major drinking water sources and maintaining the quality of these waterbodies is an important long-term concern. In this study, we investigated broad-scale effects of fires on North American lakes using the Monitoring Trends in Burn Severity (MTBS) dataset, which includes remotely sensed fires across the USA over 1984-2015, and LAGOS (LAke multi-scaled GeOSpatial and temporal database), which includes various water quality metrics for lakes > 4 ha in the conterminous USA. We asked the following: 1) what are the spatial and temporal patterns of lake exposure to fire during 1984-2015 and 2) what are the effects of fire on lake ecosystem properties? We used water clarity as an indicator of lake ecosystem properties because it is widely collected and integrates effects of lake productivity and nutrient concentrations.
Results/Conclusions
During 1984-2015, 8702 lake watersheds in the conterminous USA experienced at least one fire (6.3% of all lakes > 4 ha). Exposure to fire (number of watershed fires and area burned) was concentrated in 1) Florida, 2) the southern Great Plains and 3) California, the inland Northwest and Northern Rockies. Exposure has increased significantly with time (r = 0.70, p < 0.001), particularly since 2005, and total area burned was dominated by wildfire. Water clarity declined in the subset of lakes for which in-lake data were available when watershed area burned exceeded 30% (r = -0.47, p = 0.040, n = 14 lakes). These results suggest that the strength and direction of fire-lake ecosystem relationships may vary according to fire characteristics such as percent watershed burned. Ongoing steps include assessing effects of fire on additional lake ecosystem properties and across more diverse fire regimes as more lake data are assembled, and developing a conceptual framework for effects of fire on the physical, chemical and biological properties of lake ecosystems.