This project assessed whether stream biogeochemistry can indicate the ecological state of the contributing area, assess ecosystem resilience, and/or provide early warning of regime shifts caused by fire and permafrost thaw. The Department of Defense (DoD) maintains extensive lands in the boreal forest that are managed to provide both suitable training grounds and ecosystem services including carbon storage and wildlife habitat. These lands are vulnerable to thawing permafrost and changing fire regime, which can replace the previously dominant black spruce forests with deciduous forests that do not insulate permafrost. Stream chemistry integrates signals originating from throughout the contributing area and could therefore assess ecosystem state at larger scales than ground-based measures. 

High-frequency (15-min) observations of stream chemistry were collected by instream sensors in a space-for-time design that included five catchments arrayed across orthogonal contrasts in spatial extent of permafrost and fire history within Interior Alaska. Statistical approaches were applied to deconvolute resulting temporal patterns at diel, storm, and seasonal time scales. A synoptic survey of stream chemistry across Interior Alaska provided context for the high-frequency data. Finally, analysis of multi-decadal datasets from experimentally harvested catchments in the temperate zone assessed the utility of long-term records of stream chemistry in detecting ecological regime shifts.

Spatial and temporal patterns contrasted among catchments varying in fire history and spatial extent of permafrost. Greatest temporal variation at all time scales occurred in an unburned, high-permafrost catchment, and therefore flashiness in discharge and stream chemistry provides an indicator of permafrost integrity. Storms were the dominant source of variation in all catchments, and larger burned areas resulted in stronger source-limitation of nitrate export from catchments during storms. Catchments draining less extensive permafrost delivered nitrate to streams at a constant rate through each storm, whereas nitrate was diluted during storms in spatially extensive permafrost. Daily mean nitrate concentration was negatively correlated with temperature in burned catchments, which could result from nutrient uptake by regrowing vegetation. A broad-scale survey demonstrated that extensive fires also reduced export of dissolved organic matter from catchments to streams. Finally, leading indicators of regime change applied to multi-decadal observations of stream chemistry in experimentally harvested and reference catchments in the temperate zone demonstrated the utility of stream monitoring in detecting regime changes. 

Monitoring stream chemistry on boreal lands managed by the DoD could aid in management by identifying catchments vulnerable to or currently undergoing loss of permafrost and carbon stores. High-frequency monitoring to quantify temporal variation and concentration-discharge relationships during storms could identify catchments subject to permafrost thaw and training or construction activities could be mitigated or redirected to less vulnerable locations. Decadal-scale change in storm dynamics could signal loss of resilience and lands within such catchments might be then used less intensively. Similarly, lower-frequency (e.g., monthly) monitoring over multiple decades could provide early warnings of regime change when temporal variance exceeds a pre-disturbance baseline. Finally, surveys of dissolved organic matter concentration conducted every several years could indicate recovery of soil organic matter pools, a key contributor to resilience of boreal ecosystems, and distinguish catchments that are unlikely returning to a spruce-dominated state that insulates permafrost.