Water quality following extensive beetle-induced tree mortality: Interplay of aromatic carbon loading, disinfection byproducts, and hydrologic drivers

Abstract

The recent bark beetle epidemic across western North America may impact water quality as a result of elevated organic carbon release and hydrologic shifts associated with extensive tree dieback. Analysis of quarterly municipal monitoring data from 2004 to 2014 with discretization of six water treatment facilities in the Rocky Mountains by extent of beetle impact revealed a significant increasing trend in total organic carbon (TOC) and total trihalomethane (TTHM) production within high (≳50% areal infestation) beetle-impacted watersheds while no or insignificant trends were found in watersheds with lower impact levels. Alarmingly, the TTHM concentration trend in the high impact sites exceeded regulatory maximum contaminant levels during the most recent two years of analysis (2013-14). To evaluate seasonal differences, explore the interplay of water quality and hydrologic processes, and eliminate variability associated with municipal reporting, these treatment facilities were targeted for more detailed surface water sampling and characterization. Surface water samples collected from high impact watersheds exhibited significantly higher TOC, aromatic signatures, and disinfection byproduct (DBP) formation potential than watersheds with lower infestation levels. Spectroscopic analyses of surface water samples indicated that these heightened DBP precursor levels are a function of both elevated TOC loading and increased aromatic character. This association was heightened during precipitation and runoff events in high impact sites, supporting the hypothesis that altered hydrologic flow paths resulting from tree mortality mobilize organic carbon and elevate DBP formation potential for several months after runoff ceases. The historical trends found here likely underestimate the full extent of TTHM shifts due to monitoring biases with the extended seasonal release of DBP precursors increasing the potential for human exposure. Collectively, our analysis suggests that while water quality impacts continue to rise nearly one decade after infestation, significant increases in TOC mobilization and DBP precursors are limited to watersheds that experience extensive tree mortality.

Analysis of Anthropogenic, Climatological, and Morphological Influences on Dissolved Organic Matter in Rocky Mountain Streams

“…Similarly, a qualitative comparison of the gray-phase areas affected by the MPB with predicted DOC concentrations in the region of interest does not reveal any strong pattern (see Appendix C- Figures A10 and A11). A stronger correlation between MPB-infested areas and DOM has been observed during months with significant precipitation and individual runoff events [25,27,38,90], but we note that these studies did not control for the additional potential predictors considered in our study. Moreover, there may exist an important lag between the time at which the needles fall and the time by which their decomposition alters DOC concentrations, largely due to their recalcitrant chemical composition, especially in high-altitude and low-temperature climates [91,92].…”

“…From the three different stages followed by a forest after an MPB outbreak, only the gray phase is a significant predictor in the LASSO regression. When beetle-killed trees transition to the gray phase, they lose their needles, increasing the input of degradable organic matter to the soil surface [38]. Consequently, the observed positive relationship is expected, but it is quite weak.…”

“…Additionally, because the set of potential predictors used here may not be comprehensive, as evidenced by time being a significant predictor, we hypothesize that important predictors not included in the LASSO regression are mainly associated with the abundance, diversity, spatial distribution, and dynamics of the microbiology in the soils of the Rocky Mountains. For example, soil microbiology has been found to respond to forest disturbances, and, especially for gradual forest changes, this response can act as a buffer for water quality changes, possibly explaining why the MPB infestations, which develop over multiple years, are some of the weakest significant predictors for DOC in the streams of the Rocky Mountains [38,103,104] (contrasted to fires, which occur over weeks). Additionally, the spatial heterogeneity of microbial community composition in the Rocky Mountains is highly complex [105] and has been shown to change over time [106].…”

“…The base-flow months used here are August, September, October, and November. Carbon concentrations from late base-flow months (December to March) are not included because more significant fluctuations in the concentrations have been observed during these late months, possibly because of larger snow-melt contributions [38]. Although an analysis of DOC loads during runoff events would be of interest, the instances when stream-flow and DOC are sampled rarely match in the historical records.…”

Ecosystem Resilience and Limitations Revealed by Soil Bacterial Community Dynamics in a Bark Beetle-Impacted Forest

“…Despite the impact these perturbed forests have on human health and well-being, we still do not fully understand why some beetle-killed forests elicit a strong hydrologic or biogeochemical response ( 2 , 5 – 8 ) while others do not ( 9 , 10 ). Tree mortality on this scale has the capacity to shift a forest from a carbon sink to a source ( 1 ) or adversely alter the quality of water we consume ( 2 , 11 ). Hence, we need to better understand the ecosystem response from the watershed scale down to the microscale to enable predictions of when and where detrimental impacts will occur.…”