Analysis of Anthropogenic, Climatological, and Morphological Influences on Dissolved Organic Matter in Rocky Mountain Streams
In recent decades, the Rocky Mountains (RM) have undergone significant changes associated with anthropogenic activities and natural disturbances. These changes have the potential to alter primary productivity and biomass carbon storage. In particular, dissolved organic carbon (DOC) in RM streams can affect heterotrophic processes, act as a source for the nutrient cycle, absorb sunlight radiation, alter metal transport, and can promote the production of carcinogenic byproducts during water
... ent. Recent studies have focused on the relationship between bark beetle infestations and stream organic matter but have reached conflicting conclusions. Consequently, here we compile and process multiple datasets representing features of the RM for the period 1983-2012 with the purpose of assessing their relative influence on stream DOC concentrations using spatial statistical modeling. Features representing climate, land cover, forest disturbances, topography, soil types, and anthropogenic activities are included. We focus on DOC during base-flow conditions in RM streams because base-flow concentrations are more representative of the longer-term (annual to decadal) impacts and are less dependent on episodic, short-term storm and runoff/erosion events. To predict DOC throughout the network, we use a stream network model in a 56,550 km 2 area to address the intrinsic connectivity and hydrologic directionality of the stream network. Natural forest disturbances are positively correlated with increased DOC concentrations; however, the effect of urbanization is far greater. Similarly, higher maximum temperatures, which can be exacerbated by climate change, are also associated with elevated DOC concentrations. Overall, DOC concentrations present an increasing trend over time in the RM region. Columbia [6, 12] . Potential impacts associated with MPB disturbances include modifications of the hydrologic processes and alterations of the biogeochemical cycle    . For example, water storage and supply can be affected by changes in snow accumulation and melt derived from variations in the surface energy fluxes  , and carbon and nitrogen soil concentrations can increase in areas impacted by the MPB    . Although evidence exists that surface water contributes as a regulator of the carbon cycle and that the temporal change of these contributions is associated with a changing environment [20, 21] , little research has been published regarding the effects of MPB infestations on the carbon concentrations of RM streams. Understanding how MPB infestations affect carbon concentrations in streams is relevant because in addition to being a source for the nutrient cycle, dissolved organic matter (DOM) plays a central role in other surface water quality processes  . First, it absorbs sunlight radiation, influencing biochemical processes that depend on ultraviolet radiation and temperature  . Second, it can increase the solubility of metals by acting as a ligand, which has implications for the restoration of polluted ecosystems and the mitigation of the impact of metal pollutants  . Third, high concentrations of DOM during water treatment represent a risk because DOM can react during disinfection processes to form carcinogenic byproducts  . DOM in soils and water originates from the degradation of organic matter from plants and microorganisms, and therefore, processes altering forest and vegetation life cycles may have an indirect effect on surface water quality  . At the hillslope scale, increases in DOM and nitrogen have been observed in MPB-impacted areas [17, 19] . Mikkelson et al. observed increased concentrations of water disinfection by-products associated with DOM in water treatment facilities where the contributing watershed had suffered MPB disturbances  . Increased particulate and metal transport have also been observed in impacted watersheds  . However, the biogeochemical dynamics of the soil and the streams of the RM cannot be directly attributed to MPB disturbances without also accounting for other factors that contribute to a dynamic environment, such as climate change (e.g., temporal trends in precipitation and temperature) and anthropogenic disturbances (e.g., land cover changes associated with urbanization). Examples include MPB disturbances and forest fires: As MPB outbreaks have increased, fire activity has also increased in the western U.S., suggesting a possible link. Nonetheless, recent research suggests that increased fire activity does not depend on MPB infestations; instead, both are consequences of climate change and other factors    . Consequently, although a relationship has been observed between MPB infestations and DOM alterations at the hillslope and watershed scales, very little is understood about the causal relationship between MPB infestations and these environmental alterations because they are both highly influenced by multiple factors, including climate change and anthropogenic disturbances. Further evidence of the difference in impact between climate change and local factors on DOM is revealed by conflicting reports at different locations and scales. Trahan et al. reported reduced DOM and nutrients in soils with high tree mortality during the first 4 years after MPB disturbance  . Contrasting results were obtained by Norton et al. in an analog study, where nutrient concentration increases were observed  . Furthermore, Beudert et al. reported no long-term effects in a 28-year period study on water quality associated with bark beetle infestations in forests of southeastern Germany  . Anthropogenic disturbances and other processes associated with climate change were not accounted for in these three studies. Furthermore, the spatial domains in these studies corresponded to one or two small watersheds with areas of less than 40 km 2 . DOM change in streams is potentially one of the most practically relevant impacts of bark beetle and forest disturbances. Thus, we present an unprecedented large-scale analysis in space and time of the effects of beetle infestations, along with relevant climatological, forest disturbance, anthropogenic, and morphological processes on DOM in the RM streams. Our 56,550 km 2 study area includes highly heterogeneous watersheds with respect to scale, attributes, and hydrological processes during a 30-year period. We account for these heterogeneous features and other potentially contributing factors associated with other forest disturbances, anthropogenic activities, and the morphology of the Water 2018, 10, 534 3 of 46 region by selecting and processing extensive spatial-temporal datasets and products with appropriate attributes for the scale and resolution of the analysis. In short, we employ a statistical approach to model the relationship between DOM and many features, and this model incorporates the physical connectivity of the stream network in the region of interest for analysis and prediction. We then interpret the statistical significance and relative importance of each anthropogenic, climatological, and morphological feature for predicting DOM in the RM streams and investigate the possible existence of a temporal trend in DOM concentrations. The remainder of this article is organized as follows: We describe the region of interest, the compilation and preparation of DOM stream data, and the selection and processing of the datasets and products representing the predictive features in Section 2. Section 3 presents the necessary considerations for a proper analysis of the data's correlated features and spatial dependence in the network and introduces the associated statistical models appropriate for these considerations. Finally, the results and analysis regarding importance of the different features for DOM in the RM streams and the prediction of DOM in space and time are presented in Section 4. We conclude in Section 5. Materials and Methods In this section, we describe the region of interest and the compilation of the observed DOM dataset and the different potential predictors representing anthropogenic, climatological, and morphological features and processes in the RM. Region of Interest The region of interest consists of the section of the Rocky Mountains in the U.S. that has been most impacted by forest disturbances and anthropogenic development, as shown in Figure 1 . More than 6500 km 2 in the region have been affected by bark beetle outbreaks (~12% of the region of interest), wildfires have affected more than 1750 km 2 in the last decades (~3% of the region of interest), and approximately 1500 km 2 have been urbanized in the 1983-2012 period (~2.7% of the region of interest) [1,    . Figure 1 also presents the stream network, consisting of sixteen separate basins and covering an area of 56,550 km 2 . The network was developed by the National Stream Internet Project by reconditioning the NHD-Plus Version 2 (Rocky Mountain Research Station, Fort Collins, CO, USA) hydrography, and it has a scale of 1:100,000  . We corrected some topology issues in the network such as isolated watersheds. The stream network consists of 17,166 stream segments with a total stream length of 3,598,413 km. Specific features of the region are described in detail in Sections 2.2 and 2.3. Dissolved Organic Matter Data A statistical analysis on the strength of the dependence of DOM on anthropogenic, climatological, and morphological features of the RM involves accounting for possible correlation among the predictors, expected temporal changes, and potential spatial dependence in the response associated with a stream network setting. Therefore, a comprehensive database representing the response and potential predictors in space and time is needed. Experimentally, DOM is measured as dissolved organic carbon (DOC)  . Therefore, we first compile DOC data for the study area from the USGS Water-Quality Data for the Nation because it provides a considerable spatial and temporal coverage and uses certified sampling and processing standards  . The data complied corresponds to 12,407 DOC samples in streams for the study area during the period 1983-2012.