Electronic Thesis and Dissertation Repository

Thesis Format



Master of Science



Collaborative Specialization

Global Health Systems in Africa


Creed, Irena F.


Global change scientists seek sentinels of change. On forested landscapes, first-order catchments serve as sentinels of global stressors and their effects on downstream surface waters. Here, I explored global stressors – including climate warming, hydrological intensification, and recovery from atmospheric acidic deposition – and their effects on nutrient exports in 22-year stream chemistry records from 41 forested first-order catchments in a network of North American long-term monitoring sites. First, I used multivariate autoregressive models to establish relationships between changes in global stressors and changes in catchment nutrient exports. Second, I analyzed the residuals of these relationships to determine if there was evidence of instability in the catchment nutrient exports. I found that changes in global stressors affected the nutrient exports of these catchments but that the global stressors having the largest impacts varied geographically, and that changes in these global stressors were leading to changes in the stability of these nutrient exports.

Summary for Lay Audience

Global change scientists seek early warning systems to explore the effects of global atmospheric changes on ecosystems. First-order catchments with small, intermittent or ephemeral streams may be excellent early warning systems, as their signals are unencumbered by the confounding influences of the catchments into which they drain. However, their uniqueness in time and space create challenges in developing a predictive understanding of their responses to global changes.

I explored the effects of climate warming, hydrological intensification, and recovery from atmospheric acid deposition on first-order catchment nutrient exports in the temperate forest biome of North America. I asked two questions: Are global changes modifying catchment nutrient exports? and Are global changes leading to an increase in instability of catchment nutrient export magnitude and composition. To answer these questions, I mined 22-year records from a network of long-term monitoring sites where sulfur, nitrogen, and phosphorus exports were generally declining. I modeled the relationships between global changes and catchment nutrient exports, and then analyzed the residuals of these relationships for early warnings of changes in the stability of catchment nutrient exports. I found that global changes modify catchment nutrient exports, but that their effects were geographically dependent, with climate warming effects being greatest on northern sites, hydrological intensification effects being greatest on eastern sites and effects of recovery from acidic deposition being greatest near coastal sites, and with some sites responding to the interactive effects of climate change and the recovery from atmospheric acidic deposition. I also found that global changes were creating higher risks of changes in the magnitude and composition of catchment nutrient exports at all sites, particularly in nitrogen and phosphorus exports.

Development of a predictive understanding of global change effects on ecosystems is difficult to be generalized. Continued access to data from the network of long-term monitoring sites will be essential to reveal if the instabilities are indeed early warning of shifts to an alternative stable state in catchment nutrient exports that could have fundamental consequences on the productivity and diversity of downstream waters.