Interactive Effects of Climate Change, Wetlands, and Dissolved Organic Matter on UV Damage to Aquatic Foodwebs

Funded by Environmental Protection Agency STAR program

Principal Investigators: Scott D. Bridgham1, Gary L. Lamberti2, Carol A. Johnston3, David M. Lodge2, Patricia A. Maurice2, and Boris A. Shmagin3

Postdoctoral Associates:  Paul C. Frost4 and Kangsheng Wu3

Technicians:  Christy Cherrier1

Graduate Students:  James H. Larson2, Kathryn Docherty2, and Z. Zheng3

Institution:1University of Oregon, 2University of Notre Dame, 3South Dakota State University, 4Trent University

Project Period: July 1, 2002 – June 30, 2006

Project Overview

Understanding the factors controlling ultraviolet radiation (UVR) flux into aquatic ecosystems is critical given its deleterious effects on many ecological processes. UVR is strongly attenuated in aquatic ecosystems by dissolved organic matter (DOM), and thus we hypothesized that landscape controls over DOM would ultimately control UVR exposure to, and subsequent effects on, stream biota.  Previous research suggests that the quantity and quality of DOM at the landscape scale is controlled primarily by:  (i) vegetation community and soil type, with wetland area being of particular significance, (ii) flow paths through soil, (iii) the discharge regimes of rivers and streams, and (iv) within-stream DOM degradation and production mechanisms.  Climate change will likely affect each of these DOM control factors in complex ways.  While a significant amount of previous research has focused on the separate roles of DOM and UVR in aquatic ecosystems, much less is known about the interactive effects of climate change, landscape, DOM, UVR, and aquatic food webs.

The overarching objective of this project was to enhance the understanding of how UVR affects foodweb structure in streams and rivers through its complex interactions with DOM, landscape characteristics, and climate in a northern forested watershed (Fig. 1).

More specifically, we had five main objectives:

1. Determine the extent to which UVR exposure in streams is controlled by DOM concentration and chemistry.
2. Determine the response of stream foodwebs to the interactions among UVR intensity and DOM concentration and type.
3. Determine landscape controls over DOM concentration and chemistry (and, hence, UVR).
4. Determine how in-stream processing of DOM through biodegradation and photodegradation varies spatially and controls over that spatial variation.
5. Determine how various climate change scenarios will affect discharge and, thus, DOM concentration and UVR exposure.

Overall Experiment Design We tested our central hypothesis and the linkages at a variety of spatial scales ranging from artificial streams and laboratory experiments to an entire watershed, as was appropriate to address the project’s five main objectives. For our landscape analyses that related to Objectives 1 and 3, we sampled 60 catchments within the larger 3,460 km2 Ontonagon River watershed in northern Michigan and Wisconsin (Fig. 2) for DOM concentration and chemistry and related water chemistry variables in September, 2003. Based upon this initial sampling, we sampled 35 catchments at 11 additional times in all seasons during the next two years. We also measured discharge at each location at most time points. Additionally, we sampled soil throughout the basin to obtain their carbon and nitrogen content. We assembled a large GIS database of land cover and use, stream characteristics, wetland characteristics, soil type, and surficial geology. We then used this rich data in a variety of multivariate analyses to describ e landscape controls over DOM concentration and chemistry to fulfill Objective 3 (Frost et al. 2006a, Larson et al. 2007, Johnston et al., in press, Bridgham et al., in preparation). In a subset of these sites, we determined UVR penetration with depth in the water column and related it to DOM concentration and chemistry and other water quality variables to fulfill Objective 1 (Frost et al. 2005, 2006b).

To fulfill Objective 2, we constructed a large artificial stream facility at the University of Notre Dame Environmental Research Center, at the southern edge of the Ontonagon basin, to examine experimentally how DOM and UVR interact in controlling foodweb structure in streams. We added DOM with natural UVR exposure and with UV-B removed and examined the effects on stream periphyton and algal communities (Frost et al. 2007). We also used this facility for several other experiments to examine the effects of DOM concentration and chemistry on stream biota (Larson 2006, Frost et al., in preparation).

We performed a variety of experiments in the artificial streams, in the lab, and in situ to examine the effects of photodegradation and biodegradation on DOM concentration and chemistry to fulfill Objective 4 (Young et al. 2004, 2005, Docherty et al. 2006, Larson 2006, Cherrier et al., in preparation).

To fulfill Objective 5, we modeled discharge in the larger branches of the Ontonagon River and examined climatic and land cover controls over discharge in the basin (Wu et al. 2006, Wu and Johnston 2007, in press). We also related DOM to discharge in the 35 repeatedly sampled catchments (Bridgham et al., in preparation). However, as most of these catchments are not continuously gauged and thus we could not adequately parameterize the hydrologic model for them, we were somewhat limited in our ability to relate climate-driven changes in hydrology to DOM concentration (and hence UVR exposure) in smaller streams. Furthermore, to put the Ontonagon basin within a larger geographic context, long-term stream flow records (1956-1988) from 32 U.S. Geological Survey gauging stations within the Great Lakes Basin were analyzed using multivariate statistical techniques (Johnston and Shmagin, in preparation).

See curriculum vitae button on home page for publications resulting from this research.