THE RESPONSE OF DETRITAL AND AUTOTROPHIC RESOURCES TO LONG-TERM NUTRIENT ENRICHMENT IN A DETRITUS-BASED HEADWATER STREAM by JENNIFER LYNN GREENWOOD (Under the Direction of Amy D. Rosemond) ABSTRACT Enrichment of aquatic ecosystems with nitrogen and phosphorus is one of the most significant anthropogenic impacts to surface waters worldwide. Relatively little is known about nutrient effects on detritus-based stream systems relative to autotroph-based systems. This study examined the effects of a long-term nitrogen and phosphorus enrichment on autotrophic and heterotrophic portions of the resource base of a forested detritus-based headwater stream at the Coweeta Hydrologic Laboratory in the southern Appalachian mountains. I assessed the affects of 2 years of nutrient enrichment on the detrital and autotrophic resources: leaf litter, periphyton and bryophytes. Nutrient enrichment strongly affected processing rates of allochthonous leaf inputs. Leaf breakdown rates, microbial activity on leaf material, the nitrogen content of leaf litter, and invertebrate biomass associated with leaf litter, all increased with nutrient enrichment. Increased processing rates also accelerated the flux of nitrogen to invertebrate biomass from leaf litter standing crop. Overall, the effect of enrichment was slightly stronger on a lower quality (lower nitrogen content) leaf, rhododendron, relative to a higher quality leaf type, red maple, suggesting nutrient enrichment may not affect all detrital resources equally. The response of autotrophs to nutrient enrichment was less dramatic, primarily due to strong light limitation. Algal biomass measured as chlorophyll a and algal growth rates increased with nutrient enrichment, with strongest effects in the early spring, when light levels reaching the stream were highest. The diatom-dominated algal species assemblages were not altered by nutrient enrichment and were more related to seasonal effects. Bryophyte biomass also did not change with nutrient enrichment, potentially due to light limitation. Biomass of algal epiphytes on bryophytes, measured as biovolume, showed variable response to nutrient enrichment, and algal community patterns were more affected by substrate type (moss, liverwort or bedrock) than nutrient or light availability. Overall, results from this study show that nutrient enrichment had strong effects on primarily the detrital resources in a headwater stream, with subtle effects on the autotrophic resources. Thus, algal-based measures of nutrient impacts currently used for running waters may be inappropriate for detritus-based stream ecosystems. INDEX WORDS: Coweeta, nutrient enrichment, headwater stream, detritus, leaf breakdown, invertebrate, periphyton, aquatic bryophyte, algal epiphyte THE RESPONSE OF DETRITAL AND AUTOTROPHIC RESOURCES TO LONG-TERM NUTRIENT ENRICHMENT IN A DETRITUS-BASED HEADWATER STREAM by JENNIFER LYNN GREENWOOD B.A. Boston University, 1992 M.S. Bowling Green State University, 1998 A Dissertation Submitted to the Graduate Faculty of the University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2004 © 2004 Jennifer Lynn Greenwood All Rights Reserved THE RESPONSE OF DETRITUS AND AUTOTROPHIC RESOURCES TO LONG-TERM NUTRIENT ENRICHMENT IN A DETRITUS-BASED HEADWATER STREAM by JENNIFER LYNN GREENWOOD Approved: Major Professor: Amy D. Rosemond Committee: J. Bruce Wallace Judy L. Meyer Mary C. Freeman Darold P. Batzer Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia December 2004 iv ACKNOWLEDGEMENTS It has been a long haul and I couldn’t have done it without being surrounded by great people. First and foremost, my advisor, Amy Rosemond, who has shown me the dark side of stream ecology and has put up with me and my autotrophic bias for all of these years. Thanks to my committee Bruce Wallace, Judy Meyer, Darold Batzer and Mary Freeman for all of their time, help and support throughout this long process. Holly Weyers was a great help to me in the field, as a logistical coordinator and keeping me sane. Wyatt Cross was a great academic brother and source of “good times.” Other SNAX/Coweeta folds have provided help or data in one way or another and I am grateful to all of them: Sue Eggert, Jon Benstead, Roger Hilten, Nathan Taylor, Keller Suberkropp, Stephen Bland, Vlad Gulis, Pat Mulholland, Bob Hall and Brian Kloeppel. Thanks to Marcelo Ardon for chatting about leaf packs and statistics with me. A big bunch of thanks for everyone who helped me in the lab and field: Ted Siler, Megan Hagler, Paula Marcinek, Kristin Merritt, Heidi Wilcox, Eric Lunz, Candi Stockdale, Sally Entrekin, Catherine Evans, Andrew Paolucci. Big thanks to Becky Bixby for being a fellow algae nerd and reminding me of why I got in this business in the first place. All of the benthobabes past and present from 199B have been a great source of moral support: Cathy Gibson, Sue Herbert, Diane Sanzone, Emma Rosi-Marshall, Gretchen Loeffler, Elizabeth Sudduth, Monica Palta, and our token male, Andrew Sutherland. A shout out to fellow Rosemond labbers Laura England and Chris Anderson and other Ecology compatriots, Allison Roy, John Kominoski and Nanette Nelson. Cohorts from the Wallace Lab and Entomology have been an invaluable source of support, advice and distratction: Stephanie Haggerty, Erica Chiao, Elizabeth Reese, Erika v Krazter, Brent Johnson, Houston Joust and Damon Ely. Thanks to the staff in Ecology who have provided so much support: Janice Sand, Tom Maddox et al., Martha Roach, Winona Fields, Rich Kosvanec, Misha Boyd, Thelma Richardson, John Tougas, Jason Ritchie, Scott Crabtree and Terri Morgan. For the distraction, I thank everyone from the gone but not forgotten Ratskeller: Mike, Grampaw, Mitzi, Wages, Tracy, Ben, Heather, Carmen, Brian, Eric, Brian, Big John, Nathan, Corey, and Nickolas. Finally, I could not have gotten through this ordeal of finishing a Ph.D. if it weren’t for Mike Busbee who put up with me during my craziest and crankiest moments from long days in the field to prelims to the final push. Thank you, Sweetie. vi TABLE OF CONTENTS Page ACKNOWLEDGEMENTS........................................................................................................... iv CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW .....................................................1 2 NUTRIENTS STIMULATE BREAKDOWN RATES AND DETRITIVORE BIOMASS: BOTTOM-UP EFFECTS VIA HETEROTROPHIC PATHWAYS...13 3 PERIPHYTON RESPONSE TO LONG-TERM NUTRIENT ENRICHMENT IN A SHADED HEADWATER STREAM .....................................................................65 4 VARIATION IN PERIPHYTIC ALGAL ASSEMBLAGES ON BRYOPHYTE AND BEDROCK SUBSTRATA: POTENTIAL FOR DIFFERENTIAL RESPONSE TO ENVIRONMENTAL VARIATION .....................................................................105 5 CONCLUSIONS........................................................................................................161 CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW Nutrient loading of nitrogen and phosphorus is an important problem in aquatic ecosystems (Carpenter et al. 1998). The demands of modern civilization and an exponentially increasing human population are causing perturbations of biogeochemical cycles and inputs of nutrients to aquatic systems at unprecedented rates. For example, the amount of nitrogen fixed by industry is at least equal to natural sources (Vitousek et al. 1997; Vitousek et al. 1997), and human activity explains most of the global variation in nitrate export in world rivers (Caraco and Cole 1999). Also, mining of phosphorus has enhanced loading to aquatic systems (Caraco 1993), and increased phosphorus storage in terrestrial habitats provides a continuing future threat of phosphorus enrichment due to runoff (Bennett et al. 2001). Typical concerns with nutrient enrichment generally deal with the autotrophic response since the negative effects of enrichment are usually most obvious in the form of algal blooms or excessive plant growth (Carpenter et al. 1998). This excessive growth of autotrophs can cause problems such as interference with the use of water for recreation, irrigation or drinking water, decreased oxygen availability in the water column potentially resulting in fish kills (Carpenter et al. 1998), and toxic algal blooms presenting health hazards to the public or livestock (Paerl 1997). Thus, much work on how aquatic systems respond to nutrient enrichment have focused on lake and stream systems that are autotroph-based, where algal plankton or periphyton make up a large portion of the resource base (Schindler et al. 1973; Peterson et al. 1985; Peterson et al. 1993). 2 In spite of the focus on autotroph-based systems, many aquatic ecosystems are detritus- based, where the majority of the food web depends on allochthonous detritus as the dominant energy source. The importance of detritus has long been recognized for low-order forested streams where it plays a dominant role in production and ecosystem function (Fisher and Likens 1973; Vannote et al. 1980; Wallace et al. 1999; Moore et al. 2004). Headwater streams also play an important role in water quality and in the health of downstream ecosystems (Meyer and Wallace 2001). Headwater streams frequently retain and transform more than 50% of nitrogen inputs, preventing downstream nutrient loading (Peterson et al. 2001), and also perform a large portion of organic matter processing for the entire watershed (Meyer and Wallace 2001). In spite of the importance of