The overarching goal of our research project was to investigate the individual and combined effects of two important agents of global change, climate warming and nutrient pollution, on the structure and function of aquatic communities. To investigate how stream ecosystems respond to these factors, we conducted our study in a geothermally-influenced landscape in Iceland, i.e., the Hengill geothermal area, which contains a suite of small streams that vary in water temperature (5 to 30 degrees Centrigade) but are similar in most other physical and chemical attributes. We have been working in this system for a number of years, focused primarily on the effects of temperature on stream communities and ecosystem processes. For this study, we set out to cross nutrient enrichment and temperature to examine their interactive effects at the scale of whole stream ecosystems. To do this, we first chose 4 similar streams across the natural temperature gradient (from 8 to 16 degrees Centigrade) and conducted summer-long experimental nutrient additions in all streams. During the first summer (2015) we examined differences among streams at ambient (and low) nutrient concentrations. In 2016, we added phosphorus continuously, and in 2017 we added nitrogen. During each field season (typically April to August), we also manipulated temperature and focal nutrients in 30 streamside channels to gain more control and replication, and to investigate threshold or gradient responses in ecological processes. Our research was motivated by both applied and theoretical goals, as we sought to learn more about how environmental warming interacts with other drivers of global change, as well as whether theoretical expectations (based on the theories of metabolic ecology and ecological stoichiometry) could actually help us predict future changes to stream ecosystems.

Our research has demonstrated that how streams respond to nutrient enrichment depends on their ambient temperature. For instance, production of new biomass at the base of the ecosystem (i.e., algae, cyanobacteria, other microbes) was positively influenced by nitrogen enrichment in these streams, but this response was much more pronounced in warm streams. We also found that animal production (largely invertebrates) was positively enhanced by nitrogen enrichment, but, surprisingly, this response was strongest in cold streams. Some of this may be explained by how temperature and nutrients interact to influence the community of microbes that serve as food resources for invertebrates. For instance, our experimental channel experiments showed that the proportion of nitrogen fixing microbes (many of which are non-edible) was reduced by nitrogen enrichment, but less so in the warm streams. Our current food web analysis is aimed at disentangling this response by directly examining the pathways of food resources in the broader food web. 

From a theoretical perspective, we found that predicting responses to these global changes requires knowledge about (a) traits of the community members (e.g., their body size, thermal tolerance, nutrient requirements) and (b) how basal productivity responds to both temperature and nutrients (i.e., changes in energy and nutrient availability at the base of the food web).  Thus, our findings should be useful for refining theory that aims at making general ecological predictions about responses to global change.

Our project has supported the training and professional development of many undergraduates and two graduate students (one MS and one PhD). This project also represented a collaboration with a previous post-doctoral researcher that is now a faculty member at Ohio State University.The project also created a research blog (The Hengill Diaries), which received over 50,000 views during the duration of the project. 

Last Modified: 03/27/2020
Modified by: Wyatt F Cross