Climate warming has been identified as one of the most important threats to freshwater ecosystems globally. Because temperature is the fundamental driver of nearly all biological processes, climate warming will likely have strong effects on freshwater ecosystems and the ecosystem services they provide for humans. Despite recent theoretical advances that provide clear predictions about how organisms should respond to warming, our current understanding is hindered by (a) the short-term and small-scale nature of most previous studies and (b) the non-trivial practical challenges associated with experimental warming of freshwaters at the ecosystem scale. This study will take advantage of a unique geothermally-active watershed in Iceland that contains a steep gradient of stream temperatures and very little difference in solute chemistry. Using a landscape gradient study of ?temperature-acclimated? streams, streamside manipulations, and an ecosystem-scale experiment, this study will quantify the effects of warming on critical ecosystem processes (ecosystem metabolism and nutrient cycling) and the flow of energy and elements through stream food webs.
The proposed multi-scale design will produce much-needed data concerning the response of stream ecosystems to impending changes in temperature regime, benefitting society through increased understanding of how climate warming will influence freshwater ecosystems and the critical services they provide to humanity. The project will integrate research and education through training and mentoring of undergraduates, graduate students and a post-doctoral associate. In addition to communicating our results through traditional outlets (publications, national/international meetings), findings will be presented to local grade schools, high schools, and the community at large with presentations, an interactive research blog, and semi-annual meetings with Icelandic colleagues.
This project took advantage of natural variation in water temperature within a set of geothermally modified streams draining a valley in western Iceland. These streams differ in temperature (6-30 degrees C), but are very similar with respect to other important characteristics such as light availability and concentration of important limiting nutrients (e.g., nitrogen and phosphorus), making them ideal for independently testing the effects of temperature on stream community structure and ecosystem processes. My laboratory's component of the project examined the effects of temperature by using this site in two different ways. First, we used a gravity-fed heat exchanger placed in a warm stream to warm an adjacent, naturally cold stream by an average of 3.5 degrees C for two years. Second, we used a similar system of heat exchangers to warm a set of small, artificial stream channels in which we could conduct controlled experiments on the effects of water temperature on stream communities. The first, whole-stream warming experiment was the first of its kind and revealed some important short-term impacts of warming on stream communities. The plant community present in the summer underwent a drastic transformation, becoming dominated by a dense cover of a single species. In turn, the invertebrate community underwent a relative shift towards species that were characteristic of the warmer streams in the valley (e.g., snails and blackflies). Our analysis of samples from this large experiment is ongoing. Our artificial stream channels also revealed key responses of stream communities to temperature. The thermal gradient made possible with this experimental design was associated with steep increases in the biomass of primary producers. High biomass in warmer treatments was linked to higher rates of nitrogen fixation by algae, illustrating the important interaction between temperature and nutrient supply that is now a focus of our ongoing work in the valley. In summary, our project has allowed us to study the effects of temperature on stream communities without the confounding effects of other factors and at multiple scales. We also anticipate that our data will allow unique comparisons of the effects of temperature seen in our relative short-term (8-week to 2-year) warming experiments with those observed in natural, thermally acclimated systems (i.e., across the temperature gradient in the valley's streams). In other words, do large-scale but relatively short experiments tell us anything useful about the long-term effects of temperature shifts, such as those expected to occur as a result of climate change? Our project also trained one PhD student, one technician, and several undergraduates, who were involved in laboratory tasks and fieldwork. The information gained from the project is being disseminated via both traditional channels (conference presentations and primary research articles) and a popular project blog, which has now garnered more than 16,000 page-views.
