Jon Benstead was one of the principal investigators in the Hengill study. Lyndsie Collis at a geothermal pool MISSION: WATER 14 HEADLINE SURFACE WATER Past and Future Because of its unique geology and hydrology, Hengill provides a remarkable view of the past in the newly hewn volcanic landscape with fresh, shallow soils. It also offers a peek at the future, as Benstead, Hood and their colleagues warmed streams and enriched them with nutrients to study the effects thatglobalclimatechangecouldhaveonriversaroundtheworld. “It’s the interaction between temperature and nutrients that really drives the research,” says Benstead. He helped lead the design of an ingenious set of studies, funded largely by the National Science Foundation, to examine the impact of temperature increases and different nutrient levels on ecosystem metabolism (the growth of algae and its respiration, as well as the decomposition of organic matter) and invertebrate communities in streams. In 2010, Cross, Benstead, Huryn and Hood kicked off their research by starting to gather monthly data on two streams in Hengill, setting the baseline for respiration and life in each. They then raised the temperature of the cooler stream by about 4°C using a three-stage, gravity-fed heat exchanger system submerged in geothermally warmed water. This allowed them to study the effects on stream metabolism, nutrient cycling, flora and fauna. Zooming in from the whole-stream warming level, the team then created narrow, three-meter-long channels lined with tiles on which biofilm could grow. Fifteen channels provided three replicates of five different temperature regimes. Later, the array expanded to 30 channels and included nitrogen fertilizer dripped into the water to overlay different rates of nutrient enrichment on top of temperature changes. Tiles were removed from the channels and placed into chambers where researchers measured rates of respiration, photosynthesis, nitrogen fixation and nutrient uptake. Then the biofilm was scraped from each tile and the living community was analyzed. “We had to tease out the effects of temperature first,” Benstead explains. Now we can see that at higher temperatures we get increasing nitrogen limitation and nitrogen- fixing activity.” Tile-lined channels provided insight on stream responses to temperature changes.