It's now almost six years since we followed through on a madcap plan to warm up an entire stream reach at Hengill. It was only made possible by the ingenuity of our engineer, Philip Johnson, shown here on the right (Alex Huryn is providing the muscle behind him). Philip is now retired, but still working with us (and still just for the kicks too). Happily, all that work was worth it. This month Dan Nelson has published two papers from the project. The new one, published in Ecology, extends the results of his recent paper in Global Change Biology to show that secondary production in the manipulated stream remained constant after warming, as predicted by metabolic theory. The mechanism was surprising, however. Rather than total biomass declining and mean growth rates increasing, total biomass did not change. Instead small fast-growing taxa declined as larger slow-growing species, including newly invading taxa, increased in biomass. As shown in his previous paper, Dan's results were driven by somewhat idiosyncratic relationships between body size and thermal preference, and demonstrate how reassembly of communities under warming will depend on the range in thermal preferences and dispersal abilities in regional species pools, which may or may not be related to body size. This is the kind of result that could only be observed with an experimental manipulation of a natural ecosystem that is open to dispersal processes. Which is one reason why we go to all the trouble!
I am back from Iceland, after an ultimately successful trip, despite weather that was often difficult and that postponed the start of our five-day 15N additions to the two cold and two warm streams. The shot above is of everyone sampling food web compartments in Stream 11 during the drip. The sun did come out every now and then! The channel experiment continues to go well too. Here's a shot of our REU student Heath Goertzen and MS student Lyndsie Collis collecting and processing tiles for the biomass accumulation time series. You can just make out the diversity of growth responses that are being driven by differences in temperature and nutrient treatments. Some tiles are still effectively bare after six weeks, while a thick algal growth is threatening to overtop some channels. Maybe we'll see some interesting temperature-nutrient interactions. Time will tell.
Our field season in Iceland kicked off last month, so it's high time I posted on our progress. We started off in early May by setting up the channel experiment again, made much easier this year by a notably small snowpack. But first we had to find all the many pieces of the experimental set-up in the new storage facility, where all our gear was moved over the winter. It was all there, so we set to checking it over and packing it up in the truck.
For once, setting up the channel experiment went smoothly. Perhaps we're finally getting it down on the fourth iteration. Or maybe it's because Wyatt stayed out of the way and mostly mugged for the camera. In any case, our final channel experiment is up and running. It will explore how threshold responses of biofilm communities to relative nitrogen and phosphorus availability change with temperature.
Next up was to get the nitrogen drippers going in the two cold and two warm streams. Six hundred kilos of ammonium nitrate fertilizer will do the job, delivered by the same float-valve drippers we used last year to add phosphorus.
It's now mid-June and still early days for our nitrogen additions, but spot the difference. Here are two shots taken yesterday of one of our warm streams. The left photo is just upstream of the nitrogen dripper, while the other shot is just downstream. Note the particularly lurid green clumps of Cladophora that start directly below the dripper.
The isotope additions to the four streams will start in the next few days, so watch this space.
It's been a bit quiet on the publication front of late (we had a very sobering run of rejections last year), but Dan Nelson's first manuscript from his dissertation is now out in Global Change Biology. In it, we describe how the experimental warming of a small upland stream in Iceland by 3.8 degrees C changed the structure of its invertebrate community. Somewhat surprisingly, average body size increased. The landscape in which the stream is embedded has streams of varying temperature, which contain taxa with a wide range in thermal preference. It just happens that many of the invertebrates with higher thermal preference are relatively large-bodied (snails and black flies, for example) and these groups responded strongly to our whole-stream warming manipulation. These results show that shifts towards lower average body size with warming are not universal, and that the combination of diversity in thermal preference and dispersal ability will dictate how communities reassemble as ecosystems warm in the future. Well done to Dan for all the hard work he put in to reveal these patterns!
A large collaborative proposal on which I am lead PI just got the green light from the National Science Foundation. The project will examine the effects of temperature on organic carbon processing in forest stream networks, using a multi-scale design that includes a paired-catchment whole-stream warming experiment, an array of warmed streamside channels, laboratory studies of aquatic microbes, and reach- and network-scale modeling. The fieldwork will take place at the Coweeta Hydrologic Laboratory, North Carolina. I'm very excited about this project and now need to find a PhD student for my lab's component of the research, starting in 2017, who will study responses of the invertebrate community and food web in the whole-stream warming experiment (see the full ad on the SFS website). The ideal candidate would combine a genuine interest and background in both entomology (Coweeta's stream communities are particularly species-rich) and experimental ecosystem ecology. If this sounds like your kind of project, please take a look at the Prospective Students page and get in touch.
Some more of David Manning's PhD research just came out in Ecological Applications. The paper describes links between leaf litter stoichiometry and breakdown rate under ambient and nutrient-enriched conditions in our five study streams at Coweeta Hydrologic Laboratory. David's findings include homogenization of detrital stoichiometry across litter types under nutrient enrichment, as well as tight predictive relationships between litter stoichiometry and breakdown rates that may be useful for monitoring and management. Check it out here.
Last night Dan Nelson and Jon Benstead started the 15N additions in the four streams along our landscape temperature gradient in Iceland. So for the next five days each stream will have TWO drippers going - one adding phosphorus and one adding 15N-enriched nitrogen at tracer levels. Lots of work ahead of us as we follow where all the isotope goes and find out whether adding phosphorus has an effect on nitrogen uptake and routing through the food web. Below is a shot of Stream 11 (our coldest stream), with the phosphorus dripper on the right and metering pump delivering the isotope on the left.