This year the weather did a good job delaying the start of the planned simultaneous nitrogen isotope additions to our four focal streams (two warm, two cold), as we cannot start them if the streams are too high. We finally got a forecast for a break in the weather, so Mick and Jon started the drippers yesterday evening (in truly awful weather, as it happened). The good weather has now arrived and all 8 drippers (one for ammonium nitrate and one for 15N-enriched ammonium chloride and potassium nitrate in each stream) were behaving well when we changed the batteries after one day out of the scheduled five days of isotope addition. Fingers crossed that those five days go off without a hitch.
Tomorrow we take our first food web samples from the four streams, followed two days later by comprehensive water sampling for nitrogen concentrations and isotopes. More posts to follow.
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.
Our streamside channel experiment is up and running in Iceland. This year we're crossing the temperature treatments with varying levels of phosphorus enrichment. We have some hypotheses about what might happen, but no clue what we'll actually see. That's what science is all about, right? Below are some shots of the channels and phosphorus drippers in action.
Congratulations to lab member Dr. Dan Nelson, who defended his dissertation last week. Dan did all the macroinvertebrate community and food web work for our whole-stream warming manipulation in Iceland. His papers will be rolling out very soon. Well done, Dan!