Tracking isotopes to illuminate Nature's grand recycling program
By David Hoekman
As I stand in the sun on the shore of a beautiful Icelandic lake, the wind dies down and the midges rise from their resting places in the lakeshore grasses and wildflowers. The fog of midges quickly thickens and I am soon engulfed and surrounded by the noise of millions of tiny wings buzzing around me. Alarming as it may seem to be caught up in a cloud of flying insects, I’ve come to Iceland specifically to witness and study this extraordinary density of midges. These tiny midges are so great in number that they transport actual tons of nutrients from lake to land. I’ve come to take samples and measurements that can help me and my colleagues trace the movement of these midge-driven nutrients through and across ecosystems using isotopes. Understanding these movements helps us to predict and manage the consequences of local actions for other interconnected ecosystems. For example, we fertilize crop fields, and those nutrients flow into adjacent streams and reservoirs. The nutrients from the fertilizer may eventually create dead zones in marine systems far from the original field. Before becoming the insect ecologist at NEON, I worked on a project with several other scientists at the University of Wisconsin – Madison where we studied how aquatic insects connect food webs in lakes to food webs on land surrounding lakes. That project brought me to Iceland to stand in a cloud of midges. Midges are a type of aquatic insect that lives and grows in lakes and streams before emerging as flying adults to mate and lay their eggs back on the water. Because midges move to land for their short adult phase, they can be an important food source for terrestrial predators like birds, bats, spiders and beetles. When midge density is high far more midges arrive on shore than predators can eat. Abundant short-lived midges can deliver large quantities of nutrients to land, effectively fertilizing near shore areas with their carcasses as they decompose. We calculated midge biomass emerging from one Icelandic Lake (Mývatn – “lake of midges” in Icelandic) in 2008-2009 to be greater than 79 tons dry weight. This represents hundreds of billions of individual midges, a greater biomass than two gray whales, and a potential nutrient export to land of eight tons of nitrogen and one ton of phosphorus. Near-shore areas may receive enough midges to effectively be fertilized up to 10 times the background rates of nitrogen inputs. These tiny midges transport an incredible amount of nutrients from water to land via their own bodies. The movement of biomass and nutrients across ecosystem boundaries can have major effects in the recipient system. This is where isotope tracking comes in handy. You may have heard of paleontologists using radioactive carbon-14 to determine the age of a fossil. Stable (non-radioactive) isotopes like carbon-13 are powerful tools for tracking the movement and cycling of nutrients through ecosystems. Isotopes are versions of atoms that contain different numbers of neutrons and therefore weigh different amounts. When scientists measure stable isotopes they are looking for the ratio of heavy to light atoms. The isotope ratio of an animal can tell you about what they have been eating because when it comes to C isotopes, you literally are what you eat. For example, midges have a carbon isotope ratio that is the same as the aquatic plants they eat (they eat only as larvae in the water, not as adults). But insects that live on land have a carbon isotope ratio that reflects their terrestrial plant food sources, which happen to be very different from aquatic plants (for reasons we don’t have time to get into here). So when we look at the carbon isotope ratio in land predators like spiders, we can determine what proportion of their diet came from aquatic sources (in this case midges) versus terrestrial sources (land-living insects). My University of Wisconsin colleagues and I used carbon isotope ratios to trace midge nutrients through the food web and determine what organisms were feeding on midges and where those nutrients ended up in the system. In a recent paper, we showed that the input of midges from aquatic systems is very important for maintaining robust terrestrial food webs near water. Our carbon isotope measurements showed that midge nutrients enter terrestrial plants and animals soon after midges arrive. Detritivores, animals that feed on dead organic matter, incorporated midge nutrients the fastest and in the largest amounts. We also found carbon from midges in predators like spiders, who ate the detritivores. Through our experiment we discovered that midge nutrients can enter food webs via detritivores even if predators cannot eat live midges. We found midge nutrients in every component of the food web, from the plants to the predators. NEON isotope measurements can help scientists learn about the movement of nutrients at a very broad scale. At NEON sites, we’ll be measuring stable isotopes in atmospheric CO2, water vapor, precipitation (rain and snow), soil, plants and small mammals. Isotopes in organisms can be measured and analyzed to describe shifts in metabolism and resource use. Organisms change what resources they obtain and how they process them based on a variety of influences like the abundance of food sources, temperature and rainfall and the presence of competitors or predators. These changes are fundamental to our understanding of biology and natural resource management. Oxygen and hydrogen isotopes in water vapor and ground water can tell us how water cycles among plants, ground water, surface water and the atmosphere in a local ecosystem. Oxygen and hydrogen isotopes in precipitation at multiple sites can indicate where your water comes from and how it cycles at regional and continental scales. The water cycle regulates global and local precipitation patterns with major implications for agriculture and natural resources like drinking water. Understanding changes in the water cycle is a critical part of NEON’s mission to track and forecast environmental change. Ecosystems cannot be understood in isolation because of the important influence of organisms and nutrients that move between adjacent systems. At NEON we’re intentionally taking a landscape scale view of ecosystems. Measuring isotopes at each local NEON site will enable us to synthesize the interconnected processes of environmental change at the landscape scale. Just as the isotopes in midges helped us to understand the movement of nutrients in a local food web, the isotopes in water, air and soil at NEON sites will enable us to better understand the movement of nutrients within and among ecosystems across the continent.