Toolik Lake is a large, oligotrophic kettle lake. in the northern foothills of the Brooks Range, Alaska. The tundra surrounding the lake includes all of the common terrestrial arctic ecosystem types, as well as a number of smaller lakes, streams and rivers of various sizes. The entire region is underlain by permafrost, with continuous daylight from mid May to late July and a snow-free season normally lasting from late May to late September. The’ research camp at Toolik Lake is operated by the University of Alaska, has a current capacity of 40 scientists, and is accessible year-round by road from either Prudhoe Bay (4 hours) or Fairbanks (9 hours). Travel along the road provides access to an even wider array of arctic ecosystem types, including all of those common to central and northern Alaska.
Long-term aquatic research at Toolik Lake began in 1975, and terrestrial ecologists began long-term experiments and observations there in 1976. Since then, about 25-30 senior investigators and many more students and technicians, from a number of institutions in the United States and Europe, have worked at Toolik Lake. About 10 of these scientists have maintained their research there continuously since the mid-1970s.
The new arctic LTER program at Toolik Lake is designed to build on this extensive research base, to provide core funding for ongoing, long-term experiments, and to link terrestrial, lake, and stream studies more explicitly than has been possible in the past. The program currently includes 15 principal investigators from 7 different institutions including the Universities of Alaska, Cincinnati, Kansas, Massachusetts, and Minnesota. The lead institution is the Marine Biological Laboratory, Woods Hole, Massachusetts.
The heart of the program is a series of parallel, whole- ecosystem experiments in lakes, streams, and the major terrestrial ecosystem types. The experiments are of two kinds: “top-down” manipulations of herbivores or predators, and “bottom-up” manipulations of nutrient availability. The overall goal Is to understand and to separate the role of animal consumers versus plant/nutrient responses as controls over terrestrial and aquatic ecosystems.
Previous research at Toolik Lake has shown that arctic ecosystems respond dramatically to such experimental manipulations, and that the responses are often easier to interpret than in more complex, species-rich ecosystems at lower latitudes. The effects of manipulation can often be traced clearly through several trophic levels, In addition to major changes in productivity and species composition within trophic levels or guilds. For example, phosphorus fertilization of the Kuparuk River (near Toolik Lake) changed the entire basis of the food web from heterotrophy to autotrophy by stimulating algal growth. It changed the structure of the insect community by favoring grazers over filter feeders, and it sharply increased growth of larval and adult fish.
Parallel fertilization experiments in Toolik Lake have resulted in changes in both zooplankton and benthic community structure, and on land the growth form composition of the vegetation as well as its productivity is strongly affected by nutrient availability, air temperature, and shading. An important reason for continuing these experiments in the long-term is that not all species respond at the same rate, and there is much to be learned by observing the sequence of changes and interpreting its causes.
Additional experiments were required to determine the cause of changes in the ecosystems when nutrients were added. For example, in the Kuparuk River fertilization experiment, algal biomass was greatly elevated in the first two years but in subsequent years algal biomass in the fertilized reach has been only slightly greater than in the upstream control reach. George Gibeau and Mike Miller of the University of Cincinnati designed a bioassay experiment which showed that algal biomass increased rapidly in the fertilized reach when grazing insects were eliminated by treating substrates with the insecticide malathion. In the nutrient-poor control reach adding malathion caused a very small biomass accumulation. However, when nutrients were added either to the substrate or to the water malathion addition allowed algae to reach levels similar to those found on the river bottom in years 1 and 2 of the experiment. We conclude that grazing insects (larval chironomids and mayflies) are responsible in large part for preventing the accumulation of algal biomass when nutrients are added. This biomass is being cropped by insects when growth in turn provides an increased food supply for fish in the fertilized reach.
A second major goal of the arctic LTER program is to advance understanding of how mineral nutrients move over the arctic landscape, from terrestrial to aquatic ecosystems. This goal is especially important in the context of human disturbance, because we know that the structure and productivity of terrestrial ecosystems is strongly nutrient-limited, and that disturbance in general tends to increase nutrient cycling rates and overall nutrient availability. We also know that aquatic ecosystems are strongly dependent on nutrient inputs from the surrounding tundra. To describe the cycling of nutrients within different terrestrial ecosystems, and the movement of nutrients over the landscape, we are focusing on development of a model of nutrient transport, combined with the use of stable isotopes as tracers to identify major sources, sinks, and pathways of element cycling.
Evidence collected to date suggests that primary production in some sites may depend on nitrogen inputs from adjacent ecosystems for as much as 10-20 per cent of their annual nitrogen requirement. This conclusion is based in part on the lack of correlation between annual N mineralization in soils and the annual N requirement of primary production, with surplus amounts of N mineralized in some sites and not enough in others. The main evidence, though, is the series of changes in concentration of inorganic N In soil water, as the soil water moves down slope through a series of contrasting vegetation types. The sites with the greatest deficit in N mineralization are also those with the greatest decreases in inorganic N in soil water, suggesting that in these sites the N carried in from upslope ecosystems is being taken up and used in support of plant production.
For additional information contact Galus Shaver, The Ecosystems Center, Marine Biological Lab, Woods Hole, MA 02543, (508) 548-3705.