Our understanding of how lakes and land are linked is poor, despite the fact that lakes are important components of many terrestrial landscapes. Most research to this point has focused on how watershed characteristics or land use affect downstream surface waters, but we know little about reverse interactions. This study will investigate lake-to-land linkages at Lake Mývatn, a nutrient-rich lake in Northern Iceland with exceptional, natural, large-scale midge (aquatic insect) outbreaks that occur every 5-7 yr. During these outbreaks, millions of midges enter surrounding terrestrial ecosystems, where they affect everything from plant productivity to herbivorous insects and their spider predators. Through a series of manipulative experiments, remote sensing analyses, and empirical modeling, this system will serve as a model to study ecological linkages at the landscape scale and will address the fundamental question of how food webs on land are influenced by energy, materials, and organisms originating from separate and distinct lake ecosystems. Broadening our studies of terrestrial communities to explicitly include interactions with other elements of the landscape will allow us to better understand and manage our environment by considering multiple habitats simultaneously. Landscapes are being altered via both natural (e.g., invasive species) and anthropogenic forces (e.g., urbanization or agriculture). Determining how these dynamic patchworks of habitats affect each other will be essential to understanding how disturbances propagate through landscapes and to better managing our terrestrial ecosystems.
A hum fills the air and a thick wave-like gray fog slowly lifts from the grass and swirls like a smoke pillar toward the sky. The fog stretches over a mile along the shore and pushes inland as a ribbon 1/4 mile wide. But this is not a fire, this fog is alive. It is made of billions of tiny insects that once used to live in the muck at the bottom of the lake. Our project was focused on Lake Mývatn ("Mee-vaht", literally "midge lake" in Icelandic) in northeastern Iceland and its spectacular midge emergences. We explored how these tiny aquatic insects, by their sheer numbers and their behavior, affect the plants and insects that live in near shore. This system is expected to be broadly representative of the effects that aquatic insects have on terrestrial areas surrounding lakes even in areas where insects are just as abundant, though not always so numerous. Our findings have shown that through aquatic insects, lakes and the surrounding land are tightly connected. In four years of studying the land around lakes in northern Iceland we found evidence that midges had important and widespread effects on the terrestrial system. Most midges coming to land either die of natural causes or are eaten by insect predators. On the ground they are fed upon by a group of scavenging bugs that feed on decaying materials. The number of these small decomposers increases in response to the midges. Subsequently, the predators of these decomposers, mainly spiders and harvestmen, also increase. In the presence of live midges, however, spiders become distracted and focus all of their attention on the aquatic insects, to the benefit of the resident insects that now are ignored by the spiders. If midges are not directly eaten, their carcasses are broken down by a suite of bacteria and fungi. These microbial decomposers use carbon from midges to grow and make nitrogen available to plants. With the nitrogen grasses grow in size and start to dominate the plant community. Midges can be so abundant on land near lake Mývatn the amount of fertilization they provide is comparable to levels of fertilizers added to cropland. We estimate that in some areas around the lake midges deposit as much nitrogen in a year as farmers add to corn in Iowa. Where midges are present, plant communities are dominated by grasslands; where midges are absent, shrubby heathland dominates. Insect caterpillars and other sap-feeding herbivores also become more abundant when midges have "fertilized" the land. Our findings from the study of midges in Iceland are but one example of aquatic insects emerging from water and entering terrestrial habitats. Aquatic insects emerge from streams and rivers as well as lakes. By using computer simulation models, we extended our findings to other parts of the world. We found that aquatic insects emerging from lakes are likely to be greater contributors to land than from streams and rivers. Therefore, the relative abundance of lakes in a landscape can be a major factor affecting terrestrial communities. We predict that in areas where plant productivity is low (such as in more northern latitudes or in deserts), the contribution of aquatic insects as food for other consumers will rival the value of terrestrial insects in the same areas. That is, the lakes, through their aquatic insects, are feeding the land. As a result, what this study has shown is that although lakes were once thought of as passive recipients of materials from the surrounding land, lakes are in fact actively tied to the land by way of aquatic insects. The local ecosystem around lakes experiences both short-term and long-term effects of the aquatic insects leaving the lake. Some responses, such as those by mobile predators are short-lived with predators congregating in areas of high insect presence. Other responses, such as plant growth, take longer to develop but also last beyond when the insects are long gone. Thus, appreciating the importance of the connection between the two systems makes it possible to understand the patterns of plants and animals on land. All of this is brought about by individual movements of tiny aquatic insects whose collective actions have big effects.
