The important plant nutrient, nitrogen (N), can be transported in the atmosphere and deposited into terrestrial and aquatic habitats in rainfall. During recent decades, N deposition has greatly increased as a result of human activities, largely due to expansion of fertilizer application and of formation of nitrates (NO3) during fossil fuel burning. This study will try to determine if N deposition affects lake food webs by changing the chemical composition of the microscopic plants (phytoplankton) that form the base of lake food webs. Specifically, these investigators hypothesize that increasing inputs of N to lakes cause the phytoplankton to become strongly limited by another key element, phosphorus (P). When P-limited, phytoplankton produce biomass that has a very low P content and thus potentially become nutritionally unsuitable for growth of the next feeding level, the zooplankton. (Zooplankton, in turn, provide an important food base for fish, including those that are involved in recreational and commercial fisheries.) To test this, these researchers will measure P limitation of zooplankton growth in lakes receiving different levels of N deposition in central Colorado and in southern Norway. These investigators will also perform a larger scale field study that deliberately manipulates N inputs to experimental mesocosms deployed in a lake that receives low N deposition under normal conditions. This study will help in basic understanding of how lake food webs work while also providing potentially important information about an indirect but potentially important impact of human activity on seemingly pristine ecosystems and the food webs they support.
