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Yellowstone National Park Report

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Invasive species may alter processes by indirectly affecting other species in the ecosystem, but indirect affects are difficult to measure and often pre-invasion data were not collected. However, models may be used with empirical relationships and available past data to reconstruct past processes. The introduction of lake trout (Salvelinus namaycush) in Yellowstone Lake may have indirect effects on nitrogen cycling and provide an opportunity to study the effects of an invasive species on lake processes. To estimate how lake trout altered nitrogen cycling in Yellowstone Lake, we measured ammonium (NH4+) uptake by phytoplankton in 2005 and estimate past NH4+ uptake using empirical relationships and past data. Using 1 5N, we measured phytoplankton uptake in 4 areas of Yellowstone Lake. Phytoplankton demanded 4.9 mg N m-2 hr- 1 during the ice-free season of 2005. Uptake was higher at warmer water temperatures and shallower Secchi disk depths (measure of phytoplankton biomass). Using relationships among uptake, water temperature, and Secchi disk depth, we estimated phytoplankton uptake in the past based on historical Secchi disk depths and water temperatures. Water temperatures have increased 0.29.C/decade and Secchi disk depths became 0.53 m/decade deeper over the past 30 years. Using our multiple regression model, phytoplankton uptake did not change between 1976 and 2006. The interaction between warmer water temperatures and deeper Secchi disk depths (a sparser algal assemblage) cancelled out resulting in no changed in modeled uptake. Therefore, when estimating past processes, we should use multiple predictors.