Low-level nitrogen and phosphorus addition drives hypolimnetic oxygen drawdown in deep Arctic Lakes

Increased nutrient transport from the landscape to freshwater ecosystems is likely a secondary effect of global change in the Arctic (Hobbie 1999).  Long-term, whole-lake fertilization experiments at the Toolik Lake  ARC-LTER began in 1985 to study the effects of elevated nutrient loading on aquatic ecosystems.  From 2001-2012, shallow Lake E6 and deep/stratified Lake E5 were fertilized to examine to what extent lake response is controlled by lake morphometry.  We demonstrate that low-level fertilization with N and P (2x background) is capable of driving a decrease in hypolimnetic dissolved oxygen in Lake E5.  This oxygen depletion has important implications for sculpin and salmonid habitat availability and foodweb pathways. Lake E6 remains mixed and oxygenated throughout the summer. Despite similar benthic chlorophyll concentrations in 2011 and 2012 compared to 2000 (pre-fert), both lakes exhibit higher benthic oxygen demand.  Greater than 80% of the oxygen drawdown in the E5 hypolimnion is due to benthic respiration.  Future research will include further characterizing sediment chemistry, assessing changes in the abundance and community structure of benthos-dwelling organisms, and modeling the carbon balance of nutrient-enriched lakes.