Ohau Channel Diversion Wall  Lake Rotorua experiences anoxic bottom waters and cyanobacterial blooms (Fig. 1A). Water from Lake Rotorua flows through the Ohau Channel into Lake Rotoiti, adversely affecting Lake Rotoiti’s water quality (Fig. 1B).  The diversion will prevent 70% of the current external load from entering the main body of Lake Rotoiti annually. This is expected to improve Lake Rotoiti’s water quality within 5 years. How do lake management strategies impact denitrification capacity? A case study from Rotorua, New Zealand. Denise A. Bruesewitz 1, David P. Hamilton 1 and Louis A. Schipper 2 The University of Waikato, 1 Department of Biological Sciences, Centre for Biodiversity and Ecology Research and 2 Department of Earth and Ocean Sciences Introduction  Lakes have not typically been managed to enhance denitrification, but many are managed to mitigate poor water quality.  These management techniques include bottom water aeration, sediment capping, water diversion, and the establishment of riparian wetlands.  It is important to understand how lake management techniques will affect denitrification, as N pollution can contribute to lake eutrophication and anoxia. Figure 1: (A) Lake Rotorua water during an algal bloom (B) Lakes Rotorua and Rotoiti, linked by the Ohau Channel and (C) construction of the diversion wall, completed in July Credits: D. Burger, M. Allan and Environment Bay Of Plenty. Hypothesis Denitrification enzyme activity (DEA) will be greatest in the lake where sediment carbon (C) accumulates and oxygen levels are lower than in the diversion channel. Lakes and management of denitrification Results and Discussion  Bottom waters of the main basin of Lake Rotoiti progressed towards anoxia from Nov to Jan, while the shallower sites did not (Fig. 3).  In Nov, DEA did not differ in the main basin and the lake side of the wall but was near zero on the channel side of the diversion wall (Fig. 4).  DEA increased from Nov to Jan, with the highest rates in the main basin and the lowest rates on the channel side of the wall (Fig. 4).  Although our water chemistry data is pending, if the wall is working as it was designed to, DEA is lowest where NO 3 - concentrations should be consistently highest.  Conditions for DEA are optimal in the main basin of Lake Rotoiti.  Management efforts to improve water quality can impact denitrification capacity. Methods  In Nov 2008 and Jan 2009, we collected sediment cores (6 reps per site) on either side of the diversion wall (near the Lake Rotoiti end of the wall), and also in the deep main basin of Lake Rotoiti.  DEA was measured on a slurry of the top 5-cm of sediment from each core combined with bottom water to determine denitrification potential. Figure 3: Vertical profiles of dissolved oxygen (DO) at the 3 sampled sites: the main basin of Lake Rotoiti, the lake side of the diversion wall, and the channel side of the diversion wall on both of the sampling dates. Figure 4: Denitrification enzyme activity at the 3 sampled sites on both of the sampling dates.  Management of catchment N via load reduction or measures to intercept excess N before it enters a lake would likely be more successful than attempts to manage in-lake denitrification.  Results from a survey of DEA in lakes of the Rotorua region suggest that denitrification capacity is influenced by catchment land use (Fig. 5). Lake sediment denitrification capacity increases with external N load.  Lakes have some ability to increase denitrification potential with higher N loads, but this is tempered by seasonal stratification patterns and the catchment drainage ratio. Figure 5: Results of a broad survey of lake sediment DEA in the Rotorua region regressed with percent catchment pasture. This survey was completed Nov 08. Acknowledgements: We thank Chris Isherwood and Warrick Powrie for field assistance and Sören Warneke for laboratory assistance. This work was funded by the N.Z. Foundation of Research, Science and Technology (Contract UOWX0505). We also acknowledge the support of Environment Bay of Plenty for research on the Rotorua lakes. Figure 2: The diversion wall diverts water flowing from Lake Rotorua directly down the Kaituna River, bypassing Lake Rotoiti. C. Ohau Channel A. B.