How do lake management strategies impact denitrification capacity? A case study from Rotorua, New Zealand. Denise A. Bruesewitz1, David P. Hamilton1 and Louis A. Schipper2 The University of Waikato, 1Department of Biological Sciences, Centre for

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How do lake management strategies impact
denitrification capacity? A case study from
Rotorua, New Zealand.Denise A. Bruesewitz1,
David P. Hamilton1 and Louis A. Schipper2The
University of Waikato, 1Department of Biological
Sciences, Centre for Biodiversity and Ecology
Research and 2Department of Earth and Ocean
Sciences
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.

• 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.

• 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 NO3- 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 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 2008. Credits
D. Burger, M. Allan and Environment Bay Of Plenty.

• 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 Rotoitis
  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 Rotoitis water quality within 5 years.

Figure 4 Denitrification enzyme activity at the
3 sampled sites on both of the sampling dates.
Lakes and management of denitrification

• 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 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 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.