Transcript How does lake management for improved water quality impact

How do lake management strategies impact denitrification capacity?
A case study from Rotorua, New Zealand.
Denise A.
1
Bruesewitz ,
David P.
1
Hamilton
and Louis A.
2
Schipper
The University of Waikato,
1Department of Biological Sciences, Centre for Biodiversity and Ecology Research and 2Department 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.
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.
 It is important to understand how lake management techniques will affect
denitrification, as N pollution can contribute to lake eutrophication and
anoxia.
B.
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.
Ohau Channel
 Conditions for DEA are optimal in the main basin of Lake Rotoiti.
 Management efforts to improve water quality can impact denitrification
capacity.
A.
C.
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 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.
Figure 2: The diversion wall diverts water flowing from Lake Rotorua directly down the
Kaituna River, bypassing Lake Rotoiti.
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.
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 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 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.