Te Waihora Investigation D4:
Mechanisms of in-lake nutrient processing
Progress Report
Marc Schallenberg, Candida Savage and Josie Crawshaw*
University of Otago
Dave Kelly - Cawthron Institute
David Hamilton and Conrad Pilditch - University of Waikato
Keith Hamill - RiverLake Ltd.
* MSc student
Robertson 2008
Wriggle
Te Waihora: Stage 4
- Ruppia and other aquatic plants
are now virtually absent
- High sediment resuspension
- High levels of phytoplankton
and toxic cyanobacteria
- Low N:P ratio in lake
- Commercial eel catch has decline
since the 1970s
“…ecologically dead” – Judge Smith
“…on the brink of an ecological crisis”
– M. Mathers
“…biologically dead” – J. Fitzsimons
Planktonic algae (e.g. Nodularia)
Macroalgae (e.g. Bachelotia, Cladophora, Lingbya)
Stage 4
• Loss of aquatic plants
• Loss of kakahi
• Loss of water clarity
Suspended algae absorb around
20% of light in the water column of
Te Waihora (Hawes & Ward 1996)
300
250
Mid-lake chla: 1992 to 2012
Chla
200
150
100
50
0
12-Dec-92
12-Dec-94
11-Dec-96
11-Dec-98
10-Dec-00
10-Dec-02
9-Dec-04
9-Dec-06
8-Dec-08
8-Dec-10
7-Dec-12
Why examine in-lake nutrient processes?
Nutrient
Fraction
Inflows
Lake
Ratio Lake :
Inflows
Total N
3400
2296
0.68
Total P
50
277
5.5
Dissolved
inorganic N (DIN)
3200
92
0.03
Soluble inorganic
P (SRP)
30
7
0.23
DIN:SRP
107
13
Larned & Schallenberg (2006)
Lake losing N
Lake gaining P
Te Waihora is a likely candidate for strong in-lake
nutrient cycling and processing
• Very high algal biomass and production
• High sediment resuspension (benthic-pelagic coupling)
• Big reduction in lake N:P ratio compared to inflows
• The lake appears to be losing N and gaining P
1.2
Mid-lake nitrate: 1992 to 2012
1
NO3
0.8
0.6
0.4
0.2
0
12-Dec-92
12-Dec-94
11-Dec-96
11-Dec-98
10-Dec-00
10-Dec-02
9-Dec-04
9-Dec-06
8-Dec-08
• Intermittent large nitrate peaks at mid-lake
• Mostly winter
• Often don’t fuel algal blooms
8-Dec-10
7-Dec-12
1. Denitrification (nitrate  N2)?
● A natural self-purification process
nitrate
N2
* Lake water with
nitrate supply
Lack of oxygen
dentrification
* Sediment with
fresh organic
matter
* Oxic/anoxic boundary
How important is denitrification?
If important, what are its drivers and vulnerabilities?
Potential denitrification hotspots are found in shallow water,
in sediments of high porosity
2. Internal P fertilisation?
P
P
P
P
Lack of oxygen
P
P
P
P
P
P
P
P
P
P
P
P
1. Temporary density stratification?
• Calm, warm temperatures -> stratification + high oxygen depletion rate
• Barrier bar open -> salt wedge –> stratification + high oxygen depletion rate
2. High pH?
• High phytoplankton productivity can increase pH
Te Waihora/Lake Ellesmere
Taumutu site DO (% saturation) - late March 2014
DO logging sites
1 Sediment geochem
sites
180
160
140
120
100
80
60
40
20
Fouling
of
logger
1/04/2014 0:00
31/03/2014 0:00
30/03/2014 0:00
Wind
event
29/03/2014 0:00
28/03/2014 0:00
27/03/2014 0:00
26/03/2014 0:00
Calm period
25/03/2014 0:00
24/03/2014 0:00
23/03/2014 0:00
22/03/2014 0:00
21/03/2014 0:00
20/03/2014 0:00
19/03/2014 0:00
18/03/2014 0:00
0
• DO depletion rate = 3.3% per hour
• Time to anoxia from 100% = 30 hours
• 30h of stratification to sediment anoxia
Taumutu Site (16)
Sediment redox-exchangeable
P (+36%)
N (+26%)
3000
100
N release rate (mgN/m2/d)
P release rate (mgP/m2/d)
90
80
70
60
50
40
30
20
2500
2000
1500
1000
500
10
0
0
TP(anoxic)
TP(oxic)
TN(anoxic) TN(oxic)
Key Findings From The Field Survey
Dissolved oxygen is a key driver of N and P processing and cycling
and DO declines substantially in the bottom waters at night
The denitrification potential varies around the lake but is highest
in shallower areas (esp. at north, near the Selwyn R. inflow)
Lake sediments release more P and N under anoxic conditions
The density of invertebrates such as chironomid larvae varies
among sites and these may play a role in sediment nutrient
processing via aeration of the sediments
Work is continuing…
Work Plan
Denitrification: Josie Crawshaw
(U. of Otago MSc student)
Chironomid tube density
Sites for more intensive study:
Permanent ECan Monitoring station
Temporary station –oxygen/thermal stratification
High Organics,
High SOD
High mixing
mid depth
High Organics,
High SOD
Low mixing
mid depth
Intermediate Organics,
Mid- SOD
?? Mixing- salt wedge
Mid Depth
Low Organics,
Low SOD
Low mixing
Shallow
Intermediate Organics,
Mid SOD
High mixing
Deep
Temporary deoxygenation and thermal stratification buoy
Large marker Buoy
Sensor buoy
Hobo T sensor
Water depth
1-1.5 m
DO Opto sensor
5m sinking rope
Heavy anchor15 kg Danforth plow anchor
Cross-shaped alloy anchor
With mounting pin for
DO –Opto sensor
Lake health sensor deployment at
ECAN permanent monitoring stations
Existing sensors already recording salinity at 2 sites
Additional Sensors:
•
•
•
•
•
Dissolved oxygen and temperature – near surface and bottom
Salinity
Chlorophyll and phycoerythrin (cyanobacteria)
Turbidity or visual clarity (beam transmissometer)
pH
Denitrification
Anoxic P release
Sediment O2 balance
Lake nutrient model
1. Sediment survey to
determine sites with high
sediment O2 demand (key sites
plus transects) (DK MS)
Ellesmere/Waihora
DYRESM/CAEDYM
model (DH)
2. Select likely anoxic hotspots
3. Using lab measurements of
sediment oxygen uptake and a
benthic turbulence model,
estimate oxygen balance at
sediment-water interface at
hotspot, control sites (MS, DK)
Incorporate oxygen balance
model into DYRESM-CAEDYM
model
4. Use remote sediment profiler
to test oxygen balance model (CP)
5. Use lake buoy to test in situ
bottom anoxia at key sites (DH/DK)
6. Determine relationship
between sediment P-release
and sediment oxic state at
anoxic hotspots and control
sites (MS, CS)
Restoring
sediment
oxygen
7. Measure denitrification
potential in sediments and
water at all sites (DH MS)
8. Measure denitrification
rates in sediments and
water in relation to oxic
state (CS MS)
Incorporate rates of anoxic
P-release into DYRESM-CAEDYM
model
Protecting
ecosystem
service
Restoration and
Management
Incorporate denitrification
potential and rates of denitrificaton
into DYRESM-CAEDYM model
Scenario testing
9. Updated Ellesmere/Waihora
DYRESM/CAEDYM model (DH)
Thanks to Whakaora Te Waihora and ECAN
Outcomes of the proposed research
Key knowledge gaps addressed
1. What is the most likely cause for temporary declines in TLI?
2. Do sediments in any parts of the lake become fully anoxic?
3. Is there anoxic internal P loading and if so, at what locations, times and rates?
4. Where are the sites of denitrification and what are the rates?
Management and restoration benefits
5. Revealing the main processes affecting the nutrient status of the lake
6. Scenario testing using a dynamic model that more realistically reflects the ecology of the
lake (key interaction with other restoration efforts in the lake)
7. Indicating key management actions that could help improve the nutrient status of the lake
8. Indicating any vulnerabilities of the denitrification ecosystem service provided by the lake
Key Linkages
Science
Technical
outputs
Survey and experimental results
Updated Lake Ellesmere/
Te Waihora model (DH DK MS)
Report to ECAN
(Full research team)
Scenarios
Community
outputs
Hui with local Iwi
(SO MS DH KH)
Workshops with
local community
(MS DH CS KH)
Sharing information
and predictions from
model restoration/
mitigation scenarios
with D1, D2 and E1
groups (DH DK)
e.g. artificial habitats and
wave damping structures