Abstract
Lake Okaro is a small, warm monomictic lake
in the central North Island of New Zealand. It has remained highly
eutrophic despite an intensive catchment and in-lake restoration
program which commenced in 2003. The program has included the
implementation of a constructed wetland, riparian protection,
an alum application and application of a modified zeolite mineral
(Z2G1) to reduce internal nutrient loading. This study examines
water column and sediment nutrient dynamics; focusing on phosphorus
(P) and the ecosystem response to lake restoration designed to
reduce levels of P.
Trends in P concentrations in Lake Okaro were linked to the restoration
efforts over a six-year period (2002-08) including the period
shortly before the restoration program. Over the entire study
period, the annual average total phosphorus (TP) concentration
in the lake decreased by 56 %. Two predictive models, which derive
the annual average P concentration in the water column based on
external P loading, generally underestimated the measured TP concentrations
in the water column due to internal P loading. Of all restoration
methods, the application of Z2G1 produced the most effective reduction
in water column TP concentrations. However the lake trophic state
showed high resilience to reduced internal P loading even though
the combined effect of all restoration procedures resulted in
significantly decreased TP concentrations in the lake.
The sources and sinks of nutrients in the hypolimnion of Lake
Okaro were investigated using field measurements in a comprehensive
nutrient budget model in order to determine changes in sediment
nutrient fluxes resulting from a whole lake sediment capping trial
using Z2G1. Sediment nutrient fluxes in the hypolimnion were estimated
as the residual term in the nutrient budget model that accounted
for mineralisation of organic nutrients, nutrient uptake by phytoplankton,
nitrification, adsorption or desorption of P from inorganic particulate
material in the water column, and diffusion of dissolved nutrients
at the thermocline. The model indicated that during a period of
seasonal stratification in 2007-08 up to 60% of hypolimnetic phosphate
fluxes and 50% of ammonium fluxes were derived from bottom sediments.
Diffusion across the thermocline, adsorption/desorption of phosphate
to suspended solids, and nitrification were of relatively minor
importance (≦ 9%) to the total fluxes. Any reduction in sediment
nutrient release by Z2G1 was small compared with both the total
sediment nutrient flux and the sum of other hypolimnetic fluxes.
Sediment and settling seston organic P composition was determined
using 31P nuclear magnetic resonance
(NMR). Settling seston and sediment samples were analysed during
winter and summer, representing, respectively, a mixing period
when the water column was well oxygenated and a stratified period
when the hypolimnion was anoxic. The bottom sediments and settling
seston contained orthophosphate, orthophosphate mono- and diesters,
pyrophosphates, polyphosphates, and phosphonates with organic
P content exceeding 60% of the total extracted P occasionally.
Phosphorus content in settling seston increased 2.5-fold in winter,
with a marked increase in orthophosphate content. The 31P NMR
analyses revealed the presence of several potentially bioavailable
P compounds, which may be recycled from the sediment to the water
column. An ‘apparent half-life’ value was used to quantify the
time scales on which these compounds are degraded within the sediment
and likely being recycled to the overlying water column. Relatively
long half-life values, ranging from 8 to 23 years, indicate that
this recycling could potentially reduce the efficacy and longevity
of in-lake restoration procedures that have been applied to Lake
Okaro.
A one-dimensional process based ecosystem model (DYRESM-CAEDYM)
was used to simulate the potential effect on water quality of
Lake Okaro of separate and combined reductions in external and
internal loads of nitrogen (N) and P. The model was calibrated
against field data for a two-year period and validated over two
separate one-year periods including a year immediately following
a Z2G1 application and a year when there was an extraordinary
algal bloom from an invasive, highly buoyant, N-fixing cyanobacterium,
Anabaena planktonica. The model simulations reproduced the scale
of phosphate and ammonium concentrations at 14 m depth, corresponding
to the deeper region of the hypolimnion, both before and after
the application of Z2G1, with no adjustment of parameters, suggesting
that there was little effect of the Z2G1, at least within the
uncertainties of the model runs. The model simulations were less
successful in reproducing the Anabaena planktonica bloom. This
was attributed to a lake of flexibility in the conceptualisation
and calibration of the model, which meant that it could not encompass
this invasive species. In the model scenarios with reduced nutrient
loading, the trophic status of Lake Okaro, given quantitatively
by the Trophic Level Index (TLI), decreased to a greater extent
with a given fractional reduction of the internal load than a
reduction of the external load. The control of both N and P was
shown in simulations to be more effective in reducing phytoplankton
biomass than for N or P alone, tending to affirm an N+P control
paradigm.
Undesirable shifts in zooplankton and phytoplankton species composition
due to the application of Z2G1 were investigated by comparing
the plankton community structure before and after the Z2G1 application.
No significant differences in species composition were found at
the depths investigated (surface and 9 m). However, further analyses
showed statistically significant differences between seasons,
indicating that seasonal variations in plankton composition far
outweighed changes that occurred as a result of the Z2G1 application.
In this study, field measurements and numerical modelling provided
a comprehensive assessment methodology of testing the response
of Lake Okaro to reduced nutrient loading, with a focus on P dynamics.
Although nutrient loads to Lake Okaro were reduced using catchment
and in-lake restoration methods, further substantial and prolonged
reduction in both N and P loading appear to be required to decrease
phytoplankton biomass and the trophic state of the lake. This
study highlights the need for investigating water column P content
and sediment P composition for evaluating the potential magnitude
of internal loading, and emphasises the importance of using process
based numerical modelling as a decision support tool for lake
management.
Table of Contents
Abstract .................................................................................................................
iii
Table of Contents .................................................................................................
vi
List of Figures ........................................................................................................
x
List of Tables .......................................................................................................
xv
Acknowledgments .............................................................................................
xvii
Preface ...............................................................................................................
xviii
1 General introduction ...................................................................................
20
1.1 Motivation .............................................................................................
20
The importance of phosphorus in lakes ................................................
20
Internal nutrient loads...........................................................................
20
Management of internal nutrient loads .................................................
22
Overview of Lake Okaro .......................................................................
24
1.2 Major objectives ....................................................................................
25
Field study .............................................................................................
25
Model application .................................................................................
27
1.3 Thesis overview ....................................................................................
27
1.4 References .............................................................................................
30
2 Effect of intensive catchment and in-lake
restoration procedures on phosphorus concentrations in a eutrophic
lake ........................................ 37
2.1 Introduction ...........................................................................................
37
2.2 Methods .................................................................................................
40
Study site ...............................................................................................
40
Overview of restoration procedure .......................................................
41
Alum application ...................................................................................
41
Constructed wetland..............................................................................
42
Riparian protection ...............................................................................
42
Modified zeolite application ..................................................................
42
Farm nutrient budget ............................................................................
43
Sampling and monitoring ......................................................................
43
Data analysis .........................................................................................
44
2.3 Results ...................................................................................................
46
Temperature, stratification and dissolved oxygen distribution
............. 46
Total phosphorus trend in the bottom and surface waters ....................
48
External and internal phosphorus loads ...............................................
50
Trophic Level Index and Trophic State Index .......................................
52
2.4 Discussion .............................................................................................
54
External P loading vs internal P loading ..............................................
54
Longevity of restoration measures ........................................................
55
Criteria for improved water quality using trophic level indicators
...... 57
Relative success of restoration methods ...............................................
58
2.5 References .............................................................................................
61
3 Hypolimnetic phosphorus and nitrogen dynamics
in a small, eutrophic lake with a seasonally anoxic hypolimnion
............................................... 67
3.1 Introduction ...........................................................................................
67
3.2 Materials and methods ..........................................................................
69
Study site ...............................................................................................
69
Water column sampling.........................................................................
70
Data analysis .........................................................................................
71
3.3 Results ...................................................................................................
76
Distribution of Z2G1 on lake bed ..........................................................
76
Stratification ..........................................................................................
77
Temperature, dissolved oxygen concentration, and chlorophyll
a fluorescence ...........................................................................................
77
pH ..........................................................................................................
80
Nutrient concentrations .........................................................................
80
Hypolimnetic fluxes of SRP and NH4-N ................................................
84
3.4 Discussion .............................................................................................
88
Effects of Z2G1 on nutrient dynamics ...................................................
88
Validation of nutrient flux model ..........................................................
90
Potential P release mechanisms in Lake Okaro ...................................
91
3.5 Conclusions ...........................................................................................
92
3.6 References .............................................................................................
94
4 Phosphorus dynamics in sediments of a eutrophic
lake derived from 31P nuclear magnetic resonance spectroscopy
.............................................. 101
4.1 Introduction .........................................................................................
101
4.2 Methods ...............................................................................................
103
Study site .............................................................................................
103
Sedimentation rates .............................................................................
104
Sediment sampling ..............................................................................
104
31P NMR analysis ................................................................................
105
Data analysis .......................................................................................
106
4.3 Results .................................................................................................
106
Sedimentation rates .............................................................................
106
Vertical sediment profiles ...................................................................
108
31P NMR analysis ................................................................................
110
4.4 Discussion ...........................................................................................
115
Interpretation of P speciation .............................................................
115
Forms of phosphorus...........................................................................
117
Implications of P speciation for sediment remediation.......................
119
4.5 References ...........................................................................................
121
5 Modelling the response of a highly eutrophic
lake to reductions in external and internal nutrient loading
.................................................... 126
5.1 Introduction .........................................................................................
126
5.2 Methods ...............................................................................................
128
Study site .............................................................................................
128
Model description ................................................................................
129
Model input data .................................................................................
131
Model calibration and validation ........................................................
133
Scenarios .............................................................................................
133
5.3 Results .................................................................................................
135
Calibration and validation ..................................................................
135
Base scenario ......................................................................................
140
Nutrient loading reduction scenarios ..................................................
144
Effect of nutrient load reductions on nutrient limitation ....................
146
5.4 Discussion ...........................................................................................
147
Model performance and constraints ...................................................
148
The implications of model scenarios for lake restoration ...................
150
5.5 References ...........................................................................................
153
6 Does sediment capping have post-application
effects on zooplankton and phytoplankton? ..........................................................................................
159
6.1 Introduction .........................................................................................
159
6.2 Materials and Methods ........................................................................
161
Study site .............................................................................................
161
Sampling ..............................................................................................
162
Analytical methods ..............................................................................
163
6.3 Results .................................................................................................
164
Zooplankton dynamics ........................................................................
164
Phytoplankton dynamics .....................................................................
164
Community analyses............................................................................
167
6.4 Discussion ...........................................................................................
169
6.5 Conclusions .........................................................................................
171
6.6 References ...........................................................................................
173
7 Conclusions ................................................................................................
178
7.1 Research summary ..............................................................................
178
7.2 Recommendations for future work......................................................
182
Appendix I .........................................................................................................
185