phosphorus eutrophication in an inter-drumlin lake: causes and effects lucy crockford walsh fellow...
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Phosphorus eutrophication in an inter-drumlin lake: causes and
effects
Lucy CrockfordWalsh Fellow
Trinity College Dublin
What’s impeding this lake’s recovery?
Outline
• Policy• Pollution sources and eutrophication• Work on the lake• Results so far• Interpretation
Policy
Water Framework
Directive2000
Nitrates Directive 1991
Urban Wastewater Directive
1991
Surface Water Objectives 2009
Good Agricultural Practices
2006(Revised in 2009)
OECD and Tierney, 2008
SI 272 2009
Pollution Sources in Rural Catchments
DiffusePointUnknown
Project Objective:
To determine the main processes that are contributing to the increased P concentrations1. External P loads2. Internal P loads3. Wind induced resuspension in shallow areas
Research Questions
4. What are the main contributors of P?5. Which contributors are the primary factor in the impediment of lake
recovery despite source recovery?6. How are the dynamics of the lake contributing to the P concentrations –
biological, chemical, morphological, catchment use etc.?7. How can high frequency monitoring improve our understanding of lake
dynamics particularly the impact of episodic events, DO fluctuations, and the signal of the Chl a sensor?
Internal Lake Sources
Source: SØNdergaard et al., 2001
Work on the LakeBuoys
Data Sondes – Chlorophyll a, conductivity, dissolved oxygen, temperature, redox, pH - hourly
Manual sampling – P fractions, phytoplankton, zooplankton
Bi-monthly
Cross Section of the Lake
Neighbouring Catchment
Results so Far
Weather and external loading
Manual Sampling
P fractions, Nitrate and Chl a
Surface Sondes Chl aDeep Sonde DO
Summary Statistics Water Year Oct 2010 to Sept 2011
Conclusions• Lake is switching between meso and eutrophic
status.• The external load of P is low - inline with
source reduction?• Internal loading is occurring in anoxic water.• The lake suddenly mixing fertilises photic zone
– May 2011• Wind factor seems to not be increasing P
concentrations at eastern end.
Future Work•Chl a crash in spring not determined yet...•Lack of Chl a response to high nutrients at end of summer during fine weather also not determined•Quantify P loads and signal from sondes wrt chlorophyll a•Draw conclusions on the factors impeding lake recovery.
Algal BloomJuly 2011
References• SØNdergaard M, Jensen JP, Jeppesen E. Retention and internal loading of
phosphorus in shallow, eutrophic lakes. TheScientificWorld 2001; 1: 427-442.
• Tierney D. Water Quality of Lakes. State of the Environment in Ireland. Environmental Protection Agency, Johnstown Castle, Wexford, 2008.
Acknowledgements• The Agricultural Catchments Programme, Teagasc• Profs Philip Jordan and David Taylor• Drs Alice Melland and Per-Erik Mellander• Mr John Kennedy• GLEON technical group
Questions?
Hypotheses – Spring Chl a crash
1. Grazing pressure.2. Lake overturning and “diluting” the
Chlorophyll a concentration.3. Reduction in available nutrients so
population could not be sustained.
Hypotheses – Increased Deep Water SRP concentrations
1. Release from sediments aka internal loading.2. External inputs from the catchment.3. Wind induced resuspension of sediments.
Hypotheses – Increased Surface SRP concentrations at end of Summer
1. Lake overturning due to poor weather.2. External loading from the catchment.3. Wind induced resuspension.
1 m Data Sonde
9 m Data Sonde
1 m Data Sonde
Lake Sediment
WEST EAST
Abstraction
(245684 m3yr-1)
Lake Volume
1223389 m3
Thermocline (6-7m)
Statistical Significance Testing
N = 11
Independent T testP = 0.9243Difference in annual means is not statistically significant
1 m Data Sonde
9 m Data Sonde
1 m Data Sonde
Lake Sediment
WEST EAST
Abstraction
(245684 m3yr-1)
Lake Volume
1223389 m3
Thermocline (6-7m)
N = 11
Independent T testP = 0.9609Difference in annual means is not statistically significant
Statistical Significance Testing
1 m Data Sonde
9 m Data Sonde
1 m Data Sonde
Lake Sediment
WEST EAST
Abstraction
(245684 m3yr-1)
Lake Volume
1223389 m3
Thermocline (6-7m)
N = 11
Independent T testP = 0.9362Difference in annual means is not statistically significant
Statistical Significance Testing
1 m Data Sonde
9 m Data Sonde
1 m Data Sonde
Lake Sediment
WEST EAST
Abstraction
(245684 m3yr-1)
Lake Volume
1223389 m3
Thermocline (6-7m)
N = 11
Independent T testP = 0.1752Difference in annual means is not statistically significant – but p value much lower than other sample combinations
Statistical Significance Testing
High Frequency Turbidity
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