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