Phosphorus eutrophication in an inter-drumlin lake: causes and effects Lucy Crockford Walsh 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 Directive 2000 Nitrates Directive 1991 Urban Wastewater Directive 1991 Surface Water Objectives 2009 Good Agricultural Practices 2006 (Revised in 2009) OECD and Tierney, 2008 SI

Pollution Sources in Rural Catchments DiffusePoint Unknown

Project Objective: To determine the main processes that are contributing to the increased P concentrations 1.External P loads 2.Internal P loads 3.Wind induced resuspension in shallow areas Research Questions 1.What are the main contributors of P? 2.Which contributors are the primary factor in the impediment of lake recovery despite source recovery? 3.How are the dynamics of the lake contributing to the P concentrations – biological, chemical, morphological, catchment use etc.? 4.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 Lake Buoys 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 a Deep 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 Bloom July 2011

References SØNdergaard M, Jensen JP, Jeppesen E. Retention and internal loading of phosphorus in shallow, eutrophic lakes. TheScientificWorld 2001; 1: Tierney D. Water Quality of Lakes. State of the Environment in Ireland. Environmental Protection Agency, Johnstown Castle, Wexford, 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 ( m 3 yr -1 ) Lake Volume m 3 Thermocline (6-7m) Statistical Significance Testing N = 11 Independent T test P = Difference in annual means is not statistically significant

1 m Data Sonde 9 m Data Sonde 1 m Data Sonde Lake Sediment WEST EAST Abstraction ( m 3 yr -1 ) Lake Volume m 3 Thermocline (6-7m) N = 11 Independent T test P = Difference 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 ( m 3 yr -1 ) Lake Volume m 3 Thermocline (6-7m) N = 11 Independent T test P = Difference 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 ( m 3 yr -1 ) Lake Volume m 3 Thermocline (6-7m) N = 11 Independent T test P = Difference in annual means is not statistically significant – but p value much lower than other sample combinations Statistical Significance Testing

High Frequency Turbidity