Still phosphorus waters run deep

Ireland’s lake water quality has been under scrutiny since the Water Framework Directive was implemented in 2000. Inhabitants of north central Ireland can ascribe to the large number of lakes whose quality may be detrimentally affected by the agricultural practices and soil types in the region (Figure 1). However, another factor may be influencing the water quality in these inter-drumlin lakes which, as of yet, is not addressed by current management practices in lake catchments.

Figure 1
Figure 1 Lakes located in the drumlin belt across north- and west-central Ireland

 

Thermal stratification in moderately deep lakes, such as Lough Namachree in Monaghan (Figure 2), results in an absence of oxygen in the hypolimnion (the deep section of the lake). In hypoxic conditions, a chemical reaction with iron results in the release of a soluble phosphate compound from lake sediments, a legacy of past sedimentation of catchment P transfer (O’Dwyer et al. 2013).

Figure 2 (a) Location of study site in north-eastern Ireland; (b) Lough Namachree catchment and proxy stream catchment boundaries with location of stream bankside analyser; and (c) location of lake monitoring stations (west and east buoys) and bathymetry of lake with inflows and outflow identified. Figure produced using ArcGIS 10.1
Figure 2 (a) Location of study site in north-eastern Ireland; (b) Lough Namachree catchment and proxy stream catchment boundaries with location of stream bankside analyser; and (c) location of lake monitoring stations (west and east buoys) and bathymetry of lake with inflows and outflow identified. Figure produced using ArcGIS 10.1

The thermal separation between the photic, biologically active epilimnion and the phosphorus rich hypolimnion continues throughout the summer until a drop in air temperature causes the lake to turnover at the end of the season, and beginning of autumn. This reintroduction of the P-rich hypolimnetic waters to lake surface water has little effect on phytoplankton growth at this time of year with reduced sunlight and lower air temperatures. However, a recent study published in Inland Waters by Crockford et al. (2015) observed abrupt lake turnover in late spring and early summer in 2011 and 2012 resulting in a P load from sediment derived sources which was comparable to, and in 2011 higher than, the P load from the catchment.

Algal growth periods showed varying relationships with the two P loading mechanisms, following the reintroduction of P-charged hypolimnetic waters and increased catchment P transfer while periods of baseflow were shown to be very different from times of high P availability. With an increase in summer storms due to climate change predicted by studies (Hanna et al., 2008), the occurrence of abrupt lake turnover may increase and delay the recovery of impacted lakes from eutrophication.

Author: Lucy Crockford, Lecturer in Soil and Water Management, Harper Adams University

Lucy is a former PhD student of TCD Geography, University of Dublin where she studied the drivers of P eutrophication in rivers and lakes. She would like to thank the staff and students of TCD Geography and the School of Natural Sciences for their help in completing her fieldwork over the last 4 years and for giving her experience in teaching and supervising students which allowed her to successfully apply for her new role in Harper Adams University.

Figure3

The article is available from Inland Waters for free as a member of SIL or for a small fee.

Crockford, L., Jordan, P., Melland, A.R., and Taylor, D. (2015) Storm-triggered, increased supply of sediment-derived phosphorus to the epilimnion in a small freshwater lake. Inland Waters 5(1), 15-26. 10.5268/IW-5.1.738.

Hanna, E., Cappelen, J., Allan, R.J., Jόnsson, T., Le Blancq, F., Lilllington, T., and HIckey, K. (2008) New insights into north european and north atlantic surface pressure variability, storminess, and related climatic change since 1830. Journal of Climate 21, 6739-6766. 10.1175/2008JCLI2296.1.

O’Dwyer, B., Crockford, L., Jordan, P., Hislop, L., and Taylor, D. (2013) A palaeolimnological investigation into nutrient impact and recovery in an agricultural catchment. Environmental Management 124, 147-155.

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