Project: The application of high temporal resolution data in the management of eutrophic water-bodies in agricultural catchments
The project comprised of two specific functions: 1. Identifying the phosphorus (P) loading contributions from diffuse and point sources to a small river in Co. Louth, and 2. the apportionment of P load from catchment and internal sources to a small inter-drumlin lake in Co. Monaghan, both in Ireland.
The Athclare river catchment was located in a moderate to poorly drained area of gley soils and greywacke bedrock with a mix of arable and pastoral agricultural practices. Previous work in the region identified overland flow as a significant contributor of nutrients, in particular P, to local water-bodies (Figure 1). Observations by The Agricultural Catchments Programme, Teagasc, using high temporal resolution data (hourly; Figure 2) confirmed a sustained elevated P concentration, particularly during low flow, indicating a significant point source loading. Recently, a simple algorithm comprising of a power function was developed by Bowes et al. (2008) to apportion P loading to point and diffuse sources using P concentration and corresponding volumetric flow rates. A second model by Greene et al. (2011) aimed to fulfil a similar function but using a quadratic equation. Both of these models were developed using data collected at best 3 times per week and often only weekly, and so the primary objective of this study was to determine the effect of different sampling timings and frequencies on model outcomes and in particular the apportionment of P load to point and diffuse sources.
The main outcomes of this part of the study were that the two models were significantly different (p<0.01) from each other, and the precision and accuracy of the models reduced as sampling frequency reduced. Sampling timing during the week was also found to affect model outcomes with significant differences (p<0.05) observed between, for example, samples collected on a Monday, Tuesday, Thursday as opposed to a Monday, Wednesday, Friday. The results of this study raise uncertainty in developing management decisions on the outcomes of these models as the incorrect P source may be targeted in remediation efforts, which will impede water-body recovery to good water quality status to meet the Water Framework Directive objectives.
The lake catchment was situated in the Drumlin Belt which stretches from Donegal across north central Ireland, encompassing counties such as Fermanagh, Cavan and Monaghan. This region contains approximately 4000 lakes, with the majority of them under 50 ha in size. Lough Namachree (Figure 3) was selected for further study because although only covering 17 ha, due to lake bathmetry and orientation, a number of processes occur in the lake which are characteristic of larger lakes such as thermal stratification during summer and sudden water column mixing. High frequency sensors (Figure 4) measuring chlorophyll a concentration (indicative of algae productivity), dissolved oxygen, temperature, conductivity, turbidity, pH and redox were installed on the lake for 2.5 years at three locations. One set of sensors was located in the centre of the lake at a depth of 9m to measure the changes in the lake due to thermal stratification while a second set was located 8m directly above in the epilimnion. A third set located at the eastern end aimed to observe changes in the shallower part of the lake, which was more susceptible to mixing due to a prevailing wind, fetch and shallowing depth. Catchment P loading was measured in a proxy stream catchment located adjacent to the lake catchment.
The first outcome from this part of the project was a quantification of the contribution of P loading from sediment-derived sources which was found to be comparable to, and at times larger than, that from the catchment. While catchment sources provide P to the lake on a fairly continuous basis, a sudden lake turnover after a period of thermal stratification reintroduces P released from sediments to the eplimnion which is then used to fuel algal blooms. This mechanism is similar to a bag of fertiliser added to the lake on a single occasion. The second outcome of this study was an analysis of the effect of sampling frequency on lake, catchment and meteorological inter-relationships based on multiple linear regression. Similarly to the river component of the study, reducing sampling frequency from hourly to monthly was found to affect the variables used to describe the chlorophyll a concentration. The study also found that total particulate P was the most appropriate variable to describe chlorophyll a which was logical as the primary component of the particulate P fraction in lakes during summer is algal cells. Overall, an inadequate quantification of internal loading from sediments, and conclusions drawn from monthly sampling may impede further lake remediation which is a relevant issue across Europe and other temperate climates.
Results from this study were published in Inland Waters with two more manuscripts prepared for review. A study by Dr. Barry O’Dwyer was also completed in the early part of the PhD project. The project was funded by a Walsh Fellowship through Teagasc and by the Agricultural Catchments Programme. Lucy also received financial support from the International Society for Limnology (SIL) and the Global Lakes Ecological Observatory Network (GLEON) to attend international conferences and disseminate project findings. The PhD project was supervised by Prof. Phil Jordan (UCC), Prof. David Taylor (NUS) and Prof. Carlos Rocha (TCD) with notable supervisory support from Dr. Alice Melland (USQ).
Lucy now works as a Lecturer in Soil and Water Management at Harper Adams University, Shropshire, UK. She is actively searching for new research projects and welcomes any inquiry of collaboration on firstname.lastname@example.org.
List of Publications from Project:
Crockford, L., Jordan, P., Melland, A. R., Taylor, D., 2015. Stormtriggered, increased supply of sediment-derived phosphorus to the epilimnion in a small freshwater lake. Inland Waters, 5: 15-26.
O’ Dwyer, B., Crockford, L., Jordan, P., Hislop, L., Taylor, D., 2013. A palaeolimnological investigation into nutrient impact and recovery in an agricultural catchment. Environ Manage, 124:147-155.
Crockford, L., O’Riordain, S., Taylor, D., Melland, A., Shortle, G., and Jordan, P., 2015. Phosphorus load apportionment modelling in rivers and the effect of sampling frequency and timing.
Crockford, L., O’Riordain, S., Taylor, D., Melland, A., Shortle, G., and Jordan, P., 2015. The seasonal effect on phosphorus load apportionment modelling in rivers.
Bowes, M.J., Smith, J.T., Jarvie, H.P., and Neal, C. (2008). Modelling of phosphorus inputs to rivers from diffuse and point sources. Science of the Total Environment 395(2-3), 125-138. 10.1016/j.scitotenv.2008.01.054.
Greene, S., Taylor, D., McElarney, Y.R., Foy, R.H., and Jordan, P. (2011). An evaluation of catchment-scale phosphorus mitigation using load apportionment modelling. Science of the Total Environment 409(11), 2211-2221. 10.1016/j.scitotenv.2011.02.016.