The work focused on better understanding of spatial and temporal relationships between phosphorus and nitrogen concentrations and organic matter measurements in agricultural catchments. This evaluation was carried out based on collection of both low- and high-temporal resolution water quality data:
1. Low-temporal resolution data have been collected every two weeks for 10 agricultural streams and 9 drained observation agricultural fields, for 2 years. This sampling allowed to build a large database of phosphorus, nitrogen and organic matter responses to varying weather, flow and land use conditions.
2. High-frequency resolution data including turbidity, organic matter fluorescence, water temperature and flow discharge have been every 15 minutes for year and a half in one selected agricultural stream, with high eutrophication pressures.
Preliminary results against the original project objectives
Task 1 Building of knowledge base
• Building a database of water quality, nutrient concentrations and optical measurements for the SMPA samples collected throughout the project – The database was created in Matlab and includes 50 weeks of data on optical measurements (absorbance and fluorescence spectroscopy), nutrients, total and dissolved organic carbon and turbidity (Figure 1).
• Testing the relationship between in situ optical measurements and nutrient concentrations to account for variation in catchments’ characteristics and flow conditions – The relationships between optical measurements and nutrient concentrations are catchment-specific and their strength vary on a seasonal basis.
• Evaluation of the quenching effects from turbidity, temperature and inner-filter effect, their spatial and temporal variability and calibration and validation of the compensation equations – Quenching effects are much higher for clay catchments but in all catchments undergo significant seasonal variation driven by flow dynamics. Those effects are correctable through compensation equation for the turbidity and temperature range observed in the catchments.
Task 2 Hot-spots of nutrient delivery
• Deployment of the optical sensors for real-time measurements at sampling points in the 2 pilot catchments and collection of water samples to target storm events and agricultural practices – Initial analysis indicated that sand catchments show low quenching effects, thus the optical sensor was deployed in a clay catchment to understand how turbidity and optical measurements vary on a seasonal basis (Figure 2).
• Testing the relationship between in situ optical measurements and nutrient concentrations to account for a variation in flow, water quality characteristics, differences in catchments’ characteristics and agricultural land use – Turbidity measurements can be used as proxy for sediment and particulate phosphorus concentrations. Although CDOM and tryptophan-like fluorescence do not provide universal correlations with nutrient concentrations (since they are site- and season-specific), they provide proxy for understanding solute dynamics.
Task 3 Evaluation of the method’s potential and limitations
• Evaluation of the efficacy of the optical measurements as a proxy for nutrient concentrations based on the database built in Tasks 1 and 2 – as above.
• Determination of the method’s potential, limitations and practical requirements to be used in agricultural catchments to provide quantitative evidence of hot-spots of nutrient delivery – Optical sensors are the future of water quality monitoring. They are portable and accurate and therefore enable targeting hot-spots of nutrient delivery both in space (for different catchments and streams) and time (throughout the hydrological year). However, evaluation of the quenching effects is needed for each sampling site which will result in development of the compensation equations for turbidity and temperature. Another limitation highlighted by this project is biofouling as seen in Figure 2 for uncorrected CDOM measurements in summer 2017. Biofouling results in a build-up of microorganisms on the windows of optical sensors resulting in a gradual decrease of signal between the sensor maintenance visits. This effect is particularly observed during the summer months in clay catchment streams which show high degree of eutrophication, as can be seen in the graphical abstract. Regular maintenance, calibration and cleaning of the optical windows is mandatory.
