Understanding the relationship between total phosphorus and humic substances in organic-rich Scottish loch waters for the improvement of current EU WFD models
Item statusRestricted Access
Embargo end date31/07/2022
The interactions between aqueous humic substances (HS), the principal component of natural organic matter, and phosphorus (P), an important nutrient, are complex and poorly understood. Past research has shown that P has the potential to form complexes with HS in fresh waters. Humic complexation may change P bioavailability within such waters and, in addition, influence the transport of phosphorus within catchment settings. This project aims at examining the relationships and associations of P, HS, and iron (Fe) in Loch Meadie, Sutherland, northern Scotland, which has been assessed as reference condition with respect to P. “Reference condition” means the loch should not have been disturbed or only minimally impacted by human activities. Results will be used for identifying characteristic parameters which can enhance the performance of the model used to predict reference condition total phosphorus (TP) concentrations for highly-coloured Scottish lochs under the Water Framework Directive. Apart from Loch Meadie, samples collected from other reference condition lochs in northern Scotland and Shetland were also analysed in order to enhance our understanding of the relationship between TP and HS by including different colour types of reference condition lochs (clear water, mesohumic and polyhumic water). Significant seasonal trends in the water colour and total Fe (TFe) were observed from the Loch Meadie (Swordly) monthly samples. From mid-autumn to early winter (October-December), the water colour and TFe was typically higher than in the other seasons. The HS in Loch Meadie (Swordly) had relatively small molecular weight and low aromaticity, which was closer to fulvic acid rather than humic acid. According to the results of the 16 northern Scotland (mainland) and Shetland lochs, the total organic carbon (TOC) / dissolved organic carbon (DOC) and water colour were higher in polyhumic lochs than mesohumic lochs. The TP and total filterable P (TFP) varied in water samples collected from different lochs, whereas the TFe and total filterable Fe (TFFe) in polyhumic lochs was generally higher than that in mesohumic lochs. Ultrafiltration was used to isolate the colloidal component from loch waters, which was then fractionated by gel filtration chromatographic and centrifugal ultrafiltration methods. The gel filtration chromatographic fractionation of the colloids revealed two brown-coloured bands which had distinctive UV/Vis spectral features. The first eluting band had larger and more aromatic HS molecules than the second band, and in addition both P and Fe were primarily associated with the larger, more aromatic HS. This result demonstrated that P was able to form complexes with HS, in particular larger and more aromatic humic molecules, under the presence of Fe in the dissolved fraction of loch waters. Results from the centrifugal ultrafiltration method agreed with the gel filtration chromatographic results. The SEM-EDX results showed that P was likely bound with both the organic and inorganic particles in the particulate fraction of loch waters, however, the detailed binding mechanism between P and suspended solids was unclear and needs further investigation. Based on the main findings of this research, the current WFD models were improved by adding two new parameters, UV254nm (or SUVA254nm) and Total Suspended Solids. The new TP models, compared to the WFD non-humic model which is currently used by SEPA, gave significantly better prediction of the reference TP concentrations in most of the highly coloured Scottish lochs studied in this research.