The Role of Natural Organic Matter and Phosphorus in a Changing Environment

Abstract

Eutrophication is globally a growing concern, as freshwater bodies are becoming more and more exposed to nutrient pollution. Excessive P loading is often the main cause of eutrophication, as P is often the limiting nutrient to phytoplankton growth. P pollution to lakes and reservoirs generally comes both from non-point sources, such as runoff from agricultural areas, and point sources, such as sewage from wastewater outlets. Abatement actions regulating the application of fertilisers and ploughing regimes for agricultural areas, and re-routing of sewage outlets, greatly lowers the P loading to the waterbodies. As such, eutrophication problems can generally be easily solved given the right incentives. Lake Vansjø, located in an agricultural district in south eastern Norway, has experienced a continues worsening in water quality over the last 30 to 40 years, as a of result eutrophication. However, in the case of Lake Vansjø, over 500 million Norwegian kroners has been spent on sewage infrastructure upgrades and abatement actions over the years, without achieving the expected improvements. In 2009 the EUTROPIA project was funded, partly with the aim to improve our understanding of the catchment as a whole and better understand why the abatement actions have not had the desired affects. It is this part of the project which is the focus of this study. The study found that the decline in acid rain over the last 30 to 40 years is one of the main explanations for why the abatement actions have not had the desired affects. The decline in acid rain, has resulted in a 3 times reduction in the concentration of labile Al. The study predicts that the orthophosphate/phosphate concentration in the lake, in the 1980’s, would have been ≈ 1/5 of the concentration of what is today, due to Al co-precipitation with phosphate. Additionally, over the same period there has been a doubling in the concentration of Dissolved Organic Matter (DOM), partly explained by climate change and partly by the decline in acid rain. DOM photo- and biodegradation studies were conducted to better assess what impact an increase in DOM may have on the lake water quality. The study found that photodegradation greatly enhances the biodegradability of the DOM: Photodegradation alone after 20 hours exposure contributed to 26% mineralisation of the original Dissolved Organic Carbon (DOC). Of the remaining DOC, 3 to 39% was mineralised by micro-organism, for 0 to 20 hours exposure, respectively. The study suggests that humic substances become more biodegradable after irradiation, particularly humic acids (HA). Fulvic acids (FA) seem to be a product of HA degradation, in addition to bioavailable DOM fragments. It is postulated that the increase in the DOM concentration may have a significant impact on the microbial biodiversity in shollow lake basins, such as Vanemfjorden, Lake Vansjø’s western basin. The reduction in light attenuation, and the higher degree of low-molecular-weight bioavailable DOM, suggests migration of bacteria and phytoplankton to the surface in search for energy. However, the production of radicals from DOM photodegradation most likely will also result in a deathly environment for micro-organisms. The DGT study of the lake seems to confirm these findings, since more bioavailable P was found near the surface of the lake than any other depth. The Diffusive Gradients in Thin films (DGT) were found be useful as passive samplers for two bioavailable P fractions; Dissolved Reactive Phosphorus (DRP; approx. orthophosphate) and Low-Molecular-Weight Organic Phosphorus (LMWOP). The concentration of LMWOP is approx. equal or larger than orthophosphate in forested steams. If the concentration of LMWOP, along with the rest of the DOM, has doubled over the last 30 to 40 years; and considering that 85% of the catchment is forested area, and the largest contributor of DOM to the lake; then it is highly likely that climate change and acid rain decline have had an impact on eutrophication.