Brownification of freshwaters - the role of dissolved organic matter and iron

Abstract: The term brownification refers to the trend of increasing water color, i.e. the water becoming browner, which has been observed throughout the northern hemisphere over the last decades. Brownification has both ecological and societal implications. From an ecological point of view the impaired light climate may e.g. reduce aquatic primary production and affect predator-prey interactions within the water, while from a societal point of view brownification may obstruct drinking water purification and reduce the recreational value of lakes. Traditionally, the increasing water color has been ascribed to increasing concentrations of dissolved organic matter (DOM) from the catchment, as DOM concentrations and water color often correlate both spatially and temporally. Several mechanisms have been proposed as the driver behind the increasing DOM concentration, e.g. decreasing acidification, land-use changes and climate change with increasing precipitation and temperature. Interestingly, in many cases the water color has increased more than the DOM concentration, implying that increasing DOM concentration alone is not sufficient to explain the increase in water color. Thus, there must be other factors also affecting the water color. In this thesis I show, in a field experiment, that lower acid load results in a higher net charge of the organic matter in soils and thereby a higher solubility and mobility, which should facilitate a higher transport of DOM from the terrestrial to the aquatic system. Moreover, concurrent with the increase in mobility, there was a change in the quality of the DOM, where DOM from a lower acid load was relatively more colored, aromatic and of higher molecular weight. Thus, a reduction in acid load may contribute to brownification by increasing the export and the color of terrestrially derived DOM to the aquatic system. Experiments were performed to test if the altered quality of the mobile soil DOM may affect its reactivity in the aquatic system. It was found that the susceptibility to photodegradation increases, while the susceptibility to bacterial degradation decreases. The relative importance of each turnover process may hence be altered due to the decreasing acidification. Another factor affecting the water color of freshwaters is the concentration of iron (Fe) in the water. Still, the potential role of Fe to brownification has not previously been addressed. Data from long-term monitoring showed that water color of most Swedish rivers have increased significantly since the early 1970’s. More surprisingly, most rivers also exhibit strongly increasing iron concentrations (up to 470 %). Increases is DOM concentration were significantly lower than the increase in water color and theoretically, variations in Fe concentration could explain on average 25 % and up to 75 % of increasing water color. Fe plays a key role in aquatic systems, affecting the biogeochemical cycling of many major elements, such as carbon, nitrogen and phosphorus. Thus the increasing Fe concentrations may have profound consequences. By analyzing within-year variations in water chemistry, air temperature, and dis- charge of three Swedish rivers, I explored what control Fe concentrations. It appears that variations in Fe concentrations are primarily driven by redox dynamics in the catchment. High discharge and high temperature create conditions that favor bacterial activities and reductive dissolution of the largely insoluble Fe(III) to the more soluble Fe(II). Fe(II) may then be transported from the soil to the aquatic system. Once in the oxic stream water, interactions with DOM maintain the Fe in solution. Furthermore, long-term trends of increasing air temperature and discharge in these catch-ments may have extended the periods of reducing conditions by increasing microbial activity and soil saturation, and thus have facilitated Fe transport to the aquatic system. In summary, it appears that in south Sweden, where the acidification has been high historically, the impact of decreasing acidification on organic matter mobility may be a major factor behind brownification, whereas in the north where the acidification has been much more restricted, increasing Fe concentrations may be more important. Climate change with increasing precipitation and temperature may increase the prevalence of reducing conditions in the soils, further facilitating the export of Fe to the aquatic system and causing a continuous brownification of the freshwaters.