Increasing iron concentrations in surface waters – a factor behind brownification?

Abstract

Browning of inland waters has been noted over large parts of the Northern Hemisphere and is a phenomenon with both ecological and societal consequences. The increase in water color is generally ascribed to increasing amounts of dissolved organic matter of terrestrial origin. However, oftentimes the increase in water color is larger than that of organic matter, implying that changes in the amount of organic matter alone cannot explain the enhanced water color. Water color is known to be affected also by the quality of organic matter and the prevalence of iron. Here we investigated trends in water color, organic matter and iron between 1972 and 2010 in 30 rivers draining into the Swedish cost, and peformed a laboratory iron addition experiment to natural waters, to evaluate the role of iron and organic matter in determining water color. By comparing the effect of iron additions on water color in the experiment, to variation in water color and iron concentration in the monitoring data, we show that iron can explain a significant share of the variation in water color (on average 25%), especially in the rivers in the north of Sweden (up to 74%). Further more, positive trends for iron are seen in 27 of 30 rivers (21–468%) and the increase in iron is larger than that of organic matter, indicating that iron and organic matter concentrations are controlled by similar but not identical processes. We speculate that increasing iron concentrations can be caused by changes in redox conditions, that mean that more anoxic water with high concentrations of soluble FeII are feeding into the surface waters. More studies are needed about why iron is increasing so strongly, since both causes and consequences are partly different from those of increasing organic matter content

Deoxygenation of the Baltic Sea during the last century

“…Due to the long residence time of the Baltic Sea, elevated phosphate concentrations may still persist for decades. It is also possible that oxygen improvements anticipated from nutrient reductions have been counteracted by increases in organic matter inputs from land (22,23). Thus, the lack of recovery could be due to delayed responses and potential hysteresis in combination with shifting baselines associated with other components of global change (24).…”

Dissolved organic matter in the Baltic Sea

“…Furthermore, Fe concentrations have increased in the northeastern coastal zone and are closely linked to the Secchi depth (Fleming-Lehtinen et al, 2014), presumably also contributing to water color. In rivers in Sweden flowing into the northern Baltic Sea, Fe concentrations have increased more than the DOC concentrations (Kritzberg and Ekström, 2012).…”

The Absorption of Light in Lakes: Negative Impact of Dissolved Organic Carbon on Primary Productivity

“…Consequently, the concentration of DOC in most boreal lakes is predicted to increase under a moderate climate change scenario (Larsen and others 2011a). Being highly colored, this allochthonous DOC would lead to higher levels of CDOM, possibly also in conjunction with iron (Kritzberg and Ekströ m 2012). Increased CDOM would lead to higher light attenuation, and likely also lower PP A .…”