Many fresh waters, but also brackish coastal waters, are more or less strongly brown colored by terrestrial humic matter, both in temperate and tropical climate zones. During the last few decades there has been a marked increase in this coloring of surface waters in northern Europe and North America. This “brownification” (browning) impairs the light climate and adds organic matter that can provide a substrate for bacteria. Thus brownification is a process that has profound effects on aquatic ecosystems, including ecosystem services as recreation, fishing and drinking-water supply.
In a multidisciplinary project we study the causes of brownification and the consequences to aquatic ecosystems. We use paleolimnology (cooperation with quaternary geology, LU) to reconstruct past water color, and changes in land use that may affect the export of colored organic matter to surface waters . In field experiments the effects of acid deposition (SO4) on water color and the quantity and quality of organic matter in soil water is studied. Through comparison of lakes across a color gradient and in experimental studies we have investigated effects of brownification on lake thermal stratification, light climate and phytoplankton species composition and the balance between production and respiration.
In addition to terrestrial organic matter, iron is affecting watercolor. We study how increasing iron concentrations in surface waters may play a role in brownification. This is interesting, since both causes and consequences of increasing iron concentrations should differ from those of increasing organic matter.