Varför är Bivarödsånsvatten så brunt?
The river Bivaröd is one of the river Helgeån tributaries which has the highest and the fastest increase in water colour. The reason behind this increase is unclear but several factors have been suggested to affect this pattern. In this work, I studied the role of land use, iron and precipitation for water colour increase in river Bivaröd for the period 1976 -2011. Data from a field study along with data from SRK for river Helgeån were used for the analyses. The results showed that both water colour and iron concentration increase and that there is a strong correlation between these two variables. Land use and especially intensive forestry and short distances of the water flow are suggested as possible factors that may affect the water colour. Approximately 68 % of the river Bivarödån basin is covered by forest. Forests are a major source of humic matter production that gives the water its brown colour. The increase in iron concentration may be alarming considering that river Bivarödån has low alkalinity. Iron in conjunction with low alkalinity may affect the water ecosystem negatively. The climate seems to play an important role in the dynamics of humic substances in river Bivaröd. Water colour was significantly lower in January than in July. Ground frost which forms in January because of the low temperatures is suggested as a major reason why humic substances leaching is inhibited at this time of year. Extended periods without frost in the ground because of warmer climate can contribute to watercolor increase.