Effects of forestry ditch cleaning and supplementary ditching on water quality

The effects of ditch cleaning and additional ditching of forest land were studied at a site in central Finland. The results from the two first years demonstrate that the influence of these measures on water quality is very similar to that of new forest ditching, i.e. the increases in total solids, total phosphorus, total nitrogen, pH and buffering capacity are of the same order of magnitude. On the other hand, the amount of ammonia increased more than in most new ditchings in Finland. Nitrate nitrogen load increased due to the high cocentrations during the snow melt in April–May 1994 and the variability in iron and manganese cocentrations were more pronounced. The amount of total solids deposited on artificial surfaces (plastic plates) in the brook Ruununsuonoja increased four-fold, mineral solids five-fold and organic solids 3.5-fold after ditching. The most striking changes in the microalgal flora in the first year after ditching of the Ruununsuo drainage area were the decline in the mainly acidophilic diatoms of the family Eunotia and in the green algae and the increase in family Euglenophyceae and the diatom genera Nitzschia and Tabellaria, mainly due to the increase in pH, nutrient status and organic matter in the brook.