The periphyton index of trophic status PIT: a new eutrophication metric based on non-diatomaceous benthic algae in Nordic rivers

Eutrophication is one of the major problems for surface water quality in Norway, particularly in the lowlands near settlements and agricultural areas. Here, we present a new index based on non-diatomaceous benthic algae (Periphyton index of trophic status, PIT) which is developed on a dataset of > 500 samples from > 350 sites from the Norwegian mainland and can be used to describe trophic status at a river site. PIT indicator values for benthic algae taxa are derived from water total phosphorus concentrations and range from 1.87 for Stigonema hormoides to 68.91 for Tribonema sp. PIT site values range from 3.42 to 44.45 and cover a range from oligotrophic to eutrophic conditions. The relationship between the PIT and the total phosphorus concentration has one major threshold at 10 mu g/l TP, with a slow increase below and a steep increase above 10 mu g/l. We conclude that benthic algae species composition at nutrient poor sites reacts only slightly to small increases in phosphorus concentration, while it is most sensible to eutrophication in the range between 10 and 30 mu g TP/l. For the genus Oedogonium, we found a significant positive correlation between filament width and TP concentration, making Oedogonium an easy to use eutrophication indicator.