Ecological status assessment of clay rivers with naturally enhanced water phosphorus concentrations

• Water P and the PIT index were higher in unimpacted clay than non-clay rivers. TP was less bioavailable rivers Natural water increases with catchment cover. The good/moderate class boundary species composition of benthic algae changes as phosphorus concentrations increase, these can be used for ecological status assessment according to Framework Directive. background streams that are unaffected by anthropogenic impacts usually low. Running waters draining catchments deposits marine clay, however, may have enhanced concentrations, because is naturally rich apatite. Almost all areas been cultivated centuries, fertilization has increased soil levels. It has, therefore, difficult disentangle natural from anthropogenically areas. We compared Periphyton Index Trophic Status PIT, between non-clay, impacted Norway. found rivers, indicating indeed without clay. In addition, phosphate-P contributed 18–23% total but 33–37% affected agriculture. This indicates significantly Based on findings, we derived new boundaries Clay suggested assessed only two classes, i.e., “good or better” “moderate worse”, respectively. shown increase increasing .