Monitoring of periphyton and heterotrophic growth at 25 sites in the water district Glomma south of Øyeren 2011-2020.

Summary

This monitoring program is the first year of a 4-year study agreed between NIVA and the Water District Glomma south of Øyeren. NIVA has been carrying out monitoring here since 2011. The aim of the study was to classify ecological status according to the water regulations (“Vannforskriften”), based on the biological quality elements of benthic algae and heterotrophic growth at 25 river and stream locations, to allow improvement measures to be followed up. The current study focuses on 2020, but where previous data were available (2011, 2013, 2016, 2017, 2018) these have been included, for a bigger picture and to allow trends to be discovered. According to the eutrophication index, PIT, three sites reached the environmental target according to the water regulations for 2020 and the rest of the sites were in moderate or poor status. This is not surprising given that large areas of the catchment are agricultural. Results for organic load, based on the heterotrophic growth index (HBI2), showed that one of the sites failed the environmental target in 2020. At that site, 14 % coverage of Sphaerotilus natans (a major component of «sewage fungus») was recorded. This indicates a heavy organic load and the site was therefore classified as poor status. The rest of the sites were classified as good or high status. In the overall ecological status classification (PIT and HBI2 combined) for 2020, three sites were classified as good status, thus reaching the target set out in the water regulations, while 18 stations were classified as moderate and 4 as poor status. There were no sites where the HBI2 index class was below the PIT index class, so the overall status was the same as PIT class in all cases. Previous monitoring data have been recorded from 21 out of the 25 sites investigated in 2020. Looking at the overall classification results across all the years, only one site was in good status in every year having data. In two cases the status has declined from good to moderate, and in two cases the status has varied considerably and crossed the good-moderate boundary at least once. The 16 remaining sites have all been classified to moderate or below for all years having data. In this study, all sites were classified and included in the overall assessment, even when the results of indices were uncertain. Class boundaries for the PIT index in clay rivers have not been defined, so it is not known if the current boundaries are properly applicable. This affects 4 sites in this study. The HBI2 index, according to the guidance, should be based on a minimum of two surveys per year, preferably spring and autumn, to get a reliable status classification. In this study it is calculated based on only one, summer survey, so the results should be treated as indicative only. As only three of the river sites achieved the environmental target according to the water regulations in 2020, and only five sites achieved the environmental target in previous monitoring, the need for environmental improvement measures and follow-up monitoring is necessary.