Diatoms in Stockholm County2021: A survey of 19 streams and two lakes

In Stockholm County, water quality was examined using diatoms in 19 premises in watercourses and two in lakes in 2021. For status classification with respect to the influence of nutrients and easily degradable organic pollutants, the diatom algae index IPS was used. The support parameters TDI (quantity of nutrient-intensive species) and %PT (the percentage of pollutant-tolerant species) have been taken into account in the assessment. For acidity classification, ACID index was used. Risk flagging for other types of influences, other than those IPS and ACID developed to show, was made with the support parameters malformation rate, number of calculated tariffs and diversity. The assessment received good status from Bränningeån, Holmsjöbäcken (expert assessment), Muskån-Lillån, Orlången, Tulkaströmmen and Åkersströmmen-Holmbroån. However, the IPS index in Orlången was very close to the moderate status limit. A risk flag of the results is issued for the Tulkaströmmen and Åkersströmmen-Holmbroån as the diversity was very low and in Tulkaströmmen the number of species counted was also very low. Holmsjöbäcken and Orlången are flagged for significant and strong environmental toxins. Moderate status was found in Broströmmen, Broströmmen-Järsöströmmen, Moraån-Ogaån, Märstaån, Norrtäljeån-Balkensån (expert assessment), Norrtäljeån-Malstaån, Norrtäljeån-Vretaån, Norrviken, Oxundaån-Rosendal, Skeboån, Skeboån-Harbroholmsån and Tyresån. IPS ended up very close to the border with good status in Broströmmen, Norrtäljeån-Malstaån and Tyresån and relatively close to the same border in Märstaån and Broströmmen-Järsöströmmen. Oxundaån-Rosendal, on the other hand, ended up relatively close to the border with unsatisfactory status. Unsatisfactory status was found in Bällstaån, Turingeån and Åkerströmmen-Lillån. The IPS index in Turingeån was close to the limit of moderate status, while in Åkerströmmen-Lillån it approached poor status. On all premises, the acidity index ACID showed either alkaline or near neutral conditions. In comparison to the last two/three-year period, most premises show the same or a similar result as in 2021. A clearly better result in 2021 showed Broströmmen, Bränningeån, Muskån-Lillån and Tulkaströmmen, while Norrtäljeån-Vretaån, Åkersströmmen-Holmbroån and Åkerströmmen-Lillån clearly showed worse.