Pacheco, J.P, Larsen, S.E. & Baattrup-Pedersen, 2023. Alkalinity and its influence on benthic diatom assessments in Danish running waters. Aarhus University, DCE – Danish Centre for Environment and Energy, 30 pp. Scientific Report No. 521. http://dce2.au.dk/pub/SR521.pdf
Alkalinity co-varies with PO4 in Danish running waters, and this aff0ects the applicability of the diatom-based ecological indicator SID_TID to assess an- thropogenic eutrophication. Here, NOVANA and Omnidia databases on Danish watercourses from 2013 to 2020 were combined to i) analyse to what extent alkalinity influences the ability to meet ecological targets in running waters using SID_TID and to ii) explore the possibility of disentangling the effect of alkalinity from that of PO4 based on the abundance of specific benthic diatom species previously identified to be abundant at low PO4 and to in- crease in abundance with increasing alkalinity. The results confirm previous findings of the high co-variation of PO4 and alkalinity, and that the correla- tion between alkalinity and SID_TID was even higher than for alkalinity and PO4, indicating that alkalinity is an important interfering factor influencing SID_TID assessments. However, relationships between specific benthic dia- tom species and alkalinity could not be established, and therefore these spe- cies could not be used to disentangle the effect of alkalinity from that of PO4 in Danish running waters. Instead, another approach was chosen to identify watercourses where alkalinity might influence the chance of meeting ecolog- ical targets using SID_TID. By applying model estimates derived from a lin- ear regression analysis from empirical data, we generated a probability-based threshold for reaching good ecological status applying SID_TID along a gra- dient in alkalinity. This indicated a drastic decline in the probability of com- pliance with good ecological status based on SID_TID (< 2.39) in water- courses with alkalinities higher than 2.6 mEq/L. Based on these results, we advocate for taking into consideration the level of alkalinity in Danish run- ning waters in management planning.
