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No. 440: The importance of the alkalinity of Danish watercourses for the benthic algae communities


Baattrup-Pedersen, A., Johnsen, T.J. & Riis, T. 2021. Betydningen af danske vandløbs alkalinitet for de bentiske algesamfund. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 34 s. - Videnskabelig rapport nr. 440. http://dce2.au.dk/pub/SR440.pdf


Previously, it has been shown that values of the benthic algae index, SID_TID, in Danish streams may change as a function of both the phosphorus concentration of the stream water (PO43-P) as well as alkalinity (Andersen et al., 2018). However, it is unclear to what extent alkalinity influences the sensitivity of the SID_TID index to phosphorus, and it is therefore not possible to describe the significance of only phosphorus in the assessment of ecological state.

The purpose of this project is: i) to analyse how the benthic algae index for Danish streams, SID_TID, varies in streams with different alkalinity and ii) to examine whether high alkalinity alone may hinder fulfillment of the environmental objective of good ecological status in streams applying the index.

In this project, we used two approaches to examine the significance of the alkalinity and phosphorus concentrations for SID_TID index values. Firstly, the variation in SID_TID values is described for four different groups of streams with different levels of phosphorus and alkalinity using algae data collected within the framework of the NOVANA monitoring program. Secondly, we describe the variation in SID_TID index values in algae communities established on artificial algae substrates with and without added phosphorus after three weeks of growth in high- and low-alkaline experimental streams.

The analyses show that there is great variation in SID-TID index values in both the high-and low-alkaline streams. Thus, the index varies between 1.41 and 2.23 in the low-alkaline streams and between 1.69 and 2.47 in the high-alkaline streams, where low values are usually attributed to poor impact, while high values are attributed major impact.

The analyses also show that SID_TID index values are higher in streams with high alkalinity than in streams with low alkalinity, regardless of phosphorus levels. This means that the SID-TID index value in a stream can assume relatively high values even if the P concentration is low. Thus, in one of the experimental streams, we found that the ecological status was moderate using the SID_TID index, even though the phosphorus concentration was low.

On the basis of the analyses in this report, it can be concluded that streams can be assessed to have moderate ecological status using SID_TID, even though elevated levels of phosphorus were not recorded during the experimental period. This means that it is not possible, on the basis of the existing knowledge, to assess the extent to which an assessment of ecological state with SID_TID reflects levels of phosphorus or alkalinity, and there is therefore a need for further development of the index. This further development can naturally be based on those diatom species for which the impact of alkalinity and phosphorus can be separated. However, in its current form, the SID_TID index can be used in all Danish streams, as previously recommended in Andersen et al. (2018), but there is a need for more knowledge in order to be able to plan interventions in these if environmental objectives are not met.