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No. 296: Development of biological index for benthic algae (phytobenthos) in Danish streams

Andersen, D.K., Larsen, S.E., Johansson, L.S., Alnøe, A.B. & Baattrup-Pedersen, A. 2018. Udvikling af biologisk indeks for bentiske alger (fytobenthos) i danske vandløb. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 42 s. - Videnskabelig rapport nr. 296.  http://dce2.au.dk/pub/SR296.pdf


According to the EU Water Framework Directive, ecological status must be assessed using the biological quality elements of aquatic flora, invertebrates and fish, where the aquatic flora includes algae (phytoplankton and benthic algae) and aquatic plants. At present, Danish indices for evaluation of the ecological status of invertebrates, fish and macrophytes are implemented, while there is not yet an index for algae. The purpose of this project is to develop and intercalibrate a Danish index for the benthic algae for use in assessing the ecological status of streams.

During the period 2013-2016, samples of benthic algae were taken at 524 stations included in the National Monitoring Program on the already designated 100 m stretches where all surveys encompassed by the control monitoring are conducted. From the same sites, data are available on environmental parameters on natural, physical, chemical and area conditions. For a subset of the sites, also the water flow was measured.

Initially, the correlation between the species composition and the various environmental parameters was examined. The results showed that the benthic algae communities occurring in Danish streams vary along gradients in water chemical conditions and that these are more important than the physical and hydrological conditions for the algae communities. We found that the concentration of PO4-P, alkalinity and then the degree of organic matter were the most important parameters for the species communities.

Since most existing indices for benthic algae are based on the fact that nutrient concentrations and/or organic stress are the primary impact factors, it was examined whether one or a combination of these may be used under Danish conditions. We found that an average of the two indices  SID (Saprobic Diatom Index) and TID (Trophic Diatom Index), hereinafter referred to as SID_TID, explained the highest amount of variation (26%) relative to the primary impact factor, the PO4-P-concentration. These two indices are used in several other EU countries either alone or in combination as an index for the assessment of ecological status.

The Water Framework Directive requires that the index of the biological quality elements is intercalibrated (harmonised) with the selected index in the countries belonging to the same intercalibration group. This will ensure consistency of the ecological status assessments between the countries. It is a prerequisite for intercalibration that the index has a correlation of at least 25% with the impact factors to which the index is sensitive. The recommended index, SID_TID, has an explanation rate of 26% and can therefore be intercalibrated.

The intercalibration should follow the procedure described in connection with the intercalibration prepared by Denmark’s intercalibration group in 2008. It is based on the relationship between the selected index, SID_TID, and an "Intercalibration Common Metric" (ICM), which is an average of the two widely used indices (TID and IPS) that have shown high correlation with most of the other used indices. In connection with the joint intercalibration, intervals for ICM were determined as to the threshold between high/good and good/moderate status. When ICM shows good correlation with the selected index, these intervals can be transferred to SID_TID and the thresholds between the status classes high/good and good/moderate can be defined. Moreover, the thresholds between the different ecological status classes are determined using a Threshold Indicator Taxa Analysis (TITAN), which identifies PO4-P concentrations where a significant shift occurs in the species composition (change point).

It is recommended to use the results from the TITAN analyses to define the thresholds between high/good and good/moderate. The change points for the species responding negatively to increasing PO4-P-concentration constitute the threshold between the high and good status classes, while the change points for the species responding positively to increasing PO4-P-concentration constitute the threshold between the good and moderate status classes. The threshold can be based either on the identified change points or on the given percentiles, depending on the degree of probability/risk required for the actual change point to be included.