Water Environment and Water Services (Scotland) Act 2003 implementation: environmental standards - consultation

Consultation seeking views on our proposals for new or updated environmental standards for Scotland’s water environment, and updates to some of the assessment methods used for deriving such standards.

5. River Phytobenthos

River phytobenthos refers to a mostly microscopic group of freshwater algae found attached to submerged surfaces such as stones and plant stems. Phytobenthos are good indicators of nutrient enrichment and other pressures, and are used to assess river water quality. Diatoms are the main plant groups that are used because their silica shells are easy to identify under the microscope.

The current tool for river phytobenthos assessment is DARLEQ2 (Diatoms for Assessing River and Lake Ecological Quality), which was adopted across the UK for River Basin Management Planning cycle 2. DARLEQ2 is described in the method statement published by UKTAG (DARLEQ2 River Phytobenthos method statement), with an accompanying calculator spreadsheet, but is also available as an online tool.

DARLEQ2 uses traditional light microscopy for sample analysis, based on the standard CEN methods embedded in the Water Framework Directive. The individual taxon scores used to calculate observed Trophic Diatom Index (TDI) values for derivation of classification are referred to as TDI4 (i.e. Trophic Diatom Index version 4).

It was recognised that there were some inconsistencies in the TDI4 taxon scores, which have now been addressed. The result of this has been small adjustments to a few scores and the creation of an updated version of the TDI, referred to as TDI5 LM. Further technical details about proposed changes to the TDI are available from UKTAG.

An updated version of the DARLEQ software (DARLEQ3) has been developed that allows calculation of the new (TDI5 LM) scores. The change from TDI4 to TDI5 LM represents a method improvement resulting in minor changes to classification.

Implications of the proposal

The impact on classification of switching from the current (TDI4) to the new proposed taxon scores (TDI5 LM), is shown in Table 5.1,

Table 5.1: Comparison of the existing and proposed new river Phytobenthos assessment
Class Existing river Phytobenthos classification New proposed river Phytobenthos classification Net change
High 603 594 -9
Good 422 438 +16
Moderate 129 122 -7
Poor 4 4 0
Bad 0 0 0

*These figures are taken from draft classification results and are for indicative purposes only.

This shows that the change from TDI4 to TDI5 LM would result in a slight increase in the number of sites at Good or better status with seven waterbodies being upgraded from moderate to good. The impact on the overall classification of waterbodies would be negligible, with only one waterbody being downgraded from high to good status. There are no regulatory implications from this change.


Email: eqce@gov.scot

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