Development of Decision Making Frameworks for Managing Alterations to the Morphology of Lakes
WFD49a
October 2005

The importance of European lakes for conservation and resource use is widely recognised, yet a systematic decision-making procedure for managing lake morphology is lacking.  The Water Framework Directive (WFD) requires that all surface water bodies (including lakes) should achieve ‘Good Ecological Status’ (GES) by 2015.  GES requires hydromorphological conditions supporting at worst ‘slight changes’ in the composition and abundance of key biological quality elements (phytoplankton, macrophytes and phytobenthos, benthic macroinvertebrates and fish fauna) relative to the appropriate natural reference condition or High Ecological Status (HES).  For standing waters hydromorphology is defined by elements of morphology and hydrological regime.

This report describes the development of a pragmatic decision-making framework for regulating changes to lake morphology.  The three main objectives of the research were to:

The decision-support framework developed for this project was based around the concept of ‘thresholds of potential change’, whereby numerical thresholds and criteria of morphological change were established against a range of recognized physical pressures.  Twelve groups of specific pressures (35 in total) were classified as either ‘whole lake’ where the entire system is affected, or ‘component’, where there is modification to only part of the lake.  Hydrological regime changes which induce morphological change are also considered.  A default set of threshold values (termed sub-critical, lower and upper) are defined for each group of pressures, assigned on the basis of their nature and likely significance.  The threshold values for a particular lake are then modified using a ‘Lake Sensitivity Typology’ scheme, which adjusts default values according to those site-specific physical attributes (geology, depth, size, altitude, form and location) perceived as likely to influence lake ecology sensitivity.  Threshold values are increased where site attributes (such as increased lake surface area) lead to a greater assimilative capacity, and are reduced where attributes which are thought to increase ecological sensitivity (e.g. particular lake forms) are identified.

The decision-support scheme operates through the principle of an Alteration of Lake Morphology Score (ALMS).  Data for the ALMS must be supplied by a hydromorphological assessment, e.g. by completion of a Lake Habitat Survey (LHS), which quantifies all specific pressures with the exception of some hydrological regime data that can be added into the analysis as it becomes more widely available.  The ALMS can be used to determine whether the morphological pressures at a site are sufficiently small to permit the site to be classified as HES.  Thereafter, higher ALMS values indicate a progressively greater risk that a site will fail to achieve GES due to morphological alteration.  This approach readily enables identification of HMWBs, the eventual designation of which are subject to tests (Article 4.3), and ALMS can also help prioritize the mitigation measures required to achieve the environmental objective of Good Ecological Potential (GEP).

Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 35 .00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website