Development of Lake-MlmAS as a Decision-Support Tool for Managing Hydromorphological Alterations to  Lakes
September 2008


SNIFFER Report WFD49f (April 2007) reviewed the effects of engineering and related pressures on the physical condition of lakes and resulting impacts on ecological status and presented a prototype risk assessment tool, termed Lake-MImAS (Morphological Impact Assessment System).  This report supersedes the April report by presenting the first operational release of the tool, and associated documentation, revised in the light of international peer review, public consultation and an extensive national trialling and validation exercise completed in the autumn of 2007.

Lake-MImAS (Morphological Impact Assessment System) builds on the original MImAS scheme developed for rivers (SNIFFER Project WFD49, 2006) and which was adapted for application in transitional and coastal (TRaC) waters.  It operates on the principle that the physical response of a water body (or part thereof) to an engineering, or related pressure, is predictable for the type of lake under consideration and further that the ecological response depends on the sensitivity of the aquatic ecosystems in the lake which is also type-specific.  The first release version of Lake-MImAS uses a composite typology of six groups of lake types (distinguished by geology/alkalinity and depth classes) capturing most large (> 50 ha) lakes in Great Britain (Ecoregion 18) and Ireland (Ecoregion 17).  

Morphological Condition Limits (MCLs) represent thresholds of alteration in morphological conditions beyond which it is understood that ecological and/or morphological conditions could be altered in ways that could result in deterioration in status.  The limits are expressed in percentage terms in relation to the amount of ‘system capacity’ used, where this is a measure of a lake’s capacity to assimilate morphological alterations.  As system capacity is lost (consumed) it follows that there is an increased likelihood (or risk) that morphological and ecological conditions will degrade.  MCLs are specified for two discrete, but inter-connected lake zones.  The ‘pelagic-profundal’ zone represents off-shore open-water environments extending to the lake bed which will be profundal in the case of deep lakes.  The ‘shore zone’ extends from the riparian zone down to the sub-littoral (lakeward limit of rooted vegetation).

MCLs were derived using the expert judgement of the contractors in consultation with the project steering group and in keeping with the equivalent values used in the Rivers- and TRaC-MImAS tools.  MCL values of 5 % are proposed as the boundary between high ecological status (HES) and good ecological status (GES), and 15 % as the boundary between GES and moderate ecological status (MES).  The 5 % MCL representing the boundary between HES/GES is critical because the hydromorphological condition of a lake only contributes to classification at HES.  At GES and below, hydromorphological conditions are not defined but are ‘to be consistent with the achievement of the values specified... for the biological quality elements’ of each class e.g. GES, MES, poor ecological status (PES) and bad ecological status (BES).  Nevertheless in the absence of biological data a measure of hydromorphological alteration (allied to other risk assessment tools) may support water body classification for those lakes below HES and GES as required in the 2009 River Basin Management Plan.  In this spirit MCLs at 30 % and 45 % are recommended as class boundaries of hydromorphological alteration analogous to MES/PES and PES/BES respectively.  In a risk assessment context these values should not be regarded as absolute rather that a change in status is likely to occur at some unknown point above these thresholds.  

A two-stage process is envisaged for the regulatory applications of the Lake-MImAS tool.  Stage 1 assessments, whilst not required for WFD compliance, offer a screening mechanism to monitor the nature, scale and significance of multiple smaller scale developments which can incrementally impact ecological status.  Stage 1 assessments operate over 500 m shoreline lengths (or 5 ha lake surface areas) and only consider local morphological pressures, thus excluding the indirect effects of outlet regulation and non-structural pressures such as recreation.   Detailed information is required about the physical properties of extant and proposed developments, but such information is readily available from maps and air photographs and ultimately from geospatial databases.  Stage 2 assessments operate at the water body scale (typically whole lake) and thus effectively provide a measure of overall hydromorphological condition.  Because the WFD classification process requires high ecological status (HES) lakes to demonstrate the absence, or existence of only very minor alterations to hydromorphological quality elements, Stage 2 assessments additionally capture the impacts (expressed in terms of capacity loss) associated with regulating lake outflows (both actively managed water level control and passive adjustment of the outflow sill elevation) along with catchment regulation and lake-based recreational pressures.  

Running in parallel with a peer review and public consultation process a trialling and validation programme was initiated.  Teams of practitioners from the environment agencies of England and Wales (EA), Northern Ireland (EHS) and Scotland (SEPA) were invited to assess (effectively classify using expert judgement) both the hydromorphological condition and ecological status of lakes within their respective regions. In Northern Ireland and in Scotland the judgement of the environmental practitioners was further reviewed by specialists from the national conservation agencies (EHS and SNH).

The 95 lakes thus assembled enabled the Lake-MImAS tool to be tested and calibrated against a broad spectrum of pressures and lake types.  Some minor adjustments were made to the list of pressures represented in the tool and there was some simplification of the lake typology. ‘Activity footprints’, used as part of the scoring module were also adjusted through an iterative trialling process (most significantly outflow regulation and riparian vegetation loss).  Following this fitting exercise the agreement between Lake-MImAS tool and agency experts was an encouraging 68 % for the UK as a whole (n=95).  The same morphological condition class was assigned in 84% of Scottish lochs (n=40), in 58% of cases for lakes in England and Wales (n=28) and in 57% of cases for loughs in Northern Ireland (n=27).  Agreement to within one class was found in 98% of all comparisons.  In most cases where different classes were assigned the explanation lay in the relative importance assigned to outlet regulation (either raising or lowering) and in several cases upon consideration of the Lake-MImAS output the expert groups revised their initial classifications.  The trialling exercise thus confirmed that the MCLs of 5% and 15% consistently distinguished between slightly and moderately altered lake systems and so provides the foundation for a pragmatic risk-assessment scheme to support classification and regulatory activity within the WFD.

In summary, Lake-MImAS provides a consistent and objective framework for determining the hydromorphological condition of lakes in GB and Irish Ecoregions.  As a risk assessment tool it also supports the regulatory process by helping to inform decisions about whether new proposals are likely to have minor impacts and so can be permitted or whether the impacts are potentially more significant and so should be subjected to more detailed investigation.  The modular nature of the MImAS tool means that it is adaptive and the scheme can be revised in the light of WFD monitoring and targeted scientific research. Further work is also required to strengthen the evidence base linking hydromorphological alteration and ecological response.  

Key Words
Morphological Condition Limits (MCLs), System Capacity, Lake Zones (pelagic-profundal and shore zone), Hydromorphology, Ecological Status, Stage 1 & Stage 2 regulatory assessments, Activity Impact Scores, Activity Footprints

Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 25.00, less 20% to FWR members.

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