Development of Lake-MlmAS as a
Decision-Support Tool for Managing Hydromorphological Alterations
to Lakes
WFD49f
September 2008
EXECUTIVE SUMMARY
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