Background to research
The EC Water Framework Directive (WFD) introduced the concept of ‘ecological status’, defined as ‘an expression of the quality of the structure and functioning of aquatic ecosystems…’ It is generally accepted that geomorphology and hydrology (hydromorphology) influence the ecological status of lakes, but the bio-physical linkages remain poorly understood. The first round of River Basin Management Plans used the UK Technical Advisory Group (UKTAG) core typology based on geology and water depth. However it is also widely accepted that the ecological relevance of the core typology can be improved to yield better biological assessment tools and increased confidence in regulatory decision-making and resource management. Establishing an ecologically-relevant hydromorphology typology that better characterises the types and distribution of lakes across the UK is therefore the first step in providing a more robust evidence base for explaining natural biological variation in the composition and abundances of plants and animals within and between different lake types. This research will support the work of UK environment agencies to progress hydromorphological pressure-response relationships needed to inform WFD Phase 2 intercalibration programmes.
Objectives of research
Four objectives were established as a means to realise the aims of the project:
Key findings and recommendations
The WFD core reporting typology developed in the UK combines alkalinity and mean water depth. All the biological assessment tools developed in the UK use these parameters, whilst some additionally use lake area, continentality, drift geology, perimeter length and lake altitude to provide better explanation for site-specific variability. A review of European typology schemes revealed greater significance was attributed to the role of geographically-defined ecoregions, lake-catchment relations (e.g. through the Schindler Ratio) and mixing behaviour.
A new hydromorphological typology was developed for UK lakes which distinguishes lakes into ‘behavioural types’ according to a hierarchical framework operating at ecoregion, landscape setting, lake-catchment relations and site morphometry. The resultant lake classes are strongly correlated with lake ontogeny (mode of formation and subsequent geomorphic evolution) which also shows strong spatial dependency within the UK. The validity of this heuristic scheme was tested using a comprehensive morphometric database of 624 lakes from Britain and a further 34 from Northern Ireland. UK lakes are overwhelmingly glacial in origin. Corries, glacial troughs, ice-scoured bedrock are typical examples of scour lakes resulting in distinctive shapes, with consistent lake-catchment relations and landscape settings. Underlying geology and the extent and character of glacial drift and peat mediate hydrological pathways in catchments and geochemical fluxes into the lake systems. The morphometry of an individual system influences trophic structuring e.g. ratio of littoral:pelagic habitats, mixing and shore zone energetics. Piedmont and lowland settings are typically depositional where boulder clay, fluvio-glacial sand and gravels and fen peats are common. Less common, but of national significance are coastal lakes and the special case of Norfolk and Suffolk Broads of southeast England which are Medieval peat diggings but distinguished now for their exceptional conservation value.
Multiple discriminant analysis (MDA) confirmed the role of location (latitude/longitude), altitude, basin size and form and catchment dimensions and runoff to be important distinguishing features of the major lake types. Lake Habitat Survey (LHS) data from 85 lakes across the UK demonstrated the strong control that mode of formation and geology have on substrate composition and variability. Strong evidence of the importance of morphometric parameters in structuring aquatic ecosystems was also provided from Northern Ireland, particularly in relation to the role of basin shape and water depth to predict ecologically relevant variables such as macrophyte cover, Secchi depth and colour. The revised typology performed well in distinguishing key biological indicators such as species numbers.
There is a pressing need to better connect high quality biological assessments with detailed hydromorphological analysis and promote hypothesis-led investigative research to build the evidence base for models such as Lake-MImAS. In terms of gaps in relation to geomorphological, hydrological and other data required to explain more of the biological variation, the major limitation remains adequate knowledge of hydrological regimes. More work is also needed to reconcile scaling issues relating to the temporal and spatial representativeness of biological samples collected at single or a small number of locations with hydromorphological metrics obtained at water body and catchment scales. By extension there is a pressing need for an experimental approach to be undertaken wherein a targeted body of hydromorphological and biological data will be collected with a view to quantify ‘hydromorphological pressure – hydromorphological impact – biological response’ pathways. The field programme undertaken by NIEA in Northern Ireland which involves the systematic collection of quality bathymetric data and hydromorphological surveys by accredited surveyors and synoptic collection of water quality and biological sampling is recognised a model of best practice in UK environment agencies.
Water Framework Directive, Lake, Hydromorphology, Typology, Ecoregion, Landscape Setting, Lake-Catchment Relation, Morphometry, Morphometric analysis
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