Identification of reference lakes and evaluation of palaeoecological approaches to define reference conditions for UK (England, Wales, Scotland & Northern Ireland) ecotypes
This is the final report to the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER) under project no. WFD08, ‘Identification of reference lakes and evaluation of palaeoecological approaches to define reference conditions for UK (England, Wales, Scotland & Northern Ireland) ecotypes’, funded by SNIFFER. This project forms part of the UK Strategy for the implementation of the European Council Water Framework Directive (WFD) which requires reference conditions to be determined for all water body types including lakes. This study aims to use palaeoecological techniques to identify reference sites, describe reference conditions and assess ecological status for a set of UK lakes. In contrast with previous detailed studies on a limited number of sites, we collate existing data from large numbers of lakes in the UK to aid implementation of the WFD at the national level. Furthermore, unlike earlier studies where the interest has been in determining the causes and extent of water quality change of impacted systems, this project aims to identify reference lakes and hence new sediment cores were collected from lakes thought to be relatively unimpacted.
The study focuses largely on the analysis of diatom (Bacillariophyceae: siliceous algae) remains in sediment cores. Diatoms are sensitive to water quality changes and shifts in the diatom community often correspond closely to changes in other biological elements. A number of other groups that preserve in the lake sediments, representing a range of biological elements relevant to the WFD, are examined in two of the four work packages, including chironomids (Diptera), cladocera (zooplankton), aquatic pollen and plant macrofossils. The report is comprised of four main chapters, each dedicated to a work package.
Work package 1 aims to classify lakes (from across the UK) based on diatom reference conditions and to compare the classification with the GB Lake Typology. Diatoms were analysed in the bottoms (reference condition) of sediment cores from a large set of lakes (N=219) covering the range of types found in the ecoregion. Two Way Indicator Species Analysis (TWINSPAN) was employed to classify the reference samples according to their diatom assemblages, and thereby characterise the reference floras of the different lake types. TWINSPAN identified eight site end-groups, each with a characteristic diatom assemblage although there was some overlap in the taxa present in the groups. The diatom groups were compared with the GB Lake Typology to assess the ecological relevance of the typology scheme. There is close congruence between the two datasets, indicating that the criteria of geology (reflecting alkalinity) and, to a lesser extent, lake depth used in the latter does result in ecologically meaningful typologies. The comparison highlighted a number of lakes that had been misclassified by the GB Lake Typology.
Diatoms were analysed in the surface samples (present day conditions) of the 219 cores. The degree of floristic change between the reference and surface sample of each core was assessed using a squared chord distance dissimilarity coefficient which ranges from 0 (no difference) to 2 (completely different). A score of <0.475 (5th percentile) is used here to define reference sites (i.e. those with low floristic change between the two samples).The scores for the 219 lakes range from 0.13 to 1.77, with 79 lakes (36% of the dataset) experiencing low floristic change. In the low, medium and high alkalinity lake types, approximately 40%, 30% and 30% of the lakes, respectively, have a score <0.475 and, therefore, good examples of reference lakes are available for these lake types. However, ~50% of lakes in the existing dataset exhibit considerable floristic change with scores >0.58. In the low alkalinity lakes, these changes appear to largely reflect acidification, whereas in the medium alkalinity, high alkalinity and marl groups, the diatom changes are indicative of eutrophication.
Work package 2 aims to identify a set of reference lakes for each ecotype based on analysis of diatom assemblages preserved in lake sediment cores. Sediment cores (expected to represent the period from at least ~1850 AD to the present day) were taken from 34 sites in the UK selected as potential reference lakes. Diatom assemblages were analysed in approximately four to five samples from 31 of the cores (poor preservation prevented analysis of the remaining three sites) to allow the reference condition and general trend in water quality to be determined. In the absence of a chronology, samples spanning the complete length of the cores were analysed and the bottom-most samples taken to represent reference conditions. Detrended correspondence analysis, squared chord distance dissimilarity measures and diatom transfer functions were applied to the data to assess the nature and degree of floristic and chemical change at each site. Thirteen sites provide good examples of reference lakes based on the low degree of floristic change observed throughout the sediment record: Buttermere, Crummock Water (low alkalinity, deep); Loch Lonachan, Black Loch, Ullswater (medium alkalinity, deep); Loch Cill Chriosd, Little Sea Mere (medium alkalinity, shallow); Loch Achnacloich (high alkalinity, deep); Burton Mill Pond, Frensham Great Pond, Llyn Llygeirian, Loch Watston, Loch Watten (high alkalinity, shallow). A further set of five lakes could be classified as good status, exhibiting only slight deviation from the assemblages in the bottom samples: Muckle Water, Loch Skealtar, Lough Lattone, Lough Scolban (medium alkalinity, deep); Loch Mahaick (medium alkalinity, shallow). Thirteen lakes exhibit considerable changes in the diatom assemblages and are thus considered to be at less than good status. The two examples of low alkalinity, shallow lakes have experienced significant floristic change and, therefore, reference lakes are not identified for this type. Reference lakes are not determined for the marl lakes owing to data interpretation problems caused by diatom dissolution.
Application of the diatom transfer functions enabled typical ranges of ‘reference’ total phosphorus (TP) concentrations to be determined for the main lake types: Low alkalinity lakes < 10 µg l-1, Medium alkalinity lakes 10-20 µg l-1, High alkalinity and Marl lakes 20-40 µg l-1. These values should be used as guidelines only because TP concentrations are clearly site specific.
Work package 3 aims to demonstrate the value of the multi-proxy palaeo-record for defining site-specific ecological reference conditions at lake ecotypes where reference sites cannot be found in the current UK lake population. Multi-proxy analysis of fossil remains (diatoms, chironomids, cladocera, plant macrofossils and pollen) in reference samples (~1850 AD) and surface samples of two lakes, Lake of Menteith and Llangorse Lake, demonstrated the potential of the fuller palaeoecological record for defining reference conditions for a range of biological elements and for assessing ecological change. The multi-indicator data reflect a shift in the functioning of both lakes from benthic-littoral to planktonic dominated production.
The data for the reference samples of the Lake of Menteith cores indicate a healthy ecosystem at ~ 1850 AD with diverse communities of diatoms, chironomids, cladocera and aquatic macrophytes, and assemblages typical of a relatively nutrient poor, circumneutral system. Eutrophication appears to have had a cascading effect through the whole system. The plant macrofossil and aquatic pollen records suggest a reduction in plant abundance in the open water and a shift towards more nutrient tolerant plants. This is reflected in the cladocera community where a decline in plant associated taxa in the open water was observed. The system is now plankton dominated with greater abundance of planktonic cladocera and diatoms, and a less diverse chironomid community.
The data for the reference samples of the Llangorse Lake cores indicate a healthy ecosystem in the past with numerous plant-associated cladocera taxa, attached forms of diatoms and diverse chironomid and plant communities. The assemblages are typical of an alkaline system of intermediate trophic status. The data indicate a decline in the areal coverage of plants in the open water and a change in plant composition away from charophytes toward more nutrient-tolerant species. A general shift from a littoral, plant dominated system to a more pelagic system has occurred with the planktonic diatoms and pelagic cladocera expanding at the expense of the plant-associated and bottom-dwelling taxa. A general shift towards a more nutrient tolerant chironomid fauna was observed. The data indicate that the lake was already relatively nutrient-rich around 150 years ago but that it has undergone enrichment over the last 100-150 years.
Work package 4 aims to demonstrate analogue matching as a technique for identifying the most appropriate reference sites to be used to formulate restoration targets for lakes impacted by eutrophication. An analogue matching training set was developed comprising 30 lakes, 266 diatom taxa and 41 cladocera taxa. Three subsets of the training set were applied to the reference samples (~1850 AD) of three lakes, i) diatom data, ii) cladocera data , and iii) diatom and cladocera data combined. The squared chord distance dissimilarity coefficient was employed to determine the best analogues for the ‘reference’ assemblages of three test lakes.
For Loch Davan, a medium alkalinity, shallow lake, Black Loch (diatoms), Little Sea Mere (cladocera), Bayfield Loch (combined), Loch Grogary (cladocera and combined), and Llyn Fanod (cladocera and combined) were the best matches. These are all medium alkalinity waters and are currently mesotrophic with diverse plant populations covering a large proportion of the lake bed. Their selection as potential reference sites for medium alkalinity, shallow lakes such as Loch Davan, therefore, seems appropriate. For Felbrigg Lake, a high alkalinity, shallow lake, Llyn Helyg (cladocera), Loch Watston (diatoms, cladocera), Frensham Great Pond (diatoms, cladocera, combined) and Oxwich Pool (diatoms, cladocera, combined) were the best matches. Similar matches were found for Groby Pool, another currently high alkalinity, shallow lake, with Llyn Llygeirian (diatoms), Llyn Helyg (cladocera), Broomlee Lough (cladocera), Loch Ardnave (cladocera), Frensham Great Pond (cladocera, combined), Oxwich Pool (diatoms, cladocera, combined) and Loch Watston (diatoms, cladocera, combined) providing the best matches. Oxwich Pool and Loch Watston currently support diverse, abundant macrophyte populations, and the latter has been identified as a good example of a reference lake for the high alkalinity, shallow waterbody type. Validation of the choice of reference sites for Felbrigg Lake and Groby Pool using the fuller palaeoecological record suggests that the selected analogues are appropriate for these lakes. However, there were few lakes in the current population with good matches to the Groby Pool pre-enrichment macrophyte assemblage which was comprised of relatively nutrient-poor taxa. The analogue matching technique has the potential, therefore, to be a useful tool for identifying reference sites for lakes impacted by eutrophication, although a larger training set with a greater number of reference lakes is required to improve the diatom analogues.
In conclusion, simple methods such as ordination, clustering and dissimilarity measures applied to palaeoecological data, combined with transfer functions, offer powerful techniques for characterising and validating lake types, identifying reference lakes, defining ecological and chemical reference conditions, and assessing deviation from the reference state. On the basis of this study, a protocol is recommended for using palaeoecological techniques to identify lake reference conditions according to the WFD. The use of a range of biological indicators, in addition to diatoms, such as chironomids, cladocera and macrofossils is advocated. The project illustrates that palaeolimnological studies need not always be highly detailed and thus expensive, and that valuable information can be produced from relatively low resolution studies at large numbers of lakes to aid implementation of the WFD at the national level.
Key words: chironomids, cladocera, diatoms, ecological status, lakes, macrofossils, multi-proxy, palaeolimnology, reference conditions, Water Framework Directive.
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