Ecological status assessment of European lakes: a comparison of metrics for phytoplankton, macrophytes, benthic invertebrates and fish

Data on phytoplankton, macrophytes, benthic invertebrates and fish from more than 2000 lakes in 22 European countries were used to develop and test metrics for assessing the ecological status of European lakes as required by the Water Framework Directive. The strongest and most sensitive of the 11 metrics responding to eutrophication pressure were phytoplankton chlorophyll a, a taxonomic composition trophic index and a functional traits index, the macrophyte intercalibration taxonomic composition metric and a Nordic lake fish index. Intermediate response was found for a cyanobacterial bloom intensity index (Cyano), the Ellenberg macrophyte index and a multimetric index for benthic invertebrates. The latter also responded to hydromorphological pressure. The metrics provide information on primary and secondary impacts of eutrophication in the pelagic and the littoral zone of lakes. Several of these metrics were used as common metrics in the intercalibration of national assessment systems or have been incorporated directly into the national systems. New biological metrics have been developed to assess hydromorphological pressures, based on aquatic macrophyte responses to water level fluctuations, and on macroinvertebrate responses to morphological modifications of lake shorelines. These metrics thus enable the quantification of biological impacts of hydromorphological pressures in lakes.     

The use of aquatic vegetation in lake assessment: testing the sensitivity of macrophyte metrics to anthropogenic pressures and water quality

The relationships between aquatic vegetation, anthropogenic pressures and water quality in 83 Polish lowland lakes were analysed in order to select the best responding macrophyte metrics to be used in ecological status assessment. Several metrics describing the syntaxonomic composition, abundance and spatial structure of macrophytes were tested in three morphological lake types (deep regular-shaped, deep ribbon-shaped and shallow) separately by using the Spearman rank correlation coefficients. Among all the parameters tested, only some anthropogenic pressure parameters (the percentage share of urban areas and forests in a catchment, the pollution load from point sources) and all the water quality indicators (TP, TN, Chla, SD) were significantly correlated with most of the macrophyte metrics, although for different lake types the strength and significance of relationships varied substantially. The macrophyte metrics which best responded to anthropogenic pressures and water quality changes were: (i) in deep regular-shaped lakes: the percentage share of Chara phytocenoses and rush phytocenoses in the total phytolittoral area, the maximum depth of plant distribution and the colonisation index; (ii) in deep ribbon-shaped lakes: the hydrophytes/helophytes area ratio, the percentage share of submerged and rush phytocenoses in the total phytolittoral area; (iii) in shallow lakes: the percentage share of Chara phytocenoses and rush phytocenoses in the total phytolittoral area, and the hydrophytes/helophytes area ratio. These metrics can then be used when elaborating new or refining existing macrophyte-based methods of ecological status assessment.     

Quantifying uncertainties in biologically-based water quality assessment: A pan-European analysis of lake phytoplankton community metrics

Lake phytoplankton are adopted world-wide as a sensitive indicator of water quality. European environmental legislation, the EU Water Framework Directive (WFD), formalises this, requiring the use of phytoplankton to assess the ecological status of lakes and coastal waters. Here we provide a rigorous assessment of a number of proposed phytoplankton metrics for assessing the ecological quality of European lakes, specifically in response to nutrient enrichment, or eutrophication, the most widespread pressure affecting lakes. To be useful indicators, metrics must have a small measurement error relative to the eutrophication signal we want them to represent among lakes of different nutrient status. An understanding of variability in metric scores among different locations around a lake, or due to sampling and analytical variability can also identify how best this measurement error is minimised.To quantify metric variability, we analyse data from a multi-scale field campaign of 32 European lakes, resolving the extent to which seven phytoplankton metrics (including chlorophyll a, the most widely used metric of lake quality) vary among lakes, among sampling locations within a lake and through sample replication and processing. We also relate these metrics to environmental variables, including total phosphorus concentration as an indicator of eutrophication.For all seven metrics, 65–96% of the variance in metric scores was among lakes, much higher than variability occurring due to sampling/sample processing. Using multi-model inference, there was strong support for relationships between among-lake variation in three metrics and differences in total phosphorus concentrations. Three of the metrics were also related to mean lake depth. Variability among locations within a lake was minimal (<4%), with sub-samples and analysts accounting for much of the within-lake metric variance. This indicates that a single sampling location is representative and suggests that sub-sample replication and standardisation of analyst procedures should result in increased precision of ecological assessments based upon these metrics.For three phytoplankton metrics being used in the WFD: chlorophyll a concentration, the Phytoplankton Trophic Index (PTI) and cyanobacterial biovolume, >85% of the variance in metric scores was among-lakes and total phosphorus concentration was well supported as a predictor of this variation. Based upon this study, we can recommend that these three proposed metrics can be considered sufficiently robust for the ecological status assessment of European lakes in WFD monitoring schemes.     

Strength and uncertainty of phytoplankton metrics for assessing eutrophication impacts in lakes

Phytoplankton constitutes a diverse array of short-lived organisms which derive their nutrients from the water column of lakes. These features make this community the most direct and earliest indicator of the impacts of changing nutrient conditions on lake ecosystems. It also makes them particularly suitable for measuring the success of restoration measures following reductions in nutrient loads. This paper integrates a large volume of work on a number of measures, or metrics, developed for using phytoplankton to assess the ecological status of European lakes, as required for the Water Framework Directive. It assesses the indicator strength of these metrics, specifically in relation to representing the impacts of eutrophication. It also examines how these measures vary naturally at different locations within a lake, as well as between lakes, and how much variability is associated with different replicate samples, different months within a year and between years. On the basis of this analysis, three of the strongest metrics (chlorophyll-a, phytoplankton trophic index (PTI), and cyanobacterial biovolume) are recommended for use as robust measures for assessing the ecological quality of lakes in relation to nutrient-enrichment pressures and a minimum recommended sampling frequency is provided for these three metrics.     

Incorporating invasive weeds into a plant indicator method (LakeSPI) to assess lake ecological condition

The use of lake macrophytes for ecological quality assessments usually seeks to indicate the degree of anthropogenic impact, but few of these schemes implicitly consider impacts of alien weeds. LakeSPI (submerged plant indicators) uses indicators of habitat degradation for macrophytes but also incorporates the degree of impact from alien weeds. Application of LakeSPI to 195 New Zealand lakes provided a dataset to examine how component metrics responded over gradients of anthropogenic pressures, and consider whether weed invasion was merely a ‘passenger’ of habitat degradation, or represented an additional pressure. As expected, metrics measuring depth, and diversity of native vegetation negatively correlated with independent measures of lake eutrophy and were also relatively well explained (69–78% variation) by multiple regression with lake and catchment attributes that included proxies for anthropogenic pressure. In contrast, metrics for invasive impact were largely de-coupled from eutrophication, and poorly explained (31%) by the multiple regression. The response of native vegetation metrics to invasive impact measures varied, with the strongest detected interaction relating to native displacement by increased weed occupation of the vegetated zone. Interactions between invasion and lake trophic status were also discerned, with the extent of weed occupation having a more substantial outcome for the presence of charophyte meadows in low productivity lakes than in more productive lakes. These results suggest weed invasion should be considered as an additional source of anthropogenic pressure, and incorporated in macrophyte bioassessment schemes for a more complete differentiation of lake ecological condition.     

Similarity between contemporary vegetation and plant remains in the surface sediment in Mediterranean lakes

相似文献   

Effects of sampling and sub-sampling variation using the STAR-AQEM sampling protocol on the precision of macroinvertebrate metrics

As part of the extensive field sampling programme within the European Union STAR project, replicate macroinvertebrate samples were taken using the STAR-AQEM sampling method at each of 2–13 sites of varying ecological quality within each of 15 stream types spread over 12 countries throughout Europe. The STAR-AQEM method requires the sub-sampling and taxonomic identification of at least one-sixth of the sample and at least 700 individuals. Replicate sub-samples were also taken at most of these sites. Sub-sampling effects caused more than 50% of the overall variance between replicate samples values for 12 of the 27 macroinvertebrate metrics analysed and was generally greatest for metrics that depend on the number of taxa present. The sampling precision of each metric was estimated by the overall replicate sampling variance as a percentage P_samp of the total variance in metric values within a stream type. Average over all stream types, the three Saprobic indices had the lowest percentage sampling variances with median values of only 3–6%. Most of the metrics had typical replicate sampling variances of 8–18% of the total variability within a stream type; this gives rise to estimated rates of mis-classifying sites to ecological status class of between 22 and 55% with an average of about 40%. This suggests that the precision of such metrics based on the STAR-AQEM method is only sufficient to indicate gross changes in the ecological status of sites, but there will be considerable uncertainty in the assignment of sites to adjacent status classes. These estimates can be used to provide information on the effects of STAR-AQEM sampling variation on the expected uncertainty in multi-metric assessments of the ecological status of sites in the same or similar stream types, where only one sample has been taken at a point in time and thus there is no replication.     

The role of phytoplankton diversity metrics in shallow lake and river quality assessment

Ecological water quality problems are frequently connected to increment of phytoplankton productivity and overdominance of some phytoplankton species. Metrics that show monotonously increasing or decreasing tendencies along stressor gradients is recommended for ecological state assessment. Diversity metrics are influenced by various physical disturbances and show high within-year variability; thus, there is no agreement on the usefulness of these metrics as state indicators.To test the usefulness of phytoplankton diversity in ecological state assessment we investigated the productivity–diversity relationships for lakes and rivers in the Carpathian Basin (Hungary). We demonstrated that the shape of productivity–diversity relationship depends on the investigated water body type. Regarding lakes, hump-shaped relationship was found for all computed metrics. Parallel with the increase in phytoplankton productivity values, diversity metrics showed monotonously increasing tendencies in rhithral and decreasing tendencies in large potamal rivers. We found no systematic relationship in the case of small lowland rivers.Changes of diversity metrics calculated for species and functional groups showed similar tendencies within the types, only the slopes of regression lines differ each other.The use of diversity metrics as ecological state indicators should be restricted to water body types where diversity decreases or increases monotonously with phytoplankton biomass. Regarding the lakes the use of diversity metrics is not recommended for ecological state assessment. In rhithral and large potamal river assessment, application of diversity metrics should be strongly considered. We demonstrated that diversity metrics can be useful components of multimetric indices proposed to use by the Water Framework Directive.     

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

Resource Availability and Spatial Heterogeneity Control Bacterial Community Response to Nutrient Enrichment in Lakes

The diversity and composition of ecological communities often co-vary with ecosystem productivity. However, the relative importance of productivity, or resource abundance, versus the spatial distribution of resources in shaping those ecological patterns is not well understood, particularly for the bacterial communities that underlie most important ecosystem functions. Increasing ecosystem productivity in lakes has been shown to influence the composition and ecology of bacterial communities, but existing work has only evaluated the effect of increasing resource supply and not heterogeneity in how those resources are distributed. We quantified how bacterial communities varied with the trophic status of lakes and whether community responses differed in surface and deep habitats in response to heterogeneity in nutrient resources. Using ARISA fingerprinting, we found that bacterial communities were more abundant, richer, and more distinct among habitats as lake trophic state and vertical heterogeneity in nutrients increased, and that spatial resource variation produced habitat specific responses of bacteria in response to increased productivity. Furthermore, changes in communities in high nutrient lakes were not produced by turnover in community composition but from additional taxa augmenting core bacterial communities found in lower productivity lakes. These data suggests that bacterial community responses to nutrient enrichment in lakes vary spatially and are likely influenced disproportionately by rare taxa.     

Assessing the condition of lake habitats: a test of methods for surveying aquatic macrophyte communities

The European Union requires member states to monitor the conservation status of water bodies designated under the Habitats and Species Directive. In the UK, macrophytes were identified as useful indicators of conservation status but it was recognised that sampling methods required assessment before nationwide survey work began. At eight lakes, which varied in size (0.9–2797 ha) and trophic status (oligotrophic–eutrophic), we tested a method which uses a combination of three techniques (boat transects, wader transects and perimeter strandline searches) to characterise repeat sectors (six or fewer). The combination of techniques used at a particular lake depended on the accessibility of each sector, the safety of applying the method, expediency and target flora. Species richness and abundance were recorded. By applying indicator metrics/rules to data produced by the method it was possible to categorise the conservation and trophic status of the lakes. Power analysis revealed that the data were statistically robust and could potentially be used to track significant, lake-specific changes in macrophyte flora through time. This analysis also revealed a decreasing return in statistical certainty with increased sampling effort (number of sectors sampled). Little additional certainty was added when more than 4 sectors were sampled per lake. The wader transects proved the most practical means of collecting quantitative data while perimeter surveys were an efficient means of adding useful supplementary semi-quantitative data. Boat surveys, although useful in some situations, particularly in assessing deeper water vegetation, were more time consuming than wader surveys. The potential applications and limitations of these methods are discussed and further tests suggested.     

Consequences of Fish Kills for Long-Term Trophic Structure in Shallow Lakes: Implications for Theory and Restoration

Fish kills are a common occurrence in shallow, eutrophic lakes, but their ecological consequences, especially in the long term, are poorly understood. We studied the decadal-scale response of two UK shallow lakes to fish kills using a palaeolimnological approach. Eutrophic and turbid Barningham Lake experienced two fish kills in the early 1950s and late 1970s with fish recovering after both events, whereas less eutrophic, macrophyte-dominated Wolterton Lake experienced one kill event in the early 1970s from which fish failed to recover. Our palaeo-data show fish-driven trophic cascade effects across all trophic levels (covering benthic and pelagic species) in both lakes regardless of pre-kill macrophyte coverage and trophic status. In turbid Barningham Lake, similar to long-term studies of biomanipulations in other eutrophic lakes, effects at the macrophyte level are shown to be temporary after the first kill (c. 20 years) and non-existent after the second kill. In plant-dominated Wolterton Lake, permanent fish disappearance failed to halt a long-term pattern of macrophyte community change (for example, loss of charophytes and over-wintering macrophyte species) symptomatic of eutrophication. Important implications for theory and restoration ecology arise from our study. Firstly, our data support ideas of slow eutrophication-driven change in shallow lakes where perturbations are not necessary prerequisites for macrophyte loss. Secondly, the study emphasises a key need for lake managers to reduce external nutrient loading if sustainable and long-term lake restoration is to be achieved. Our research highlights the enormous potential of multi-indicator palaeolimnology and alludes to an important need to consider potential fish kill signatures when interpreting results.     

Zooplankton as indicators in lakes: a scientific-based plea for including zooplankton in the ecological quality assessment of lakes according to the European Water Framework Directive (WFD)

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.     

Littoral benthic macroinvertebrates of mountain lakes in the Tatra Mountains (Slovakia,Poland)

Littoral benthic macroinvertebrates of 45 mountain lakes in the Tatra Mountains were sampled using a semi-quantitative method in September 2000. A total of 32,852 specimens were identified to 93 taxa belonging to 14 higher taxonomic groups. Multivariate statistics (CCA, RDA) and nine biotic metrics (AQEM/STAR) were used to explain relationships between macroinvertebrate assemblages and environmental variables. Up to 57% of the ecological position of littoral macroinvertebrate assemblages were explained by variance of environmental variables divided into chemical, trophic, physical, catchment and location. Five types of Tatra lakes were recognized using CCA: A — strongly acidified lakes (small catchment, low pH, high concentration of TP, DOC, highest amount of POM in littoral); B — alpine acidified lakes (low amount of POM, low values of biotic metrics); C — alpine non-acidified lakes (high value of diversity index, predominance of Diptera); D — subalpine acidified lakes (high values of biotic metrics: number of families, proportion of crenal and rhithral taxa/total taxa); E — subalpine non-acidified lakes (high values of biotic metrics: number of families, number of genera, BMWP score, number of taxa and abundance of EPT taxa). RDA was used to design five levels of macroinvertebrate taxa acidification tolerance. The Tatra Acidification Index (TAI) was established to assess the acidification status of the lakes in the Tatra Mts.     

What does resilience of a clear-water state in lakes mean for the spatial heterogeneity of submersed macrophyte biovolume?

Short-term variability of spatial heterogeneity of submersed macrophyte biovolume (percent of water column occupied by vegetation) was evaluated over 3 years along a gradient of productivity in four north temperate glacial lakes in Minnesota, USA. We hypothesized we would observe the lowest among-year variability in spatial heterogeneity of biovolume in our undisturbed, moderately productive lake and high variability in our more locally disturbed productive lakes. Our analysis involved three major steps: first, we removed negative trends of biovolume across depth with non-parametric regression smoothers; second, we examined spatial pattern in residuals using variograms; finally, we compared spatial pattern of biovolume among lakes seasonally, over 3 years. Lake productivity negatively correlated with water clarity and the depth range of macrophyte growth, and positively correlated with the variability of spatial patterns. In the least disturbed moderately productive lake, vegetation grew over a large range of depths (up to 7.5 m), and spatial pattern across the littoral zone was similar for each survey. In contrast, in the more turbid, productive lakes, depth and spatial patterns of biovolume varied greatly from survey to survey. Factors that increase productivity and weaken resilience in lakes may lead to unstable spatial patterns of macrophyte biovolume.     

What does resilience of a clear-water state in lakes mean for the spatial heterogeneity of submersed macrophyte biovolume?

Short-term variability of spatial heterogeneity of submersed macrophyte biovolume (percent of water column occupied by vegetation) was evaluated over 3 years along a gradient of productivity in four north temperate glacial lakes in Minnesota, USA. We hypothesized we would observe the lowest among-year variability in spatial heterogeneity of biovolume in our undisturbed, moderately productive lake and high variability in our more locally disturbed productive lakes. Our analysis involved three major steps: first, we removed negative trends of biovolume across depth with non-parametric regression smoothers; second, we examined spatial pattern in residuals using variograms; finally, we compared spatial pattern of biovolume among lakes seasonally, over 3 years. Lake productivity negatively correlated with water clarity and the depth range of macrophyte growth, and positively correlated with the variability of spatial patterns. In the least disturbed moderately productive lake, vegetation grew over a large range of depths (up to 7.5 m), and spatial pattern across the littoral zone was similar for each survey. In contrast, in the more turbid, productive lakes, depth and spatial patterns of biovolume varied greatly from survey to survey. Factors that increase productivity and weaken resilience in lakes may lead to unstable spatial patterns of macrophyte biovolume.     

Exploring the robustness of macrophyte-based classification methods to assess the ecological status of coastal and transitional ecosystems under the Water Framework Directive

Identifying and quantifying the factors that contribute to the potential misclassification of the ecological status of water bodies is a major challenge of the Water Framework Directive (WFD). The present study compiles extensive biomonitoring data from a range of macrophyte-based classification methods developed by several European countries. The data reflect spatial and temporal variation as well as inter-observer variation. Uncertainty analysis identified that factors related to the spatial scale of sampling generally contributed most to the uncertainty in classifying water bodies to their ecological status, reflecting the high horizontal and depth-related heterogeneity displayed by macrophyte communities. In contrast, the uncertainty associated with temporal variation was low. In addition, inter-observer variation, where assessed, did not contribute much to overall uncertainty, indicating that these methods are easily transferable and insensitive to observer error. The study, therefore, suggests that macrophyte-based sampling schemes should prioritize large spatial replication over temporal replication to maximize the effectiveness and reliability of water body classification within the WFD. We encourage conducting similar uncertainty analyses for new/additional ecological indicators to optimize sampling schemes and improve the reliability of classification of ecological status.     

Spatial, seasonal and long-term variability of phytoplankton photosynthesis in lakes

A cross-system, worldwide approach has been used to ascertainthe spatial, seasonal and long-term variability of areal phytoplanktonphotosynthesis (PP) in lakes using published data sets. Also,the average fraction of annual PP occurring under ice is calculated.The lakes considered embrace a range of properties (depth, mixing,flushing rate, latitude and trophic status). The overall yearlyPP distribution is skewed to the left, suggesting the dominanceof low PP rates in the data set. When comparing lake types,no differences in average PP have been found among them. Inparticular, there are no clear areal PP differences among lakesof different trophic status on yearly, averaged basis, suggestingthat environmental limitations to PP also exist in lakes ofhigher trophic status. Volumetric-based PP can be better usedto outline PP-based trophic differences, but some degree ofoverlap is also apparent. Across all lake types (except in tropicallakes), the PP seasonal course experiences only one peak inthe year, but its timing is clearly different for each laketype. The seasonal variability of PP is lower in tropical lakes,as previously reported, but the variability of the other laketypes is roughly the same. Therefore, the effects of depth,mixing regime, flushing rate and nutrient status on PP seasonalityare difficult to ascertain since they appear to be counterbalancedby other more pervasive, local effects. Particularly, thereis no increase in temporal variability with the trophic statusof lakes, suggesting that PP seasonal control by physical variablesoverrides that of nutrients. Also, no significant relationshipbetween average PP and latitude has been found. Seasonal variabilityincreases as the yearly PP increases. On a relative basis, thereis a spatial gradient of seasonal variability of PP, which isweaker when seasonal variability of PP is considered in interyearcomparisons. Long-term (i.e. interannual) variability of PPis clearly related to increasing yearly averaged PP. Specifically,in temperate, stratifying lakes the seasonal time course ofPP is clearly different from that of phytoplankton biomass,suggesting an uncoupling of both variables as a result of differingP_max and losses throughout the year. On an average basis, environmentalvariables are poor predictors of areal daily PP, thereby implyingthat the interplay of factors is complex and changing throughoutthe year. PP under ice averages 10% of yearly PP, but its variabilityis high enough to make its measurement advisable.     

An application of canonical correspondence analysis for developing ecological quality assessment metrics for river macrophytes

1. Aquatic macrophyte composition and abundance is required by the European Union's Water Framework Directive for determining ecological status. Five metrics were produced that can be combined to determine the deviation of aquatic macrophytes from reference conditions in Northern Ireland's rivers. 2. Species optima and niche breadths along silt, nitrate, pH, conductivity and dissolved oxygen gradients were generated from aquatic macrophyte and water quality surveys conducted at 273 sites throughout Northern Ireland using Canonical Correspondence Analysis (CCA). Five metric scores based on these environmental gradients were determined at new monitoring sites using the mean optima of the species occurring at the site, weighted by percentage cover and niche breadth of each species. 3. A preliminary reference network of 32 sites of high physico‐chemical and hydromorphological quality, and representative of the range of river types in Northern Ireland, enabled reference metric scores to be produced for each river type. Five unimpacted and twenty impacted sites were used for testing the performance of the metrics. By subtracting reference metric scores from metric scores at a monitoring site measures of ecological impact could be determined along five different impact gradients. Metrics were also combined to give a measure of total ecological change. 4. The metrics system distinguished unimpacted from impacted sites and correctly identified 77% of the known impacts. The metrics distinguished different types of impact, e.g. silt and nitrate. 5. Aquatic macrophyte occurrence and abundance has high natural variability at a site, both temporally and spatially. This method was designed to be sensitive to ecological change whilst reducing noise caused by natural variation.     

Using aquatic macrophyte community indices to define the ecological status of European lakes

Defining the overall ecological status of lakes according to the Water Framework Directive (WFD) is to be partially based on the species composition of the aquatic macrophyte community. We tested three assessment methods to define the ecological status of the macrophyte community in response to a eutrophication pressure as reflected by total phosphorus concentrations in lake water. An absolute species richness, a trophic index (TI) and a lake trophic ranking (LTR) method were tested at Europe-wide, regional and national scales as well as by alkalinity category, using data from 1,147 lakes from 12 European states. Total phosphorus data were used to represent the trophic status of individual samples and were plotted against the calculated TI and LTR values. Additionally, the LTR method was tested in some individual lakes with a relatively long time series of monitoring data. The TI correlated well with total P in the Northern European lake types, whereas the relationship in the Central European lake types was less clear. The relationship between total P and light extinction is often very good in the Northern European lake types compared to the Central European lake types. This can be one of the reasons for a better agreement between the indices and eutrophication pressure in the Northern European lake types. The response of individual lakes to changes in the abiotic environment was sometimes represented incorrectly by the indices used, which is a cause of concern for the use of single indices in status assessments in practice.