Importance of scalar and riparian habitat effects for assessment of ecological status using littoral diatoms

The classification of waterbodies under the Water Framework Directive is dependent on the ability of monitoring programmes to reflect habitat quality using biotic elements including benthic diatom communities. This study investigated the influence of specific riparian habitats, of mixed woodland, grassland and lake artificial structures such as jetties and slipways, on benthic diatom assemblages in nine lakes across gradients of total phosphorus, alkalinity and in the presence or absence of Dreissena polymorpha. The heterogeneity of the benthic diatom assemblages at riparian and lake scale was assessed by taking three replicates per site category per lake, following standard European Union protocols. Canonical correspondence analysis (CCA) and mixed effect modelling was used to investigate the main environmental controls on assemblage structure. Non-metric Multidimensional Scaling (NMDS) was used to examine patterns in assemblage structure. No single environmental gradient was found to control benthic diatom composition, with differences among assemblages influenced both by riparian habitat type within lakes and interaction of multiple environment gradients, including presence of D. polymorpha. Greater control was exerted on community structure at the lake than local riparian scale. The influence of scalar factors on diatom assemblages increased with increasing scale. We recommend that for effective monitoring and assessment of ecological status, standard sampling protocols should include localised littoral habitats with individual samples pooled across riparian habitat types, thereby accounting for both multiple environmental and spatial controls on community structure.     

A phytoplankton trophic index to assess the status of lakes for the Water Framework Directive

Despite improvements in wastewater treatment systems, the impact of anthropogenic nutrient sources remains a key issue for the management of European lakes. The Water Framework Directive (WFD) provides a mechanism through which progress can be made on this issue. The Directive requires a classification of the ecological status of phytoplankton, which includes an assessment of taxonomic composition. In this paper, we present a composition metric, the plankton trophic index, that was developed in the WISER EU FP7 project and demonstrate how it has been used to compare national phytoplankton classification systems in Northern and Central Europe. The metric was derived from summer phytoplankton data summarised by genus from 1,795 lakes, covering 20 European countries. We show that it is significantly related to total phosphorus concentrations, but that it is also sensitive to alkalinity, lake size and climatic variables. Through the use of country-specific reference values for the index, we demonstrate that it is significantly related to other national phytoplankton assessment systems and illustrate for a single European (intercalibration) lake type how it was used to intercalibrate WFD boundaries from different countries.     

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.     

Cladocerans respond to differences in trophic state in deeper nutrient poor lakes from Southern Norway

We investigated the role of trophic state in structuring the cladoceran assemblages in 35 relatively deep lakes of low nutrient levels in Southern Norway. The lakes cover gradients of altitude, latitude, longitude, area and total phosphorus concentration. The environmental control of the cladoceran assemblages was analyzed using canonical correspondence analysis. Total phosphorus was the most important variable in the minimal adequate model. Hence, the cladoceran assemblage changed with trophic state. Typical clear water species were displaced by indicators of eutrofication with increasing trophic levels. Littoral species constituted the majority of the species recorded. Littoral and pelagic species richness showed a unimodal relationship with trophic state. Along with the change in trophic state and shift in cladoceran assemblage, we observed an increase in the ratio of pelagic to littoral species as well as in the ratio of pelagic efficient bacterial feeders to total pelagic filter feeders. The study indicates that zooplankton could provide a valuable indicator of ecosystem’s structure and function in deeper nutrient poor lakes if included in water quality assessments according to the EU Water Framework Directive. Including sampling of the littoral zooplankton would improve diversity estimates and incorporate the response of the littoral zone to eutrophication.     

Chlorophyll–nutrient relationships of different lake types using a large European dataset

In Europe there is a renewed focus on relationships between chemical determinands and ecological impact as a result of the Water Framework Directive (WFD). In this paper we use regression analysis to examine the relationship of growing season mean chlorophyll a concentration with total phosphorus and total nitrogen using summary data from over 1,000 European lakes. For analysis, lakes were grouped into types with three categories of mean depth, alkalinity and humic content. The lakes were also divided into broad geographic regions covering Atlantic, Northern, Central/Baltic and for some types the Mediterranean areas of Europe. Chlorophyll a was found to be significantly related to both total phosphorus and total nitrogen, although total phosphorus was almost always found to be the best predictor of chlorophyll. Different nutrient chlorophyll relationships were found for lakes according to mean depth and alkalinity, although no significant effect of geographic region or humic content was found for the majority of lake types. We identified three groups of lakes with significantly different responses. Deep lakes had the lowest yield of chlorophyll per unit of nutrient, low and moderate alkalinity shallow lakes the highest and high alkalinity lakes were intermediate. We recommend that the regression models provided for these three lake groups should be used for lake management in Europe and discuss the limitations of such models.     

Water‐level fluctuations regulate the structure and functioning of natural lakes

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Efficient sampling methodologies for lake littoral invertebrates in compliance with the European Water Framework Directive

Lake shores are characterised by a high natural variability, which is increasingly threatened by a multitude of anthropogenic disturbances including morphological alterations to the littoral zone. The European Water Framework Directive (EU WFD) calls for the assessment of lake ecological status by monitoring biological quality elements including benthic macroinvertebrates. To identify cost- and time-efficient sampling strategies for routine lake monitoring, we sampled littoral invertebrates in 32 lakes located in different geographical regions in Europe. We compared the efficiency of two sampling methodologies, defined as habitat-specific and pooled composite sampling protocols. Benthic samples were collected from unmodified and morphologically altered shorelines. Variability within macroinvertebrate communities did not differ significantly between sampling protocols across alteration types, lake types and geographical regions. Community composition showed no significant differences between field composite samples and artificially generated composite samples, and correlation coefficients between macroinvertebrate metrics calculated with both methods and a predefined morphological stressor index were similar. We conclude that proportional composite sampling represents a time- and cost-efficient method for routine lake monitoring as requested under the EU WFD, and may be applied across various European geographical regions.     

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.     

A fish-based index for the assessment of the ecological quality of temperate lakes

A fish – based index for the assessment of the ecological quality of natural temperate lakes was developed, in accordance to the requirements of the Water Framework Directive (WFD) 2000/60/EC. As a case study, 11 natural lakes located at northern and western Greece were selected. Fish surveys were conducted during mid summer to mid autumn in 2010, 2011 and 2012 using Nordic gillnets and electrofishing. Environmental parameters and anthropogenic pressures were assessed for each lake. Fish species richness, abundance, trophic, reproductive and habitat functional guilds were used for extracting a set of 107 metrics, meeting the requirements of the WFD. All metrics were initially tested as candidates for the index development. A stepwise linear regression of each metric against environmental parameters (lake area, altitude, maximum depth, alkalinity) and anthropogenic pressures (drainage area covered by non-natural land uses – NNLC, water total phosphorus concentrations – TP, Lake Habitat Modification Score – LHMS) was initially conducted for ensuring pressure-response relationships. Reference conditions for each lake were estimated by the hindcasting procedure and the ecological quality for each lake was expressed as the ecological quality ratio (EQR) by a value ranging from 0 (poor quality) to 1 (excellent quality). Two fish fauna metrics, the relative numerical abundance of introduced species (Introduced_a) and the relative biomass of omnivorous species (OMNI_b) were finally extracted as the most significant, responding to LHMS and TP, respectively. The final index was expressed as the mean values of the EQRs of these two metrics. The multimetric fish index presented herein could serve as a tool for assessing the ecological quality of natural lakes at broad geographical scale and generally, in the Mediterranean temperate lakes with similar hydromorphological characteristics.     

Site-specific chlorophyll reference conditions for lakes in Northern and Western Europe

The Water Framework Directive (WFD) requires EU Member States to assess the “ecological status” of surface waters. As a component of ecological status, many European countries are developing a classification scheme for chlorophyll concentrations as a measure of phytoplankton biomass. The chlorophyll classification must be based on the degree of divergence of a water body from an appropriate baseline or ‘reference condition’. This article describes the development of a series of regression models for predicting reference chlorophyll concentrations on a site-specific basis. For model development, a large dataset of European lakes considered to be in reference condition, 466 lakes in total, was assembled. Data were included from 12 European countries, but lakes from Northern and Western Europe dominated and made up 92% of all reference lakes. Data have been collated on chlorophyll concentration, altitude, mean depth, alkalinity, humic type, surface area and geographical region. Regression models were developed for estimating site-specific reference chlorophyll concentrations from significant predictor ‘typology’ variables. Reference chlorophyll concentrations were found to vary along a number of environmental gradients. Concentrations increased with colour and alkalinity and decreased with lake depth and altitude. Forward selection was used to identify independent explanatory variables in regression models for predicting site-specific reference chlorophyll concentrations. Depth was selected as an explanatory variable in all models. Alkalinity was included in models for low colour and humic lakes and altitude was included in models for low colour and very humic lakes. Uncertainty in the models was quite high and arises from errors in the data used to develop the models (including natural temporal and spatial variability in data) and also from additional explanatory variables not considered in the models, particularly nutrient concentrations, retention time and grazing. Despite these uncertainties, site-specific reference conditions are still recommended in preference to type-specific reference conditions, as they use the individual characteristics of a site known to influence phytoplankton biomass, rather than adopt standards set to generally represent a large population of lakes of a particular type. For this reason, site-specific reference conditions should result in reduced error in ecological status classifications, particularly for lakes close to typology boundaries.     

A water level drawdown index for aquatic macrophytes in Nordic lakes

Many northern lakes are regulated to enhance hydropower production and flood protection. This bears hydromorphological pressures which are important factors causing lowered ecological status. Water level fluctuation triggers erosion on the shoreline and, depending on fluctuation range, also affects species composition or disappearance of sensitive aquatic macrophytes. We developed a water level-drawdown index (WI_c) for Nordic lakes using macrophyte data from 73 lakes with varying water level fluctuation in Finland, Norway and Sweden. The index is based on the ratio between sensitive and tolerant macrophyte species. The sensitive and tolerant species are identified based on a percentile approach, analysing the presence or absence of species along the winter drawdown range. The index correlates well with winter drawdown in Finnish and Norwegian lakes with strongest correlations with winter drawdown in storage lakes (lakes regulated for hydroelectric power and with a considerable winter drawdown). The WI_c-index is applicable in low alkalinity, oligotrophic and ice-covered lakes, and is suggested to be a useful tool to identify and designate heavily modified water bodies in Nordic lakes according to the European Water Framework Directive.     

Lake depth and geographical position modify lake fish assemblages of the European 'Central Plains' ecoregion

1. Classification of European lake fish assemblages can be based on fish‐assemblage structure or morphological, geographical, physical and chemical lake attributes. However, substantial gaps in knowledge exist with respect to the correspondence between both classification approaches. 2. Here, we compiled fish assemblage data from 165 lakes situated in the European ‘Central Plains’ ecoregion. Cluster analysis of fish abundances was performed to compare fish assemblage types of the entire ecoregion with those from previous country‐specific studies. Nonparametric group comparisons, classification trees and partial canonical ordinations were used to infer the correspondence between fish assemblage types and morphology, geographical position and nutrient concentration of the lakes. 3. Three distinct fish assemblages were revealed: vendace (Coregonus albula), ruffe (Gymnocephalus cernuus) and roach (Rutilus rutilus) lake types. Both latitude and lake depth were the best determinants of lake type, but total phosphorus (TP) concentrations were also important. Vendace lakes were deep and had low TP concentrations, whereas the shallower ruffe and roach lakes had higher TP values. Roach lakes were more frequent in the north‐west area of the ecoregion, whereas ruffe lakes were more often found south of the Baltic Sea. 4. Controlling for the influence of nutrient concentration showed that lake morphology and geographical position were important determinants of fish assemblages. However, the variance explained was low (<20%), implying that biological interactions may also be important in forming the lake‐specific fish assemblages. 5. The results suggest that fish assemblages differ between deep and shallow lakes, and between the north‐west and south‐east locations within the Central Plains ecoregion. Accordingly, establishment of depth‐related lake morphotypes is needed, and the European ecoregions recommended to be used in evaluation systems according to the Water Framework Directive seem to be too coarse to reflect the subtle differences of fish species richness along geographical gradients.     

Potential conflicts between environmental legislation and conservation exemplified by aquatic macrophytes

It is important that legislation on water quality issues of freshwaters is not in conflict with nature conservation purposes. So far, it is however unknown how the assessment of ecological status according to for example the Water Framework Directive (WFD) of the European Community relates to the status of lakes according to the Habitat Directive (HD) or to national environmental objectives including, e.g., the protection of important wetland areas and red-listed species. We used lake macrophyte classification schemes of Norway, Sweden, and Finland and a total of 1,014 lakes to evaluate the possible conflict between these directives and national legislation. The classification schemes represent mainly trophic indices penalizing lakes with elevated phosphorous concentrations. In general, high ecological status according to the WFD did not mean high number of red-listed species or high status according to the HD or other national environmental objectives. In Sweden 78%, in Norway 47%, and in Finland 29% of lakes with red-listed species were classified as lakes of moderate or worse ecological status based on the macrophyte classification scheme. These lakes thus did not fulfill the demands of the WFD. Restoration of surface water toward fulfilling the demands requires in practice a reduction of the trophic status. This might potentially result in for example the loss of red-listed species. To avoid such potential conflicts, we primarily suggest revising the national quality assessment systems toward implicitly incorporating nature conservation aspects, e.g., the number of red-listed species in a multi-metric assessment system.     

Bacterial Diversity and Geochemical Profiles in Sediments from Eutrophic Azorean Lakes

In the Azores, the advanced trophic state of the lakes requires a fast intervention to achieve the good ecological status prescribed by the Water Framework Directive. Despite the considerable effort made to describe the phytoplankton growing on the water column, the lack of information regarding the microbial processes in sediments is still high. Thus, for the successful implementation of internal management actions, the present work explored the relationships between geochemical profiles and dominant members of the bacterial community in sediments from eutrophic Azorean lakes. Lake Azul geochemical profiles were quite homogeneous for all parameters, while in lake Furnas the total iron profile presented a peak below the aerobic layer. For lake Verde, the concentrations of all studied parameters (20 ± 2% loss-on-ignition; 2.10 ± 0.08 mg g^?1 total phosphorus; 1.31 ± 0.50 mg g^?1 total nitrogen; 8.06 ± 0.13 mg g^?1 total iron) in the uppermost sediment layer were approximately two times higher than the ones in sediments from other lakes, decreasing with sediment depth. The higher amounts of phosphorus and organic matter in lake Verde suggested a higher internal contribution of phosphorus to eutrophication. The dominant members of the sediment bacterial community, investigated by denaturing gradient gel electrophoresis, were mostly affiliated to Proteobacteria phylum (Alpha-, Delta-, and Gamma-subclasses), group Bacteroidetes/Chlorobi and phylum Chloroflexi. The Cyanobacteria phylum was solely detected in sediments from lake Verde and lake Furnas that presented the highest amounts of nitrogen and phosphorus both in the water column and sediments, while the other phyla were detected in sediments from the three studied lakes. In conclusion, management measurers to achieve the good ecological status until 2015 should be distinct for the different lakes taking into account the relative magnitude of the nutrient sources and the bacterial diversity in sediments.     

Patterns in the Distribution of Cladocerans (Crustacea: Branchiopoda) in Lakes Across a North–south Transect in Alaska, USA

The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a north–south transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.     

Mining lakes in a disturbed landscape: Application of the EC Water Framework Directive and future management strategies

Some of the several hundred Lusatian mining lakes originating from lignite mining will be among the largest and deepest lakes in Germany. A special problem of many of these lakes is severe acidification. According to the EC Water Framework Directive (WFD), artificial lakes larger than 0.5 km² have to be monitored and managed in order to establish or maintain a state of so-called good ecological water quality. Therefore the WFD demands the definition of type-specific reference conditions as those of the closest comparable surface water body type in nature, such as natural acidic volcanic crater lakes or oligo- to mesotrophic neutral hard water lakes. We propose a classification scheme for mining lakes applying acidity and planktonic colonization as criteria. Five types of lakes are distinguished ranging from extremely acidic to neutral hard water lakes. Phytoplankton and especially metazoan zooplankton are introduced as intriguing and simple indicators for different states of ecological quality in highly acidic mining lakes. We recommend that extremely and very acidic lakes should remain in the acidic state to protect these as valuable ecosystems, whereas moderately and weakly acidic lakes can be managed by external flooding, chemical or ecotechnological measures to achieve neutral water quality conditions necessary for recreation or fishery use.     

Biological indicators track differential responses of pelagic and littoral areas to nutrient load reductions in German lakes

External nutrient loading was reduced over the past decades as a measure for improving the water quality of eutrophic lakes in western Europe, and has since been accelerated by the adoption of the European Water Framework Directive (WFD) in 2000 (EC, 2000). A variety of eutrophication-related metrics have indicated that the response of biological communities to this decreased nutrient loading has been diverse. Phytoplankton, a major component of the pelagic community, often responded rapidly, whereas a significant delay was observed for submerged macrophytes colonizing littoral areas. In this study we tested whether assessment methods developed for phytoplankton and macrophytes in lakes during Germany's implementation of the WFD reflect this differential response. An assessment of 263 German lakes confirmed that a lower ecological state was recorded when based on the biological quality element (BQE) for macrophytes than the BQE for phytoplankton during the investigated period (2003–2012). On average, lakes had a moderate ecological status for both phytoplankton and macrophyte BQEs, but differences of up to three classes were observed in single cases. Long-term data were available for five lowland lakes subject to strong reductions in phosphorus loading. Their phytoplankton-based assessments indicated a constant improvement of the ecological status in parallel to decreasing water phosphorus concentrations. In contrast, macrophyte-based assessments indicated a 10–20 year delay in their ecological recovery following nutrient load reduction. This delay was confirmed by detailed data on the temporal development of macrophyte species diversity and maximum colonization depths of two lakes after nutrient load reduction. We conclude that the available WFD assessment methods for phytoplankton and macrophyte BQEs are suitable to track the differential response of pelagic and littoral areas to nutrient load reductions in German lakes.     

Recommendations for sampling littoral diatoms in lakes for ecological status assessments

This review summarises the existing literature and outlines the theoretical basis for using standard methods for sampling diatoms from rivers to sample littoral diatoms and other phytobenthos from standing waters. The European Union's Water Framework Directive has created a statutory obligation for EU Member States to monitor macrophytes and phytobenthos in lakes. Although there has been a considerable amount of work using phytobenthos (especially diatoms) to monitor river water quality in Europe, there are fewer studies on the use of phytobenthos for monitoring in lakes. European standard methods for sampling diatoms from rivers should be suitable, with only minor modifications, for sampling littoral diatoms from lakes and other standing water bodies. These recommendations should be applicable to all temperate regions.     

A comparative review of recovery processes in rivers, lakes, estuarine and coastal waters

The European Water Framework Directive aims to improve ecological status within river basins. This requires knowledge of responses of aquatic assemblages to recovery processes that occur after measures have been taken to reduce major stressors. A systematic literature review comparatively assesses recovery measures across the four major water categories. The main drivers of degradation stem primarily from human population growth and increases in land use and water use changes. These drivers and pressures are the same in all four water categories: rivers, lakes, transitional and coastal waters. Few studies provide evidence of how ecological knowledge might enhance restoration success. Other major bottlenecks are the lack of data, effects mostly occur only in short-term and at local scale, the organism group(s) selected to assess recovery does not always provide the most appropriate response, the time lags of recovery are highly variable, and most restoration projects incorporate restoration of abiotic conditions and do not include abiotic extremes and biological processes. Restoration ecology is just emerging as a field in aquatic ecology and is a site, time and organism group-specific activity. It is therefore difficult to generalise. Despite the many studies only few provide evidence of how ecological knowledge might enhance restoration success.