Water colour, phosphorus and alkalinity are the major determinants of the dominant phytoplankton species in European lakes

Analysis of phytoplankton data from about 1,500 lakes in 20 European countries has revealed that two-thirds of the species that dominate lakes during the summer are dominant right across Europe. Using Canonical Correspondence Analyses, we have examined how both habitat conditions within lakes and environmental factors over broad geographical scales explained the distribution of the 151 most common summer dominant species. The distributions of these species were best explained by water colour and latitude, although alkalinity and total phosphorus also appeared to be important explanatory factors. Contrary to our original hypothesis, summer water temperatures had a negligible impact on the distribution of dominants, although, due to the restricted summer season we examined, only a limited temperature gradient was present in the dataset. Cryptophytes occurred more frequently among dominants in Northern Europe whereas cyanobacteria and dinophytes dominated more in Central and Southern Europe. Our analyses suggest that besides nutrient concentrations, other water chemistry variables, such as alkalinity and the content of humic substances, have at least as important a role in determining the distribution of the dominant phytoplankton species in European lakes.     

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.     

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.     

Phytoplankton indicator taxa for reference conditions in Northern and Central European lowland lakes

Phytoplankton data from 606 lakes were used to characterize indicator taxa of near-pristine reference conditions in clearwater and humic lowland lakes of Northern and Central Europe. Reference lakes were selected based on low pressure from catchment land-use, low population density and the absence of point sources. Reference lakes had low phytoplankton biomass and taxa richness compared to non-reference lakes. In low alkalinity lakes of Northern Europe, the reference communities had high biomass proportions of chrysophytes and low proportions of cyanobacteria; in the Central European high alkalinity lakes, the biomass was distributed more evenly among algal groups. Indicator species analysis and similarity analysis listed 5–29 taxa indicating reference conditions. Indicator taxa differed especially between the low alkalinity and the high alkalinity lakes, but there were also country-specific differences. Most common indicator taxa for the northern reference lakes were chrysophytes (e.g. Bitrichia, Dinobryon). In the Central European reference lakes, diatoms (e.g. Cyclotella) were more characteristic. Despite the differences, there was a general finding that taxa present in reference lakes were often also present in non-reference lakes, but typically in lower biomass proportions; another characteristic of the reference communities is the absence of many taxa typically found in non-reference lakes.     

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.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

A comparison of the European Water Framework Directive physical typology and RIVPACS-type models as alternative methods of establishing reference conditions for benthic macroinvertebrates

The EU Water Framework Directive requires European Union Member States to establish ‘type-specific biological reference conditions’ for streams and rivers. Types can be defined by using either a fixed typology (System-A), defined by ecoregions and categories of altitude, catchment area and geology, or by means of an alternative characterisation (System-B) that can use a variety of physical and chemical factors. Several European countries also have existing RIVPACS-type models that give site (rather than stream type) specific predictions of benthic macroinvertebrate communities. In this paper we compare the Water Framework Directive (WFD) System-A physical typology and three existing European multivariate RIVPACS-type models as alternative methods of establishing reference conditions. This work is carried out in Great Britain – using RIVPACS, Sweden – using SWEPAC_SRI and the Czech Republic – using PERLA. We found that in all three countries, all seasons and season combinations, and for all biotic indices tested, RIVPACS-type models were more effective (lower standard deviations of O/E ratios) than models based solely on the WFD System-A variables or null models (based on a single expectation for all sites). We also investigated the explanatory power of whole groups of WFD System-A variables and RIVPACS-type model variables, and the explanatory power of individual variables. We found that variables used in the RIVPACS-type models were often better correlates of macroinvertebrate community variation than the WFD System-A variables. We conclude that this is primarily because while the latter use very broad categories of map-derived variables, the former are based on continuous variables selected for their ecological significance.     

Productivity of clear and humic lakes: nutrients,phytoplankton, bacteria

The relationships between long-term surface average concentrations of humic acids measured as water colour, dissolved organic carbon (DOC) or Secchi disk transparency and trophic state variables were studied with literature data from more than 600 freshwater lakes. The geometric means of summer surface average nutrient (phosphorus and nitrogen) concentration, phytoplankton biomass (chlorophyll concentration), and hypolimnetic anoxia (anoxic factor) were significantly higher in coloured than in clear lakes. The regressions of colour or DOC on these trophic state variables were positive and significant throughout a range of three orders of magnitude. Phytoplankton or primary productivity was higher in coloured lakes, when expressed per volume of epilimnion. Annual integral primary productivity expressed on an areal basis was smaller in coloured lakes, probably a reflection of shallower phototrophic depths in these lakes. There is evidence that annual integral bacteria productivity is much higher in coloured lakes for two reasons: first, epilimnetic bacteria production was ca. four times higher in coloured lakes, second, other studies have shown that hypolimnetic bacteria production is commonly higher than epilimnetic production, especially in anoxic hypolimnia that are frequent in coloured lakes. All volumetrically expressed variables indicated higher productivity in coloured lakes. In addition, high bacteria productivity reflects a different food chain involving mixotrophs, possibly compensating for low light conditions. Our analyses indicate that primary and secondary productivity is as high as or higher than in clear lakes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure of chironomid assemblages along environmental and geographical gradients in the Bohemian Forest lakes (Central Europe): An exploratory analysis

Eight glacial lakes of the Bohemian Forest (Czech Republic and Germany) were characterised by the distribution of chironomids collected as pupal exuviae. Twenty-eight taxa were identified, including some faunistically interesting species of the region. Two-way indicator species analysis (TWINSPAN) was used to classify lakes according to their taxonomic composition. Canonical correspondence analysis (CCA) and multiple regression were used to relate the chironomid assemblages to two sets of explanatory variables: (i) local environmental variables, and (ii) broad-scale spatial variables. The TWINSPAN classified the lakes into four groups, whereas presence/absence of three taxa was indicative for this classification. The CCA of assemblage composition on environmental variables showed that chironomids respond significantly to altitude and alkalinity. The ordination of composition data on geographical variables revealed strong longitudinal gradient in chironomid distributions. Altitude and alkalinity accounted for 36.2% of the total variation, while the geographic gradient explained 20.5%. As revealed by the variation partitioning procedure, the significant effect of these variables was, in large part, independent of each other. Overall taxonomic richness appeared to be governed by altitude only. Causal ecological and historical factors underlying these results are discussed. This paper may provide a basis for hypothesis testing in future research of the Bohemian Forest lakes.     

Bacterioplankton in the littoral and pelagic zones of subtropical shallow lakes

We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting area (from 13 to 80,800 ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon (DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than in the pelagic zone of shallow lakes.     

Relationships between morphometry,geographic location and water quality parameters of European lakes

I addressed the question how lake and catchment morphometry influences water chemistry and water quality over a large scale of European lakes, and developed the regression equations between most closely related morphometric and water quality indices. I analysed the data of 1,337 lakes included in the European Environment Agency (EEA) database, carrying out separate analyses for three basic lake types: large lakes (area ≥100 km², 138 lakes), shallow lakes (mean depth ≤3 m, 153 lakes) and large and shallow lakes (area ≥100 km² and mean depth ≤8 m, 35 lakes). The study revealed that in Europe, the lakes towards North are larger but shallower and have smaller catchment areas than the southern lakes; lakes at higher altitudes are deeper and smaller and have smaller catchment areas than the lowland lakes. Larger lakes have generally larger catchment areas and bigger volumes, and they are deeper than smaller lakes, but the relative depth decreases with increasing surface area. The lakes at higher latitudes have lower alkalinity, pH and conductivity, and also lower concentrations of nitrogen and phosphorus while the concentration of organic matter is higher. In the lakes at higher altitudes, the concentration of organic matter and nutrient contents are lower and water is more transparent than in lowland lakes. In larger lakes with larger catchment area, the alkalinity, pH, conductivity and the concentrations of nutrients and organic matter are generally higher than in smaller lakes with smaller catchments. If the lake is deep and/or its residence time is long, the water is more transparent and the concentrations of chlorophyll a, organic matter and nutrients are lower than in shallower lakes with shorter residence times. The larger the catchment area is with respect to lake depth, area and volume, the lower is the water transparency and the higher are the concentrations of the nutrients, organic matter and chlorophyll as well as pH, alkalinity and conductivity. The links between lake water quality and morphometry become stronger towards large and shallow lakes. Along the decreasing gradients of latitude, altitude and relative depth, the present phosphorus concentration and its deviation from the reference concentration increases.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Assessing lake typologies and indicator fish species for Italian natural lakes using past fish richness and assemblages

In order to establish a fish-based typology of Italian lakes and identify possible reference and indicator fish species for each lake type, we analysed historical data on fish assemblages of all Italian natural lakes >0.5 km² from the period prior to the major decline in water quality in the 1950s. General linear regression models showed the ecoregion and lake altitude being the best predictors of fish species richness. The number of species was significantly higher in the Alpine than in the Mediterranean ecoregion. Among Alpine lakes, the number of fish species increased significantly with lake volume whilst decreased with altitude. In the Mediterranean lakes, none of the selected parameters was significant. Cluster analysis of fish assemblages (presence/absence) divided the lakes of the Alpine and Mediterranean ecoregions into four and two types, respectively. Pike (Esox lucius), rudd (Scardinius erythrophthalmus) and tench (Tinca tinca) were the main indicator species for the small and mostly shallow lakes in both the Alpine (Type 1) and Mediterranean (Type 6) ecoregions, minnow (Phoxinus phoxinus) for the alpine high altitude lakes (Type 2) and landlocked shad (Alosa fallax lacustris), European whitefish (Coregonus lavaretus) and burbot (Lota lota) for the large and very deep alpine lakes (Type 4). The European whitefish was the only indicator species for the deep Mediterranean lakes (Type 5). These species and associated fish assemblages may be useful indicators in future assessments of the ecological status of Italian lakes according to the European Directives (2000/60/EC and 2008/105/EC).     

Viral and Bacterioplankton Dynamics in Two Lakes with Different Humic Contents

Viral and bacterioplankton dynamics were investigated, together with the temporal variation of phage-infected bacterioplankton in two oligotrophic lakes, one humic and the other clearwater. Bacterial abundance was significantly higher in the humic lake, while the abundance of virus-like particles (VLP) was significantly higher in the clearwater lake. There were no differences in either the frequency of infected bacterial cells (FIC), or in burst size between the lakes. Because of the higher bacterial abundance in the humic lake, a larger number of bacteria were lyzed in this lake. FIC showed large seasonal changes, varying between 9 and 43%, which covers almost the entire range of previously published data from both lacustrine and marine environments. The temporal changes in VLP abundance and FIC were slow in both the humic and clearwater lakes. The burst size was low in both lakes (average value, nine in each case), probably because of the oligotrophic status of the lakes. The chlorophyll a concentrations were higher and positively correlated with VLP numbers in the clearwater lake, indicating that a significant proportion of the viruses in this lake may be phytoplankton viruses.     

Assessment of eutrophication pressure on lakes using littoral invertebrates

Until the E.U. Water Framework Directive listed benthic invertebrates as a biotic element to be used for ecological classification of lakes, techniques for the assessment of the response of littoral invertebrates to anthropogenic pressures were extremely limited compared with those of rivers and lake profundal zones. We describe here the development of an ecological classification model based on changes of littoral invertebrate assemblages across a gradient of eutrophication, which is the most widespread anthropogenic pressure on lakes across Europe. The model comprises three derived parameters, two of which were developed from taxon-specific optima along a total phosphorus gradient calculated using canonical correspondence analysis, and the third based on invertebrate abundance. Combining the parameter metrics, we can estimate the ecological quality ratio (EQR), relative to those from paleolimnologically-confirmed reference lakes. The model was tested using independent samples collected from both hard and soft substrata and across two seasons from 45 lakes, comprising three alkalinity groups (n = 15 in each), and across gradients in water column total phosphorus concentrations. For hard substrata, EQRs were related consistently and highly significantly to water column concentrations of total phosphorus, accounting for the majority of the variance in every alkalinity group. For samples taken from soft substrata, a significant relationship was found only for high alkalinity lakes, accounting for a moderate proportion of the variability in water column total phosphorus concentrations. Our results compare highly favourably with those from other aquatic ecological assessment methods, irrespective of the faunal or floral group upon which they are based, demonstrating that littoral invertebrate assemblages can provide a statistically robust prediction of nutrient status when samples are collected from hard substrata. While the method was developed specifically to assess nutrient pressures on littoral invertebrates, many lakes are subject to multiple pressures. The development of classification models that incorporate multiple pressures presents a particularly significant challenge for the implementation of the Water Framework Directive, requiring both reliable identification of minimally-impacted reference states and incorporation of pressures that are unlikely to interact in predictable ways.     

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.     

Biological assessment of European lakes: ecological rationale and human impacts

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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.