The effect of land use on the diatom communities in lakes

Data on recenr diatom community structure and relevant environmental characteristics from the lakes and their catchments have been collected from 151 oligotrophic lakes in eastern Finland. The pattern of frequency distribution of diatoms as a function of environmental variables, including land use in the catchment, differs between diatom taxa and indicates the optimum conditions and amplitude of occurrence for particular species. This kind of study should lead to increased understanding of the environmental requirements of diatom species and will be useful in the interpretation of historical changes in lakes as well as in forecasting possible future changes.     

Climate drivers of diatom distribution in shallow subarctic lakes

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The relationship between surface sediment diatom assemblages and pH in 33 Galloway lakes: Some regression models for reconstructing pH and their application to sediment cores

The species composition of surface sediment diatom assemblages in 33 Galloway lakes, pH range c. 4.5 to 7.4, is related in a statistically significant manner to water acidity (or to factors closely associated with pH). Predictive models of summer mean pH, using simple regression equations of Index B (Renberg & Hellberg, 1982) with both Scandinavian and Galloway data sets, and multiple regression equations using diatom pH preference groups and individual species, are described and applied to fossil diatoms in six sediment cores. Although multiple regression of individual species gives the highest correlation coefficient (r²=0.87) in the modern data set this method is least appropriate for reconstructing pH values from fossil material where predictor species are often absent. Of the four methods examined here it is suggested that multiple regression of diatom preference groups is probably the most suitable for pH reconstructions from sediment cores.     

Effects of nutrient (N,P, C) enrichment upon the littoral diatom community of an oligotrophic high-mountain lake

The effects of nutrient additions upon the epilithic diatom communities and the algal standing crop were investigated in the oligotrophic, softwater Lake Piccolo Naret, situated in the Swiss alps. Nutrient-diffusing flower pot substrates were filled with either N (0.15 mol NaNO₃), P (0.015 mol Na₂HPO₄) or C (0.15 mol NaHCO₃) or combinations of them. Twenty-five pots representing eight treatments were placed into the lake in July 1991 and sampled after 42 days of exposure.On the surfaces of all pots containing P we measured higher algal biomasses as on the control pots. The chlorophyll-a maximum of 12.9 µg cm^{– 2} was obtained on NPC pots (0.47 µg cm^{– 2} on control pots). On pots with P, NP or NPC supply high amounts of green algae were detected, also reflected in an increased chl-b/chl-c ratio related to the controls which showed algal communities dominated by diatoms.The diatom communities on the control pots as well as on the pots with N, P and NP had a structure similar to the epilithic community in Lake Piccolo Naret (dominance identity > 58 %). However, the community structures of the diatoms from pots with C addition (C, NC, PC and NPC) differed considerably. This is discussed in view of the cell densities of dominant diatom species. For further comparisons the results of two additional high-mountain lakes are used. By means of a cluster analysis it could be shown that epilithic diatom communities were considerably influenced by C addition, while N and P supply were of minor importance.     

Changes in diatom assemblages in the profundal sediments of two large oligohumic lakes in eastern Finland

Sedimentary diatom assemblages in two large oligotrophic clear-water lakes were analysed, to assess their present ecological state and possible eutrophication due to diffuse nutrient loading. The lakes Pyhäjärvi and Puruvesi (Finnish lake district) are proportionally large for their catchment areas which accounts for their long retention times (ca 7 and 11 yr) and oligohumic character. Pyhäjärvi was studied by pairwise comparison of surface sediment diatom assemblages collected in 1985 and 1990 at 12 sites from different parts of the lake. In Puruvesi, the stratigraphy of diatoms was analysed in two short cores from 8 m and 32 m depths.The diatom assemblages of the two lakes are rather similar, and quite distinct from the assemblages of the mesohumic large lakes of the area. Cyclotella kuetzingiana is the most common planktonic dia- tom, but Aulacoseira ambigua abounds in Pyhäjärvi at sites with local sources of eutrophication. A diverse assemblage of benthic forms, especially Fragilaria and Achnanthes spp. characterizes the shallow bottoms in both lakes.There was little change within the short-core diatom profiles of Puruvesi, but the floral composition of the 8-m and 32-m sites differed markedly. The 8-m site, with 60–70% of benthic forms, represents illuminated bottom, on which much of the buried algae have lived in situ, while the deeper site is true profundal, dominated by sedimented planktonic algae.In Pyhäjärvi there was a slight increase in the benthic diatoms from 1985 to 1990, coinciding with increased phosphorus and chlorophyll concentrations as well as Secchi depth lowering. We interprete this observation as a very early step of eutrophication, of which first the sessile algal communities of the illuminated bottom areas have benefited.     

Phosphorus dynamics in Danish lakes and the implications for diatom ecology and palaeoecology

SUMMARY 1. Seasonal phosphorus variability may be considerable in eutrophic lakes and patterns are influenced by internal, as well as external, nutrient loading. The strong seasonality of planktonic diatoms, with main growth periods in the spring and also in the autumn in meso‐ to eutrophic lakes, means that the measure of total phosphorus (TP) that is most relevant for diatom ecology may not be adequately assessed by single or few measurements during the year. 2. The diatom species assemblage of surface sediments can be used to infer in‐lake nutrient concentrations. Weighted averaging (WA) and weighted averaging partial least squares (WAPLS) regression and calibration models for diatoms and annual mean TP were developed for a 29‐site data set of Danish lakes based on (a) all diatom species and (b) planktonic species only. Jack‐knifed error statistics were: =0.37, root mean squared error of prediction (RMSEP)=0.28 log₁₀μg TP L^?1 and mean bias=0.04 log₁₀μg TP L^?1 for the WAPLS 2‐component model based on all species; =0.23, RMSEP= 0.32log₁₀μg TP L^?1 and mean bias=0.07 log₁₀μg TP L^?1 for the WA model with tolerance downweighting based on planktonic species only. These are comparable with similar, published data sets. 3. A subset of 23 sites was used to develop models based on seasonal TP measurements. Mean spring TP concentrations gave only slightly improved RMSEP values for models based on all species and plankton‐only (0.24 log₁₀μg TP L^?1 and 0.29 log₁₀μg TP L^?1, respectively). 4. Weighted averaging models derive environmental optima, for individual species, which are not necessarily of ecological relevance. However, good water chemistry data are required to model species' responses adequately and to develop calibration data sets.     

Multivariate analysis of diatoms and water chemistry in Bolivian saline lakes

Diatom assemblages are described from surface sediments in thirteen salt lakes located in the southern Bolivian Altiplano. Factor analysis of correspondences and cluster analysis are used to classify the diatom assemblages. New methods are proposed to establish the qualitative and quantitative relationships between diatom floras and ecological parameters. Diatom assemblages are linked more to the ionic elements than to the salinity, pH, depth, temperature or elevation. Environmental variables are divided into three modalities which allow considerations of many different variables not under the same units.     

Sedimentary diatom assemblages and their utility in computing diatom-inferred pH in Sudbury Ontario lakes

Surface sediment diatoms from 30 Sudbury lakes were analyzed to establish a relationship between diatoms and limnological characteristics of these lakes. Factor analysis of various chemical and physical variables suggested that most of the variance was associated with the pH, pH-metal, and conductivity factors. Factor analysis of common diatom taxa indicated that the distribution of most of these diatoms is closely related to lake water pH and/or pH related factors. Regression analysis of various pH indicator assemblages with measured lake water pH also demonstrated significant relationships. Among the transfer functions employed to compute diatom-inferred pH, multiple regressions using pH indicator assemblages proved to be the best. Using this method an accuracy of about 0.3 pH unit can be achieved, even for lakes which have received large trace metal inputs concurrent with increase in acidity. The presence of planktonic diatoms in acidic lakes suggests that the widely accepted hypothesis that planktonic diatoms are greatly reduced in waters at pH below 5.6 is not always true.     

Methods of pH calibration of sedimentary diatom remains for reconstructing history of pH in lakes

The pH history of lakes can be inferred from diatom remains in dated sediment cores. To derive transfer functions for pH inference in acidic lakes, we counted diatoms in surface-sediment from 31 soft-water lakes in n. New England (NE) and 36 in Norway (N), covering pH 4.4–7.1. Cluster analysis of each data set indicates that pH 6 is an upper limit for a group of similar diatom assemblages. For each set, we developed multiple linear regressions to relate three versions of the diatom data to pH of surface-waters: (1) relative frequencies of selected diatom taxa, (2) the first principal component (1 PC) of these frequencies, and (3) the frequencies of Hustedt pH groups. Also, simple linear regressions were developed for two versions: (1) Index B and (2) Index Alpha, both based on pH groups. Regressions were run separately for lakes with pH 6; these are most relevant for pH inference in acidic lakes. The best regressions (N: taxa & 1 PC taxa) have r² 0.69–0.91 and S_e 0.24–0.31 pH units, the worst (NE: log alpha) have r² 0.27–0.57 and S_e 0.51. In all cases, errors for NE are greater than N, partly due to greater diversity of NE lakes. Regressions based on pH groups (directly & by indices) have smaller r² and larger S_e than those based on taxa and 1 taxa. The Index Alpha is least useful because its requirement for alkaline diatom units is unsatisfied at many acidic lakes. Regressions based on taxa may give erratic pH inferences due to sensitivity to unusual frequencies of individual taxa; this effect is reduced by using 1 PC taxa. Four regressions based on pH 6 lakes were used for inferring pH in a ²¹⁰Pb dated core from Nedre Målmesvatn, N (now pH 4.6). There is good agreement among three of the four (not for the regression based directly on taxa) that there has been a decrease of ca. 0.6 pH units starting in the late 1800's.     

Recent environmental change in North African wetland lakes: diatom and other stratigraphic evidence from nine sites in the CASSARINA Project

All nine wetland lakes in the CASSARINA North African suite of sample sites have been disturbed strongly by human activity during the 20th century. Dated lake sediment core were used to provide evidence of the extent of recent environmental change at each site. Sedimentary diatoms at seven sites were useful for inferring salinity change trends during the last century. At two sites preservation problems severely degraded the sedimentary diatom record. Sediment core integrity was otherwise established.Lithostratigraphic measurements indicated some site specific changes in soil erosion and sediment composition but, for the Egyptian Delta lakes, no physical signal synchronous with Aswan High Dam construction was found. Sedimentary diatom assemblages were generally site characteristic and halophilous taxa were common. At two sites planktonic diatoms indicated some recent eutrophication but generally the assemblages were more indicative of salinity changes. Diatom-inferred salinity trends for the seven sites typically indicated that reductions in water salinity occurred sometime during the early or mid 20th century.Rather than climate, hydrological modification of water resources is implicated as the primary driver of salinity changes during most of the 20th century. In the western North African region these modifications were mainly local land drainage and water diversion programmes to alleviate winter flooding and/or promote summer water availability. In the Delta region, the Nile has been intensively exploited since antiquity and intensively so from the late 19th century to release more fresh water for agriculture. Here, diatom records indicate that freshening began well before the Aswan High Dam but salinity fluctuations have tended to diminish during the latter part of the 20th century. A small reversal in the water freshening trend in the 1980/90s was possibly a response to land subsidence/sea-level change or to reduced freshwater supply.Freshwater supply to the sites is generally diminishing as former freshwater surpluses switch to deficit. One site (Merja Bokka, Morocco) became completely dry in 1998 as agriculture encroached and Megene Chitane, the only acid lake in Tunisia, is currently affected by excessive inflow abstraction. At the beginning of the 21st century, eight of the nine CASSARINA sites persist as viable but modified aquatic ecosystems. They nevertheless continue to support valuable aquatic biodiversity, especially in the Delta sites and in Chitane. The modern diatom communities are clearly tolerant of considerable environmental change but the remaining sites are increasingly threatened by major hydrological disturbance. Base-line floristic data for the late 20th century are given but continuous biomonitoring combined with effective management is needed urgently to help conserve North Africa's diminishing natural wetland lake resources.     

Diatom distribution and diatom inferred pH in the sediment of four alpine lakes

The diatom distribution in the sediment of four alpine lakes in the central alpine area (Ötztaler Alpen, Tyrolia, and Mölltal, Carinthia) has been analysed in order to assess possible influences of atmospheric pollutants on remote lakes. The lakes are located more than 2000 m above sea level on poorly buffered bedrock (granite, gneiss and mica schist) and are therefore susceptible to acidification by acid deposition. The pH history of the lakes has been calculated by using Index B from Renberg & Hellberg (1982), which has been adjusted to central European conditions and applied in this region for the first time. The results indicate that there have been no pH decreases in three of the lakes, while in one lake diatom composition and distribution change significantly towards the sediment surface. But taxonomical problems and difficulties in the pH classification of the rarely found diatoms complicate the interpretation of the results. Important taxa will be discussed in view of their ecological significance.     

Some aspects of the seasonal distribution of flagellates in mountain lakes

In a larger regional survey in Tyrol, phytoplankton species composition and biovolume of mid-altitude and high-mountain lakes was studied. Results from eight lakes showed that flagellates (mainly Chrysophyceae, Dinophyceae, and Cryptophyceae) are important components of the phytoplankton.In the mid-altitude lakes a spring and an autumn maximum of Chrysophyceae as well as a summer maximum of large dinoflagellates are observed, whereas Cryptophyceae and Dinophyceae show irregular distributions. In the high-mountain lakes the seasonal variations of phytoplankton, including flagellates, are limited by the long duration of the winter situation. However similar sequences of phytoplankton assemblages as in the midaltitude lakes can be observed. Flagellates in high-mountain lakes are important to sustain phytoplankton standing crop under the winter snow and ice cover.In order to show similarities and differences of high-mountain and mid-altitude lakes, vertical profiles of phytoplankton from three lakes and seasonal patterns of Gymnodinium uberrimum from two lakes are compared. In addition the patterns of cryptomonads differing in their ecological requirements (Cryptomonas spp. and Rhodomonas minuta) are shown for a meromictic mid-altitude lake.     

Associations of diatoms in the surficial sediments of lakes and peat pools in Atlantic Canada

The distribution of diatom frustules in the surficial sediments of lakes and ombrotrophic peat pools in Atlantic Canada are examined especially in relation to pH and bog lake succession. Sedimentary diatom associations for peat pools, strongly acidic lakes, and weakly acidic lakes are characterized. These associations and the derived diatom-pH calibration functions will facilitate improved paleoecological interpretation of natural ontogenetic pathways and assessments of recent anthropogenic impacts.     

Prodrome of the Italian vegetation: A new alliance for the high-mountain chamaephytic communities of central and southern Apennines

The new alliance Cerastio tomentosi-Globularion meridionalis and the new association Helianthemo alpestris-Globularietum meridionalis are here proposed as a contribute to the prodrome of Italian vegetation. The new syntaxa are related to the chamaephytic high-mountain vegetation and they are characterized by a wide presence of endemics, southern European orophytes, mountain-Mediterranean, and amphi-Adriatic species. The proposal aims to differentiate, on a structural basis, the high-mountain vegetation of the Apennines at higher syntaxonomical levels.     

Broad‐scale environmental response and niche conservatism in lacustrine diatom communities

Aim (1) To resolve theoretical debates regarding the role of environment versus dispersal limitation, the conservatism of niches across distances and the prevalence of environmental specialists in diatom communities. (2) To provide guidance on the use of diatom communities and other microbial analogues to analyse ecological response to environmental change. Location Eight hundred and ninety‐one lakes in five regional datasets from north‐western Europe and four regional datasets from north‐eastern North America. Methods Lacustrine diatom communities were analysed at three scales: inter‐continental, intra‐continental and regional. Nested partial redundancy analyses (RDAs) were used to determine spatial versus environmental components of community variation. Weighted‐averaging (WA) regression and calibration, as well as logistic and quadratic regressions, were used to detect niche conservatism and the prevalence of environmental specialists. Results Community patterns indicate that dispersal limitation acts predominantly at the inter‐continental scale, while at the regional (less than c. 1,000,000 km²) scale, a single environmental variable (pH) explains more than five times the community variation as spatial (dispersal‐related) variables. In addition, pH niche components appear to be conserved at the inter‐continental scale, and environmental specialization does not impose relative rarity, as specialists apparently readily disperse to suitable environments. Main conclusions Analysis at multiple scales is clearly important in determining the influences of community variation. For diatom communities, dispersal limitation acts most strongly at the broadest scales, giving way to environment at the scales considered by most analyses. The availability of a wide variety of propagules with consistent niches across regions indicates that diatom communities reflect the succession of taxa according to local environmental conditions, rather than disequilibrium with the environment or adaptation of local populations. While multi‐scale analyses must be undertaken for other groups to resolve debates over community drivers and determine appropriate scales for prediction, for diatoms (and probably other microbial communities), responses to environmental change can be inferred using analogue datasets from large geographic areas.     

Establishing realistic restoration targets for nutrient-enriched shallow lakes: linking diatom ecology and palaeoecology at the Attenborough Ponds, U.K.

Eutrophication is a major problem for shallow lakes in the U.K. lowlands. Over the last few decades issues relating to the management and restoration of these lakes have come to the fore, including the need to develop methods for establishing realistic restoration targets. One group of lakes for which restoration is a key concern are the Attenborough Ponds in the English Midlands, an interconnected series of shallow, nutrient-enriched gravel-pit lakes. In November 1972, the highly polluted River Erewash was diverted into the Attenborough Ponds. To determine the ecological effects of this event and to establish restoration goals for this lake system, two complimentary studies were undertaken here; (i) monitoring of the ecology and sedimentary representation of diatom communities in a disturbed lake connected to the R. Erewash (Tween Pond: high nutrient concentrations, no submerged macrophytes) and a relatively undisturbed lake isolated from the R. Erewash (Clifton Pond: lower nutrient concentrations, abundant submerged macrophytes), and; (ii) examination of the sedimentary diatom record in a short sediment core collected from the chosen disturbed lake (Tween Pond). The species composition, seasonality, relative productivity and sedimentary representation of planktonic and periphytic diatom communities were very different in Tween and Clifton Ponds. In Tween Pond two major phases of planktonic diatom production/sedimentation were observed during March–May and July–September. By contrast, planktonic diatoms were restricted to a single spring peak in Clifton Pond and after May there was a switch to the dominance of periphytic diatoms associated with the development of submerged macrophytes. The 1972 diversion of the River Erewash was clearly reflected in the sedimentary diatom record from Tween Pond, by; (i) the abrupt shift to planktonic diatom dominance, and; (ii) increasing percentages of late summer–autumn associated planktonic diatom species. These changes suggest both significant nutrient enrichment and the switch from submerged macrophyte to phytoplankton dominance. Numerical matching of pre-1972 diatom assemblages with surface sediment assemblages in the gravel-pits using Principal Components Analysis (PCA) and a squared chord distance measure revealed no close analogues. Nevertheless, similarities between; (i) percentages of different diatom habitat and seasonality groups, and; (ii) pre-1972 and contemporary macrophyte survey data suggested that Clifton Pond is probably a good pre-diversion analogue for habitat structure (e.g. macrophyte biomass, composition and architecture) and phenology (e.g. diatom and macrophyte seasonality) in the Ponds. The practical value of combining space-for-time substitution and palaeoecological approaches in restoration ecology studies of shallow lakes is discussed.     

Eutrophication decreases distance decay of similarity in diatom communities

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A multi-lake comparison of epilithic diatom communities on natural and artificial substrates

In a multi-season, multi-lake study of epilithic diatom communities, glass slide artificial substrate samplers provided poor representation of communities on natural substrates. Percent similarities between the two communities averaged only 37 over the course of the study. Overall, natural substrates exhibited greater species richness than artificial samplers, particularly during the summer, although this difference was slight. Patterns of selection and inhibition by artificial substrates of individual diatom species varied by both season and lake. Members of the genus Cymbella, particularly C. microcephala, appeared to be the most consistently inhibited, while Achnanthes minutissima was often selected for. In spite of the great differences between the two substrate types, replicability of artificial substrates was very high, and could prove to be a more important quality in monitoring applications than accurate representation of the natural community.     

Fossil diatom inferred reconstruction of the pH history of two acidic,clear water lakes from insular Newfoundland,Canada

Palaeoecological research has been used to evaluate the impact of acidic deposition on lakes in insular Newfoundland. Terrestrial organic deposits in the region have considerable influence on freshwaters and have placed constrains on interpretation of the degree of anthropogenic acidification. In this paper, a region-specific calibration equation unique to clear water lakes (colour values 15) is developed from diatom assemblages in surface sediments from 22 lakes. The inferred pH history of two acidic, clear water lakes is then developed with a view to eliminating the influence of organic acidity from the interpretation of historical acid-base chemistry. The pH histories of the two lakes suggest modest declines (0.3 to 0.4 unit) in the most recent strata (i.e. since the 1930's). Both lakes demonstrate an increase in inferred pH in the surface horizon, which is consistent with declines in acidic deposition in the region since the mid 1970's. The magnitude and timing of the pH trends in these two lakes is common to those previously developed from more highly coloured lakes. The similar magnitude and onset of pH declines in lakes with varying amounts of organic influence provides no palaeolimnological evidence to suggest a contribution to, or modification of, lake acidification by organic acids.