A study of the salt lakes and salt springs of Eyre Peninsula, South Australia

An 18-month-study of 40 saline wetlands, ranging from 6 to 336 g l⁻¹, on the west and southern coasts of Eyre Peninsula yielded 88 species of invertebrates, some aquatic plants and a fish. The invertebrates are taxonomically diverse and include 38 crustaceans, 28 insects, 12 molluscs and significantly an aquatic spider, a nemertean, two polychaetes, two sea anemones, a sponge and a bryzoan. Most were tolerant of wide fluctuations in salinity, there being 51 halobionts, 21 halophils and only 16 salt-tolerant freshwater species. Many invertebrates are restricted to the thalassic springs where marine molluscs dominated. Athalassic wetlands were dominated by crustaceans and were of two basic types—coastal and continental. There is evidence of the former evolving biologically into the later, and for some lakes to be still in transition. There is also evidence of increasing salinity in recent decades and already two lakes exhibit severe secondary salinity. Like other salt lakes in Australia the fauna is regionally distinctive. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.     

Comparison of Freshwater Diatom Assemblages from a High Arctic Oasis to Nearby Polar Desert Sites and Their Application to Environmental Inference Models

Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (<5 km²) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO₂ as explaining significant (P < 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.     

Development of diatom-based salinity models for paleoclimatic research from lakes in British Columbia (Canada)

Diatoms were identified and enumerated from the surface sediments of 65 lakes located on the Cariboo and Chilcotin Plateaux (British Columbia, Canada). These lakes span a large gradient in lakewater ionic concentration (fresh through hypersaline) and composition, as well as other physical/chemical variables. Almost all of the study lakes had higher salinities in the late-summer than in the spring. The lakes with spring salinities >8 g l^–1 showed the largest seasonal increases in salinity. Ionic composition was similar in the spring and late-summer for most lakes. Both ionic concentration (i.e. salinity) and composition were important environmental variables that could account for the different diatom floras in the lakes. Diatom assemblages characteristic of carbonate-dominated and sulfate-dominated waters were identified. Other variables such as water depth and phosphorus concentration were also important.The majority (87%) of diatom taxa had estimated salinity optima < 3 g l^–1 Halophilic diatom taxa had broader tolerances to salinity when compared to the fresh water taxa, however taxa with narrow and broad tolerances could be identified across the salinity gradient. Species diversity was weakly but significantly correlated to lakewater salinity (r ² = 0.18 to 0.3, P < 0.05).Salinity inference models were developed based on the relationship between the diatom assemblages and the spring, late-summer and average salinity. The correlations between the measured and diatominferred salinity, based on the spring (r = 0.95), late-summer (r = 0.94) and average (r = 0.95) salinity data, are high because there was an extremely strong correlation (r = 0.98) between the log transformed spring and late-summer measured salinities. These salinity reconstruction models provide a tool that can be used to infer past climatic changes as part of paleolimnological studies from appropriate closed-basin lakes in British Columbia.     

Seasonal dynamics of zooplankton in a shallow eutrophic,man-made hyposaline lake in Delhi (India): role of environmental factors

Old Fort Lake, a small (1.6 ha), shallow, and recreational water body in Delhi (India) was studied through monthly surveys in two consecutive years (January, 2000–December, 2001). Precipitation is the major source of water for this closed basin lake. In addition, ground water is used for replenishing the lake regularly. This alkaline, hyposaline hard water lake contains very high ionic concentration, especially of nitrates. Based on overall ionic composition, this lake can be categorized as chloride–sulfate alkaline waters with the anion sequence dominated by SO₄ ²⁻ > Cl⁻ > HCO₃ ⁻, and the cations by Mg⁺⁺ > Ca⁺⁺. The overall seasonal variability in physicochemical profile was largely regulated by the annual cycle of evaporation and precipitation, whereas the ground water largely influences its water quality. The lake exhibited phytoplankton-dominated turbid state due to dominance of the blue green alga, Microcystis aeruginosa. The persistent cyanobacterial blooms and the elevated nutrient levels are indicative of the cultural eutrophication of the lake. This study focuses on the relative importance of eutrophic vis-à-vis hyposaline conditions in determining the structure and seasonal dynamics of zooplankton species assemblages. A total of 52 zooplankton species were recorded and rotifers dominated the community structure qualitatively as well as quantitatively. The genus Brachionus comprised a significant component of zooplankton community with B. plicatilis as the most dominant species. The other common taxa were B. quadridentatus, B. angularis, Lecane grandis, L. thalera, L. punctata, Mesocyclops sp., and Alona rectangula. Multivariate data analysis techniques, Canonical Correspondence Analysis (CCA) along with Monte Carlo Permutation Tests were used to determine the minimum number of environmental factors that could explain statistically significant (P < 0.05) proportions of variation in the species data. The significant variables selected by CCA were NH₃–N followed by percent saturation of DO, COD, SS, BOD, NO₂–N, rainfall, silicates, and PO₄–P. The results indicate that the seasonal succession patterns of the zooplankton species were largely controlled by physicochemical factors related directly or indirectly to the process of eutrophication, whereas hyposaline conditions in the lake determined the characteristic species composition. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

Diatom–environmental relationships and nutrient transfer functions from contrasting shallow and deep limestone lakes in Ontario, Canada

Diatom assemblages were analysed in the surface sediments of 44 alkaline lakes in south-western Ontario, Canada, and combined with a pre-existing 58 south-eastern Ontario lake set: (1) to determine if shallow, polymictic Ontario lakes contain different diatom assemblages from deeper, dimictic lakes, and if so, which environmental variables most influence assemblages; (2) to improve the existing transfer functions; (3) to construct and compare transfer functions separately for dimictic, deep lakes and for polymictic, shallow lakes. Polymictic and dimictic lakes covered a similar nutrient range (spring total phosphorus (TP)=4–54 g/l, spring total nitrogen (TN)=200–927 g/l; n=101) and spring pH levels (7.6–9; n=101). However, polymictic lakes were shallower (median mean depth = 2.9 m vs. 7.3 m in dimictic lakes). Benthic diatoms (average 60% relative abundance) dominated the polymictic lakes, whereas planktonic diatoms (average 60%) dominated dimictic lakes. A Canonical Correspondence Analyses with forward selection (p < 0.05, 999 Monte Carlo permutations) identified TP, alkalinity, watershed to volume ratios and lake depth as the most important measured environmental variables influencing diatom distribution in both polymictic and dimictic lakes. Additionally, pH was identified as an important variable in polymictic lakes, whereas TN was also forward selected in the dimictic lakes. Adding more lakes to the original southern Ontario calibration set improved the TN transfer function (r² _jack=0.42, root mean squared error of prediction (RMSEP)_jack=0.11 [log g TN/l]), although there was a high systematic error in the revised model (r² _residual = 0.48). However, the strongest TP model was derived from the polymictic lakes (r² _boot =0.44, RMSEP_boot=0.20 [log g TP/l]), which was the smallest lake set (n=30) with the lowest number of diatom species. The stronger TP model from the polymictic lakes may be partly due to the relatively low macrophyte cover in our polymictic lakes, which may lead to stronger benthic–pelagic coupling than in lakes with large macrophyte populations. Additionally, our study suggests that the Chrysophyceae cyst:diatom frustule ratio may be useful for indicating trends in TP levels of 35 g/l in alkaline lakes that are dimictic, but is not necessarily indicative of trophic state changes in shallow, polymictic lakes. Our study demonstrates that it may be important to construct separate diatom-based nutrient transfer functions for polymictic and dimictic lakes.     

Total phosphorus inference models and indices for coastal plain streams based on benthic diatom assemblages from artificial substrates

We investigated the potential for using diatoms to monitor and assess nutrient enrichment in coastal plain streams using weighted-averaging inference models and diatom trophic indices. Samples were collected from low-gradient, clay- to sand-bottom streams in New Jersey (NJ), USA, using artificial substrates (diatometers). Multivariate analysis showed that conductivity was overall the most important explanatory variable. Total phosphorus (TP) explained a significant proportion of the variation in diatom species composition. There was statistical justification for development of inference models for TP but not for total nitrogen (TN). We developed and tested models for inferring TP using weighted-averaging (WA) and weighted-averaging partial least squares (WA-PLS) regression and calibration techniques. We also created a diatom TP index by rescaling the inferred TP values. WA-PLS provided the best model (n = 38), which showed moderate predictive ability (r _boot² = 0.43; RMSEP_boot = 0.30 log₁₀ μg l⁻¹ TP); it performed best at lower TP concentrations and tended to underestimate values above 100 μg l⁻¹. The TP index performed well; it assigned the majority of the index scores to the correct nutrient category. TP models and indices developed for the Coastal Plain had lower predictive ability than those developed for northern NJ and streams in other comparable geographic regions of the US. This lower performance can be attributed primarily to a data gap in the TP gradient in the calibration dataset (lack of sites with TP concentrations between 240 and 560 μg l⁻¹), and a smaller number of samples. We conclude that diatom-based TP inference models and artificial substrate sampling are useful for assessing and monitoring nutrient enrichment in coastal plain streams. Given the worldwide distribution of streams similar to those in this study, these tools should be widely applicable. Handling editor: D. Ryder     

Land-use influence on stream water quality and diatom communities in Victoria, Australia: a response to secondary salinization

Diatom communities were analyzed in 39 streams located in drainages with varied land-use practices throughout Victoria, Australia. Thirteen water quality parameters were also measured in each stream. Most streams had low HCO₃ ^1- concentrations (low buffering capacity) with >90% of the waters dominated by Na¹⁺ and Cl^1-. Phosphate concentrations ranged from 0.003 to 2.0 mg/L. Diatom communities (245 taxa) were strongly correlated with land-use practices, i.e. historic clear cutting, and secondary salinization. Streams influenced by heavy irrigation practices and dryland farming had reduced species diversity and richness compared to systems with low to moderate land use. A nonmetric multidimensional ordination of diatom communities in the 39 streams was conducted. An ANOSIM on the ordination showed that diatom communities in upland watersheds with native forest canopies and low salinization, lowland streams in watersheds with cleared forest canopies, moderate agricultural utilization and salinization, and lowland streams in areas with high irrigation and salinization were all significantly different (p<0.001) from one another. Community ordination techniques showed that both specific conductance (salinity) and phosphorus interacted to determine stream diatom community structure in drainages with high secondary salinization. Drainages with low to moderate agricultural activity and low nutrients, but with a wide range of salinities showed strong associations with the diatom taxa Amphora coffeaeformis, Cymbella pusilla and Entomoneis paludosa, whereas, streams in regions with heavy agricultural practices and high phosphorus had Bacillaria paradoxa, Nitzschia hungarica, N. frustulum and Aulacoseira granulata as numerically important diatoms. In contrast, Rhizosolenia eriensis, Frustulia rhomboides, Eunotia pectinatus and Tabellaria flocculosa were strongly associated with upland streams with fast current, relatively low O-PO₄ ^3- concentrations, low pH, low salinity, and low temperature. In general, the diatom communities in saline streams (3 mS) were similar to those previously reported in saline lakes in Victoria.     

DIATOM ASSEMBLAGES AS INDICATORS OF LAKE TROPHIC STATUS IN SOUTHEASTERN ONTARIO LAKES1

Canonical correspondence analysis (CCA) was used to explore the relationship between measured environmental variables and surficial diatom (Bacillariophyceae) assemblages in alkaline lakes from southeastern Ontario. Total nitrogen (TN), watershed area, alkalinity, and maximum depth each explain significant (P ≤ 0.05) directions of variance in the distribution of diatom taxa. TN was highly correlated to total phosphorus (TP) (r = 0.92), chlorophyll a (r = 0.86), and Secchi depth (r =0.77). When a series of CCAs were run with the first axis constrained to each of these variables in turn, the ratio of the eigenvalue of the first axis to that of the second axis (λ1/λ2) was highest for TN, indicating that TN best explained the distribution of the diatom assemblages in this set of lakes. Furthermore, results of Monte Carlo permutation tests indicated that these four variables did not act independently on the diatom assemblages. Therefore, TN was selected to represent these four closely related variables to infer lake trophic status. Weighted-averaging regression and calibration (with classical deshrinking) were used to develop transfer functions to infer TN from the relative abundances of 83 diatom taxa recovered from the surficial sediments of 51 lakes. There was a good correlation between diatom-inferred TN concentrations and measured TN concentrations (r²= 0.75, n = 51). The weighted-averaging regression and calibration model was used to infer lake trophic status (represented by TN) from diatom assemblages presented in the sediments from Little Round Lake, Ontario. These data were used in conjunction with historical land-use data in order to quantify the sequence and extent of nutrient enrichment related to human activity in the watershed area.     

Dietary flexibility in three representative waterbirds across salinity and depth gradients in salt ponds of San Francisco Bay

Salt evaporation ponds have existed in San Francisco Bay, California, for more than a century. In the past decade, most of the salt ponds have been retired from production and purchased for resource conservation with a focus on tidal marsh restoration. However, large numbers of waterbirds are found in salt ponds, especially during migration and wintering periods. The value of these hypersaline wetlands for waterbirds is not well understood, including how different avian foraging guilds use invertebrate prey resources at different salinities and depths. The aim of this study was to investigate the dietary flexibility of waterbirds by examining the population number and diet of three feeding guilds across a salinity and depth gradient in former salt ponds of the Napa-Sonoma Marshes. Although total invertebrate biomass and species richness were greater in low than high salinity salt ponds, waterbirds fed in ponds that ranged from low (20 g l⁻¹) to very high salinities (250 g l⁻¹). American avocets (surface sweeper) foraged in shallow areas at pond edges and consumed a wide range of prey types (8) including seeds at low salinity, but preferred brine flies at mid salinity (40–80 g l⁻¹). Western sandpipers (prober) focused on exposed edges and shoal habitats and consumed only a few prey types (2–4) at both low and mid salinities. Suitable depths for foraging were greatest for ruddy ducks (diving benthivore) that consumed a wide variety of invertebrate taxa (5) at low salinity, but focused on fewer prey (3) at mid salinity. We found few brine shrimp, common in higher salinity waters, in the digestive tracts of any of these species. Dietary flexibility allows different guilds to use ponds across a range of salinities, but their foraging extent is limited by available water depths. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

The Ethiopian Rift Valley lakes: chemical characteristics of a salinity-alkalinity series

The study on 10 lakes within the Ethiopian Rift Valley during March–May 1991 covered a range of conductivity (K₂₅) between 286 and 49100 µS cm^–1. HCO₃ ^– — CO _inf3 ^sup2– and Na⁺ were the dominant ions in all the lakes. Concentrations of K⁺, Cl^– and SO _inf4 ^sup2– increased with increasing salinity and alkalinity, whereas Ca²⁺ and Mg²⁺ decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12–222 mg SiO₂ 1^–1) and increased with increasing salinity, except for Lake Chamo which showed SiO₂ depletion (to < 1 mg SiO₂ 1 ^–1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a — total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.     

Diatom species–environment relationships and inference models from Isachsen, Ellef Ringnes Island, Canadian High Arctic

Surface sediment diatom assemblages were examined from 26 freshwater sites near Isachsen (78°47N, 103°32W), Ellef Ringnes Island, a region of diverse and atypical water chemistry for high arctic sites. One hundred and sixty eight diatom taxa were identified from these samples, over 50% of which had not previously been recorded in the Canadian High Arctic. Variations in diatom assemblages were related to changes in measured environmental variables using multivariate techniques. Canonical correspondence analysis (CCA) indicated that five variables contributed significantly to explaining patterns of diatom variation (i.e., COND, DIC, Mn, TPF, TPU). The first CCA axis (=0.44) was primarily controlled by conductivity-related variables, while CCA axis 2 (=0.21) was related to particulate concentrations. Diatom-based inference models were generated for the reconstruction of conductivity (RMSEP_jack=0.32, r²_jack=0.76) and pH (RMSEP_jack=0.40, r²_jack=0.69). The strengths of these models indicate that it will be possible to reliably infer past trends in conductivity and pH from diatom assemblages preserved in dated sediment cores from the Isachsen region.     

Sensitivity and responses of diatoms to climate warming in lakes heavily influenced by humans

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Limnology of two lake systems of Katmai National Park and Preserve, Alaska: Part II. Light penetration and Secchi depth

Seven large lakes in the Naknek River drainage and four in the Alagnak River drainage within the Katmai National Park and Preserve, Alaska, were surveyed once a summer during the period 1990–92 to determine baseline limnological conditions. All of the lakes are oligotrophic based on Secchi depth (SD) transparency and light penetration. Overall, SD transparency varied from 4.4 m to 17 m, the vertical light extinction coefficient (K _d) ranged from 0.411 m^-1 to 0.070 m^-1 and the depth of 1% light penetration (I_1%) varied from 11 m to 67 m. However, because of greater light scattering, the percent of photosynthetic radiation (PAR) at SD was nearly twice as much in Battle Lake (30.4%) and Naknek Lake (32.8%), compared with the other nine lakes (mean 16%). Consequently, the ratio of I_1% to SD was about 4 in these two lakes compared to a mean value of 2.6 for the other lakes. However, Battle Lake is a deep blue calcium sulfate lake with little phytoplankton, whereas Naknek Lake contains some inorganic glacial flour and volcanic ash, as well as planktonic algae, but where sampled exhibits minimal turbidity. Biomass of planktonic algae (indexed by total chlorophyll concentration) explained most of the variation in SD (r ²=0.66), K _d (r ²=0.75), and I _1% (r ²=0.85). In contrast, neither color nor turbidity were significant predictors of any optical variable. Considering all 11 lakes, there was a significant linear relationship between SD and both K _d (r ²=0.80) and I _1% (r ²=0.72); however, most of the unaccounted for variation was attributed to Battle Lake and Naknek Lake. Although changes in water transparency are often linked to changes in algal biomass (chlorophyll), simple measures of SD transparency alone may not be appropriate for assessing whole-scale watershed or regional changes toward oligotrophication or eutrophication in lakes of the remote and pristine Katmai National Park and Preserve.     

Diversity of Carbon Monoxide-Oxidizing Bacteria in Five Lakes on the Qinghai-Tibet Plateau,China

Carbon monoxide-oxidizing (COX) bacteria play an important role in controlling the flux of carbon monoxide among natural reservoirs, and thus studying their diversity in natural environments is of great significance to understanding the carbon cycle. In this study, the COX bacterial diversity was investigated in five lakes (Erhai Lake, Gahai Lake1, Gahai Lake2, Xiaochaidan Lake, Lake Chaka) on the Qinghai-Tibet Plateau and its correlation with environmental variables of the lakes was explored. Phylogenetic analyses showed that the CO-oxidizers were dominated by Proteobacteria and Actinobacteria in the Qinghai-Tibet Plateau lakes, and their relative abundance varied with salinity: in the freshwater Erhai Lake, the COX bacteria in the water were dominated by the Betaproteobacteria, in contrast to the Actinobacteridae dominance in the sediment; in the saline and hypersaline lakes of Gahai Lake1, Gahai Lake2 and Xiaochaidan Lake, alphaproteobacterial COX bacteria were dominant in the water, whereas Actinobacteridae and alphaproteobacterial COX bacteria were dominant in the sediment. In the hypersaline Lake Chaka, an unknown COX bacterial clade and alphaproteobacterial COX bacteria were dominant in the water and sediment, respectively. Statistical analyses showed that salinity, pH, and major ions (e.g., K⁺, Na⁺, Ca²⁺, Mg²⁺, SO₄ ^2-, and Cl^-) were important factors affecting the COX bacterial community compositions in the investigated lakes. Overall, our results provided insights into the COX bacterial diversity in Qinghai-Tibetan lakes.     

Epiphytic,littoral diatoms as bioindicators of shallow lake trophic status: Trophic Diatom Index for Lakes (TDIL) developed in Hungary

Littoral diatoms are important contributors of the primary production in shallow aquatic ecosystems and they can be used as indicators of the trophic status. The aim of the study was to develop an index to assess trophic status of Hungarian lakes as suggested by the Water Framework Directive. In spring of 2005 and 2006, epiphytic diatom samples were collected from 83 shallow lakes. Weighted average method was used to develop and test the TP model. In the developed TP model correlation between the observed and diatom inferred TP was high (r ² = 0.96, n = 67). The optimum and tolerance TP parametrics of 127 species were determined and trophic indicator and sensibility values were defined for the Trophic Diatom Index for Lakes (TDIL). The TDIL was applicable to assess the ecological status of Hungarian shallow lakes. According to the TDIL the ecological status of 4 lakes were in excellent, 25 in good, 21 in medium, 21 in tolerable and 12 in bad status. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: K. Martens     

Developing diatom-based transfer functions for Central Mexican lakes

This paper is the first attempt to produce diatom-based transfer functions for the northern tropical Americas. A dataset of 53 modern diatom samples and associated hydrochemical variables from 31 sites in the volcanic highlands of central Mexico is presented. The relationship between diatom species distribution and water chemistry is explored using canonical correspondence analysis (CCA) and partial CCA. Variance partitioning indicates that ionic strength and ion type both account for significant and independent portions of this variation. Transfer functions are developed for electrical conductivity (r ² = 0.91) and alkalinity (as a percentage of total anions) (r ² = 0.90), reflecting ionic strength and ionic composition respectively. Prediction errors, estimated using jack-knifing, are low for the conductivity model, but the carbonate transfer function performs less well. This study highlights the potential for diatom-based quantitative palaeoenvironmental reconstructions in central Mexico. However, a number of key diatom species found in fossil material are not represented in the modern flora. Sampling of additional sites may resolve this, but it is thought that the lack of modern analogues may reflect the high degree of anthropogenic disturbance in many of the catchments. This highlights the problem of trying to reconstruct pre-disturbance environmental changes in highly modified ecosystems. One possible solution is to merge the central Mexican data with the African dataset, which includes sites of similar chemical composition, but which have not suffered the same degree of disturbance.     

A multi-proxy paleolimnological reconstruction of trophic state reference conditions for stratified carbonate-rich lakes in northern Germany

This study aims to identify reference conditions (nutrient status and diatom assemblages) as required by the European Water Framework Directive (WFD) for stratified, carbonate-rich lowland lakes with a large watershed area (watershed area to lake volume ratio (WV) > 1.5 km² 10⁻⁶ m⁻³) and a retention time (RT) from 0.1 to 10 years (Central Baltic Lake-Type 1, German Lake-Type 10) in European ecoregion 14. Diatoms, pollen and geochemistry were analysed from sediment cores of six lakes from northern Germany representing different subtypes of Lake-Type 10 (varying WV and RT) and covering the past 290–1,750 years. Historic total phosphorus levels were inferred using diatom-based transfer functions selected from a merged European data set and from optimised data sets identified with the moving-window approach. Pollen and geochemical proxies were used to identify occurrence and intensity of anthropogenic catchment usage. Lake trophic state reference conditions and associated diatom assemblages were identified for three of the six study lakes. In contrast, according to fossil pollen assemblages, two lakes were already strongly impacted by intensive catchment usage when the oldest investigated sediments were laid down. Thus, reference conditions of these already eutrophic lakes could not be identified. Similarly, the lowermost samples of a core from the sixth lake showed signs of impact, and it remains unclear whether the identified dystrophic conditions occurred naturally or if they were due to the drainage of wetlands in Medieval times. Lakes with a relatively small WV (1.5–5.0 km² 10⁻⁶ m⁻³) and RT > 1 year were naturally oligotrophic to low mesotrophic and a typical, representative diatom assemblage was identified. In contrast, typical reference conditions or diatom assemblages for lakes with higher WV (5–18.6 km² 10⁻⁶ m⁻³) and RT < 1 year could not be identified as chemical precipitation and upstream lakes (nutrient sinks or sources) additionally influenced natural nutrient levels. Therefore, the reference situation of both trophic state and diatom assemblages in a lake may be strongly influenced by other modifying, limnological processes in addition to WV and RT. Overall, this study helps to implement the WFD by identifying reference conditions and by discussing the level of differentiation of lake types required to set reference conditions. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of many Great Basin lakes has resulted from the diversion of tributary streams for agricultural or municipal uses. At Mono Lake, an alkaline salt lake in eastern California, salinities have risen from 50 to 100 g·L^?1 in just 50 years. Experimental mesocosms were established to simulate some of the potential ecological effects that could have accompanied this change. The influence of salinity on diatom diversity, taxonomic structure, and primary production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0.5 m deep, with open tops covered by 1 mm mesh net. Five treatments (50, 75, 100, 125, and 150 g·L^?1) with four replicates per treatment were used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g·m^?2) and diversity (from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities ≥75 g·L^?1. Photosynthetic oxygen production also was significantly lower at salinities ≥75 g·L^?1. Diatom indicator taxa for these shifts included Denticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Stephanodiscus oregonicus increasing in relative abundance in higher salinity treatments, accompanied by decreases in Achnanthes minutissima, Cymbella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting dominance at moderate salinity levels (75 to 125 g·L^?1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophyte, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occurred only in salinity treatments from 50 to 100 g·L^?1. Diversion of tributary stream flow and resulting salinity increases in this lake threaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision of the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to around 75 g·L^?1 and is expected to increase the diversity and productivity of the benthic algae of this ecosystem.     

Salmon‐derived nutrients drive diatom beta‐diversity patterns

1. Pacific salmon are a textbook example of migratory animals that transfer nutrients between ecosystems, but little is known about how salmon‐derived nutrients (SDN) affect the biodiversity of recipient freshwater ecosystems. We examined paleolimnological records from six Alaskan lakes to define how changes in SDN from sockeye salmon (Oncorhynchus nerka) influenced sedimentary diatom community structure and beta‐diversity among lakes and through time. 2. Using an isotopic mixing model, we showed that SDN loading could account for >80% of the lake total nitrogen budgets and strongly regulated diatom community composition. Spatial dissimilarity in diatom communities was positively related to differences in SDN among lakes (r² = 0.69, P < 0.01, n = 10). Likewise, temporal dissimilarity in diatom communities was positively related to differences in SDN in a sediment core with substantial variation in salmon spawner dynamics between 1700 and 1950 AD (r² = 0.34, P < 0.01, n = 19). Finally, beta‐diversity metrics quantifying temporal turnover within each lake’s sediment record were also positively related to the variance in SDN loading among lakes (r² = 0.88, P < 0.05, n = 5). Mean SDN was only negatively correlated to temporal diatom beta‐diversity. 3. Spatially subsidised systems often receive temporally variable resource inputs, and thus, it is not surprising that, unlike previous studies, we found that resource variability was the key driver of community composition and beta‐diversity. In habitats that receive strongly fluctuating external nutrient loads, environment heterogeneity may overweigh stochastic community processes. In addition, freshwater diatoms are characterised by great dispersal capabilities and short life cycles and therefore may be a more sensitive indicator for evaluating the role of resource variability than previously used model organisms. These results suggest that productivity–diversity relationship vary with the nature of nutrient loading and the life history of the community studied. 4. Overall, our study highlights that the transport of nutrients by sockeye salmon across ecosystem boundaries is a significant driver of algal community and biodiversity in nursery lakes, mainly through changing the magnitude of nutrient variation. As such, freshwater species diversity in regions like the U.S. Pacific Northwest may become impoverished where there have been long‐term declines in salmon populations and decreases in nutrient variability among lakes.