Diatoms In Lakes Research Articles

Eutrophication weakens the positive biodiversity–productivity relationship of benthic diatoms in plateau lakes

Context Freshwater primary productivity is threatened by the decline in biodiversity associated with nutrient enrichment, but there is still uncertainty about how the biodiversity and productivity relationship (BPR) varies with the trophic states. Aims We aimed to examine the variation of benthic diatom BPRs in three plateau lakes with different trophic states and their underlying driving mechanisms. Methods We examined the relationship between diatom taxonomic and functional diversity, niche width, and niche overlap with productivity. Key results The taxonomic and functional diversity, niche width and productivity of benthic diatoms were highest in the mesotrophic lake. The benthic diatom BPRs were linear and positive, with the slope of BPRs being the lowest in eutrophic lake. Motile, non-attached and small-sized diatoms were dominant in eutrophic lake. Nutrient concentrations indirectly affected primary productivity by influencing algal community structure, niche width and biodiversity change. Conclusions Diatom productivity and diversity showed a positive relationship, but nutrient enrichment weakened this relationship. By combining taxonomic and functional diversity indices, supplemented by niche analysis, we can further understand the variation of diatom productivity. Implications The results provide a basis for predicting the changes in BPRs of benthic diatoms in the littoral zone with different trophic states.

Detecting climate-driven ecological changes in high-altitude lakes in the Sierra Nevada, California

Over the past several decades, increasing climate instability in the Sierra Nevada, California, expressed primarily as reduced winter precipitation and higher temperatures, has led to more frequent drought. High-altitude lakes in this region have been characterized as pristine ecosystems, but growing evidence suggests that they are responding acutely to climate change. To address this, we analyzed the diatom assemblages of two 210Pb dated sediment cores (Gull and June Lakes) from the eastern Sierra Nevada with the aim of assessing their sensitivity to and timing of responses to climate change at the end of the neoglacial (~1450 CE to ~1850 CE) and identifying how climate drivers can impact diatom communities. The nutrient cycles of both lakes have been disrupted by changes in thermal stratification, driven by increasing temperatures, as interpreted from the shift from a Stephanodiscus corruscus dominated ecosystem to a Stephanodiscus minutulus dominance. In this case, the June Lake (the deeper lake) diatom assemblage shifted from an assemblage representative of well mixed conditions to one representative of a stratified system before Gull Lake as a response to increasing temperatures and a strengthened thermocline. We relate the asynchronous change in the thermocline stability to basin morphology, where the deeper lake with a deeper thermocline is more sensitive to increasing temperatures. Further, generalized additive models (GAMs) allowed us to identify the onset of ecological change in both lakes, pointing to the termination of the Little Ice Age (LIA; ~1850 CE), revealing an acute response to changing climate.

Effects of nitrogen on benthic diatom assemblages in high-elevation central and eastern alpine lakes

ABSTRACT We explored patterns of benthic diatom composition across sixty-two high-elevation alpine lakes spanning a wide range of nitrogen (N) concentrations due to atmospheric deposition and background variation in lake and watershed characteristics. Our goals were to (1) assess the effect of lake water N concentration on benthic diatom composition during late summer or fall conditions and (2) identify policy-relevant response thresholds. The analyses were carried out on a large set of diatom and water chemistry data, integrated with new data. Multivariate and correlation analyses revealed associations between pH, N concentration, and benthic diatom composition, but the effects of pH and N were confounded. However, partial correlation analysis allowed us to identify “N-responsive diatoms”; that is, diatom taxa with nonspurious associations with N. Focusing on these taxa, we detected a decline in the abundance of taxa preferring low N concentrations and an increase in the abundance of taxa preferring high N concentrations starting at NO3 concentrations of approximately 5 µmol L−1. We interpreted this shift as an effect of watershed N saturation due to atmospheric deposition. Based on the results, we suggest a late-summer or fall concentration threshold of 5 µmol NO3 L−1 to prevent change in benthic diatoms in high-elevation alpine lakes.

The distribution of diatom assemblages with the effects of environmental parameters and ecological status assessment based on diatom indices in the lentic systems of the Akarçay and Asi Basins (Türkiye).

In order to detect the effects of different stressors on the littoral diatom assemblages and biodiversity of the lentic systems in two different basins in Türkiye, 15 lentic systems in the Akarçay and Asi basins were sampled three times (spring, summer, and fall) for littoral diatoms and environmental variables. The biological assessment of these sites was also evaluated using 9 different river and lake diatom indices. Based on the total phosphorus (TP), total nitrogen (TN), electrical conductivity (EC), salinity (SAL) values, and dominant diatom species, three lentic systems in Akarçay Basin (Lake Eber, 26 Ağustos Pond, and Lake Karamık) were more eutrophic and under the pressure of anthropogenic and climatic (precipitation) stressors. Trophic diatom index for lakes (TDIL) showed a higher correlation with TN and TP and was the most effective index in explaining ecological quality in 15 lentic systems. The species richness of diatoms was also negatively affected by EC and SAL values. The lentic systems of the two basins were separated from each other based on the species composition and the distribution of the dominant species with the effect of altitude (ALT). Considering the distribution of diatom assemblage of the lentic systems in these two basins, anthropogenic, climatic, and geographical factors have become prominent to shape the diatom community structure.

Development of novel diatom metrics to assess ecological status of phytobenthos in Greek lakes

Mediterranean lakes present particular challenges when developing ecological assessment techniques, due partly to long histories of human activity in their catchments but also to strong interactions with climate, intensifying in recent decades due to global warming. A survey of diatoms in lakes in Greece revealed both a strong nutrient gradient and a salinity gradient. It is necessary to disentangle these when assessing ecological status in order that appropriate measures for restoring good ecological status are applied. Two approaches were used to develop a Water Framework Directive-compatible metric for assessing ecological status using diatoms. The strongest relationship was obtained using weighted averaging, but an alternative metric, generated by rescaling a well-established and widely used diatom metric performed only slightly less well. The limited number of lakes in Greece resulted in a small dataset, creating a risk of overfitting models and the rescaled metric has the advantage of drawing on a broader autecological knowledge base. Supplementary metrics to evaluate salinity are also proposed in order to ensure that lakes subject to this pressure are clearly differentiated from those where nutrient impacts predominate. Approaches to defining reference conditions and status class boundaries draw on established criteria developed for Water Framework Assessments in Europe to ensure that status assessments in Greece reflect similar levels of ambition to those established elsewhere in the European Union.

Microbiome and hydraulic performance changes of drinking water biofilters during disruptive events-media replacement, lake diatom bloom, and chlorination

Little is known about filter media microbiome and characteristics during disruptive events and drastic hydraulic performance decline in drinking water biofilters.

Resolving 500 Years of Anthropogenic Impacts in a Mesotrophic Lake: Nutrients Outweigh Other Drivers of Lake Change.

Interactions among multiple stressors, legacies of past perturbations, and the lack of historical information make it difficult to determine the influence of individual anthropogenic impacts on lakes and separate them from natural ecosystem variability. In the present study, we coupled paleolimnological approaches, historical data, and ecological experiments to disentangle the impacts of multiple long-term stressors on lake ecosystem structure and function. We found that the lake structure and function remained resistant to the impacts of catchment deforestation and erosion, and the introduction of several exotic fish species. Changes in ecosystem structure and function were consistent, with nutrient enrichment being the primary driver of change. Significant and sustained changes in the lake diatom community structure (and their nutrient requirements), bacterial community function, and paleolimnological proxies of ecosystem function coincided with nitrogen and phosphorus fertilizers in the catchment. The results highlight that the effects of increased nutrient inputs are much stronger than the influence of other, potentially significant, drivers of ecosystem change, and that the degree of nutrient impact can be underestimated by environmental monitoring due to its diffuse and accumulative nature. Delineating the effects of multiple anthropogenic drivers requires long-term records of both impacts and lake ecosystem change across multiple trophic levels.

Ecological and habitat preferences of diatoms in lakes of Lützow‐Holm Bay, East Antarctica

Lakes and ponds along the Antarctic coastline provide an opportunity for palaeoclimate reconstructions to complement ice core records from the interior of the continent, including via their diatom records. Diatom palaeoecological studies in remote Antarctica are partially undermined by uncertainty concerning the relative influence of climate, water quality and substrate on diatom species assemblages. The relative abundance and distribution of diatoms across a range of substrates and water depths were analysed from nine lakes in the coastal ice-free regions of Lützow-Holm Bay, East Antarctica. Specific conductivity and sample position (whether from the shallow, littoral zone, or from deeper parts of the lake) were identified as significant variables in explaining diatom distribution, using non-metric multidimensional scaling and regression tree analysis to identify statistically significant groupings of diatom assemblages and associated environmental variables. In contrast to the significant difference between the diatom assemblages from the shallow littoral region and the lake floor at most sites, the diatom flora from different substrates in the littoral zone were not significantly different, which may be related to the extent of ice cover. Humodophila australis was the most common species in the lakes with lowest specific conductivity, whereas Halamphora vyvermaniana was associated with lake-floor habitats and Psammothidium papilio with samples from littoral zones at several sites. These observations hold significance for the interpretation of lake sediment diatom assemblages in Lützow-Holm Bay as archives of past climate and indicate that sampling strategies should consider a range of locations in lakes, with samples from the lake floor prioritised for the interpretation of sediment core diatom records.

Responses of lake diatoms to rapid 21st century warming on the southeastern Tibetan Plateau

Many paleolimnological studies from the Northern Hemisphere have shown how diatom assemblages preserved in dated lake sediment cores have responded to anthropogenic climate warming over the past ~100 years. In contrast, diatom records from lakes in the southeastern Tibetan Plateau have typically registered minimal compositional change during this recent period of warming. Site-specific conditions can dampen the biological response to regional climate change in this topographically complex mountain landscape. In this study, we used a 210 Lead/ 137 Cesium-dated sedimentary diatom record collected in 2018 from a remote, oligotrophic, alpine lake (Cuomujiri Lake) in the southeastern Tibetan Plateau to track environmental changes during the past ~100 years. Change-point analysis on air temperature trends from the nearby Nyingchi meteorological station and from across the Tibetan Plateau (composite of 150 stations) showed exceptionally rapid and statistically significant warming since 2004 CE, although localized variations exist in this heterogeneous region. Concurrent with this pronounced temperature rise, the Cuomujiri diatom record registered an abrupt shift from a benthic to a planktonic-dominated assemblage for the first time in the record, with a clear increase in Lindavia ocellata since ca. 2004 CE. Despite substantial warming in the southeastern Tibetan Plateau over the past several decades, diatom assemblages in Cuomujiri Lake did not register a notable response until the significantly high temperatures experienced during the past ~14 years. These results from Earth’s highest plateau signify that, with sustained anthropogenic warming, exceedance of ecological thresholds will continue in lakes throughout the Tibetan Plateau and likely in other high latitude regions of the world.

Influence of Selected Environmental Factors on Diatom β Diversity (Bacillariophyta) and the Value of Diatom Indices and Sampling Issues

Human impacts and environmental climate changes have led to a progressive decline in the diversity of diatoms in lakes in the recent past. The components of β diversity (e.g., species turnover and nestedness) and underlying factors are still poorly understood. Here, we report an investigation of two alternative approaches—beta diversity (β diversity) partitioning and local contribution to β diversity (LCBD)—including their responses to selected environmental factors and representativeness of samples in estimating the ecological fitness of a lake. The β diversity of diatoms and their local contributions could be explained by the effects of environmental variables (p < 0.01). The random forest method showed the most contribution to the variance for NO3−, Cl−, and SO42−. PERMANOVA as well as a network analysis in JASP (Jeffrey’s Amazing Statistics Program) showed significant differences between the seasons in diatom assemblages and in the diatom index for Polish lakes (IOJ). Our findings provide insights into the mechanisms responsible for community organizations along environmental gradients from the perspective of β diversity components, and mechanisms of the indication value of diatoms for lakes; the results could be used especially by countries implementing ecological assessments.

Diatom-inferred centennial-millennial postglacial climate change in the Pacific Northwest of North America

A diatom record from Moss Lake, Washington, USA spans the last 14,500 cal year and revealed Holocene climate change in the Pacific Northwest (PNW), including evidence for periodicities related to ocean-atmosphere teleconnections and/or variations in solar output. Three main climate phases were identified: (i) Late Pleistocene to early Greenlandian (until 10,800 cal year BP, spanning GI-1, GS-1), with a cold climate and low diatom abundance; (ii) early Greenlandian to Northgrippian (10,800–7500 cal year BP), shifting to a warmer climate; and (iii) late Northgrippian and Meghalayan from 7500 cal year BP onwards, with a cooler, moist climate. These climate shifts are in good agreement with the pollen record from the same core and other regional studies. Fluctuations in Discostella pseudostelligera and Aulacoseira taxa suggest climate cycles of different frequency and amplitude throughout the record. Spectral and wavelet analyses revealed periodicities of approximately 1400 and 400–500 years. We interpret the ~ 1400-year and ~ 400–500-year cycles to reflect alternating periods of enhanced (and reduced) convective mixing in the water column, associated with increased (and decreased) storms, resulting from ocean–atmosphere teleconnections in the wider Pacific region. The ~ 1400-year periodicity is evident throughout the Late Pleistocene and late Northgrippian/Meghalayan, reflecting high-amplitude millennial shifts from periods of stable thermal stratification of the water column (weak wind intensity) to periods of convective mixing (high wind intensity). The millennial cycle diminishes during the Greenlandian, in association with the boreal summer insolation maximum, consistent with suppression of ENSO-like dynamics by enhanced trade winds. Ocean–atmosphere teleconnection suppression is recorded throughout the PNW, but there is a time discrepancy with other records, some that reveal suppression during the Greenlandian and others during the Northgrippian, suggesting endogenic processes may also modulate the Moss Lake diatom record. The large amplitude of millennial variability indicated by the lake data suggests that regional climate in the PNW was characterised over the longer term by shifting influences of ocean–atmosphere dynamics and that an improved understanding of the external forcing is necessary for understanding past and future climate conditions in western North America.

Benthic-Pelagic state changes in the primary trophic level of an ancient tropical lake

Many large, ancient lakes support primarily planktonic-driven food webs. In contrast, the lowest trophic levels of the Malili Lakes of Sulawesi Island, Indonesia are dominated by benthic diatom communities composed mainly of endemic species. Centric diatoms are not observed in the current diatom assemblage and pennate pelagic species are rare. Using two deep drill-cores from Lake Towuti, we investigated whether the lake has always been dominated by benthic primary production. Despite the ultra-oligotrophic conditions observed in the lake today, we observed state changes characterized by productive planktonic communities and less productive, benthic-dominated diatom assemblages. Two periods of planktonic production, each lasting approximately 50 kyr, were dominated by a complex mixture of Aulacoseira spp., with valve densities >1.0 × 10 9 valves/g with a maximum biovolume of 3.5 × 10 12 μm 3 /g. A similar planktonic assemblage was observed at much lower abundances (<10 7 valves/g) in the deeper sediments (>1 Ma), during the earliest stages of lake formation. Two periods of increased benthic primary production of approximately ~11 kyr in duration, originating from the littoral zone of the lake, were also observed, one between the two planktonic phases and one above the last planktonic maxima. The benthic periods were dominated by Cymbopleura spp., with valve densities of approximately 2.0 × 10 8 valves/g and a maximum biovolume of 1.2 × 10 11 μm 3 /g. Nonmetric multidimensional scaling analysis of sediment chemistry revealed that the benthic and pelagic states clustered with distinct chemical environments. The benthic phases were associated with well-mixed conditions in the lake whereas the planktonic phases were related to rapidly reoccurring water column mixing followed by intense stratification that generate nutrient recycling events. We conclude that lake mixing and nutrient cycling regimes regulated, in part, the switch between benthic and pelagic states in Lake Towuti. • Major switches between benthic and pelagic states in lake's diatom fossil record • Long-term planktonic diatom dominance in current benthic Lake Towuti • Planktonic communities established and persisted naturally for ~50 ky each • Both planktonic diatom communities had similar composition and abundance • Switches in state potentially linked to shifts in lake mixing regime

Seasonal variations of diatoms in a low-latitude mountain lake: a case study from Douhu lake-Southeast China

In the scientific literature on diatom ecology whether temperature drives changes in diatom assemblage directly or indirectly remains a subject of debate. In order to clarify the relationship between temperature and diatom assemblage composition, we focused on a region with significant seasonal changes but without a freezing period. From March 2016 to March 2018, sediment trap samples were collected monthly to monitor the seasonal succession of diatoms in Douhu Lake, in subtropical southeast China. The results show that in 2016 the variations in abundance of Aulacoseira granulata and Aulacoseira ambigua followed an obvious seasonal pattern, corresponding to the seasonal variation in water and air temperature. In 2016, the thermophilic taxon A. granulata was dominant in the warm season, and in the cold season its abundance declined, while that of A. ambigua, a taxon better adapted to cooler conditions, increased relatively. In 2017, however, there was no such typical seasonal variation. In the cold season of 2017, A. granulata did not decline and there was no significant increase in the cold-preferring A. ambigua. This was related to the large drop in the lake level associated with human activity (substantial water extraction) that occurred during the winter of 2017, which disturbed the natural seasonal fluctuation in water temperature. Thus, in 2017, while air temperature followed the regular seasonal cycle, lake water temperature did not decrease significantly in winter, which allowed A. granulata to maintain a relatively high abundance in the cold season. From these results and an extensive survey of the literature, we conclude that water temperature, more than the seasonal succession, directly drives the fluctuation in abundance of these two Aulacoseira species.

РЕАКЦИЯ ДИАТОМОВЫХ ВОДОРОСЛЕЙ ОЗЕРА ГРЯЗЕВОЕ (МАГАДАНСКАЯ ОБЛАСТЬ) НА ИЗМЕНЕНИЯ ОКРУЖАЮЩЕЙ СРЕДЫ СЕВЕРНОГО ПРИОХОТЬЯ В ПОЗДНЕМ ПЛЕЙСТОЦЕНЕ-ГОЛОЦЕНЕ

The study of diatoms from the sediments of well Gz-1 (Gryazevoe Lake, Magadan Region) allowed us to identify the response of diatom communities to environmental changes in the Late Pleistocene and Holocene and establish the main stages of the transformation of the lake ecosystem. Three diatom zones and four subzones were identified based on changes in the concentration of diatom floras in sediments, the ratio of representatives of ecological groups and dominants, reflecting the evolution trend of the lake diatom flora during the transition period from the last glacial maximum to the Holocene. This trend is similar to the changes in the lake diatom communities of the Arctic regions noted for the last millennium in terms of the change of taxonomic composition and dominants. The data of diatom studies, lithological and petrophysical characteristics of sediments indicate that the most significant changes in the natural environment occurred at the Pleistocene-Holocene boundary from 12.6 to 11.1 thousand years ago.

Ecology and climate sensitivity of a groundwater-fed lake on subtropical North Stradbroke Island (Minjerribah), Queensland, Australia over the last 7500 years

Lake sediments are important archives of past climate variability and lake responses to climate. In order to accurately infer past climates, it is necessary to understand, and account for, the ecological processes that affect the record of indicators preserved in lake sediment. This is particularly the case with respect to the concentration of carbon and nitrogen (TOC, TN, and calculated C:N), and the stable isotope composition of organic matter preserved in lake sediments. These are common, yet ambiguous, tracers of environmental change. Ideally, palaeoenvironmental reconstructions using the concentration and isotope composition of organic matter should be grounded in a detailed understanding of the sources of the organic material. This study documents the history and evolution of Blue Lake, an environmentally and culturally important oligotrophic, groundwater window lake on North Stradbroke Island, Queensland, Australia. We utilise organic matter δ13C, TOC, TN, and C:N from a 2.4 m sediment core with a basal age of 7.5 cal kyr BP, to investigate changing organic matter sources as a measure of the climate sensitivity of Blue Lake. This interpretation is supported by data from contemporary algae, aquatic and terrestrial plants, and catchment soils. We show that lake nutrient dynamics drove an increase in algal biomass at 4.2 cal kyr BP. This change coincides with a widely documented intensification of the El Niño-Southern Oscillation, which we infer to have influenced lake nutrient concentrations by reducing groundwater throughflow. Climatic changes resulted in marked changes in lake primary productivity, despite relatively little turnover of the lake diatom flora and catchment vegetation. This suggests that south-east Queensland dune lakes are sensitive to climate changes and helps to refine past and future palaeoclimate research using sediments from these lakes. It also indicates that increased nutrient concentrations in Blue Lake may result from projected changes in 21st Century climate.

River connectivity and climate behind the long-term evolution of tropical American floodplain lakes.

This study presents the long‐term evolution of two floodplains lakes (San Juana and Barbacoas) of the Magdalena River in Colombia with varying degree of connectivity to the River and with different responses to climate events (i.e., extreme floods and droughts). Historical limnological changes were identified through a multiproxy‐based reconstruction including diatoms, sedimentation, and sediment geochemistry, while historical climatic changes were derived from the application of the Standardised Precipitation‐Evapotranspiration Index. The main gradients in climatic and limnological change were assessed via multivariate analysis and generalized additive models. The reconstruction of the more isolated San Juana Lake spanned the last c. 500 years. Between c. 1,620 and 1,750 CE, riverine‐flooded conditions prevailed as indicated by high detrital input, reductive conditions, and dominance of planktonic diatoms. Since the early 1800s, the riverine meander became disconnected, conveying into a marsh‐like environment rich in aerophil diatoms and organic matter. The current lake was then formed around the mid‐1960s with a diverse lake diatom flora including benthic and planktonic diatoms, and more oxygenated waters under a gradual increase in sedimentation and nutrients. The reconstruction for Barbacoas Lake, a waterbody directly connected to the Magdalena River, spanned the last 60 years and showed alternating riverine–wetland–lake conditions in response to varying ENSO conditions. Wet periods were dominated by planktonic and benthic diatoms, while aerophil diatom species prevailed during dry periods; during the two intense ENSO periods of 1987 and 1992, the lake almost desiccated and sedimentation rates spiked. A gradual increase in sedimentation rates post‐2000 suggests that other factors rather than climate are also influencing sediment deposition in the lake. We propose that hydrological connectivity to the Magdalena River is a main factor controlling lake long‐term responses to human pressures, where highly connected lakes respond more acutely to ENSO events while isolated lakes are more sensitive to local land‐use changes.

Application of a Diatom Transfer Function to Quantitative Paleoclimatic Reconstruction — A Case Study of Yunlong Lake, Southwest China

Although diatom records from lake sediments have been used for quantitative paleoclimatic reconstruction, their validity and sensitivity have rarely been tested rigorously. At Yunlong Lake, an alpine lake in Southwest China, we studied the seasonal succession of diatom assemblages to produce a mean surface water temperature (MSWT) transfer function. In addition, based on the spatial distribution of surface diatom assemblages with water depth, we produced a diatom-water depth (WD) transfer function. Combined with the analysis of diatom assemblages in a sediment core (YL2013-A), changes in surface water temperature and water level over the last ∼100 years were quantitatively reconstructed using the diatom-based transfer functions. Comparison with records of regional meteorology and reservoir water capacity revealed that the diatom-based lake water level reconstruction is a sensitive indicator of short-term fluctuations in precipitation, and it also reflects a long-term stepwise rise in water level caused by the impounding and large-scale extension of the reservoir. In addition, the diatom-inferred MSWT is consistent with the changes in air temperature prior to large-scale human disturbance of the site. However, after the extension of the reservoir, although the regional air temperature continued to increase, the water temperature decreased substantially. This suggests that the large increase in lake water volume in the short term led to a decrease in the average water temperature, which in turn led to the occurrence of a diatom bloom in the cold season. The results demonstrate that diatom transfer functions based on modern observations of the same lake has a high environmental sensitivity and can be used for the quantitative reconstruction of regional climate change. Overall, our findings provide a foundation for the use of lake diatom records for quantitative paleoclimatic reconstruction on various timescales.

Use of seasonal epilithic diatom assemblages to evaluate ecological status in Irish lakes

Periphyton is increasingly used to assess the biological condition of lakes. The predominance of relatively shallow exposed waterbodies in Ireland implies that epilithic diatoms should have a prominent role as bioindicators for the WFD; but epilithic diatom ecology in these Irish systems is poorly described. We characterised spring and summer diatom populations in diverse periphyton communities in 91 Irish lake waterbodies. Six assemblage clusters of diatom species were identified, and variation in their assemblages was predominately associated with environmental gradients of acidity-alkalinity, total phosphorus, and true colour. Diatom composition differed amongst high status lakes in three a priori defined lake alkalinity groups commonly used as a diatom typology for lake classification. However, when waterbodies were evaluated with a lake trophic diatom index (LTDI) their distribution of ‘expected-values’ was equivalent. The LTDI was highly correlated with total phosphorus but also had similar responses in the three lake alkalinity types. These findings indicated that lake typology had a limited role for diatom classification with the LTDI in Ireland. Matched spring and summer sampling across wide environmental gradients allowed the role of seasonality to be investigated. Relatively few benthic diatoms had a significant seasonal indicator value suggesting that unbiased monitoring of the epilithon is possible during any month of the growing season in comparable temperate waterbodies. However, temporal variability was lower at low values of the LTDI and increased unpredictably on a lake by lake basis thereafter. Given that lake diatom classification has been validated we suggest that the approach could be developed and used for more targeted assessment objectives.

A 450-year record of environmental change from Castle Lake, California (USA), inferred from diatoms and organic geochemistry

A 39-cm sediment core from Castle Lake, California (USA) spans the last ~ 450 years and was analyzed for diatoms and organic geochemistry (δ15N, δ13C, and C:N), with the goal of determining sensitivity to natural climate variation and twentieth century anthropogenic effects. Castle Lake is a subalpine, nitrogen-limited lake with ~ 5 months of annual ice cover. Human impacts include light recreational use, past fish stocking, and experimental use by the Castle Lake Research Station. The base of the core (below 32 cm; pre mid-1700s) represents the period of maximum ice cover. In contrast, the end of the Little Ice Age (mid 1700s–early 1800s) is dominated by cyclotelloids (mostly Discostella stelligera), indicating significant open-water periods, a condition that persisted into the early 1900s. Cyclotelloids began to decline in the 1960s and were replaced by the Fragilaria tenera grp. (peak in 1970s), succeeded by Asterionella formosa (peak ~ 2010), and accompanied by a reduction in δ15N values and a decrease in C:N that may represent increased atmospheric nitrogen deposition. Another anthropogenic signal was discerned in the core and was interpreted to be the result of an ammonium nitrate fertilization experiment of the epilimnion that was conducted in 1980 and 1981. This signal was manifested in the core largely by a negative excursion in δ15N, possibly caused by fractionation during denitrification in surface sediment. A phytoplankton monitoring dataset collected by the Castle Lake Research Station from 1967 to 1984 corroborates the timing of increased araphid euplanktonic species in the 1970s, and increases in two benthic diatoms (Staurosirella pinnata and Tabellaria fenestrata), entrained in the phytoplankton tows during the experimentation years. Both ice cover and nitrogen addition appear to be strong drivers that affected the lake diatoms, although additional drivers, such as fish stocking and associated cascade effects need further exploration. These data will be helpful for interpreting longer core records from Castle Lake, should the opportunity arise, as well as cores from similar systems in the region.

Spatial and Temporal Ecological Uniqueness of Andean Diatom Communities Are Correlated With Climate, Geodiversity and Long-Term Limnological Change

National Socio-Environmental Synthesis Center (SESYNC) from the US NSF DBI-1639145 National Geographic Society 8672-09 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT LIFE14/IPE/FI/023 project FRESHABIT LIFE IP LIFE14/IPE/FI/023 National Science Foundation (NSF) EAR-1338694