Discostella Stelligera Research Articles

Sediment records from Lake Nipissing (ON, Canada) register a lake-wide multi-trophic response to climate change and reveal its possible role for increased cyanobacterial blooms

Recurring cyanobacterial blooms have been reported in Lake Nipissing (Ontario, Canada) since at least 2000. However, monitoring data indicate that surface water nutrient concentrations have not increased over the past ∼45 years in this large, shallow, mesotrophic lake. Applying paleolimnological methods, we establish baseline environmental conditions and extend the Lake Nipissing water quality data to pre-industrial times. Lake-wide assessment of changes in nutrient status, algal production, and effects of climate warming are provided by examining: 1) modern and pre-industrial sedimentary diatom assemblages from nine locations; 2) temporal trends in sedimentary photosynthetic pigments over ∼200 years from five locations; and 3) detailed sedimentary profiles of diatom and chironomid assemblage composition from Callander Bay, spanning the past ∼200 years. A pronounced shift in diatom assemblage composition was registered at all sites with pre-industrial sediments dominated by Aulacoseira spp. and modern samples dominated by Discostella stelligera. The detailed diatom analysis for Callander Bay indicated that this shift occurred ca. 1990. Since ca. 1975, cyanobacterial production (spectrally inferred from sediments) exceeded the ∼200-year mean and matches the observed increase in bloom reports. Twentieth century shifts in chironomid assemblages in Callander Bay from Procladius to Chironomus spp. dominance indicate lower hypolimnetic oxygen in recent decades. Collectively, these changes suggest that previously polymictic Lake Nipissing is now experiencing periods of thermal stratification and development of transient anoxia in bottom waters. These conditions are more prevalent and widespread in modern Lake Nipissing, consistent with a response to recent warming, decreased wind speed, and a longer ice-free period.

Diatoms and water quality indices in Thamirabarani river help environmental impact assessment of stretch between Naranammalpuram and Punnakayal, Tamil Nadu, India

The present study reports the occurrence of diatom taxa, their distribution and ecology in Thamirabarani River between the study areas Naranammalpuram and Punnakayal, covering the districts of Tirunelveli and Thoothukudiin Tamil Nadu, India. The study employs diatoms collected from 10 different locations in the Thamirabarani River to monitor its quality. A total of 40 diatom taxa belonging to 20 genera were recorded in the study area. The dominant presence of diatom taxa, such as Aulacoseira ambigua, Cocconeis placentula, Cyclotella meneghiniana, Cymbella tropica, Discostella stelligera, Gyrosigma acuminatum, Navicula cryptonella, Nitzschia amphibia, Planothidium lanceolatum, Staurosirella pinnata, and the water quality index values (29–48) recorded at sites 1 to 7 indicate oligotrophic due to good flow of water. The abundant of diatom taxa, such as Diadesmis confervacea, Gomphonema parvulum, Nitzschia palea and Pinnularia gibba, were recorded at sites 8–10, and the water quality indices (100–110) indicate eutrophication attributable to less flow of water, high dispersion of cremation ashes (the casting ceremony) and anthropogenic activities. Particularly, locations around Punnakayal are most polluted due to the dumping of fish wastes as well as stagnant water.

Drivers and ecology of Ceratium furcoides invasion of a Brazilian subtropical reservoir and its interaction with the phytoplankton community

We analyzed the phytoplankton interactions during a Ceratium furcoides invasion along with the environmental variables contributing to its dominance and blooms in Marrecas, a south-Brazilian reservoir. We monitored Marrecas weekly/monthly (n = 116) from 2014 to 2018. C. furcoides avoided summer and preferred spring, which displayed optimal temperatures for the species (15–23 °C), exhibiting in October 2014 the maximum density ever reported (15,865 ind. mL-1). It showed direct correlation with iron, manganese, and turbidity; evidencing dependency of a mixing regime which triggers cyst resuspension (interrupting cysts’ dormancy). Cryptomonas spp., Komma caudata, Discostella stelligera and after 2016 also Chlamydomonas sp. were the only species present during C. furcoides blooms. Komma caudata, Cryptomonas sp., and Aulacoseira alpigena correlated positively with the invader, as all prefer turbulent waters. C. furcoides correlated negatively with Chlorophyta (mainly Chlamydomonas spp.) which is vulnerable to mixing, and Bacillariophyta (Urosolenia eriensis and Discostella stelligera) that prefers clear waters. It also showed a consistent negative correlation with Ochrophyta (Mallomonas spp.), Cyanobacteria (Microcystis), and Urosolenia eriensis; suggesting competition for resources. Furthermore, it correlated negatively with species richness. Notwithstanding, based on the decrease of blooms, the phytoplankton community appears to have reached a more stable state (homogeneous species representation). However, further monitoring is required for confirmation.

Distribution of the Diatoms and Water Quality Assessment of the Thamirabarani River, Tamil Nadu, South India


 
 
 
 
 
 The diatoms are one of the most significant organisms beneficial for biomonitoring investigations. The present study deals with the diatom and water samples collected from 10 different localities in the Thamirabarani River, Southern India. Diatom samples were collected by scraping stones with a toothbrush and squeezing macrophytes. Study of diatom valve morphology and measurements of diatoms were carried out using an optical miccroscope. The present study recorded 58 diatom taxa belonging to 25 genera. Among the diatoms, the taxonomical characters of 20 abundant diatom taxa viz. Aulacoseira granulata, Discostella stelligera, Navicula cryptocephala, Encyonema minutum, Gomphonema gandhii, Eunotia asterionelloides, Gomphonema parvulum, Brachysira neglectissima, Aulacoseira ambigua, Diadesmis confervacea, Nitzschia palea, Nitzschia amphibia, Sellaphora Americana, Staurosirella pinnata, Gomphonema affine, Fragilaria pararumpens, Eunotia incisa, Navicula erifuga, Navicula cryptotenella and Navicula rostellata are described and illustrated. In the present study abundant diatom genera were observed such as Aulacoseira, Discostella, Navicula, Encyonema, Gomphonema, Brachysira, Nitzschia, Sellaphora, Staurosirella, Fragilaria and Euno- tia. In abundant diatom taxa such as Aulacoseira granulate, Discostella stelligera, Gomphonema gandhii, Fragilaria pararumpens, Navicula cryptocephala, Navicula rostellata, Staurosirella pinnata water quality index values between 21-42 were recorded at Sites 1-9, which indicates unpolluted water. The abundant diatom taxa Diadesmis confervacea, Nitzschia palea, and Gomphonema parvulum were present at Site 10 and a water quality index value of 101 was recorded, indicating water polluted due to anthropogenic activity. The diatom assemblage of the present study was compared with various river systems to understand the environmental condition assessment. The water samples were analysed for assessment of the water quality.
 
 
 
 
 
 
 

Diversity and distribution of lacustrine diatoms along the Trans‐Mexican Volcanic Belt

Abstract The Trans‐Mexican Volcanic Belt is recognised as a highly biodiverse zone and we present a study of diatom diversity and species distributions in its lakes. We aimed to identify diatom taxa with the highest regional occupancy, explore diversity at a regional level, identify the main variables associated with species distributions, and present weighted average optima for variables that could provide a valuable reference for future palaeoenvironmental research. We used the relative abundances of diatoms in surficial lake sediments to estimate different diversity metrics and alpha, beta and gamma diversities. Groups of lakes with similar diatom assemblages were identified by means of cluster analysis whereas canonical correspondence analysis (CCA) was used to evaluate the association between diatom composition and climatic, morphometric, hydrochemical and trophic status related variables. Weighted average optima were estimated for salinity and temperature. From a total of 184 diatom taxa, three were endemic to one or two lakes and eight had the largest regional occupancy, two of which were associated with high salinity environments (Stephanocyclus meneghinianus, Nitzschia frustulum) and the rest with freshwater conditions (Aulacoseira ambigua, A. granulata, A. granulata var. angustissima, Discostella stelligera, Fragilaria crotonensis, Achnanthidium minutissimum). Surprisingly, endemic taxa as well as those with high regional occupancy were mostly planktic or epipelic motile forms, with high dispersal potential. Our results showed 13 clusters of lakes with similar diatom assemblages, in close agreement with beta diversity which predicted the presence of 12 communities. There was high species turnover (βSIM = 0.97) and CCA showed not only that the main environmental variable defining these groups was salinity, but also that temperature, precipitation, depth, and soluble reactive phosphorus were meaningful variables. There was no evidence of an altitude–diversity pattern. This study emphasises the strong influence that climatic variables have on species distribution when relatively broad geographical areas, with large climatic gradients are analysed.

Using palaeolimnology to guide rehabilitation of a culturally significant lake in New Zealand

Abstract Lakes are becoming degraded at an accelerating rate owing to human activity, and understanding their past ecology is necessary for lake management and rehabilitation. Palaeolimnology provides numerous methods that enable the historical state of lakes to be determined. New Zealand provides an ideal setting in which to do this as human modification of the landscape occurred later here than in most regions of the world (approx. 1300 CE). Lake Oporoa is a shallow lake that is highly significant to the local indigenous Māori community. This study used multiple proxy palaeolimnology to explore how lake ecology shifted following Māori and European settlement in the catchment, and how palaeolimnological data can be used to inform lake rehabilitation and conservation measures, alongside the desires of the indigenous community. Sedimentary pollen, diatoms, bacterial communities, and elemental and hyperspectral imaging scanning were used to infer ecological changes in the lake and catchment from pre‐human times to present. Following Māori settlement (approx. 1620 CE) there was gradual vegetation change and a rapid shift in diatom and bacterial assemblages, but not in phytoplankton pigments or sediment geochemistry. An increasing abundance of diatom taxa Discostella stelligera and Staurosirella cf. ovata indicates early nutrient enrichment. European pastoralism from approximately 1840 CE resulted in further deforestation, and all proxies show evidence for enhanced primary productivity driven by a combination of nutrient enrichment and changing lake levels, particularly since the 1960s. This has caused degradation in water quality and is likely to have contributed to the decline in populations of tuna (eel, Anguilla spp.). Conversations with local Māori, together with the palaeolimnological results, indicate that a culturally acceptable and realistic rehabilitation target for Lake Oporoa aligns with ecological conditions in the 1950s. The palaeoecological data provide information to guide catchment and lake revegetation and other methods of nutrient abatement, with the eventual aim of restoring culturally important tuna and native fish populations.

Unraveling centric diatoms from the Caatinga: Coscinodiscophyceae and Mediophyceae in northwestern Ceará, Brazil

Abstract The Caatinga is the largest tropical dry forest region in South America and harbors an impressive biological diversity. However, efforts in the studies of many groups of organisms, especially aquatic ones, are very small compared to other Brazilian regions. The state of Ceará stands out due to the major concentration of surface water in the semi-arid region, and the diatom flora is virtually unknown. We performed a pioneering and extensive taxonomic study on the centric diatoms from the Caatinga, documenting the morphological variation of the species found, expanding their geographic distribution, and reviewing taxonomic and nomenclatural aspects when relevant. The study was based on planktonic and periphytic samples collected in four hydrographic basins located in the northwestern region of Ceará, northeastern Brazil. Fifteen infrageneric diatom taxa were identified, described, and illustrated using light and scanning electron microscopy. Discostella stelligera was first recorded for northeastern Brazil and D. woltereckii is a new record to the country. Diminutive centric diatoms are well represented in the samples and their taxonomy is discussed. Terpsinoë musica occurred in a population with high phenotypic plasticity and we discussed the related taxonomic implications.

Diatom Response to Global Warming in Douhu Lake, Southeast China

AbstractA large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere. However, due to the intensification of human activities since 1850, some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities. The debate is ongoing. In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes, we chose a lake located at a low latitude, where the relationship between diatoms and temperature is not indirect but direct. The diatom record spans the past ca. 100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985, replacing the previously dominant Aulacoseira ambigua. These changes are in agreement with the increasing trend in global temperature. Since 1985, the percentages of the small‐celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased, while Aulacoseira granulata has decreased. This latest shift is caused by further global warming. We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.

Surface Sediment Diatom as A Water Quality Indicator: Case Study: Cilalay and Cibuntu Ponds, Cibinong

Diatom is classified as algae within the Division of Bacillariophyta. They are unicellular eukaryotic organisms characterized by siliceous cell walls that can be long preserved in sediments. Therefore, diatom analysis in sediment records is a potential water quality indicator for present or paleo studies. The current knowledge on the distribution and diversity of diatoms in the sediment in the urban pond is poorly known. This study aimed to identify the distribution and diversity of diatoms from the sediments of the pond. We expect to obtain a primary database of a variety of diatoms. The sediment samples were taken from Cibuntu and Cilalay Ponds in Cibinong Botanical Garden. Sediments were digested using HCL and H2O2. The resulting diatom solution was dried and transferred onto glass coverslip, which subsequently mounted onto microscope slides using Naphrax (Refraction index 1.7). Diatom identification was examined using a light microscope at 1,000x magnification. Diatom communities in Cibuntu Pond were dominated by species Aulacoseria ambigua, Eunotia bilunaris, Cymbopleura sp, Discostella stelligera, and Rossithidium sp with diversity index of 2.4 and species evenness of 0.8. Whereas, species Fragilaria sp, Eunotia monodon, Navicymbula pusilla, Eunotia bilunaris, and Pinnularia viridis were predominant in Cilalay Pond with diversity index of 1.6 and species evenness of 0.5. Based on the diatom community, Cibuntu Pond is eutrophic indicated by the occurrence of Aulacoseria ambigua, whereas Cilalay Pond is meso-eutrophic indicated by the dominance of Fragillaria. This exploratory survey provides the first inventory of diatom assemblage in Cibuntu and Cilalay Ponds for roughly inferring the environmental changes in a shallow lake ecosystem.

Patterns of phytoplankton structure in response to environmental gradients in a macrotidal estuary of the Equatorial Margin (Atlantic coast, Brazil)

The structure of phytoplankton community was studied in a tropical macrotidal estuary (Paciência river estuary), which is socially and ecologically important because is surrounded by one of the most extensive mangrove forests of the world and plays a key role in the local economy, through artisanal fishing and bivalve extraction activities in natural environments. To understand the regulation mechanisms driving the phytoplankton dynamics and the key indicator species for monitoring ecological change, the study considered seasonal and spatial analysis of environmental and biological variables in thirteen sampling points during the three seasonal periods in 2017. The ecological patterns of phytoplankton were explored by Cluster Analysis and key species were selected by the indicator value method (IndVal). From 171 identified taxa, Bacillariophyta was the most representative division (128 taxa). The phytoplankton structure was well-defined by a salinity gradient governed by rainfall and river discharge. Thus, three scenarios related to seasonality and salinity gradient were identified: tidal river zone, mixing zone and coastal zone. During the rainy and transitional seasons, it was observed a spatial gradient of salinity, characterized by the gradual increase in phytoplankton abundance and biomass from the freshwater zone towards the coastal zone, following an opposite nutrient gradient. The highest salinity and light availability (low turbidity and total particulate matter) in the dry season favored a significant growth of phytoplankton (cell abundance and biomass). Thus, the key species selected by IndVal method was Discostella stelligera, Coscinodiscus oculus-iridis and Trieres sinensis in the rainy period, Nitzschia reversa, Thalassiosira sp. and Bellerochea horologicalis in the transitional period, and Protoperidinium sp., Thalassiosira subtilis, Cyclotella striata and Thalassiosira rotula in the dry period. Hence, this work contributes to the understanding of the macrotidal estuaries functioning, reinforcing the importance of phytoplankton monitoring for the identification of ecological changes.

Differing limnological responses to late Holocene climate variability in the Cordillera Vilcanota, Peruvian Andes

In recent decades, climate change in Peru’s Cordillera Vilcanota has resulted in massive reductions to its cryosphere and the upslope migration of species and agriculture. Little, however, is known about the response of the region’s many lakes that support local biodiversity and supply water to downstream populations. We analyzed fossil diatom assemblages in dated sediment cores from three lakes with differing morphometry and catchment characteristics to document the limnological response to climate variability over the late Holocene. The study lakes contained similar dominant diatom taxa but recorded markedly different assemblage shifts over time. The two deeper lakes, Laguna Sibinacocha (zmax = 92 m) and Chaca Cocha (zmax = 18 m), contained diatom assemblages that oscillated in dominance between benthic fragilarioids (Staurosirella pinnata, Pseudostaurosira brevistriata) and the planktonic Discostella stelligera. The Chaca Cocha diatom record closely matched the mean state changes in climate inferred from independent proxy records, whereas the record from the glacially influenced Laguna Sibinacocha did not match independent records. In contrast, the shallow study site, Lado del Quelccaya (zmax = 5 m), recorded a complacent diatom profile dominated by benthic fragilarioids with negligible planktonic taxa. The disparate diatom trajectories among the study lakes reflect variations in morphometry and glacial influence and the assemblage shifts appear best explained by climate-driven alterations to thermal stratification. These data offer insight into the primary mechanisms driving limnological change in this region and how different lake types throughout the Andes may respond to future warming.

Regional gold mining activities and recent climate warming alter diatom assemblages in deep sub-Arctic lakes

Previous paleolimnological investigations have examined the effects of gold mining operations, local land-use changes, and regional climate warming on the aquatic biota from shallow lakes in Yellowknife, Northwest Territories, Canada. However, the long-term impacts of these multiple environmental stressors on the biota of deeper lakes that support large-bodied fish species have not been investigated. In this study, we examined multiple sedimentary proxies from two deep lakes around Yellowknife to assess the long-term effects of metalloid contamination, development of the city, and recent warming over the past ~ 200 years. The sedimentary metalloid profiles tracked the influence of mining operations and local land-use changes in the Yellowknife area and there were some similarities in the diatom responses to multiple stressors across the two lakes. However, the increases in sedimentary metalloid concentrations, eutrophic diatom taxa, and whole-lake primary production were more pronounced at Grace Lake relative to Alexie, likely because Grace is located nearer to the gold mines, as well as local city development. The overall lake primary production and the relative abundance of the planktonic diatom Discostella stelligera increased at both sites suggesting that some of the biological changes may be influenced by changes in thermal stratification, as has been documented in a wide spectrum of lakes across the Northern Hemisphere. Furthermore, the diatom assemblage changes in these deep lakes differed from those observed from shallow lakes in the region, suggesting that site-specific limnological characteristics will influence the biological responses to multiple environmental stressors through time.

Spatial and temporal variation of the phytoplankton structure in cascading oligotrophic reservoirs of southeast Brazil

Abstract: Aim Structural changes in phytoplankton was evaluated in one ultraoligotrophic and two oligotrophic cascading reservoirs during two climatic periods (austral winter and summer) of 2014. Changes in phytoplankton biomass, species composition, and codons (functional groups) were evaluated along a vertical and a longitudinal profile. Methods Four sampling sites were selected for the first reservoir in the cascade (Cachoeira do França) and three for each of the other two reservoirs (Cachoeira da Fumaça and Serraria). Samples were collected along a vertical and a longitudinal profile. Species biomass and codons ‘sensu’ Reynolds were identified and expressed in terms of biomass. Results 18 codons were identified and associated to the reservoirs’ limnological conditions. Composition and biomass of descriptor species changed among seasons. Codon B consisting mainly of Discostella stelligera was the most frequent in all reservoirs in both climatic seasons. Conclusions Our findings showed that seasonality was the key factor for the species composition and phytoplankton codons changes. However, the cascade effect on phytoplankton structure was shown by the continuous contribution of Ceratium furcoides (codon LM) during the winter, and of Discostella stelligera (codon B) in both sampling periods in all three reservoirs.

Plankton variation in Río Cuarto Lake, Costa Rica (2013-2017)

Lake Rio Cuarto is a meromictic maar lake located in the low plains of northern Costa Rica. Its plankton composition has been studied several times since the decade of 1970. Yet most studies made short term surveys, and at least one study lasted for one year. Here we present data on phytoplankton and zooplankton composition from a four year study of the lake, from 2013 to 2017. Phytoplankton was studied from subsurface samples, preserved in Lugol’s solution and zooplankton from 20 m vertical hawls of a 64 um mesh net, and preserved with formaline and later transferred to 95% etanol. Phytoplankton composition included 54 taxa, most of them from the were Chlorophyta, followed by Cyanobacteria and Bacillariophyta. The most abundant were Synechococcus spp., Aphanocapsa sp. Coenochloris fotii, Chlorella sp. and Discostella stelligera. Total phytoplankton decreased from 2013 to 2017, mainly due to a decrease in Cyanobacteria. Zooplankton comprised 14 taxa, 5 cladocerans, 4 copepods, 1 diptera and 4 rotifers. The most abundant were Keratella cochlearis, Brachionus falcatus, Eubosmina tubicen and Arctodiaptomus dorsalis. There was a substitution of dominant species through time, at first (2013-2014) K. cochlearis and E. tubicen alternated in dominance, and later (2015-2016), A. dorsalis, Diaphanosoma sp. and Ceridaphnia sp. became dominant, with a return of K. cochlearis by 2017. The relationship between both components of the plankton is a complex result of the responses of individual species of both microalgae and zooplankton. However, most zooplankton taxa should be regarded as omnivores, so those relationships are not direct causal effects. The changes that occurred during the time of the study seem to be related to changes in the conditions of the lake, since its thermocline deepened with time and became more transparent. Those changes were not cyclical on an annual basis, but seemed to occur more on a long-term basis, as has been recognized earlier in the literature about tropical lakes.

Assessing the degree of ecological change and baselines for reservoirs: challenges and implications for management

Tropical reservoirs are sensitive to eutrophication but long-term impacts of impoundment on their productivity and biota are poorly understood. Here, we employ a palaeolimnological approach to assess whether ecological baselines can be defined for reservoirs, and examine the challenges and management implications. We studied the environmental history of five reservoirs in Brazil with different productivities, using sediment records covering the period since reservoir construction (~ 50–90 years). Our main goals, based on the analysis of organic geochemistry (TOC, TN, TP, C:N), stable isotopes (δ13C, δ15N), and diatoms, were to reconstruct and compare the magnitude of environmental changes, to determine the conditions prior to any eutrophication and discuss the implications for reservoir management. We inferred that Ribeirão do Campo reservoir has remained oligotrophic since its construction, Itupararanga and Paineiras have both remained mesotrophic with an improvement in water quality around 1970, and Salto Grande has been eutrophic since its construction in 1949. In Rio Grande reservoir, which was originally oligotrophic, eutrophication began in the 1950s, with a slight improvement in water quality after its separation from Billings Reservoir, followed by a subsequent decline in quality since ~ 2001. We found that the studied reservoirs have unique environmental histories and there are clearly challenges associated with defining ecological baselines for reservoirs against which the extent of degradation can be assessed. Nonetheless, when the data from all reservoirs were compared, a coherent pattern in the diatom assemblages emerged, reflecting the trophic gradient. The diatom assemblages prior to enrichment were composed of two groups. The oligotrophic baseline was characterized by several benthic species with low abundances, mainly Eunotia and Brachysira, while in three reservoirs the early assemblages were characterized by planktonic taxa associated with mesotrophic conditions, namely planktonic species Aulacoseira ambigua, Aulacoseira tenella, Discostella stelligera, and Spicaticriba rudis. This work provides information on the baseline conditions, the natural variability of non-degraded reservoirs, and the degree of ecological change in degraded ecosystems. This study contributes to an improved understanding of the timing and extent of eutrophication in these systems and provides information to help better inform the management of tropical reservoirs vulnerable to anthropogenic pressures.

The long-term impacts of climate and fire on catchment processes and aquatic ecosystem response in Tasmania, Australia

The impacts of fire and climate on freshwater ecosystems are not well understood, masking the potential impacts of anthropogenic climate change on these systems. A 9200 year Holocene record of sedimentary Carbon/Nitrogen, x-ray fluorescence, charcoal, pollen, and diatoms preserved within a freshwater lake in Tasmania was used to understand the influences of climate variability and fire on aquatic ecosystem response. Western Tasmania is a cool temperate environment where fire occurrence is driven by hydroclimate. High rainfall during the early to mid-Holocene drove an increase in rainforest and peat in the absence of fire, resulting in an oligotrophic and turbid aquatic environment. This also resulted in leaching of humic acid from the catchment, increasing acidity and dystrophy. The onset of a drier, more variable hydroclimate from the mid-to late Holocene drove lower lake levels and a shift to the dominant planktonic diatom species, Discostella stelligera, the result of the unusual bathymetry of Lake Vera where planktonic diatoms increase with lower lake levels. Further drying caused burning of the rainforest (at ca. 2.3 ka) and increased terrigenous deposition into the lake, leading to a productive, alkaline and disturbed diatom community. Repeated fire disturbance resulted in increased inorganic material deposition, the removal of nutrient rich peat, and an invasion of ferns and sclerophyll vegetation. These fire-driven catchment changes caused a shift in the diatom community to low productivity, oligotrophic and acidic assemblages, likely due to restricted light availability and nutrient uptake by increased deposition of terrigenous material. Therefore, the aquatic ecosystem is responding to climate-mediated changes in the terrestrial environment consistent with regional trends in nearby terrestrial-aquatic Holocene records.

Environmental and spatial drivers of diatom assemblages in the water column and surface sediment of tropical reservoirs

The relative contribution of environmental and spatial drivers of community structure has been the focus of many ecological studies. In the context of metacommunity theory, diatoms have been associated mainly with local environmental factors, but in some cases spatial factors have also been identified as important drivers. This study aimed to determine the relative importance of environmental and spatial drivers as predictors of surface sediment and water column diatom assemblages in tropical reservoirs. Sampling was carried out in 33 reservoirs in southeast Brazil, which range in trophic state from oligotrophic to hypereutrophic, and span five hydrological basins. We performed a partial RDA between the predictor matrices (environmental and spatial predictors) and the response matrix (either surface sediment or water column diatom assemblages) to identify the main environmental drivers, and used variation partitioning to assess the relative contribution of environmental and spatial predictors of diatom assemblages. In general, diatom assemblages were structured by a combination of environmental and spatial drivers. Comparisons of results between surface sediment and water column ordinations demonstrated that the assemblages were structured by similar environmental variables (productivity-related variables such as total phosphorus, water transparency and pH) and by a similar set of diatom species, mainly centric diatoms like Aulacoseira granulata var. granulata, A. tenella, Discostella stelligera and Spicaticribra kingstonii. Overall, surface sediment diatoms were more strongly associated with environmental factors than were water-column assemblages, probably because they integrate information in space and time, confirming that surface sediment assemblages are robust indicators of environmental conditions.

Contemporary abundance patterns of Cyclotella sensu lato diatom taxa in Lake Superior: Assessing responses to physical and chemical gradients and potential links to climate change

Abstract Many studies attribute shifts in relative abundances of Cyclotella sensu lato diatom taxa in freshwater lakes over the past 150 years to climate change. Although abundances of several of these taxa have also changed in Lake Superior over the last century, the specific drivers are currently unclear. This study used multiple linear regression to analyze long-term planktonic monitoring data to identify drivers of changing cell densities of six Cyclotella sensu lato species, four shown to be responsive to climate-driven change in smaller lakes and two species with similar morphology to two previously assessed taxa: Discostella stelligera and the morphologically similar D. pseudostelligera, Lindavia comensis and the morphologically similar L. delicatula, Lindavia ocellata, and Lindavia bodanica. Assessed variables associated with climate-driven physical change included surface water temperature and light availability, with turbidity and thermocline depth as indicators of the light environment. One to three variables explained 8–61% variation for each of 5 taxa from 2001 to 2011, significant predictive variables were not identified for D. stelligera. Multiple linear regression identified nutrients, including nitrate, phosphorus, and silica, as essential in explaining distributions of all the other assessed taxa. A physical variable associated with climate explained changes of one taxon: L. ocellata, which increased with lower water temperatures. Climate-driven species responses were not as apparent as in other systems; specifically, changes in the light environment of this clear lake did not explain abundance changes in any of the taxa. Multiple and complex environmental variables drive Cyclotella sensu lato abundance patterns in Lake Superior, possibly suggesting ecotype formation.

An 11,000‐yr record of diatom assemblage responses to climate and terrestrial vegetation changes, southwestern Québec

AbstractA high‐resolution diatom sequence was developed from continuously laminated lake sediments in southwestern Québec, which spanned the Holocene (~11,000 yr). General and species‐specific changes in the diatom record were synchronous with patterns of vegetation succession inferred from regional palynological studies, demonstrating significant influences of terrestrial ecosystems on lake water chemistry and aquatic biota. Principal components analysis was used to identify three major diatom biostratigraphic zones corresponding broadly to early, mid‐, and late Holocene climate intervals. Benthic alkaliphilus diatom taxa were abundant when the landscape was a Picea‐dominated woodland in the early Holocene (11.1–8.0 ka), and when climate conditions were inferred as relatively cool and dry. In the mid‐Holocene (8.0–3.6 ka), warmer temperatures resulted in a stabilization of soils and forests in the catchment area. Shade‐tolerant tree species (Tsuga, Fagus, and Betula) increased during this time, and planktonic diatom taxa (Discostella stelligera and Fragilaria tenera) were abundant in the lake. The well‐known decline in Tsuga across eastern North America between 6.0 and 5.0 ka corresponded to a shift toward more nutrient‐rich lake conditions for several centuries, indicated by an increase in benthic diatom taxa and a lower ratio of chrysophytes relative to diatoms. Neoglacial cooling in the late Holocene (3.6 ka to present) triggered an initial decline in D. stelligera, which later recovered as a result of nutrient inputs that were associated with increased abundances of hardwood, deciduous species (Betula and Alnus) in the catchment. In addition to providing a high‐resolution record of lake ontogeny for the region, the full Holocene record from this lake adds a new perspective on the spatial extent of regional aridity during the Medieval Warm Period (1.2–0.8 ka).

Seasonal diatom variability of Yunlong Lake, southwest China – a case study based on sediment trap records

Yunlong Lake, a subtropical alpine lake in southwest China, was selected for a sediment trap diatom study between September 2013 and August 2015. Combining the results of meteorological records, water temperature monitoring and water chemistry sampling, we draw the following conclusions: water temperature monitoring indicated that the entire upper 7 m of the water column was mixed throughout the year, reflecting the lake’s strong mixing characteristics; this ensured the year-round growth of planktonic species and the high abundance of benthic species. In addition, the diatoms exhibited a pronounced biannual warm-cold seasonal pattern, which may be related to water temperature, Si nutrient concentration and thermal stratification. During the warm season (May–November) when the lake was almost stratified, with overall higher water temperature and Si concentration, the representative species were Aulacoseira granulata var. angustissima and Achnanthidium catenatum. During the cold season (December–April), the water temperature and Si nutrient concentration were lower, but stronger lake mixing occurred, and the blooming species included Aulacoseira ambigua, Discostella asterocostata and Lindavia balatonis. After a detailed analysis, we conclude that the seasonal diatom succession was primarily driven by water temperature, and comparison with other lake records indicates that the strong mixing mechanism and Si were not the determinants of the lake diatom seasonal succession. Our data also show that during the first cold season, diatoms bloomed in the following sequence: A. ambigua – D. asterocostata – Discostella stelligera – L. balatonis – Discostella pseudostelligera, as Si decreased over the season, indicating that smaller species were more competitive in obtaining limited nutrients. Finally, there were interannual differences between diatom assemblages in the second year, especially in the abundance of L. balatonis and the occurrence of Asterionella formosa, which were probably related to the significantly increased lake trophic status.