Diversity Of Diatom Assemblages Research Articles

Fossil diatom assemblage changes due to paleoenvironment change, tsunami, and typhoon in southern Japan

AbstractTracking paleoenvironmental change and past event deposits is very important to evaluate the natural hazard spatially. This paper presents how the environment changes and implies the event deposit depending on the diatom assemblage change. To investigate paleoenvironmental change and identify the difference between tsunami and typhoon deposits, we analyze diatoms from the sediments in two coastal lakes in southern Japan where flood deposits have been linked to historical typhoon and tsunami events (Lakes Kawahara and Ryuo). The sediment cores extend from B.C.E. 500 to approximately C.E. 1000 and the downcore variation in diatom assemblages indicates a series of transitions from saline to fresher conditions in both Lake Kawahara and Lake Ryuo between approximately C.E. 500 and 1700. We observe an obvious deviation in diatom assemblages in event deposits previously identified to be either of tsunami or typhoon in origin. For the most prominent event deposit preserved in Lake Ryuo by the Hoei tsunami of C.E. 1707, the deposition of marine diatoms serves as evidence of marine flooding, while the subsequent deposition of soil and freshwater diatoms indicates the mobilization of terrigenous sediment during returning seaward flows. In contrast, the most prominent event deposit in Lake Kawahara is associated with freshwater flooding by the Kamikaze typhoon of C.E. 1281 and contains very low diatom abundances and a peak of freshwater taxa, followed by a peak in diatom counts potentially due to greater biological activity induced by a resultant influx of nutrients and re‐oxygenation during the event.

Diatoms of the intertidal environments of Willapa Bay, Washington, USA as a sea-level indicator

An understanding of the modern relationship between diatom species and elevation is a prerequisite for using fossil diatoms to reconstruct relative sea level (RSL). We described modern diatom distributions from seven transects covering unvegetated subtidal environments to forested uplands from four tidal wetland sites (Smith Creek, Bone River, Niawiakum River, and Naselle River) of Willapa Bay, Washington, USA. We compared our diatom dataset (320 species from 104 samples) to a series of environmental variables (elevation, grain-size, total organic carbon (TOCSOM), and porewater salinity) using hierarchical clustering and ordination. While no single variable consistently explains variations in diatom assemblages at every site, elevation, salinity, and substrate (mud fraction and TOCSOM) are variables affecting diatom distribution along our transects. Elevation was the major environmental control of diatom variability (explained 27–39% variance) at four transects (Bone River Transect 1, Niawiakum River Transect 2, Naselle River Transect 1 and 2). Salinity and substrate were the major environmental controls (explained 12–32% variance) of diatom variability at three transects (Niawiakum River Transect 1: salinity; Smith Creek Transect and Bone River Transect 2: TOCSOM). Analyses of a combined regional dataset of all transects suggest that elevation is the driver of regional diatom variability in Willapa Bay, with salinity and substrate co-varying along an elevation gradient. We identify species with narrow elevation tolerances that can serve as indicator species of specific environments. Despite the site-specific variability of our modern diatom distribution, the regional dataset provides an important dataset that can be used to reconstruct RSL in Willapa Bay.

Trace Metals and Diatom Stratigraphy along the Sill between Lakes Telaga Warna and Telaga Pengilon, Dieng, Central Java, Indonesia

This study examines the spatiotemporal variations in diatom assemblages and selected metal concentrations (Pb, Cr, Cd, Al, and Zn) in bed sediments of lakes Telaga Pengilon and Telaga Warna in Dieng, Indonesia to document natural and/or anthropogenic changes in the local aquatic and terrestrial environment. The analyses focused on sediments collected from a 150-cm core taken from a sill between the two lakes, which exhibit significant differences in water chemistry. The core was subdivided into 14 stratigraphic intervals allowing for an analysis of the vertical (and temporal) variations in diatom composition and selected metal concentrations. A total of 103 taxa from 25 genera were identified in the core. Diatom assemblages were dominated by Eunotia (56%), Pinnularia (17.2%), and Frustulia (4.6%). The most abundant species was Eunotia, a diatom that can tolerate a wide range of pH conditions. Given that pH within waters of the two lakes differs significantly, the abundance of Eunotia suggests that pH in the area between Telaga Pengilon and Telaga Warna varied through time, potentially ranging from about 2.5 to 8. Changes in pH were likely related to alterations in hydrological conditions. Metal concentrations varied with depth/time of deposition within the core. Peak metal concentrations appear to be related to the influx of debris from a volcanic eruption. Based on the principle component analysis (PCA), the input of volcanic materials also influenced diatom assemblages and resulted in a distinct layer of broken diatom frustules. Relatively low metal concentrations in surface sediments suggest that the erosion of hillslope soils in response to agricultural activity did not significantly impact the lakes.

Large-scale geographical and environmental drivers of shallow lake diatom metacommunities across Europe

Disentangling the relative role of species sorting and dispersal limitation in biological communities has become one of the main issues for community ecologists and biogeographers. In this study, we analysed a data set of epiphytic diatoms comprising 34 lakes from six European countries. This data set covers a relatively large latitudinal gradient to elucidate which processes are affecting the distribution of diatom communities on a broad spatial extent. Our results show strong environmental effects on the composition of the diatom communities, while the spatial factor effects were weak, emphasising that compositional variation was mainly due to species turnover. Our data support information from the literature that local abiotic factors are the main predictors controlling the compositional variation of diatom assemblages in European shallow lakes. More specifically, changes in species composition were driven mainly by nutrient content in Northern Europe, whereas lakes located in Southern Europe were more affected by conductivity and lake depth. Our results solve pending questions in the spatial ecology of diatoms by proving that species turnover is stronger than nestedness at any spatial scale, and give support to the use of epiphytic diatoms as biological indicators for shallow lakes.

Ocean surface warming in Krossfjorden, Svalbard, during the last 60 years

A high-resolution marine sediment core NP16-Kro1-MCB from Krossfjorden, Western Svalbard is studied to investigate changes in sea surface conditions in the fjord during the last 60 years (1953–2014). The diatom-based reconstruction of August sea surface temperature (aSST) demonstrates a clear warming trend of 0.6 °C through the record. As inferred from Marginal Ice Zone (MIZ) diatoms, surface warming occurs in parallel with a decline in sea ice extent (SIE) during recent decades. Factor analysis identified variations in diatom assemblages representing different water masses, showing a dominance of Arctic water diatoms throughout the period and decadal variations in the sea ice assemblage during periods of peak sea ice extent. The strong dominance of Arctic water diatoms along with increasing aSST suggest prolonged open-water conditions and increased sea ice melting in the region throughout the observed period. The reconstructed ocean surface changes are in line with the background warming occurring over the Arctic region. A comparison with instrumental records from neighboring regions supports the quality of the reconstructions, including the average reconstructed aSST and the magnitude of the warming trend. We suggest that increased CO2 forcing together with ocean–atmospheric interaction have caused the increasing SST trend and decreasing sea ice presence in Krossfjorden rather than an increasing influence from Atlantic Water, which has amplified changes in many regions of Svalbard.

Water usage and seasonality as primary drivers of benthic diatom assemblages in a lowland reservoir

Abstract In general, reservoirs are multi-purpose objects and they often have to fulfil different tasks at the same time. The compliance with these different expectations is more manageable in reservoirs with well-separated heterogeneous basins than in homogenous ones. The Kiskore Reservoir is the second largest standing water in the Carpathian Basin. Its water level is artificially regulated which creates unique habitats with high nature conservation value. Furthermore, the Reservoir is also considered as an important recreation centre in the region. It consists of four basins with different protection degree and management strategy. Here, we tested whether spatial (usage-dependent) or seasonal (time-dependent) segregations characterized the benthic diatom composition in the Reservoir. We also tested the influence of different water usage of basins using diatom metrics and diversity indices. We hypothesized that spatial heterogeneity in diatom composition will be more pronounced than seasonal ones. We also supposed that composition and diversity of diatom assemblages as well as diatom based ecological status will clearly reflect to different management strategies in the basins, whereas we expected moderate ecological status and low diversity in basins with high level of human impact. Our first hypothesis was not confirmed by the results. While diatom composition was clearly heterogeneous in time, surprisingly no usage-dependent segregation was found. Furthermore, our second hypothesis was also only partially confirmed by the results. In early summer, diversity was significantly lower in basins with higher level of human impact, than in the other basins. In late summer, however, diversity was rather directly controlled by nutrients and light not by water usage. Moreover, diatom based ecological status positively correlated to the intensity of recreation activities and negatively to protection degree. Although these results were surprising at the first time, they clearly confirm that a balanced ecological-economical relation can be maintained with properly designed and performed strategies in artificial reservoirs. But protection strategies, because of their exclusive interest to habitat conservation of macroscopic not microscopic organisms, require careful revisions.

Holocene environmental change in Tonle Sap, Cambodia based on fossil diatoms

Tonle Sap Lake (“Great Lake”, Cambodia) is a biggest inland freshwater body. The size of the lake is changed dynamically following monsoon via connected to the Mekong river, especially the flood pulse. The flood pulse on Tonle Sap has affected considerably the lake’s ecological property as well diatom assemblages. The present study aimed to assess the impact of the flood pulse to diatom assemblages by time. Two short sediment cores from Tonle Sap Lake with the depth of 1.54 m and 1.27 m respectively below the lake floor were collected in May 2013 and 2015 and one short sediment core with the depth of 1.68 m was collected from the confluence of the Mekong River and Tonle Sap River in May, 2013. The sedimentations were dated by using radiometric dating (210Pb and 137Cs). Succession of fossil diatom assemblages was calculated by Rarefaction index (ES) and species richness is by Hill's N2 index. A total of 70 diatom species was released, and the diversity of diatom assemblages was extremely fluctuated in function of time (p-value = 0.0045***). Especially, 6 diatom taxa: Aulacoseira distans, Aulacoseira granulata, Aulacoseira granulata var. angustissima, Gyrosigma acuminatum, Gyrosigma attenuatum and Paralia sulcata characterized by the highest relative abundance (>1 %). In term of ecology, these species are the epipelic diatoms living commonly in eutrophication and high suspendid solid conditions. In fact, it is clear that the onset of flood pulses affected considerably the studied diatom assemblages in particular, and made sense to bio-community in general; also the lacustrine environment of Tonle Sap lake was changed very strongly in response to this shift of hydrological regime.

New insights into the termination of the African Humid Period (5.5 ka BP) in central Ethiopia from detailed analysis of a diatom record

The termination of the African Humid Period in northern Africa has been described as abrupt, occurring within centuries, as well as gradual, in response to incremental decreases in summer insolation. This study examined the rapidity of the change in diatom assemblages over the period from 6.5 to 4.5 cal ka BP, in a core studied previously at a coarser resolution. This transition was characterized by high variability of assemblages, which could be related, in part, to changes in water conductivity, and potentially enhanced by a site-specific hydrological threshold or ecological salinity threshold. We hypothesize that the variations in diatom assemblages reflect climate fluctuations, which may have been an early warning signal of an impending climate regime shift.

Geothermal Diatoms: Seasonal Variability in the El Tatio Geothermal Field (Altiplano, Chile)

Diatom floras were examined in a high-altitude geothermal field, 4200 - 4500 m (29°19'S 68°W'), located in the Central Andean dry Puna ecoregion or southern Altiplano. These locations include hostile environments subjecting living organisms to extreme conditions. The aim of the present study was to investigate the spatial patterns and describe the response of diatom assemblages to differences in physical and chemical variables. Different shallow (<10 cm) aquatic systems with variable chemical and physical conditions (fumaroles, freshwater-saline rivers and swamps) were studied seasonally during 2011-2012. The conductivity exhibited high variability (360 to 18340 μS cm-1) among the systems studied, temperature was lower in rivers and swamps (6.8°C - 10°C) and high in fumaroles stations (30°C - 37.5°C), and pH was lower in fumaroles than freshwater systems (3.25 to 8.97). Statistical analyses suggest that the diatoms cluster into three major groups. The most common taxa include: Achnanthidium exiguum (Grunow) Czarnecki, Cocconeis placentula var. lineata (Ehrenberg) Van Heurck, Eolimna minima (Grunow) Lange-Bertalot, Staurosirella pinnata (Ehrenberg) Williams and Round, Navicula gregaria Donkin, Nitzschia inconspicua Grunow, Nitzschia palea (Kutzing) Smith, Nitzschia perminuta (Grunow) Peragallo, and Planothidium lanceolatum (Brebisson ex Kutzing) Lange-Bertalot. As expected, the 20 to 200 μm-size fraction contained the highest numbers of diatom taxa (53 species), although an unexpectedly high number (47 species) were also found in the smaller 5 to 20 μm-size fraction, more associated to fumaroles and saline systems. The 180 to 2000 μm size fraction contained only two species, including rosette-forming diatom Ulnaria ulna (Nitzsch) Compere, and the unicellular species Surirella chilensis Janisch, both species exclusively reported in freshwater systems. Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed clear correlations between species, conductivity, TP (total phosphorous), NO3- , HO3- , Mg2+, temperature and dissolved oxygen (DO). The gradient of ionic composition values explaining most variation in diatom assemblages ranged from waters dominated by Ca2+ and SO4- to waters with higher proportions of Na+, K+, F-, Li+, Mg2+ and Cl-. Other factors include substrate type, presence of macrophytes, current velocity and other local environmental conditions. The results presented here enhance our understanding of diatom richness/composition in hostile environments from a high-altitude arid and semi-arid geothermal region.

Multi-proxy evidence for small historical tsunamis leaving little or no sedimentary record

There has been considerable progress in tsunami research in recent years, yet most work has been focussed on identifying and understanding the evidence of large events. This study aimed to identify the evidence of small historical tsunamis in Lyttelton Harbour, New Zealand. The study area has been inundated by multiple relatively small historical tsunamis that left atypical sedimentary evidence (no sand) or no sediment. Shallow trenches revealed discontinuous layers of small grey mud clasts at various depths across the study area, most likely transported from the nearby tidal flats. The origin of these mud clast layers was investigated using a multi-proxy approach comprising sedimentological, geochemical and diatom analyses complemented by dating from 137Cs activity profiles and historical data. Subtle variations consistent with inclusions of marine mud such as a decrease in organic content and magnetic susceptibility and increases in geochemical markers (e.g. Ca/kcps, K/kcps, K/Rb, Si/Rb and Sr/Rb) were found in the sediment profile. Variations in diatom assemblages suggesting a marine influence were also recorded at similar depths, aligning with layers of mud clasts. Using 137Cs activity profiles and historical data, these deposits were attributed to the 1960 Chile Valdivia and 1964 Alaska tsunamis, as well as a possible earlier event. Sedimentary evidence for the 2010 Chile Maule tsunami was not found at the study site, but geochemical analysis of surface samples revealed marked changes in Ca, Cl, Sr and Ti concentrations, indicative of a change from a terrestrial to marine influence. This was used to identify the landward extent of inundation by the 2010 event. Ultimately, this study shows that a broad multi-proxy analysis can distinguish even the subtle signatures of an inconspicuous deposit laid down by a small tsunami.

Distribution and diversity of diatom assemblages in surficial sediments of shallow lakes in Wapusk National Park (Manitoba, Canada) region of the Hudson Bay Lowlands.

The hydrology of shallow lakes (and ponds) located in the western Hudson Bay Lowlands (HBL) is sensitive to climate warming and associated permafrost thaw. However, their biological characteristics are poorly known, which hampers effective aquatic ecosystem monitoring. Located in northern Manitoba along the southwestern coast of Hudson Bay, Wapusk National Park (WNP) encompasses numerous shallow lakes representative of the subarctic zone. We analyzed the distribution and diversity of diatom (microscopic algae; class Bacillariophyceae) assemblages in surficial sediments of 33 lakes located in three different ecozones spanning a vegetation gradient, from NE to SW: the Coastal Fen (CF), the Interior Peat Plateau (IPP), and the Boreal Spruce Forest (BSF). We found significant differences (P < 0.05) in diatom community composition between CF and IPP lakes, and CF and BSF lakes, but not between IPP and BSF lakes. These results are consistent with water chemistry measurements, which indicated distinct limnological conditions for CF lakes. Diatom communities in CF lakes were generally dominated by alkaliphilous taxa typical of waters with medium to high conductivity, such as Nitzschia denticula. In contrast, several IPP and BSF lakes were dominated by acidophilous and circumneutral diatom taxa with preference for low conductivity (e.g., Tabellaria flocculosa, Eunotia mucophila, E. necompacta var. vixcompacta). This exploratory survey provides a first detailed inventory of the diatom assemblages in the WNP region needed for monitoring programs to detect changes in shallow lake ecosystems and ecozonal shifts in response to climate variations.

Similar small-scale variation of diatom assemblages on different substrates in a mesotrophic stream

Abstract The aim of the present study was to analyze if small-scale spatial variation of benthic diatom assemblages has consequences for biomonitoring. Benthic diatom samples were collected at one sampling site in a mesotrophic stream in Middle-Sweden from stone and plant (macrophytes and mosses) substrate. Our results showed that spatial variation of both the diatom species composition and the calculated bioindices were similar on both small (distance of centimeter) and medium (distance of decimeters) scales. Spatial variation was also similar on both studied substrates. This implies that it does not matter if a small or a larger area is sampled for biomonitoring as long as no major environmental factors impact certain sites systematically. Diatom assemblages and indices were significantly different between substrates. Spatial variation did not contribute much to this variation, and variation on a slide was unimportant. These results confirm earlier findings that smallscale spatial variation is not a problem when using diatoms to detect anthropogenic impacts to a stream or lake.

A preliminary assessment of the effects of Didymosphenia geminata nuisance growths on the structure and diversity of diatom assemblages of the Restigouche River basin, Quebec, Canada

In June 2006, blooms of Didymosphenia geminata (Lyngbye) M. Schmidt were observed for the first time in the Matapedia River, Quebec. This species can produce nuisance-level epilithic algal mats in oligotrophic rivers with stable flows. The lack of knowledge surrounding the potential impacts of nuisance growths on lotic ecosystems has raised concerns. Periphyton communities were sampled in three rivers of the Restigouche watershed to assess the effects of D. geminata nuisance growths on surrounding diatom assemblages. This study provides a preliminary assessment of the effect of D. geminata on the species composition of diatom assemblages and the influence of increased biofilm complexity on species diversity. Our results show that D. geminata mats have an effect on the structure and diversity of diatom assemblages. Conversely, we found no evidence that an increase in Standing Crop Index (SCI) further impacted diatom diversity or structure of these assemblages. This preliminary study contributes to the growing knowledge of the biology and impact of D. geminata in lotic environments.

The diversity of diatom assemblages in ships' ballast sediments: colonization and propagule pressure on Canadian ports

This research focused on the specific diversity of diatoms carried in ballast sediments of ships reaching Canadian ports on the Pacific, Atlantic and the Great Lakes during 2007 and 2009. The inventory of 180 taxa included Thalassiosira, Chaetoceros spores, Cyclotella, Actinocyclus, Aulacoseira, Melosira and Pseudo-nitzschia as the most species-rich genera found in 142 samples of the matter settled at the bottom of ship tanks. We also tested and showed evidence that diatom species composition identified in any given tank was the product of: (i) species intrinsic survival strategies, (ii) the most recent ballast water exchange (BWE), and (iii) the cumulative end result of past ballast operations. Multivariate analysis reduced our data set to three assemblages. Propagule pressure of these potentially colonizing assemblages may be enhanced due to repeated introduction attempts by ships belonging to different ballast management categories (transoceanic, intra-coastal with our without BWE) but that actually carried the same assemblage type. A non-linear relationship between colonization and propagule pressure confirms that the early stages of invasion (ballast uptake, survival in the tank) structure the later stages. Coastal floras in tanks that underwent offshore BWE (particularly persisting freshwater species) corroborate concern with the low efficiency of this management option.

Epibryic diatoms from ombrotrophic mires: diversity, gradients and indicating options

The present study focuses on comparison of recent diatom diversity within the unique Central European ombrotrophic wetlands in two regions with contrasting levels of past atmospheric depositions (Jizerske hory Mts. and Jeseniky Mts.). Further, two methods of mire diatom sampling (squeezing and mineralization of the entire moss sample) are compared at the same localities to determine whether bryophyte herbarium specimens could be used for studies on the historical development of peat bog areas. There were a total of 47 diatom species at 59 sites within 13 ombrotrophic mires along the CzechPolish-German border areas. The regions did not differ in diatom species richness. The two-group pattern along the Axis 1 in NMDS ordination analysis did not correspond with the regional distribution. The inter-site variation in diatom assemblages was lower than the intra-site variation. The combination of three environmental variables (conductivity, pH, and height above water table – HWT) appears to be able to explain the diatom distribution patterns. No significant differences were found between the composition of diatom species of moss and squeezed samples. However, moss samples can be characterized by significantly greater species richness, most likely because the mineralization of the whole bryophyte minimizes the losses of diatom frustules caused by the squeezing method. These results allow us to reccommend herbarized bryophytes as a source of missing information on the sub-recent diatom diversity of mires.

Temporal variation of diatom assemblages in oligotrophic and eutrophic streams

Diatoms are used as indicators of water quality as they respond predictably to many water chemistry variables and their ecology is relatively well known. However, temporal variation in diatom assemblages remains poorly resolved and studies covering large temporal and spatial scales are urgently needed. Here, we studied how diatom assemblages vary within and between three years at eight boreal stream sites located in Finland using Mantel tests and ordination analyses. Our study streams differed greatly in their trophic level and size: Vantaanjoki is mesotrophic to eutrophic and large, whereas Luutajoki and Evojoki are oligotrophic and smaller. We found that species composition reflected well the status of the sampled streams. In Vantaanjoki, the most common diatoms were all typical species for mesotrophic to eutrophic streams, whereas oligotrophic streams were characterized by species with narrow ecological niches confined to nutrient-poor water. According to Non-metric Multidimensional Scaling (NMDS), the between-year variation was nearly comparable at all sites, eutrophic sites generally showing slightly higher between-year variation. The reasons for large between-year differences in assemblages were probably related to different weather conditions between the years. The Mantel tests also showed a notable degree of within-year variation in all assemblages. Therefore, we suggest that ecosystem size and productivity act in different directions in the data, resulting in no major differences in the turnover rates at sampled sites. However, NMDS analyses conducted simultaneously for the whole diatom data showed that temporal variation does not confound the large differences in assemblages between eutrophic and oligotrophic streams and that temporal variation may add slightly less noise in spatial assemblage variation in oligotrophic streams. This suggests that site classifications based on diatoms may be overall slightly more robust in oligotrophic waters, whereas in mesotrophic to eutrophic waters, misclassifications may be more likely due to large temporal variation in assemblages.

A diatom record of late Pliocene cooling from the Ross Sea continental shelf, AND-1B, Antarctica

A late Pliocene – early Pleistocene, 2.9–2.0Ma, diatom record from the Antarctic Geological Drilling Program (ANDRILL) MIS drillcore AND-1B is presented. This core, recovered from beneath the Ross Ice Shelf south of Ross Island, comprises multiple diatomaceous-sediment units deposited during interglacial periods with open water over the core site. These represent interglacial phases of orbitally paced climate cycles and are punctuated by glacial advances. Extant diatom assemblages have limited presence in the late Pliocene record, which makes environmental interpretation less straight forward. We employ modern ecological data in combination with late Pliocene to present variation in diatom assemblages across the Southern Ocean oceanic fronts based on DSDP/ODP diatom biostratigraphic data to evaluate paleoenvironmental change for the 2.9–2.0Ma interval of the AND-1B core. The diatom assemblages from AND-1B record a progressive environmental change through the late Pliocene–early Pleistocene. A relatively warm period with potential SST of up to 4°C at ca. 2.9Ma was succeeded by a reduction of warm water species and an increase of taxa associated with more southerly water masses until 2.58Ma (at isotope stage G1). Younger, early Pleistocene, diatomaceous units are dominated by extinct Rouxia, Thalassiosira species and newly described Fragilariopsis species indicative of cold open water and drift ice. The last recorded cooling step occurs at the top of the interval studied (ca 2.0) Ma indicated by the trace abundance of Fragilariopsis and Actinocyclus species present in modern sea ice assemblages but absent in the late Pliocene–early Pleistocene AND-1B record, even though many of its species had their first occurrences during the Pliocene. The extant sea ice assemblage occurs with an abundance of 2–29% (average 10%) in the late Pliocene–early Pleistocene AND-1B record implying that the modern sea ice and ice shelf polar conditions were not established within the studied interval. The most frequently applied sea ice indicator, Fragilariopsis curta, is consistently present in low to moderate abundance (1–22%) together with Chaetoceros resting spores (2–30%) through the 2.9–2.0Ma interval. The diatom assemblage shifts indicate a dynamic environment with an overall trend towards colder conditions after ca. 2.6Ma but does not reach the Holocene configuration.

Hidden diversity in diatoms of Kenyan Lake Naivasha: a genetic approach detects temporal variation

This study provides insights into the morphological and genetic diversity in diatoms occurring in core sediments from tropical lakes in Kenya. We developed a genetic survey technique specific for diatoms utilizing a short region (76-67 bp) of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (rbcL) gene as genetic barcode. Our analyses (i) validated the use of rbcL as a barcoding marker for diatoms, applied to sediment samples, (ii) showed a significant correlation between the results obtained by morphological and molecular data and (iii) indicated temporal variation in diatom assemblages on the inter- and intra-specific level. Diatom assemblages from a short core from Lake Naivasha show a drastic shift over the last 200 years, as littoral species (e.g. Navicula) are replaced by more planktonic ones (e.g. Aulacoseira). Within that same period, we detected periodic changes in the respective frequencies of distinct haplotype groups of Navicula, which coincide with wet and dry periods of Lake Naivasha between 1820 and 1938 AD. Our genetic analyses on historical lake sediments revealed inter- and intra-specific variation in diatoms, which is partially hidden behind single morphotypes. The occurrence of particular genetic lineages is probably correlated with environmental factors.

Exploration of sample size and diatom-based indicator performance in three North American phosphorus training sets

Three large training sets were investigated to determine optimal sample sizes for diatom-based inference models. The sample sets represented (1) assemblages from Great Lakes coastlines, (2) phytoplankton from the pelagic Great Lakes and (3) surface sediment assemblages from Minnesota lakes. Diatom-based weighted average models to infer nutrient concentrations were developed for each training set. Training set sample sizes ranging from 10 to the maximum number of samples were created through random sample selection, and performance of each model was evaluated. For each model iteration, diatom-inferred (DI) nutrient data were related to stressor data (e.g., adjacent agricultural or urban development) to characterize the ability of each model to track human activities. The relationships between model performance parameters (DI-stressor correlations and model r2, error and bias) and sample size were used to determine the minimum sample size needed to optimize models for each region. Depending on the training set, at least 40–70 samples were needed to capture the variation in diatom assemblages and environmental conditions to such a degree that non-analog situations should be rare and so should provide an unambiguous result if the model was applied to any sample assemblage from the region. It is recommended that one exercises caution when dealing with smaller training sets unless there is certainty that the selected samples reflect the regional variability in diatom assemblages and environmental conditions.

Spatial factors contribute to benthic diatom structure in streams across spatial scales: Considerations for biomonitoring

Diatoms are widely used in the biological monitoring of streams because they are strong responders to environmental change, but dispersal and spatial factors can play important and potentially confounding roles in the presence, absence, and abundance of species along with characterizing species–environment relationships. To examine how spatial factors affect diatom community structure and biomonitoring, multiple scales were sampled including the Western Allegheny Plateau ( n = 58), Leading Creek watershed ( n = 18), and the adjacent Shade River watershed ( n = 21) in southeast Ohio. Partitioning of spatial, environmental, and spatially-structured environmental variation was conducted on diatom assemblages and on diatom metrics used in biomonitoring. At the regional scale, diatom assemblages and metrics had strong relationships with agricultural ( e. g., significant correlations with nutrients, conductivity, and pasture/row crops in the watershed) and alkalinity gradients. Diatom assemblages and metrics in both watersheds were strongly associated with acid mine drainage (AMD) impacts, and when spatial factors were set as covariables in CCAs, relationships with AMD gradients became even stronger, indicating the need to consider how spatial factors could reduce the strength of diatom-environment relationships. Metrics calculated at all scales had very little variation explained exclusively by spatial factors, likely because multiple species are combined into a simplified metric that reduces the effects of species dispersal. Local environmental variables accounted for 57, 42, and 42% of the total variation explained (TVE), and spatial variables accounted for 28, 31, and 37% of the TVE in the regional, Leading Creek, and Shade River datasets, respectively. The amounts of variation in diatom assemblages explained solely by spatial factors at these scales were substantial and similar to what has been reported at continental, national, and large regional (Level I Omernik ecoregions) scales (approximately 1/3 of TVE). Although amounts of variation explained are similar across scales, processes underlying the spatial structure likely differ. In addition to describing ecological patterns, recognizing the potential influence of spatial factors could improve the identification and management of environmental problems at a range of scales, as well as aid in the development of new research questions and hypotheses aimed at exploring factors that could explain portions of the spatially explicit variation.