Comparison of attached algal communities on natural and artificial substrata along a thermal gradient

Colonization rate and community structure of periphyton assemblages was examined on aluminium and glass substrata and compared to populations on four submerged macrophyte species in three temperature zones in Cholla Lake, Arizona, U.S.A. Higher densities were achieved over shorter incubation intervals in the warmer zones (26–35° C). Representatives from the planktonic diatom community were first to colonize artificial substrata during the initial two hour incubation period in all temperature zones. Two periphyton diatom representatives, Amphora coffeiformis and Cocconeis placentula var. lineata were the numerical dominants after one week. Cocconeis placentula var. lineata was most competitive on natural substrata at temperatures <26°C, while Amphora coffeiformis dominated temperature zones >26°C with no significant preferences for artificial or natural substrata. The significance of temperature, specific conductance and availability of living hosts is discussed with respect to regulating populations of these two common periphytic diatom species in alkaline waters in southwestern U.S.A. Similarity indices (SIMI) were used to compare algal assemblages on various natural and artificial substrata pairs. Periphyton assemblages were very similar on all natural substrata within similar temperature zones, with little or no preference for macrophyte species displaying similar leaf morphology. Diatom assemblages were quite similar on aluminium and glass substrata throughout the incubation period in all temperature zones, while blue-green algal populations were significantly different, particularly in the higher temperature zones (>28°C). Natural periphyton communities were best represented after four weeks incubation with aluminium substrata in warmer temperature zones (>28°C) or where filamentous blue-green algae dominated. The selection of adequate incubation time when employing artificial substrata to evaluate natural assemblages for different environmental conditions and algal populations is discussed.     

Benthic diatoms of the Vistula River estuary (Northern Poland): Seasonality,substrata preferences,and the influence of water chemistry

Seasonal studies of water bodies in separate branches of the Vistula River estuary in Northern Poland were conducted over 30 months from May 2005 to October 2007. Diatom samples were collected from different substrata from two sites located in the river mouth, which has physical characteristics as a result of a complex interplay of natural and human influences that have taken place over the last 100+ years. The diatom flora growing on hard surfaces near the river banks such as sand, muddy sand, rocks, macroscopic green algae and/or vascular aquatic plants was recorded. All diatom taxa collected were distinguished based on their morphological features using light (LM) and scanning electron microscopy (SEM). A total of 187 species belonging to 61 genera were identified. A canonical correspondence analysis indicated that the diatom assemblages were definitely associated with salinity as well as nutrient concentrations and substratum type. During the study, malformations of diatom valves were observed in 48% of the microphytobenthos samples. Abnormal outline morphology and ornamentation deformities in several individuals of nine different species were recorded, which were mainly in epilithic and epiphytic communities collected from spring to autumn.     

EFFECTS OF CHIRONOMID (INSECTA: DIPTERA) TUBE-BUILDING ACTIVITIES ON STREAM DIATOM COMMUNITIES1,2

Chironomid retreats, constructed out of sand grains upon submerged wood debris, increase the surface area available for diatom colonization. The three dimensional substratum afforded by chironomid tubes supports up to twelve times the diatom biavolume found upon adjacent, unmodified substratum in a northern Michigan stream. Diatom enumeration within scrapings from small defined areas on artificial substrata, combined with examination of intact natural substrata through scanning electron microscopy (SEM), reveals distinct, microdistribution patterns. The larval retreats of two major taxa of tube-dwelling chironomid's (Micropsectra sp. and Pseudodiamesa cf. pertinax Garrett) display significantly different diatom communities relative to adjacent masonite substratum. Substratum without chironomid tubes is primarily colonized by Achnanthes minutissima Kütz. and Cocconeis placentula Ehr., exhibiting the lowest species diversity of microhabitats examined. The diatom flora upon sand tubes of Micropsectra sp. is dominated by Opephora martyi Herib., as is the flora of sand grains collected from the stream sediment load. These two micro-habitats exhibit a high community similarity (SIMI). The SIMI index also suggests that the flora of P. pertinax tubes is highly similar to that of sand grains. Diversity, however, is almost three times greater on P. pertinax tubes and SEM observations reveal that this microhabitat is characterized by a more spatially complex flora; Nitzschia and Navicula spp. dominate the upperstory, and O. martyi is located on underlying sand grains. Results indicate that tube-building chironomids in Carp Creek affect diatom microdistribution by: (1) stabilizing sand grains and associated flora within their retreats, (2) providing a ‘refugium’ for upperstory diatom taxa from the mayfly grazer, Baetis vagans McDunnough (Insecta: Ephemeroptera), and (3) through local nutrient enrichment.     

The Role of Substrate Type on Benthic Diatom Assemblages in the Daly and Roper Rivers of the Australian Wet/Dry Tropics

The selection of one or more river substrata for the collection of benthic diatoms is fundamental to any monitoring or research programme because it can potentially bias the diatom data set. In the wet/dry tropics of Australia, where the use of benthic diatoms for river health assessment is in its infancy, the comparability of diatom assemblages on river substrata has been assessed. Benthic diatoms were sampled from seven river sites, with a range of ionic chemistries (conductivities 27–6500 μS cm⁻¹) but low nutrient concentrations. At each site, triplicate samples were collected from 3 to 6 substrata. The diatom assemblages sampled were: epilithon (assemblages on rock), epiphytes on macroalgae and macrophytes, epidendron (assemblages on wood), epipsammon (assemblages on sand), epipelon (assemblages on mud) and bacterial slime. The variability between substrate assemblages, at each site, were assessed according to the following: (1) a multivariate analysis of diatom assemblages, (2) ANOVA tests of species richness, (3) ANOVA tests of the relative abundance of common species (defined by an abundance of at least 10% in any one sample), and (4) a comparison of the number of species unique to a substrate. A total of 198 taxa were identified, with some taxa common to temperate Australia. Common species were found on all substrata, with sometimes statistically significantly different relative abundances. Taxa unique to a substrate had low relative abundances (0.1–2%), were most often found on only one replicate, and are unlikely to be substrate specific because many are known to occur on other substrata. The assemblages on hard substrata, epilithon and epidendron, were found to be most similar. Diatom assemblages on macroalgal and macrophyte substrata, compared to other substrata, were highly variable. This is attributed to the loss of diatoms from grazing and sloughing, followed by recolonisation of newly exposed substrate. Other assemblages, notably epipsammon, were similar to epilithon and epidendron but sometimes differed in their relative abundance of common species. The principal finding of the study was the similarity of the epilithon and epidendron, which are considered to be indistinguishable. Rock and wood hard substrata can be substituted for one another during field surveys, thereby increasing the number of potential sample sites available for monitoring activities that standardise to a hard substrate.     

Colonization rates and community structure of diatoms on three different rock substrata in a lotic system

Diatom assemblages were monitored at weekly intervals over a 5 week period on Verde limestone, Supai sandstone, and Andesitic basalt substrata in a mountain stream in northern Arizona, U.S.A. Density, Shannon-Weiner diversity, evenness, and community similarity (SIMI) were used to compare colonization patterns and community structure between individual substratum types. Average standing crop values were nearly two-fold higher on sandstone than on either basalt or limestone substrata after the first week of the study. It is proposed that differences in micro-surface features between substrata and possibly the rate of substratum solubilization may cause these differences in density early in the colonization period. Following the initial week, standing crop and community structure were significantly similar on all substrata for the remainder of the study period. Maximum densities were attained by the third week and remained relatively constant on all substrata for the remainder of the study.

SEM micrographs demonstrated that surfaces of submerged substrata in streams are modified after the first week by the accumulation of organic aggregates. The establishment of an “organic matrix” early in the colonization process may provide relatively similar attachment surfaces for microbial invasion. This appears to reduce the initial microtopographic differences displayed by substrata and allows for a more uniform colonization pattern.     

Multivariate analysis of diatoms and water chemistry in Bolivian saline lakes

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

Diatom changes in two Uinta mountain lakes,Utah, USA: responses to anthropogenic and natural atmospheric inputs

Diatom assemblages in sediments from two subalpine lakes in the Uinta Mountains, Utah, show asynchronous changes that are related to both anthropogenic and natural inputs of dust. These lakes are downwind of sources of atmospheric inputs originating from mining, industrial, urban, agricultural and natural sources that are distributed within tens to hundreds of kilometers west and south of the Uinta Mountains. Sediment cores were retrieved from Marshall and Hidden lakes to determine the impacts of atmospheric pollution, especially metals. Paleolimnological techniques, including elemental analyses and ²¹⁰Pb and ^239+240Pu dating, indicate that both lakes began receiving eolian inputs from anthropogenic sources in the late 1800s with the greatest increases occurring after the early 1900s. Over the last century, sediments in Marshall Lake, which is closer to the Wasatch Front and receives more precipitation than Hidden Lake, received twice the concentrations of metals and phosphorus as Hidden Lake. Comparison of diatom and elemental data reveals coeval changes in geochemistry and diatom assemblages at Marshall Lake, but not at Hidden Lake; however, a major shift in diatom assemblages occurs at Hidden Lake in the seventeenth century. The change in diatoms at Marshall Lake is marked by the near disappearance of Cyclotella stelligera and C. pseudostelligera and an increase in benthic, metal-tolerant diatoms. This change is similar to changes in other lakes that have been attributed to metal pollution. The marked change in diatom assemblages at Hidden Lake indicates a shift in lake-water pH from somewhat acidic to circumneutral. We hypothesize that this change in pH is related to drought-induced changes in input of carbonate-rich desert dust.     

Biofilm diatom community structure: influence of temporal and substratum variability

Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.     

Assessing the short-term response of stream diatoms to acidity using inter-basin transplantations and chemical diffusing substrates

1. Acid‐base status has major effects on diatoms, but there is little information on their short‐term response to changing acidity. This is despite the use of diatoms as bioindicators in streams where acid episodes are important during rainstorms (hours to days) or snowmelt (days to weeks). In the Llyn Brianne experimental catchments (Wales, UK), we attempted to mimic the effects of short‐term acidification by (i) reciprocally transplanting diatoms between two streams of contrasting acidity and (ii) using acid‐diffusing substrates. 2. Diatom diversity decreased rapidly on substrata transplanted from the circumneutral into the acidic stream, and increased in the reciprocal transplantation. Changes in dominant taxa occurred within three days in the acidic stream because of the rapid growth of Eunotia exigua, and by nine days in the circumneutral stream because of the proliferation of Achnanthidium minutissimum. Transplants were near indistinguishable from ambient assemblages by day 12. 3. There were no effects of enclosures on assemblage composition, but diatoms responded more rapidly to altered chemistry in enclosures with coarse mesh (26 × 50 mm) than finer mesh (320 μm). 4. Chemical diffusing substrates comprised terracotta tiles attached to dosing reservoirs that created locally acid (using H₂SO₄) or metal‐rich conditions (using MnSO₄) in the circumneutral stream over 26 days. Diatom responses were compared with reference substrates dosed with deionised or circumneutral stream water, and we also assessed whether effects were moderated by macroinvertebrate grazers. 5. Surface pH was lower by 1–2 pH units on acid‐dosed substrates than on reference tiles or in surrounding streamwater. Grazed assemblages on acid‐dosed substrates differed significantly from ungrazed reference assemblages, acquiring significantly greater relative abundance of Eunotia spp. However, the magnitude of response was less than in the between‐stream transplantations either because (i) metal exposure and base cation concentrations differed between the transplants and dosing substrates or (ii) diatom response to reduced pH on the diffusing substrates was restricted by the scarcity of acidobiontic diatoms in the circumneutral stream. Similar filter, founder or dominance effects might also affect diatom responses to real acid episodes. 6. These data show that diatom assemblages can respond rapidly and directly to changes in acid‐base status, but short‐term acidification might affect diatoms more rapidly than subsequent recovery. Because the experimental methods used were imperfect representations of episodic effects, diatom response to real acid events requires further field evaluation.     

A comparison of diatom community structure on natural and artificial substrata

Artificial substrata have been used in diatom studies for almost 100 years. However, concern still exists over whether diatom communities developing on artificial substrata accurately represent communities developing on natural substrata. This study compares the diatom communities colonising glass slides and clay tiles in two coastal dune lakes, and compares these communities to the naturally occurring communities in the epipelon, epilithon, and epiphyton. Both glass microslides and clay tiles, incubated for three separate periods ranging from 29 to 68 days, resulted in replicate substratum samples supporting similar diatom community compositions at each site. The degree of variation between artificial substrata communities at different sites, and between the two artificial substrata types, was generally no more than the degree of variation between communities on different types of natural substrata. Additionally, the composition of the diatom communities on the artificial substrata was representative of the community composition on the natural substrata. The effects of incubation period and siting are discussed.     

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

Small scale variability of benthic assemblages: biogenic neighborhood effects

In this study, patterns of community development were investigated within vs. outside 'habitats'. These habitats represented five different monospecific assemblages of one of the following species: the brown alga Fucus serratus, the red alga Delesseria sanguinea, the green alga Enteromorpha intestinalis, the seagrass Zostera marina and the blue mussel Mytilus edulis. Natural assemblages were allowed to develop on paired artificial substrata-separated by ca. 1 m-within (treatment) vs. outside (control) of habitats. The same colonizer species settled on treatment and control substrata for given habitats. However, after 5 months of settlement and post-settlement dynamics, their proportional abundance and the structure of treatment and control assemblages differed in many instances. Variability among replicates of a given treatment, seperated by up to 50 m, was large, indicating a patchy spatial distribution of organisms. Despite this spatial heterogeneity among within-treatment replicates, analysis of similarity revealed that in most instances significantly different assemblages developed between treatments on a small spatial scale depending on whether substrata were positioned within as compared to outside a given habitat.Consequently, the algae, seagrass or mussels constituting a habitat seem to control the structure of the benthic assemblage developing in their vicinity by one or more possible mechanisms: reduction of larval advection, exudation of metabolites that influence settlement and/or post-settlement survival, and/or-in the case of mussel assemblages-predation on larvae.In addition to spatial variability in larval supply, stochasticity in succession, substratum heterogeneity, competition and predation effects, this investigation reveals the potential of a further assemblage structuring factor: the impact of neighboring organisms.     

Optimal colonization and growth of marine benthic diatoms on artificial substrata: protocol for a routine use in bioindication

Benthic diatoms growing on hard substrata are used for their bioindication ability in freshwater quality monitoring. Artificial substrata are needed in cases where any natural substrate is present or to achieve similar sampling conditions between sites. Prior to use marine benthic diatoms for monitoring, a standardized protocol for sampling on artificial substrata must be set up. Two major types of information are required: (1) the time needed for a diatom community to be well developed and mature (climax stage); (2) the optimal growth conditions, given that the substrataum nature and texture are important parameters for the initial phase of biofilm development and can influence the future diatom assemblage. Three substrataum types were tested: frosted Plexiglass®, frosted glass, and rough enameled tiles. They were submerged for 8 weeks and sampled weekly. The experiment was conducted at five sites of distinct morphology and water chemistry, along the coastal area of Martinique Island, French West Indies. Development of diatom community was studied through biofilm dry weight, valve density, species richness, and species relative abundances. Globally, substratum type had no significant effect on any parameter. Frosted Plexiglass® was found to be the most interesting substratum because of higher valve densities and practical use. The asymptotic phase of biofilm development was encountered between 5 and 8 weeks depending on site and parameter. A compromise between community development and vandalism or loss through time was fixed to 5 weeks. This period is longer than for stream environments and is valid for tropical oligotrophic marine environments.     

EFFECTS OF LIGHT AND WAVE DISTURBANCE ON VERTICAL ZONATION OF ATTACHED MICROALGAGE IN A LARGE RESERVOIR1

Experimental field manipulations of artificial substrata were used to examine the mechanisms controlling attached algal zonation down the face of the dam in Lake McConaughy, a large reservoir in western Nabraska. Sets of clay tiles were incubated in the upper (2.5 m depth) and lower (8 m depth) growth zones for two weeks. Five sets tiles were then switched from the upper to the lower growth zone and vice versa. Five additional sets of tiles were switched to the lower zone and artificially disturbed. Diatom cell densities increased rapidly in both the zones; however, wind-induced turbulence caused dramatic declines (up to 61%) in densities in the upper zone. Consequently, cell densities in the upper and lower growth zones were not significantly different after four weeks, despite the 17–30% higher light levels in the upper zone. Based on cell densities and relative abundances on clay tiles and naturally occurring rocks, 26 of the 32 most common diatom taxa had a significant upper (10) or lower (16) zone preference. Of these, 15 taxa exhibited a consistent response to one or both switching manipulations, confirming a growth zone preference, and two showed a clear preference for disturbed substrata. Diatom growth form appeared to play a major role in determining the vertical zonation of attached communities, since actively motile taxa exhibited a lower zone preference and stalked forms occurred primarily in the upper zone. The present study indicates that light attenuation and wave disturbance are primary mechanisms that control the vertical zonation of freshwater epilithic algae.     

Biofilm diatom community structure: Influence of temporal and substratum variability

Abstract

Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.     

Regional distribution of diatom assemblages in the headwater streams of Luxembourg

The aim of this study was to determine the influence of environmental variables on the structure of benthic diatom assemblages, and to propose type assemblages of diatoms characterizing unpolluted headwater streams of Luxembourg. A total of 289 diatom samples were collected in the headwater streams of Luxembourg. At each sampling site, physical and chemical variables were also measured. The relationships between environmental variables and the distribution of the taxa were estimated using univariate and multivariate statistical analyses. On the basis of geology and stream water chemistry, Luxembourg could be separated in two regions. The streams in the schistose northern part of Luxembourg presented a low carbonate hardness (median 3.5° F); the streams in the southern part of Luxembourg presented a high carbonate hardness (median 21.8° F) notably connected to the presence of sandstone and limestone substrata. A Twinspan classification carried out on diatom assemblages defined two groups of samples corresponding to these two regions. The carbonate hardness that is related to the nature of the geological substratum appeared to be the major structuring variable for the assemblage composition. Anthropogenic pollution was a secondary structuring variable for diatom assemblages since each group could be subdivided in subgroups presenting statistically different nutrient and organic matter concentrations. The diatom assemblages of these subgroups were characterised by differences of saprobic and trophic preferences. On the basis of these results, two type assemblages of diatoms are proposed for the unpolluted headwater streams in the two regions of Luxembourg. The presence of these different type assemblages in Luxembourg shows the necessity to adapt diatom bioindication to the different regions of the country.     

Comparative algological and bacteriological examinations on biofilms developed on different substrata in a shallow soda lake

According to the European Water Framework Directives, benthic diatoms of lakes are a tool for ecological status assessment. In this study, we followed an integrative sample analysis approach, in order to find an appropriate substratum for the water qualification-oriented biomonitoring of a shallow soda lake, Lake Velencei. Six types of substrata (five artificial and one natural), i.e., andesite, granite, polycarbonate, old reed stems, Plexiglass discs and green reed, were sampled in May and in November. We analysed total alga and diatom composition, chlorophyll a content of the periphyton, surface tension and roughness of the substrata and carbon source utilisation of microbial communities. Water quality index was calculated based on diatom composition. Moreover, using a novel statistical tool, a self-organising map, we related algal composition to substratum types. Biofilms on plastic substrates deviated to a great extent from the stone and reed substrata, with regard to the parameters measured, whereas the biofilms developing on reed and stone substrata were quite similar. We conclude that for water quality monitoring purposes, sampling from green reed during springtime is not recommended, since this is the colonization time of periphyton on the newly growing reed, but it may be appropriate from the second half of the vegetation period. Stone and artificially placed old reed substrata may be appropriate for biomonitoring of shallow soda lakes in both spring and autumn since they showed in both seasons similar results regarding all measured features.     

DIATOM COLONIZATION ON ARTIFICIAL SUBSTRATA IN POOL AND RIFFLE ZONES STUDIED BY LIGHT AND SCANNING ELECTRON MICROSCOPY1

Light and scanning electron microscopy were utilized to quality diatom colonization in Oak Creek, Arizona. Aluminum SEM stubs with and without plexiglass discs were anchored into rocks. Early colonization on five stub microzones was examined at hourly intervals; weekly intervals of up to 3 wk were employed to record community development in pool find riffle. Plexiglass was more suitable for microbial colonization than aluminum. Organic matter and bacteria were important surface pre-conditioning agents while fungi were instrumental in trap/ting cells during early stages of colonization in the riffle. Diatom colonization was initialed within 1 h on the upstream side of substrata in riffles, while the tap face was colonized first in pools. Colonization moved rapidly to the perimeter in each system. Early colonization of-side microzones was considerably more asymmetric in the riffle than, pool. At Idler stages (2 wk) diatoms with their associated mucilage and algal filaments contributed to the stability of the microbial communities. Horizontally positioned species (Achnanthes, Cocconeis) were early colonizers in both systems while vertically positioned species (Gomphonema, Nitzschia) were more important in later successional stages (3 wk) in the riffle. Horizontally positioned species remained dominant throughout the 3 wk period in the pool. After 3 wk, diversity was normally greater in the pool while density was higher in the riffle. Detrital microcosms containing viable microbiol assemblages frequently collected on tin-upstream face of substrata in the riffle. The random nature by which these detrital microcoms contact downstream substrata greatly contribute to the spatial variation of periphyton in streams. These detrital microcosms expedite repeated colonization in lotic systems.     

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.     

Substratum‐dependent responses of ciliate assemblages to temperature: a natural experiment in Icelandic streams

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