The influence of substrates and physicochemical factors on the composition of diatom assemblages in karst springs and their applicability in water-quality assessment

The methodological standards of monitoring programs recommend collecting material from stones, but this community is often absent in slow-flowing waters, whereas a great number of localities do offer bryophytic material for sampling. In order to investigate whether the kind of substrate (epilithic or bryophytic) influences the diatom-based biomonitoring results the structure of diatom assemblages from 13 anthropogenically altered springs were investigated. To examine differences between diatom assemblages, the species richness of diatom assemblages, H′ (Shannon-Wiener diversity index), Trophic Diatom Index (TDI) and Specific Pollution sensitivity Index (SPI) were compared. The TDI and SPI differed significantly (P = 0.002 and P = 0.004) between springs located in villages and springs away from villages, and did not differ significantly between epilithic and bryophytic samples (P > 0.05). No significant differences in species richness or the diversity index (H′) (P > 0.05) were found between the epilithic and bryophytic diatom assemblages. This study suggests that both the kinds of substrates can be successfully used for diatom-based water-quality biomonitoring in mesotrophic and eutrophic environments.     

Effects of temperature and light on cyst germination and germinated cell survival of the noxious raphidophyte Heterosigma akashiwo

The effects of temperature and light on the germination of Heterosigma akashiwo cysts were examined using bottom sediments collected from Hakata Bay, Japan. In a suspension of mixed sediment and seawater in the temperature range of 5–30 °C, motile cells emerged within 3 weeks, but at ≤12 °C the cell numbers were markedly lower and the emergence of motile cells delayed. When suspension samples incubated at various temperatures were moved to 20 °C and incubated, only a few additional motile cells emerged. The number of motile cells germinated in the dark was significantly lower than under light conditions. When suspension samples incubated in the dark were exposed to light, only a few additional motile cells emerged. These results indicate that the initiation of germination in Heterosigma cysts suspended in seawater is not dependent on temperature and light conditions, although the speed of the germination process is affected by temperature, and cell survival just after germination is strongly affected by temperature and light.     

Using diatoms to assess the biological condition of large rivers in Idaho (U.S.A.)

SUMMARY 1. Diatoms were collected from 49 sites located on 23 rivers and identified to species. We defined 26 attributes of the diatom assemblage on the basis of tolerance and intolerance, autecological guild, community structure, morphological guild and individual condition. 2. We grouped sites into three geographical regions on the basis of altitude, land cover and use. Within each region, we tested for an association between diatom attributes and human disturbance. 3. We selected nine attributes for inclusion in a multimetric index, the river diatom index (RDI): percentage of valves belonging to species sensitive to disturbance, percentage of valves belonging to species very tolerant of disturbance, eutrophic species richness, percentage of valves belonging to nitrogen heterotrophic species, percentage of valves belonging to polysaprobic species, alkaliphilic species richness, percentage of valves belonging to species that require high oxygen, percentage of valves belonging to very motile genera and percentage of deformed valves. 4. The RDI was significantly correlated with measures of human disturbance made at the site (conductivity, percentage of fine sediments and number of human activities) and at the catchment level (percentage of urbanisation and agriculture in the upstream catchment). 5. The percentage of the total variance of RDI related to transect location was very small (1%) compared with among‐site differences (73%) and time of sampling (26%). The RDI could reliably detect approximately three categories of biological condition based on annual monitoring and potentially more if sampling were restricted to the same month each year.     

Algal assemblages and their relationship with water quality in tropical Mexican streams with different land uses

This study analyzes the relationship between physical and chemical factors and the algal communities in tropical streams in micro-watersheds where >70% of their area has different land uses, specifically, cloud mountain forest, coffee plantations, and livestock pastures. Physical, chemical, and biological variables were measured monthly in each stream over a 1-year period. The concentrations of nitrates + nitrites, total suspended solids (TSS), and silica in the streams were found to differ during the dry and rainy seasons. Coffee-plantation streams showed the highest levels of suspended solids, nitrates + nitrites, and sulfates. Based on chlorophyll a concentration, the forest and coffee-plantation streams are oligo-mesotrophic, while pasture streams are meso-eutrophic. Forest streams displayed the lowest levels of richness and algal diversity, followed by coffee-plantation streams, whereas pasture streams were the most diverse. Chlorophyll a concentration and species richness depended on land use and season. Forest coverage was positively correlated with acidophilous and oligo-eutraphentic diatom species. Coffee coverage displayed a significant positive correlation with motile species and a significant negative correlation with pollution-sensitive diatom taxa. The results show that diatom assemblages responded to micro-watershed conditions and can be used to monitor the effects of land use on streams in tropical regions.     

Epiphytic diatom communities on intertidal seaweeds from Iceland

Epiphytic diatom communities on macroalgae from Iceland coastal waters were investigated during July 2005. Ten species of seaweeds have been collected belonging to brown, red and green algae. The analysis of epiphytic diatom community was carried out under scanning electron microscopy. The epiphytic diatom abundances varied from 7 ± 5 to 7524 ± 3491 cells mm⁻². Erect growth forms were the most abundant, representing on average 50% of the total diatoms (Achnanthes cf. brevipes var. parvula, Tabularia investiens, T. fasciculata, Hyalosira cf. delicatula, Gomphoseptatum aestuarii, Pseudogomphonema plinskii), followed by adnate (29%) (Cocconeis stauroneiformis, C. scutellum) and motile forms (21%) (Nitzschia cf. amphibia and Navicula perminuta). Highly branched seaweeds with articulated thallus surface, offering a number of microenvironments to be occupied by the epiphytes, showed a high level of colonization, mainly due to erect and motile diatoms. Flat thalli with smooth surface allowed for the growth of mainly erect diatoms.     

MORPHOLOGY OF THE DIATOM GENUS CAMPYLONEIS (BACILLARIOPHYCEAE,BACILLARIOPHYTA), WITH A DESCRIPTION OF CAMPYLONEIS JULIAE SP. NOV. AND AN EVALUATION OF THE FUNCTION OF THE VALVOCOPULAE1

A revision of the monoraphid pennate diatom genus Campyloneis Grunow was carried out based on LM and EM observations. The material examined originated from various herbarium collections and from extant epiphytic diatom communities on leaves of Posidonia spp. We also examined the generitype C. grevillei (Smith) Grunow and the fossil material of C. gheyselinchi Reinhold from which the author extracted the type. Our results clarified the fine structure of C. grevillei and C. gheyselinchi. Of the various varieties of C. grevillei, only the variety argus (Grunow) Cleve was retained. This differs from the nominate variety in the arrangement and shape of the areolae adjacent to the sternum of the araphid valve. The newly described taxon Campyloneis juliae De Stefano differs from all Campyloneis species in areolae ultrastructure and morphology of the valvocopulae. As for the fossil species C. gheyselinchi, the sternum valve areolae are similar to those of C. grevillei, but scarcity of frustules in the type material prohibited evaluation of its variability. For this reason we provisionally maintained its rank of species. The elaborate linking systems among the valvocopulae and valves in Campyloneis species appear to provide structural reinforcement against pressure from neighboring epiphytic diatoms and scouring of seagrass leaves.     

Benthic diatoms as indicators of eutrophication in tropical streams

Diatoms are frequently used as indicators of eutrophication in temperate systems, but little is known about their application to impacted African tropical systems. Five streams located within Gombe Stream National Park and five streams supporting human settlements draining into Lake Tanganyika, East Africa, were investigated for species composition, richness and diversity of epilithic algae. In addition, a trophic diatom index (TDI) developed for monitoring European rivers was applied to these tropical systems. 54 specific and infraspecific diatom taxa representing 20 genera were identified for all sites with Achnanthes s.l., Gomphonema and Navicula s.l. being the most common genera. Species richness varied between 10 and 21 in disturbed streams and 13 and 19 in undisturbed streams. Nutrients were significantly enriched in streams draining the deforested watersheds but indices of diversity and evenness (Shannon H, J and Simpson–Yule D, E) did not show any significant differences between streams in forested and deforested watersheds. Significant differences were observed between pooled data for the TDI between forested and deforested watersheds. Analysis of percent pollution tolerant diatom taxa indicates that organic pollution of streams in deforested watersheds may be contributing to eutrophication. This study shows that African diatoms, cosmopolitan or resembling well-known North American and European taxa, allows for trophic indices tailored to the autecological preferences of species to be applied to new regions, although intensive studies on these African taxa will lead to more accurate results. Measures of species-richness and diversity, historically used to describe the state of an ecosystem, may not be suitable to evaluate streams which are not grossly polluted. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

The genus Pseudo-nitzschia (Bacillariophyceae) in continental shelf waters of Argentina (Southwestern Atlantic Ocean, 38–55°S)

The distribution pattern of Pseudo-nitzschia species, associated phytoplankton flora and its relationships with main environmental factors were studied for the first time in continental shelf surface waters of the Argentine Sea (Southwestern Atlantic Ocean, 38–55°S). Both qualitative and quantitative samples, collected during summer and fall 2003, were examined using light and scanning electron microscopy. Results indicated that the genus Pseudo-nitzschia has a wide distribution along the studied area. It was present at low densities, with infrequent peak abundances and appeared most frequently as a minor component of the diatom populations that typically develop on the continental shelf of the Argentine Sea. Moreover, phytoplankton communities were numerically dominated by unidentified phytoflagellates (≤5 μm) throughout almost all samples analyzed. Eight Pseudo-nitzschia species were identified in our study: P. australis, P. fraudulenta, P. heimii, P. lineola, P. pungens, P. cf. subcurvata, P. turgidula and P. turgiduloides. Of these, P. heimii, P. lineola and P. turgiduloides are new records for the Argentine Sea. Their presence in the area is attributable to the influence of southerly cold water masses. Spatial and temporal variations of the environmental parameters recorded in the study area generally determined the distribution of Pseudo-nitzschia species. P. pungens and P. australis were widely distributed and reached high densities, especially in waters with elevated temperatures and salinities (around 15 °C, 33.8 psu) and low nutrients concentrations. On the other hand, P. heimii, P. lineola, P. turgidula and P. turgiduloides showed a more restricted distribution, with lower densities in relatively cold, less saline (8 °C, 32.45 psu) and nutrient-rich waters. From the Pseudo-nitzschia species found throughout this survey, P. australis, P. fraudulenta, P. pungens and P. turgidula are known as domoic acid (DA) producers around the world, but there is little information on the potential toxicity of these species in Argentina.     

Effects of dilution and centrifugation on the survival of spermatozoa and the structure of motile sperm cell subpopulations in refrigerated Catalonian donkey semen

The aim of this work was to study the effects of dilution and centrifugation (i.e., two methods of reducing the influence of the seminal plasma) on the survival of spermatozoa and the structure of motile sperm cell subpopulations in refrigerated Catalonian donkey (Equus asinus) semen. Fifty ejaculates from nine Catalonian jackasses were collected. Gel-free semen was diluted 1:1, 1:5 or 1:10 with Kenney extender. Another sample of semen was diluted 1:5, centrifuged, and then resuspended with Kenney extender until a final dilution of 25 × 10⁶ sperm/ml was achieved (C). After 24 h, 48 h or 72 h of refrigerated storage at 5 °C, aliquots of these semen samples were incubated at 37 °C for 5 min. The percentage of viable sperm was determined by staining with eosin-nigrosin. The motility characteristics of the spermatozoa were examined using the CASA system (Microptic, Barcelona, Spain). At 24 h, more surviving spermatozoa were seen in the more diluted and in the centrifuged semen samples (1:1 48.71%; 1:5 56.58%, 1:10 62.65%; C 72.40%). These differences were maintained at 48 h (1:1 34.31%, 1:5 40.56%, 1:10 48.52%, C 66.30%). After 72 h, only the C samples showed a survival rate of above 25%. The four known donkey motile sperm subpopulations were maintained by refrigeration. However, the percentage of motile sperms in each subpopulation changed with dilution. Only the centrifuged samples, and only at 24 h, showed exactly the same motile sperm subpopulation proportions as recorded for fresh sperm. However, the 1:10 dilutions at 24 and 48 h, and the centrifuged semen at 48 h, showed few variations compared to fresh sperm. These results show that the elimination of seminal plasma increases the survival of spermatozoa and the maintenance of motility patterns.The initial sperm concentration had a significant (P < 0.05) influence on centrifugation efficacy, but did not influence the number of spermatozoa damaged by centrifugation. In contrast, the percentage of live spermatozoa in the fresh semen significantly influenced the number of spermatozoa damaged by centrifugation, but not centrifugation efficacy.     

Microalgal–meiofaunal interactions in a sublittoral site of the Gulf of Trieste (northern Adriatic Sea, Italy): A three-year study

Monthly sediment sampling was carried out from February 1999 to December 2001 at a 21-m deep station. Benthic diatom biomass and meiobenthic abundance were estimated together with abiotic parameters (PAR irradiance, temperature, salinity and dissolved oxygen). During the three-year study average microphytobenthic abundance (ABU) and biomass (BIOM) were 4.7 ± 2.6 × 10⁴ cell cm^− 3 and 14.3 ± 8.1 µg C cm^− 3, respectively, while the mean meiofaunal abundance was 923 ± 210 ind 10 cm^− 2. The microphytobenthic community was mainly composed of Bacillariophyceae (99.3%) with a smaller percentage of phytoflagellates (0.7%). Among diatoms 39 genera were observed with a total of 110 taxa and 100 species. The main benthic diatom genera were Navicula, Paralia, Nitzschia and Diploneis. A total of 18 meiobenthic taxa were detected: 7 taxa belonged to permanent meiofauna (Nematoda, Copepoda Harpacticoida and their nauplius stages, Kinorhyncha, Turbellaria, Gastrotricha, Ostracoda and Acarina). The remaining 11 taxa belonged to temporary meiofauna (juvenile macrofauna: Polychaeta, Cumacea, Amphipoda, Isopoda, Nemertea, Decapoda, Sipunculida, Ophiurida, Gastropoda, Bivalvia and Cnidaria). Positive correlations between PAR and the biomass of the major diatom genera were revealed. Temperature did not seem to influence the microphytobenthic community as a whole, although we found statistically significant correlation between temperature and the genera Nitzschia and Amphora. Salinity showed significant positive correlation only with the genus Pleurosigma, in contrast, dissolved oxygen did not seem to affect the microalgal community. The major meiofaunal taxa were not correlated with abiotic variables, but were positively correlated with most diatom genera. Paralia was negatively correlated with the three main meiofaunal taxa. The three-point moving average applied to our biotic data allowed us to better the signal without the background noise which hid the actual variations of the studied communities. Applying the three-point moving average to the main taxa, it resulted that microphyto- and meiobenthos were in phase. The principal component analysis (PCA), constructed considering both biotic and abiotic variables, accounted for 58% of the total variance. PC1 axis explained 39.74% of the total variance and was correlated with Navicula, Diploneis and Nitzschia. PC2 axis explained 18.40% of the remaining variance and was correlated with Copepoda, Paralia and PAR. In contrast to 1999 and 2001, a mucilage event occurred in 2000, which resulted in the formation of a false bottom and a decrease in the abundance of microphyto- and meiobenthos.     

Metagenomic analysis of historical herbarium specimens reveals a postmortem microbial community

Advances in DNA extraction and next‐generation sequencing have made a vast number of historical herbarium specimens available for genomic investigation. These specimens contain not only genomic information from the individual plants themselves, but also from associated microorganisms such as bacteria and fungi. These microorganisms may have colonized the living plant (e.g., pathogens or host‐associated commensal taxa) or may result from postmortem colonization that may include decomposition processes or contamination during sample handling. Here we characterize the metagenomic profile from shotgun sequencing data from herbarium specimens of two widespread plant species (Ambrosia artemisiifolia and Arabidopsis thaliana) collected up to 180 years ago. We used blast searching in combination with megan and were able to infer the metagenomic community even from the oldest herbarium sample. Through comparison with contemporary plant collections, we identify three microbial species that are nearly exclusive to herbarium specimens, including the fungus Alternaria alternata, which can comprise up to 7% of the total sequencing reads. This species probably colonizes the herbarium specimens during preparation for mounting or during storage. By removing the probable contaminating taxa, we observe a temporal shift in the metagenomic composition of the invasive weed Am. artemisiifolia. Our findings demonstrate that it is generally possible to use herbarium specimens for metagenomic analyses, but that the results should be treated with caution, as some of the identified species may be herbarium contaminants rather than representing the natural metagenomic community of the host plant.     

Surface-associated diatoms from marine habitats at Cape Evans,Antarctica, including the first record of living <Emphasis Type="Italic">Eunotogramma marginopunctatum</Emphasis>

The diversity of diatom communities found epiphytically on red macroalgae collected under the sea ice at Cape Evans, Antarctica, was studied microscopically. Comparison of communities along a depth profile showed that species diversity decreased as depth below sea ice increased. Dominant taxa also changed with depth, with Cocconeis fasciolata dominant at 10 and 15 m, Porosira glacialis at 20 m and Eunotogramma marginopunctatum at 25 m. Epiphytic communities were also compared to sympagic and sediment-associated communities collected from a single depth. Species richness was greatest for communities associated with surface sediments while the poorest was found in sympagic, or sea-ice, communities. E. marginopunctatum, previously only described from fossil material, was found associated epiphytically on macroalgae from 20 to 25 m below sea ice.     

Structure and diversity of intertidal benthic diatom assemblages in contrasting shores: a case study from the Tagus estuary1

The structure of intertidal benthic diatoms assemblages in the Tagus estuary was investigated during a 2‐year survey, carried out in six stations with different sediment texture. Nonparametric multivariate analyses were used to characterize spatial and temporal patterns of the assemblages and to link them to the measured environmental variables. In addition, diversity and other features related to community physiognomy, such as size‐class or life‐form distributions, were used to describe the diatom assemblages. A total of 183 diatom taxa were identified during cell counts and their biovolume was determined. Differences between stations (analysis of similarity (ANOSIM), R = 0.932) were more evident than temporal patterns (R = 0.308) and mud content alone was the environmental variable most correlated to the biotic data (BEST, ρ = 0.863). Mudflat stations were typically colonized by low diversity diatom assemblages (H′ ~ 1.9), mainly composed of medium‐sized motile epipelic species (250–1,000 μm³), that showed species‐specific seasonal blooms (e.g., Navicula gregaria Donkin). Sandy stations had more complex and diverse diatom assemblages (H′ ~ 3.2). They were mostly composed by a large set of minute epipsammic species (<250 μm³) that, generally, did not show temporal patterns. The structure of intertidal diatom assemblages was largely defined by the interplay between epipelon and epipsammon, and its diversity was explained within the framework of the Intermediate Disturbance Hypothesis. However, the spatial distribution of epipelic and epipsammic life‐forms showed that the definition of both functional groups should not be over‐simplified.     

Using diatoms to assess human impacts on streams benefits from multiple-habitat sampling

Benthic diatoms are important indicators of human impacts on streams. Epilithic diatoms are collected most often for bioassessments, but potentially important ecological information from other habitats could be missed. Within our study region, substrata ranged from 100% rock to 100% sandy silt at 61 sites, leading us to question the appropriateness of sampling solely epilithic surfaces in riffles. We compared two protocols for collecting diatoms: (1) epilithic habitat samples (EHS) within riffles and (2) multi-habitat samples (MHS), which proportionately included different habitats (e.g., riffles and pools) and substrata (e.g., sand, silt, and rock). Three streams were not compared because rocks were absent. Nonmetric multidimensional scaling indicated that diatom communities from EHS and MHS responded similarly to alkalinity and agricultural gradients, and Procrustes analysis showed that ordinations were similar (P < 0.01). Percent motile diatoms and metrics indicating high or low P and N from MHS showed stronger relationships with land use variables than those metrics from EHS. Bray-Curtis (BC) similarity between the protocols increased as diversity and richness decreased in MHS, as agricultural impacts increased, and as motile diatoms became more abundant in EHS, which likely indicated greater habitat homogeneity or an overriding effect of water chemistry. The two protocols classified 78% of sites concordantly as minimally impacted or impacted. Multi-habitat sampling is preferred where stream habitats vary greatly because it assessed land use impacts more effectively than EHS, while indicating water chemistry impacts as effectively. Multiple habitats also contained greater species diversity, which is important to documenting species distributions, and were always available for sampling, whereas epilithic habitats could be sparse or absent.     

Diatoms from Indonesian mangroves and their suitability as sea-level indicators for tropical environments

We collected modern diatom samples from two mangrove environments of Sulawesi, Indonesia to provide a much needed dataset for the reconstruction of sea level from tropical environments. The diatom assemblages are dominated by mesohalobous species (e.g. Amphora coffeaeformis, Amphora turgida, Achnanthes delicatula, Nitzschia sigma and Tryblionella balatonis) and oligohalobous (e.g. Amphora veneta, Diploneis ovalis and Progonoia didiomatia) taxa. Both study sites show strong vertical zonations, which suggests that duration and frequency of intertidal exposure are important factors in controlling the relative abundance of diatoms. The assemblages can be generally divided into a mixed assemblage of mesohalobous, oligohalobous–halophilous and oligohalobous-indifferent diatoms that are found from the dense mangrove vegetation towards the landward edge of the transects, and mesohalobous diatom assemblages that are located within the fringing Rhizophora and tidal flat environments. We subsequently developed a diatom-based transfer function, which is a quantitative approach to sea-level reconstruction. The relationship between observed and diatom-predicted elevations suggests accurate and precise reconstructions are possible. The error estimate (± 0.15 m) is comparable to diatom-based transfer functions from temperate marshes.     

Short-term fluctuations in benthic diatom numbers on an intertidal sandflat in the Westerschelde estuary (Zeeland,The Netherlands)

During the period March–May 1991, sediment samples were taken every two or three days at one intertidal station in the brackish part of the Westerschelde estuary. Quantitative cell counts were made in order to investigate the short-term temporal changes in diatom numbers and assemblage structure.Throughout the whole sampling period, the diatom assemblage was dominated by epipsammic diatoms. Three species, Achnanthes delicatula, Opephora cf. perminuta and Catenula adhaerens on average accounted for almost 67% of all valves counted. The epipsammic diatom fraction showed no significant changes in absolute numbers; its species composition appeared relatively stable. In contrast, epipelic diatom densities significantly increased towards the end of the study period. Species composition within this fraction was less stable. Multivariate analysis (Principal Components Analysis), in combination with multiple regression, indicated that total sky irradiance (on the second and third day preceding sampling) and percentage organic matter were related to the short-term fluctuations of the epipelic diatom fraction.     

A synoptic revision of Dacryodes (Burseraceae) in Africa, with a new species from Central Africa

Summary  A synoptic revision is presented for the genus Dacryodes Vahl in Africa. The studies are based on examination of herbarium material. Eighteen species are recognised, including two not well known due to poor material. The new species Dacryodes villiersiana Onana is described and illustrated. The conservation status of the species is discussed following the categories and criteria of IUCN (2001).     

Historic DNA reveals genetic consequences of fragmentation in an endangered,endemic mustard

Understanding how anthropogenic disturbance affects genetic diversity is essential to appropriately incorporating genetic considerations into conservation plans. Unfortunately, we rarely have information about a population’s genetic diversity before it becomes imperiled. Here we reconstruct the historic range of the naturally rare annual mustard Streptanthus glandulosus subsp. niger (Sgn) and use herbarium specimens to quantify pre-disturbance genetic diversity. We compare this to the genetic diversity in the contemporary plant populations and to plants in the seed bank. We conclude that Sgn was recently a single, panmictic population composed of orders of magnitude more plants than exist today but experienced recent and abrupt declines following housing development. Today Sgn persists as two disjunct populations, the larger of which has retained historic levels of diversity although there is a downward trend in all measures. The smaller population has lost 21–28% of the diversity that was present only 50 years ago with an N_e?~?5–16. The contemporary populations have differentiated from each other due to drift. The seed bank contained no novel alleles and had high levels of homozygosity, indicating that it is incapable of providing genetic rescue. This novel combination of hDNA, the aboveground plant population and the seed bank can be used to design high impact conservation plans that appropriately incorporate genetic diversity for this and other imperiled species.

     

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g⁻¹ vs. 107 μg g⁻¹, respectively) and soil TP (284 μg g⁻¹ vs. 145 μg g⁻¹). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m⁻²). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly promote the alternate community. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Degrees of polarization of reflected light eliciting polarotaxis in dragonflies (Odonata), mayflies (Ephemeroptera) and tabanid flies (Tabanidae)

With few exceptions insects whose larvae develop in freshwater possess positive polarotaxis, i.e., are attracted to sources of horizontally polarized light, because they detect water by means of the horizontal polarization of light reflected from the water surface. These insects can be deceived by artificial surfaces (e.g. oil lakes, asphalt roads, black plastic sheets, dark-coloured cars, black gravestones, dark glass surfaces, solar panels) reflecting highly and horizontally polarized light. Apart from the surface characteristics, the extent of such a ‘polarized light pollution’ depends on the illumination conditions, direction of view, and the threshold p* of polarization sensitivity of a given aquatic insect species. p* means the minimum degree of linear polarization p of reflected light that can elicit positive polarotaxis from a given insect species. Earlier there were no quantitative data on p* in aquatic insects. The aim of this work is to provide such data. Using imaging polarimetry in the red, green and blue parts of the spectrum, in multiple-choice field experiments we measured the threshold p* of ventral polarization sensitivity in mayflies, dragonflies and tabanid flies, the positive polarotaxis of which has been shown earlier. In the blue (450 nm) spectral range, for example, we obtained the following thresholds: dragonflies: Enallagma cyathigerum (0% < p* ≤ 17%), Ischnura elegans (17% ≤ p* ≤ 24%). Mayflies: Baetis rhodani (32% ≤ p* ≤ 55%), Ephemera danica, Epeorus silvicola, Rhithrogena semicolorata (55% ≤ p* ≤ 92%). Tabanids: Tabanus bovinus, Tabanus tergestinus (32% ≤ p* ≤ 55%), Tabanus maculicornis (55% ≤ p* ≤ 92%).