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Diatoms constitute a diverse lineage of unicellular organisms abundant and ecologically important in aquatic ecosystems. Compared to other protists, their biology and taxonomy are well-studied, offering the opportunity to combine traditional approaches and new technologies. We examined a dataset of diatom 18S rRNA- and rDNA- (V4 region) reads from different plankton size-fractions and sediments from six European coastal marine sites, with the aim of identifying peculiarities and commonalities with respect to the whole protistan community. Almost all metabarcodes (99.6%) were assigned to known genera (121) and species (236), the most abundant of which were those already known from classic studies and coincided with those seen in light microscopy. rDNA and rRNA showed comparable patterns for the dominant taxa, but rRNA revealed a much higher diversity particularly in the sediment communities. Peculiar to diatoms is a tight bentho-pelagic coupling, with many benthic or planktonic species colonizing both water column and sediments and the dominance of planktonic species in both habitats. Overall metabarcoding results reflected the marked specificity of diatoms compared to other protistan groups in terms of morphological and ecological characteristics, at the same time confirming their great potential in the description of protist communities.

The lakes around Yellowknife (Northwest Territories, Canada) have been impacted by multiple environmental stressors throughout the 20th and early 21st centuries. Here, we have synthesized diatom assemblage data from ten lake sediment cores from the Yellowknife area and used a landscape-scale paleolimnological approach to investigate the cumulative impacts of past gold mining activities, urbanization, and climate warming on aquatic biota. Our investigations indicated that diatom species turnover (measured using detrended canonical correspondence analysis) was highest at lakes closer to the city and mines, as these sites were more severely impacted by land-use changes (e.g., sewage disposal, run-off from waste disposal sites) and roaster stack emission from the gold mines. Diatom assemblage shifts indicative of climate-induced changes to lake thermal properties were also observed across the gradient of human activities. The inclusion of remote sites was useful to disentangle the effects of climate-mediated changes from impacts related to mining and urbanization. This investigation suggests that the diatom assemblages of the lakes around Yellowknife have changed markedly over the last ∼80 years and there are no signs of biological recovery since the cessation of mining activities around the turn of the 21st century. The biota of the subarctic lakes around Yellowknife are now strongly influenced by climate-mediated changes to lake thermal properties and the urban lakes are also influenced by the legacies of past land-use changes.

Over the last decades, river lentification processes and droughts have been dramatically spreading worldwide, due to global and local drastic changes due to human activities. Under this scenario, the evaluation of physical disturbance caused by intermittency and droughts has become more and more relevant. In this research, we compare samples collected in Mediterranean streams following both traditional and experimental approaches with the aim of understanding if diatom indices calculated from a new sampling strategy could provide additional information for the physical disturbance assessment. Moreover, we also evaluated the response of functional metrics. Our results demonstrated that even though an enhanced sampling method better reflects hydrological disturbance than the traditional one, diatom indices do not detect it. Conversely, functional traits proved to be important metrics for the hydrological disturbance assessment. In particular, benthic diatom chlorophyll a showed significantly lower values in sections more subject to droughts. With respect to ecological guilds, the motile taxa proved to be linked to depositional areas, which resulted important microhabitats (MHs) to be explored in rivers affected by lentification. Including different MHs in the sampling process improves the information we can obtain from the analysis of the diatom community and presents important implications in documenting species distribution and autecology. Au cours des dernières décennies, les processus de ralentissement des rivières et les sécheresses ont été considérablement répandus dans le monde entier, en raison de changements drastiques globaux et locaux dûs aux activités humaines. Selon ce scénario, l'évaluation des perturbations physiques causées par intermittence et les sécheresses est devenue de plus en plus pertinente. Dans cette recherche, nous comparons les échantillons prélevés dans des rivières méditerranéennes suivant des approches traditionnelles et expérimentales dans le but de comprendre si les indices diatomées calculés à partir d'une nouvelle stratégie d'échantillonnage pourraient fournir des informations supplémentaires pour l'évaluation des perturbations physiques. De plus, nous avons également évalué la réponse des paramètres fonctionnels. Nos résultats ont montré que, même si une méthode d'échantillonnage améliorée reflète mieux les perturbations hydrologiques que la méthode traditionnelle, les indices diatomées ne les détectent pas. À l'inverse, les traits fonctionnels se sont avérés être des paramètres importants pour l'évaluation des perturbations hydrologiques. En particulier, la chlorophylle a des diatomées benthiques a montré des valeurs significativement plus faibles dans les sections plus exposées à la sécheresse. En ce qui concerne les guildes écologiques, les taxons motiles se sont avérés être liés à des zones de dépôt, qui sont des microhabitats importants à explorer dans les rivières touchées par le ralentissement. Inclure différents microhabitats dans le processus d'échantillonnage améliore les informations que nous pouvons obtenir de l'analyse de la communauté de diatomées et présente des implications importantes dans la connaissance de la répartition et de l’autécologie des espèces.

The main objective of the present study is to describe the structure of benthic diatom communities in 23 water reservoirs in Slovakia classified as heavily modified water bodies. Environmental variables together with biological data obtained during the routine biomonitoring of water reservoirs in Slovakia were explored and analysed to understand variability of benthic diatom communities and their relationships with environmental variables in order to obtain an integrated knowledge about their relevance as bioindicators for the Water Framework Directive-compliant ecological potential assessment. This study summarizes results from a four-year monitoring programme of water reservoirs surveyed during the period of 2011–2014. The performed survey and statistical analyses revealed the following: (i) two main groups of reservoirs could be distinguished based on the purpose of their main use (multipurpose or drinking water-supply use); (ii) multipurpose and drinking water-supply reservoirs differed in benthic diatom community structure, diatom water quality indices as well as in the principal environmental gradients structuring the diatom communities; (iii) 5 distinct sub-groups of reservoirs could be identified differing in terms of diatom species composition and several environmental parameters; (iv) the most significant environmental variables in explaining differences in diatom species composition in multipurpose reservoirs were mean depth and mean annual flow; in drinking water-supply reservoirs conductivity and water transparency. L’objectif principal de la présente étude est de décrire la structure des communautés de diatomées benthiques dans 23 réservoirs en Slovaquie classés comme masses d’eau fortement modifiées. Les variables environnementales ainsi que des données biologiques obtenues au cours de la biosurveillance de routine des réservoirs en Slovaquie ont été explorées et analysées afin de comprendre la variabilité des communautés de diatomées benthiques et leurs relations avec les variables environnementales et d’obtenir une connaissance intégrée sur leur pertinence comme bioindicateurs pour la directive cadre sur l’eau – conforme à l’évaluation du potentiel écologique. Cette étude résume les résultats d’un programme de suivi de quatre ans des réservoirs au cours de la période 2011–2014. Le suivi effectué et les analyses statistiques ont révélé : (i) deux principaux groupes de réservoirs peuvent être distingués en fonction du but de leur utilisation principale (polyvalente ou d’approvisionnement en eau potable); (ii) les réservoirs à usages multiples et d’alimentation en eau potable différaient dans la structure des communautés de diatomées benthiques, aussi bien les indices diatomée de qualité de l’eau, que les principaux gradients environnementaux qui structurent les communautés de diatomées; (iii) 5 sous-groupes distincts de réservoirs pourraient être identifiés différant en termes de composition des espèces de diatomées et de plusieurs paramètres environnementaux; (iv) les variables environnementales les plus importantes dans l’explication des différences dans la composition des espèces de diatomées dans les réservoirs polyvalents étaient la profondeur moyenne et débit moyen annuel; dans les réservoirs d’alimentation en eau potable la conductivité et la transparence de l’eau.

This study aims to contribute to the understanding of the impact of Didymosphenia geminata massive growths upon river ecosystem communities' composition and functioning. This is the first study to jointly consider the taxonomic composition and functional structure of diatom and macroinvertebrate assemblages in order to determine changes in community structure, and the food web alterations associated with this invasive alga. This study was carried out in the Lumbreras River (Ebro Basin, La Rioja, Northern Spain), which has been affected by a considerable massive growth of D. geminata since 2011. The study shows a profound alteration in both the river community composition and in the food web structure at the sites affected by the massive growth, which is primarily due to the alteration of the environmental conditions, thus demonstrating that D. geminata has an important role as an ecosystem engineer in the river. Thick filamentous mats impede the movement of large invertebrates especially those that move and feed up on it and favor small, opportunistic, herbivorous organisms, mainly chironomids, that are capable of moving between filaments and are aided by the absence of large trophic competitors and predators -prey release effect-. Only small predators, such as hydra, are capable of surviving in the new environment, as they are favored by the increase in chironomids, a source of food, and by the reduction in both their own predators and other midge predators -mesopredator release-. This change in the top-down control affects the diatom community, since chironomids may feed on large diatoms, increasing the proportion of small diatoms in the substrate. The survival of small and fast-growing pioneer diatoms is also favored by the mesh of filaments, which offers them a new habitat for colonization. Simultaneously, D. geminata causes a significant reduction in the number of diatoms with similar ecological requirements (those attached to the substrate). Overall, D. geminata creates a community dominated by small organisms that is clearly different from the existing communities in the same stream where there is an absence of massive growths.

We investigated regional diatom species distribution, local diversity, and spatial variation in body size from 105 boreal stream sites in Finland. Stream diatom body size distributions ranged from a slightly right-skewed pattern at the largest study extent, comprising the whole country, to symmetrical distributions within watersheds. Compared to earlier findings for larger organisms, diatom body size distributions were more symmetrical, implying either narrower study extent or fundamental differences between unicellular and multicellular organisms. There was a weak positive correlation (r = 0.239; P < 0.05) between latitude and average maximum cell size in stream diatom assemblages. Multiple linear regression showed that average maximum cell size was best accounted for by a model incorporating moss cover and latitude. There was also a weak (r = 0.290; P < 0.01; n = 105) negative correlation between local species richness and average cell size corroborating the earlier findings for other organism groups.

Plankton: Diatoms and other Phytoplankton.