Benthic Diatom Community Structure Research Articles

Benthic diatom diversity in microtidal mangrove estuaries

Mangrove habitats support taxonomically diverse benthic communities, yet their effects on microphytobenthos remain understudied. This study investigated the benthic diatom community structure of 17 microtidal mangrove estuaries situated along the east coast of South Africa. High sediment organic content (≤ 18.1 %) and clay fraction (≤ 17.5 %) measured in the Avicennia marina and Rhizophora mucronata habitats favoured diverse communities (H′ > 3) and the presence of unique benthic diatom species such as Gyrosigma balticum. The habitat complexity and stabilisation provided by the mangrove forests served as microrefugia for colonisation of benthic diatoms. Taxa displayed a broad range of nutrient tolerance, with some nutrient tolerant species (e.g., Halamphora acutiuscula, Navicula clausii, Navicula gregaria, and Navicula radiosa) dominating the communities in the Mbashe and Mnyameni estuaries. The presence of these nutrient tolerant taxa in relatively pristine systems serves as an early warning for possible anthropogenic nutrient enrichment.

Spatial patterns of benthic diatom community structure in the largest northwestern river of Cambodia (Sangker River)

To date, very little is known about the relative importance of physicochemical factors structuring the benthic diatom community in the flood-pulse tropical ecosystems. This paper aims to investigate the spatial distribution of the benthic diatom community in a poorly studied river catchment of the Tonle Sap Basin in Cambodia. Diatom samples were collected at nineteen (19) monitoring sites from 2018 to 2023 along the Sangker River, and the hierarchical clustering technique was applied to characterize the variation in diatom community structure based on the similarity of diatom genera composition. Seventy-eight (78) diatom genera belonging to 36 families, 18 orders, and 5 classes of diatom flora were recorded. Gomphonema Ehrenberg, Nitzschia Hassall, and Navicula Bory were the most dominant diatom genera in the catchment area. Accordingly, three main diatom assemblages (Ia, Ib, and II) were classified, and 11 indicator genera were identified. Indeed, the identified assemblages reflect the change of diatiom community composition from the upstream assemblage (Ia), middle course (Ib), and floodplain assemblage (II). Dissolved oxygen (DO), pH, conductivity (EC), total phosphorus (TP), and silicon dioxide (SiO2) were the physico-chemical determinants of benthic diatom communities, while physical factors (DO, pH, and nutrients) seemed to be more important than chemical factors. Consequently, any disturbances to the physical condition of the river would drive an abrupt shift in the diatom community’s diversity and structure. These findings pave an essential step towards the establishment of sustainable biomonitoring programs in developing countries such as Cambodia to protect aquatic ecosystems from global change.

Effects of mesohabitat, grazing and substratum roughness on locally common and rare diatom species

Abstract Habitat type and grazing are key factors controlling benthic diatom communities. Nonetheless, the strength of grazing may depend on the habitat complexity at fine (substratum roughness) and meso scales (mesohabitat type). The interaction between grazers and physical conditions may thus be an important factor shaping benthic diatom communities. Several investigations have evaluated how habitat type, grazing and substratum roughness separately affect the biomass, species richness and community structure of algal communities. However, how these factors act together remains poorly understood, particularly for sets of common and rare species. The latter is important for understanding whether common and rare species are affected differently by environmental filters at the local scale. We investigated the effects of mesohabitat type, grazer occurrence and substratum roughness as well as their interactions on benthic diatom communities in a split‐split‐plot field experiment consisting of hierarchical treatments: (i) mesohabitat (pools and riffles); (ii) grazer occurrence (presence or absence of grazers >1 cm in size); and (iii) substratum roughness (smooth and rough substrata). We analysed the entire community and also datasets of common and rare species. Here, we define common and rare species based on their abundance–occupancy relationships, with rare species presenting low mean abundance per experimental unit (between 0.025 and 8.2) and low occupancy (between 2.5% and 25%). Substratum roughness was the main factor explaining differences in species richness, with higher species richness found on rough than on smooth substrata for both the complete dataset and for common and rare species. There was evidence that the interactions between mesohabitat and grazer occurrence and between mesohabitat and substratum roughness affected community structure (relative abundances) for the complete dataset and for common species, whereas the three‐way interaction between mesohabitat, grazer and substratum roughness affected community structure of rare species. Our findings emphasise that crevices are important microhabitats that allow the establishment of both common and rare species. Furthermore, our results suggest that physical differences between mesohabitats influenced the effect of grazers on the community structure of common and rare species. For common species, the effects of this interaction were difficult to depict, but we found a change in the second‐most‐common species between mesohabitats and depending on the occurrence of grazers. For rare species, grazers more clearly affected community structure on smooth substrata in riffles than in pools. Our results demonstrate that local patterns of common and rare diatom species resulted from distinct combinations of the evaluated environmental filters. Together, our results highlight how multifactorial field experiments are key to understanding the main environmental filters driving the species richness and community structure of benthic diatom communities.

Characteristics of Benthic Diatom Community Structure and Water Ecological Health Evaluation in the Lalin River Basin

With the improvement of public requirements for the health status of aquatic ecosystems, there have been innovative assessment methods developed for aquatic ecosystems. In this study, benthic algae assemblages and water quality variables were analyzed to develop a benthic diatom-based index of biotic integrity (D-IBI) for the assessment of the aquatic environment in the Lalin River. In addition, using redundancy analysis (RDA) based on dominant species and physicochemical indexes, the ecological distribution characteristics of the benthic diatom community were revealed, and the key influencing factors were identified. The results showed that the benthic diatom community structure in the Lalin River basin had obvious spatial differences. The application of the index revealed that the water quality could be described as excellent condition in the upper reaches of the Lalin River, good to common condition in the middle of the sites, and moderate to poor condition in the downstream. The assessment further revealed that the main reason for the degradation of the Lalin River ecosystem was nutrient enrichment through agricultural land use.

Composição e estrutura da comunidade de diatomáceas bentônicas de marismas do sul do Brasil e sua relação com variáveis ambientais

Abstract: Aim This study focused on the spatiotemporal variation of the benthic diatom community structure in salt marshes from the Patos Lagoon estuary and their relationship with environmental variables. Methods Samplings were carried out in the winter of 2010 and summer of 2011 (during El Niño and La Niña) in sites with different sediment granulometry, salinity, and distances from the Atlantic Ocean. The surface sediment was collected using a core (10 cm in diameter and 2 cm in depth) and the benthic diatoms were removed following the Trapping method, allowing the observation of live diatoms. Results The richness values (18 and 48 taxa), evenness (0.41 and 0.68), and Shannon diversity indices (2.02 and 3.31 bits/ind.) variations were not significative between the sites and seasons, although temperature and salinity differed significatively between winter and summer. However, the diatom's composition and distribution were related to temperature, salinity, and sediment particle size. Mainly the species Hippodonta hungarica, Luticola simplex Navicula cf. cryptotenelloides, N. erifuga, N. jacobii, Nitzschia filiformis var. conferta, Planothidium frequentissimum and Tryblionella calida were associated with lower temperature and lower salinity in the winter. The species Navicula cf. cryptocephala, N. phylleptosomaformis, Nitzschia pusilla, N. frustulum, N. scalpelliformis and Pseudostaurosiropsis geocollegarum were associated with higher temperature and higher salinity in the summer. Birraphid diatom taxa, mainly species of the genus Navicula and Nitzschia, were frequent or abundant in sandy sediments, and monoraphids, such as Planothidium frequentissimum, were frequent in sites where silt and clay predominated. Conclusions The composition of the benthic diatoms revealed variation between the sites in the winter and summer seasons, during El Niño and La Niña episodes. In winter, high rainfall and freshwater runoff maintained oligohaline condition in the marshes, while in summer, flooding with meso-poly-euhaline waters changed the diatom composition. The granulometry was an important factor in explaining the distribution of the birraphid and monorhapid taxa.

The influence of estuary eutrophication on the benthic diatom community: a molecular approach

The occurrence of harmful algal blooms (HABs) is increasing in frequency and intensity in South African estuaries because of eutrophication. This study used DNA metabarcoding to identify the benthic diatom community structure in the eutrophic Sundays Estuary. The recurrent HAB phytoplankton species Heterosigma akashiwo was recorded at bloom concentrations (∼100 µg Chl a l−1) in the brackish, vertically stratified middle reaches of the estuary. In contrast, microphytobenthos biomass increased towards the upper reaches in response to elevated availability of total oxidised nitrogen (NOx) and increased sediment stability. A total of 76 diatom species were identified using the molecular technique. Seven known nutrient-tolerant diatom species indicate a preference for either ammonium or NOx enrichment. Benthic diatom community diversity (H′ < 1) and evenness (J′ < 0.25) were low in the brackish middle reaches of the estuary, where the HABs decreased available light. Heterosigma akashiwo has been found in previous studies to suppress the growth of co-occurring taxa. Future research efforts should focus on verifying this relationship over seasonal timescales given the dominance of H. akashiwo in spring/summer. As the persistently nutrient-enriched state and stable flow conditions of the Sundays Estuary is the key driver of change, management efforts should be geared towards re-establishing the estuary’s natural flow variations and to measures that can mitigate nutrient pollution.

Shift in benthic diatom community structure and salinity thresholds in a hypersaline environment of solar saltern, Korea

The community dynamics of benthic diatoms in the hypersaline environment are investigated to advance our understanding how salinity impacts marine life. Diatoms were sampled in the two salterns encompassing salt Ponds, ditches, and seawater reservoirs (n = 11), along the salinity gradient (max = 324 psu), and nearby tidal flats (n = 2). The floral assemblages and distributions across sites and stations showed great variations, with a total of 169 identified taxa. First, not surprisingly, higher diversity of benthic diatoms was found at natural tidal flats than salterns. The saltern diatoms generally showed salinity dependent distributions with distinct spatial changes in species composition and dominant taxa. Biota-environment and principal component analysis confirmed that salinity, mud content, and total nitrogen were key factors influencing the overall benthic community structure. Some dominant species, e.g., Nitzschia scalpelliformis and Achnanthes sp. 1, showed salinity tolerance / preference. The number of diatom species at salinity of >100 psu reduced over half and no diatoms were found at maximum salinity of 324 psu. The highest salinity for the observed live diatoms was 205 psu, however, a simple regression indicated a theoretical salinity threshold of ~300 psu on the survival. Finally, the indicator species were identified along the salinity gradient in salterns as well as natural tidal flats. Overall, high species numbers, varying taxa, and euryhaline distributions of saltern diatoms collectively reflected a dynamic saltern ecosystem. The present study would provide backgrounds for biodiversity monitoring of ecologically important microalgal producers in some unique hypersaline environment, and elsewhere. Key words: benthic diatom; hypersaline; indicator species; salinity gradient; saltern

SUSTAINABLE BIOFUEL PRODUCTION FROM ESTUARINE DIATOMS

Purpose of the study: Microalgae with better carbon sequestration potential, higher biomass, and lipid productivity in comparison to terrestrial counterparts is emerging as a viable sustainable alternative to fossil fuels. Diatoms, a subgroup of a broader category of microalgae are well-known for their role as a bio-indicator in palaeo-climatological studies across historical timelines. Understanding ecology, community structure, and habitat preference of diatoms are prerequisites for prioritizing diatom strains towards sustainable biofuel production along with value-added product extraction.&#x0D; Method: Selection of appropriate strains, economically viable harvesting and environmentally sound transesterification are the challenges faced in microalgal industry. The present study focusses on understanding the variability in benthic diatom community assemblages across seasons and its affinity to different substrata at fixed sampling locations in an estuarine eco-system through field sampling for twelve months covering all seasons.&#x0D; Main Findings: The results highlight the tolerance and resilience in select diatom strains in fluctuating water qualities and seasonal variations, the insights much needed during mass cultivation under open environments. Statistical data analyses revealed distinct demarcation between sensitive and tolerant species with selective habitat preferences and resilience to fluctuating environmental conditions. Variations of benthic diatom community structure across various substrata highlights substratum affinity of diatom strains. From the inferences derived through field studies, mixed consortia of diatoms were developed, monitored, and optimized for higher biomass and lipid productivity under controlled laboratory conditions.&#x0D; Application of the Study: The results highlight the prospects of phyco-prospecting of indigenously grown diatom strains for sustainable biofuel production.

Community signals of the effect of Didymosphenia geminata (Lyngbye) M. Schmidt on benthic diatom communities in Chilean rivers

BackgroundDidymosphenia geminata (Lyngbye) M. Schmidt is a freshwater invasive benthic diatom native to oligotrophic systems of the Northern Hemisphere. Since 2010, freshwater systems of south-austral Chile, have experienced a progressive invasion of this species between 37° 18′ S and 54° 30′ S. This invaded area is characterized by biogeographic heterogeneity, anthropic pressure and lack of knowledge. The objective of this study was to analyze the effect of D. geminata on the community structure of benthic diatoms and the variability of this effect in Chilean rivers.MethodsThe data were collected between 2010 and 2017 and pre-treated using fourth-root transformation to construct a Bray-Curtis similarity matrix and a Principal Coordinate Analysis to identify groups of sites. The α diversity and β diversity were estimated and compared using univariate indexes and the Mann-Whitney test, respectively. We used Permutational multivariate analyses of variance (PERMANOVA) and PERMDISP to estimate the spatial variation of communities, and an Analysis of Percentage Similarity (SIMPER) was performed to identify the species which contributed to the similarity between the groups of sites.ResultsThe results suggest significant differences between invaded and noninvaded rivers (54% of the total variation); the univariate indexes showed greater richness (S), greater diversity (H′), and less species turnover in invaded rivers. The PERMANOVA indicated significant interaction between D. geminata and the basins; there was also a significant effect on the internal dispersion in the basins. The SIMPER analysis showed that the groups composed exclusively of invaded sites had high mean similarity in all basins and small pedunculated benthic diatoms showed a significant response to the presence of D. geminata.ConclusionWe found high variability in the community structures, that determine a strong segregation between non-invaded and invaded sites by D. geminata, and that the main effect of D. geminata on diatoms benthic was the homogenization of the communities. This is reflected in the increase in density of small stalked diatoms and the reduction of species turnover among rivers in the same basin, leading to the loss of community heterogeneity that overcomes the spatial heterogeneity in the area invaded in Chile.

Assessment of Aquatic Ecosystem Health of the Wutong River Based on Benthic Diatoms

The community structure of benthic diatoms and water environmental characteristics were extensively investigated to assess the aquatic ecosystem health of the Wutong River (Heilongjiang Province, China). Several diatom indices were calculated, and a benthic diatom index based on biotic integrity (BD-IBI) was developed. Principal component analysis (PCA), Spearman correlation analysis (CA), cluster analysis, redundancy analysis (RDA), and the box plot analysis were used to analyze the benthic diatom communities, assess the river ecosystem health, and compare the applicability of different indexes. The results indicated that Gomphonema parvulum and other tolerant species were the dominant species. Meanwhile, most sites were in “poor” or “very poor” condition according to the diatom indexes evaluation, indicating that the river has been disturbed by human activities. The sampling sites of the Wutong River were divided into three groups based on different pollution levels. The derived BD-IBI included four individual metrics of different aspects, showed strong distinguishability for three grouping and robust correlation with environmental variables. Of all the indexes selected, IBI performed the best, followed by the species-level diatom indexes and the genus-level diatom indexes.

Epipelic Diatom Communities in Pengilon Lake Dieng, Central Java

Diatom communities in the lake have an advantage of environmental assessment and paleo reconstruction study, due to the ability of diatom preserved in the sediment. There are only limited studies about taxonomy, ecology, and diversity of diatom flora in tropical areas, especially Indonesia. The main objective of the research is to describe the structure of benthic diatom communities in Pengilon Lake, together with physical chemical variables. Sediment samples were collected at an altitude of more than 2,000 msal habitats with pH around 5.40-6.86. A total of 83 diatom species identified as belonging to 16 families. The diversity index ranges from about 2.25 to 3.01 indicated the ecosystem that naturally stable with none diatom dominant. The most abundant genera were Eunotia with high diversity around 14 taxa. The composition of Eunotia assemblages was different among habitat and season variation. Eunotia has a wide tolerance of pH because of its ability to survive in an area with an acidic environment and circumneutral water. The most dominance diatom species in Pengilon Lake were Eunotia paludosa (Grunow) 39.2%, Nitzschia frustulum (Kutzing) 28.1%, Pinnularia viridis (Krammer &amp; Lange-Bertalot) and Pinnularia divergentissima (Grunow) 24.6%.

The effect of agriculture on the seasonal dynamics and functional diversity of benthic biofilm in tropical headwater streams

AbstractTropical streams are one of the most endangered ecosystems in the world due to the constant pressures from human activities. Among these activities, agriculture represents a land use that is crucial for human development but also a key driver of stream degradation and biodiversity decline in the tropics. Against this background, we investigated indirect effects of agriculture (alterations in stream flow and nutrient availability) and climate characteristics (water temperature) on benthic biofilm communities in tropical streams (São Paulo State, Brazil). Three first‐order streams draining catchments dominated by agricultural land use (sugarcane for bioenergy production, pasture) with some remaining riparian forest were studied for 1 year. We focused on the relationships of benthic biofilm biomass, algal biomass, diatom community, and functional structure with streamflow dynamics, nitrate concentrations, and water temperature. Our results indicate that these biological responses were mainly mediated by flow and water temperature and not by resource availability in the studied headwater streams. This result could be explained by the heavy rains and elevated runoff generation in these tropical catchments under agricultural influence, which might override the known effects of nutrient enrichment on benthic biofilm communities. Considering forecast climate and land‐use changes in tropical streams, our findings may suggest potential shifts in benthic biofilm communities, with functional consequences for aquatic food webs in these environments.Abstract in Brazilian Portuguese is available with online material.

Responses of the Benthic Diatom Community to Nutrients and the Identification of Nutrient Thresholds in Three Aquatic Ecoregions of the Huntai River, Northeast China

Benthic diatom communities and nutrient gradients were investigated from 287 sampling sites in three aquatic ecoregions (AE) of the Huntai River to characterize the spatial distribution of nutrients and the benthic diatom communities. Locally weighted scatterplot smoothing (LOWESS) was used to analyze the thresholds for nitrogen and phosphorus. The results showed that:① The concentration of ammonia nitrogen and total phosphorus significantly differs in the three AEs, and shows a tendency of AEⅠ < AE Ⅱ < AE Ⅲ. ② To reveal the structure of benthic diatom communities, various benthic diatom indexes, including the sportive diatom percentage, sensitive diatom percentage, stipitate diatom percentage, Pielou evenness index, specific polluosensitivity index (IPS), biological diatom index (IBD), and generic diatom index (IDG) were analyzed. The sportive diatom percentage varied significantly in AEI, AEⅡ, and AE Ⅲ, with the highest percentage observed in AEⅢ and the lowest in AEⅠ. However, the other six indexes exhibited an opposite trend. All revealed that AE Ⅲ has been seriously damaged, while AEⅠ is less disturbed than AEⅡ and AE Ⅲ. ③ LOWESS fitting curves show thresholds for ammonia nitrogen (NH4+ -N) in the three aquatic ecoregions as 0.13, 0.30, and 1.98 mg·L-1, respectively and for total phosphorous (TP) were 0.04, 0.06, and 0.20 mg·L-1. All results were tested by independent-sample T tests. This study will provide assistance for effectively protecting the benthic diatom community in different aquatic ecoregions and also provide a theoretical basis for water management.

Propidium monoazide-denaturing gradient gel electrophoresis (PMA-DGGE) assay for the characterization of viable diatoms in marine sediments

Molecular-based community profiles often overestimate species richness and diversity as they are incapable of distinguishing DNA signals derived from extracellular or moribund sources from that of viable organisms. Propidium monoazide (PMA), a double stranded DNA-binding dye, was used to selectively isolate the genetic signature of viable diatoms from marine cultures and environmental samples. Upon optimization of photoactivation time using a 600-W halogen lamp, PMA concentrations, and number of treatments, the compromised genetic signals from heat-killed samples, visualized by denaturing gradient gel electrophoresis, were either greatly reduced or removed entirely while those derived from intact, viable samples remained relatively unaffected. Subsequent tests illustrated that bands associated with extracellular and moribund DNA were removed from combinations of viable and nonviable cells initially derived from two sedimentary diatom isolates. In addition, viable and nonviable samples of Skeletonema marinoi that were spiked into fresh deposit-feeder (Balanoglossus aurantiacus) faecal-coil sediments demonstrated the effectiveness of the assay within an environmental matrix. Thus, we propose the use of PMA as a quick and necessary addition to any protocol relying on genetic profiles to discern changes in viable benthic diatom community structure.

Spatio-temporal variation of benthic diatom diversity and community structure in a sandy intertidal zone of the Nanji Islands, China

Spatio-temporal variation of benthic diatom diversity and community structure in a sandy intertidal zone of the Nanji Islands, China

Assessment of water quality based on diatom indices in a small temperate river system, Kowie River, South Africa

This study aimed to assess the impact of land use patterns on water quality and benthic diatom community structure and to test the applicability of diatom indices developed in other regions of the world to a small temperate southern African river system. Sampling was conducted at eight study sites along the length of the river on four separate occasions. Multivariate data analyses were performed on the diatom community dataset to specify the main gradients of floristic variation and to detect and visualize similarities in diatom samples in relation to land-use patterns within the catchment. One hundred and twelve (112) diatom species belonging to 36 genera were recorded during the study. Canonical correspondence analysis (CCA) demonstrated that variations in the benthic diatom community structure were best explained by ammonium, nitrate, conductivity, pH, temperature, resistivity and water flow. OMNIDIA was used for calculation of selected diatom water quality indices. A number of the indices, e.g., the trophic diatom index (TDI), eutrophication/pollution index and biological index of water quality (BIWQ), either under- or over-estimated the water quality of the system. With few exceptions, there were no significant correlations (p> 0.05) between the diatom indices’ values and the nutrient variables. The absence of any significant correlations between the diatom indices’ values and selected physico-chemical variables suggests that indices developed in other regions of the world may not be suitable for temperate southern African rivers. Keywords : biomonitoring, diatom indices, diatom community, Kowie River, nitrogen, multivariate analysis, point-source pollution, water quality

Effect of artificial macrocosms on water characteristics and benthic diatom communities in Donghu Lake, China

ABSTRACTMacrocosm experiments at various scales have been applied in lake eutrophication management worldwide. This study investigated how artificially created macrocosms affect the physicochemical characteristics of lake water, as well as the density, community structure, and biodiversity of benthic diatoms. First, we found that macrocosms were mesotrophic compared to the eutrophic control lake water from March 2012 to February 2013. Second, 148 diatom taxa were recorded across all study sites; both species richness and the number of dominant diatom taxa were higher in the macrocosms, compared with the control lake water. Mean diatom densities in the macrocosms and the control were 8.4 and 21 × 109 cells/m2, respectively. Across all sites and seasons, Achnanthidium minutissimum was the most abundant species and dominant contributor to total abundance (14%). In the macrocosms, other abundant diatom taxa were Cymbella gracilis, Cymbella affinis, Gomphonema truncatum, and Gomphoneis olivaceum, all of which are associated with high water quality. Canonical correspondence analysis indicated that the variation in benthic diatom communities was strongly related to phosphorus and nitrogen concentrations in macrocosms, but related to pH and total nitrogen in the control. This study suggests that macrocosms created to alter water characteristics have a positive effect on ecosystem biodiversity and function.

Relationships between benthic diatom assemblages’ structure and selected environmental parameters in Slovak water reservoirs (Slovakia, Europe)

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.

Distribution of benthic diatom communities in a permanently open temperate estuary in relation to physico-chemical variables

The spatial and temporal patterns in benthic diatom community structure in temperate permanently open estuaries are poorly understood. In this study, we used a combination of multivariate and diversity indices to elucidate environmental factors associated with diatom community structure in the Kowie Estuary, South Africa. Benthic diatom samples were collected from three sites corresponding to the upper, middle and lower reaches of the estuary on four occasions over the period early spring 2012 to winter 2013. Among the 89 benthic diatoms observed, Entomoneis paludosa (W Smith) Reimer, Nitzschia reversa W Smith, Nitzschia closterium (Ehrenberg) W Smith, Pleurosigma elongatum W Smith, P. salinarum (Grunow) Grunow, Staurosira elliptica (Schumann) DM Williams & Round, Surirella brebissonii Krammer & Lange-Bertalot, and Surirella ovalis Brébisson were the numerically dominant species. Principal component analysis demonstrated that the diatom community structure was determined by a variety of factors including nutrient (ammonia, nitrate) concentrations, hydrology (e.g., water depth and flow) and pH. Hierarchical cluster analysis revealed the absence of any distinct spatial patterns, although distinct benthic diatom communities were recorded during the different sampling periods. The species richness was highest in the middle reach, decreasing from early spring to summer in all reaches. The results of the study provide important insights into the various factors that structure benthic diatom community composition within a permanently open temperate southern African estuary.

Fish predation regimes modify benthic diatom community structures: Experimental evidence from an in situ mesocosm study

AbstractDiatoms are important primary producers in shallow water environments. Few studies have assessed the importance of biological interactions in structuring these communities. In the present study, benthic diatom community structure in relation to manipulated food webs was assessed using in situ mesocosms, whereby predator‐free environments and environments comprising two different fish species were assessed. Zooplankton abundance, settled algal biomass and the diatom community were monitored over a 12‐day period across each of the three trophic scenarios. Differences among treatments over time were observed in zooplankton abundances, particularly copepods. Similarly, the benthic diatom community structure changed significantly over time across the three trophic treatments. However, no differences in total algal biomass were found among treatments. This was likely the result of non‐diatom phytoplankton contributions. We propose that the benthic diatom community structure within the mesocosms was influenced by trophic cascades and potentially through direct consumption by the fish. The study highlights that not only are organisms at the base of the food web affected by predators at the top of the food web, but that predator identity is potentially an important consideration for predator–prey interaction outcomes with consequences for multiple trophic levels.