Epiphytic Diatoms Research Articles

Diatom species composition and indices for determining the ecological status of coastal Mediterranean Spanish lakes

Diatom indices have been used and tested mainly for assessing the ecological status of rivers and deep lakes, but there are scarce studies that determine their effectiveness in shallow lakes and in coastal Mediterranean lakes. This study evaluates the validity of several common diatom indices (SPI, BDI, CEC and TDIL) for the determination of the ecological quality of three coastal lakes (Valencia, Spain) and presents descriptions and ecological data of the main diatom species recorded. Diatom samples were collected from phytobenthos, both from epiphyton of the dominant submerged macrophytes and the sediment. The ecological status of the systems was determined according to different physico-chemical variables and was compared with the results obtained from epiphytic diatom communities. The results showed discrepancies among diatom indices and also with the state determined by the environmental variables. The effectiveness of the indices depended on the number of species assessed for each index with respect to the total species recorded and the suitability of the weight assigned to each species. The results reveal the need to gather more information about the composition and ecology of the diatoms and microalgae characteristic of coastal Mediterranean standing waters. This work contributes to their better knowledge.

Ecología trófica del pez exótico Guatapote del Lerma <i>Poeciliopsis infans</i> (Cyprinodontiformes: Poeciliidae) en el Lago de Pátzcuaro, Región Central de México

Exotic fish species has caused several impacts on aquatic biodiversity. The Lago de Pátzcuaro has some well-studied exotic species, except the Lerma livebearer Poeciliopsis infans. This fish species was introduced into the Lago de Pátzcuaro before 1997 and the aspects of its biology are still unknown. In this study we assessed aspects of the trophic ecology of this exotic fish, P infans, using gut content and stable isotope analysis to understand its capacity to tolerate anthropogenic environmental degradation in the Lago de Pátzcuaro. We also determined its trophic guild position (TP) using the TrophLab Program and stable isotope. Niche breadth was calculated by standardized Levins' Index (Bi). Fish was captured with a seine during wet and dry seasons at six environmentally different sites and gut contents were obtained. We analyzed a total of 239 gut contents of P. infans. The contribution of each food item in the diet was quantified using frequency of occurrence and area percentage. The importance of each prey item was determined according to the index of relative importance (IRI), and the omnivory index (OI) was used to assess the feeding behavior. Fish were categorized by size and the diet was compared between fish sizes and sites. Dorsal muscle tissue and water hyacinth tissue was obtained for nitrogen isotope signature he fish can behave as a specialist (Bi = 0.39) or generalist (Bi = 0.68) and as a primary consumer (TROPH = 2.2; TP = 2.3) with a feeding strategy that was the same at dif- f erent sizes, seasons and sites. None of the evaluated sites showed good environmental quality. We argue that P infans can tolerate changes in water quality and feeding items availability, because it can exploit resources in multiple trophic webs. However, this species could be dependent on habitat complexity, especially in the aquatic vegetation cover.

Epiphytic Diatoms along Environmental Gradients in Western European Shallow Lakes

Diatom‐based assays have been successfully associated worldwide with the tropic status of lakes. Several studies have demonstrated a correlation between epiphytic diatoms and nutrient load in shallow lakes and wetlands. We examine the relative importance of environmental factors in explaining the structure of epiphytic diatom communities in a set of Western European shallow lakes. The effects of lake physical/chemical, morphometric, and geographical variables on diatom assemblages were tested using distance‐based, canonical correspondence and regression analyses. Our results show that epiphytic diatom communities respond, mainly to physical/chemical variables, overriding the effects of lake depth, size, and location. The clustering of studied systems based along a total phosphorus concentration gradient concurs with previous classification of diatom taxa and nutrient state. Assessment of epiphytic diatoms provided a potential method for shallow lake classification.

River flooding as a driver of polygon dynamics: modern vegetation data and a millennial peat record from the Anabar River lowlands (Arctic Siberia)

Abstract. The spatial and temporal variability of a low-centred polygon on the eastern floodplain area of the lower Anabar River (72.070° N, 113.921° E; northern Yakutia, Siberia) has been investigated using a multi-method approach. The present-day vegetation in each square metre was analysed, revealing a community of Larix, shrubby Betula, and Salix on the polygon rim, a dominance of Carex and Andromeda polifolia in the rim-to-pond transition zone, and a predominantly monospecific Scorpidium scorpioides coverage within the pond. The total organic carbon (TOC) content, TOC / TN (total nitrogen) ratio, grain size, vascular plant macrofossils, moss remains, diatoms, and pollen were analysed for two vertical sections and a sediment core from a transect across the polygon. Radiocarbon dating indicates that the formation of the polygon started at least 1500 yr ago; the general positions of the pond and rim have not changed since that time. Two types of pond vegetation were identified, indicating two contrasting development stages of the polygon. The first was a well-established moss association, dominated by submerged or floating Scorpidium scorpioides and/or Drepanocladus spp. and overgrown by epiphytic diatoms such as Tabellaria flocculosa and Eunotia taxa. This stage coincides temporally with a period in which the polygon was only drained by lateral subsurface water flow, as indicated by mixed grain sizes. A different moss association occurred during times of repeated river flooding (indicated by homogeneous medium-grained sand that probably accumulated during the annual spring snowmelt), characterized by an abundance of Meesia triquetra and a dominance of benthic diatoms (e.g. Navicula vulpina), indicative of a relatively high pH and a high tolerance of disturbance. A comparison of the local polygon vegetation (inferred from moss and macrofossil spectra) with the regional vegetation (inferred from pollen spectra) indicated that the moss association with Scorpidium scorpioides became established during relatively favourable climatic conditions, while the association dominated by Meesia triquetra occurred during periods of harsh climatic conditions. Our study revealed a strong riverine influence (in addition to climatic influences) on polygon development and the type of peat accumulated.

Cocconeis Ehrenberg (Bacillariophyta), a genus dominating diatom communities associated with Posidonia oceanica Delile (monocotyledons) in the Mediterranean Sea

A fine-scale analysis of diatoms associated with Posidonia oceanica in selected locations was carried out along a longitudinal gradient in the Mediterranean Sea (00°21′W–27°07′E) to investigate the composition and diversity of epipthytic communities and to determine the distribution and relative importance of their components with special reference to the most common and abundant genus, Cocconeis. Posidonia oceanica was sampled twice (once per year for two years) at 19 sites. At each site, forty samples of P. oceanica shoots separated by several metres were taken. Epiphytic diatom communities were studied with SEM (scanning electron microscopy) and TEM (transmission electron microscopy). Fifty-two diatom genera were identified. In every studied case, adnate Cocconeis spp. cells dominated the community. Among 32 Cocconeis species and varieties found, recently described C. neothumensis var. marina and C. scutellum var. posidoniae were almost always the most numerous. The structure of Cocconeis assemblages changed across the various geographic regions both in terms of the species’ presence–absence and their relative abundance. In general, the most abundant species appeared to be broadly distributed, while less abundant species had a more distinct biogeographic pattern. In the course of this study, several Cocconeis species described as Indo-Pacific were found in the Mediterranean Sea for the first time. Potential relationships between different Cocconeis species and genetically distinct P. oceanica populations are discussed.

Epiphytic diatom communities of Terra Nova Bay, Ross Sea, Antarctica: structural analysis and relations to algal host

AbstractEpiphytic diatoms are important constituents of the Southern Ocean coastal water ecosystem, being a key element in many of the Antarctic trophic chains. However, only limited information exists relating to these microalgal communities. Here we describe our findings of a study on epiphytic diatoms from Terra Nova Bay (Ross Sea, Antarctica) based on material collected during the summer campaigns spanning from 1990–2004. Observations of diatoms associated with three rhodophyte species (Iridaea cordata(Turner) Bory,Phyllophora antarcticaGepp & Gepp, andPlocamium cartilagineum(L.) Dixon) were carried out with the use of a scanning electron microscope. A total of 73 diatom taxa (32 genera) were distinguished, of which 20 taxa exceeded 3% of total abundance.Cocconeis fasciolata(Ehrenberg) Brown,Navicula perminutaGrunow, andFragilariopsis nana(Steemann Nielsen) Paasche appeared in every sample. The analysis of similarities (ANOSIM) test as well as non-metric multidimensional scaling analysis indicated the nature of host organism as a major factor influencing associated diatom community structure, whereas depth, site, and time of sampling seemed to be less important. The epizooic communities associated with sessile fauna epiphytic on macroalgae differed significantly from those associated with macroalgal surface. A pronounced difference between the communities epiphytic on various host macroalgae species was also observed, although most of the dissimilarities occurred between diatom taxa of the same growth form.

Changes in photosynthetic pigments and species diversity of epiphytic diatoms on Myriophyllum triphyllum exposed to cadmium

With the aim to investigate alterations in diatom population when exposed to cadmium under epiphytic conditions, an experiment was conducted on samples from Myriophyllum triphyllum without removing the epiphytic diatoms. Epiphytic diatoms were exposed to cadmium in doses of 0, 2, 4, 6, 8, and 16 ppm for 96 h. While the total organism showed a decrease in all cadmium concentrations compared to the control group, the viability rate in the total organism declined from 87% to 62%. Gomphonema, Navicula, and Nitzchia were found to be the dominant species in the flora. The diversity index value, which was determined to be 1.18 in the control group, dropped to 0.51 as a result of the increase in cadmium concentrations (16 ppm). The amounts of chlorophyll-a and carotenoids decreased with increasing cadmium concentrations.

Diatom community response to an acid mine drainage gradient

Acid mine drainage (AMD) is a significant environmental issue worldwide. On the West Coast of the South Island, New Zealand, many AMD-affected streams occur within close proximity to naturally acidic streams, enabling us to compare the response of communities in naturally and anthropogenic acidified systems. We investigated epiphytic diatom communities in 39 streams along an AMD gradient that included naturally acidic and circum-neutral reference streams. There was a wide range in taxonomic richness in reference streams and those moderately impacted by AMD (8–33 taxa). Taxonomic richness was greatly reduced in severely impacted streams (1–5 taxa) at a threshold of pH 3.4 and was dominated by Pinnularia cf. acidophila (69–100% relative abundance). Community composition differed between circum-neutral reference streams and moderately and severely impacted streams. However, naturally acidic and moderately impacted streams had similar diatom communities primarily composed of Eunotia and Frustulia species. Our results indicate that diatom communities are strongly structured by pH and able to tolerate moderate conductivity and metal concentrations. This is a challenge for researchers and water managers attempting to incorporate diatoms into AMD monitoring in regions with naturally acidic streams.

The structure and species composition of the diatom communities in tropical volcanic lakes of eastern Mexico

Six nutrient-rich tropical volcanic lakes in eastern Mexico were investigated to document their characteristic diatom floras. Plankton and sedimentary diatom communities were compared to verify the representativity of the sediment assemblage by identifying the plankton species present in the sediment. Relationships between surface sediment diatom communities and 20 physico-chemical variables were evaluated using multivariate analysis. High phosphorus concentrations were measured in all the lakes sampled and chlorophyll a concentrations were found to be useful in describing lake trophic status which ranged from meso- to hypereutrophic. The results showed that the species composition and distribution of the diatom community in these lakes was most influenced by nutrient availability (P) and ionic composition. Canonical correspondence analysis showed that cation dominance explained a significant amount of the variation in the diatom community composition. It allowed the identification of three lake types: (1) Na+-dominated lakes where diatom communities had>50% Achnanthidium minutissimum (Grunow) Czarnecki; (2) Mg2+-dominated lakes, where the characteristic species were A. minutissimum (<30%) and Cymbella microcephala Grunow (<50%); and (3) one Ca2+-dominated lake, where Navicula arvensis Hustedt and Discostella pseudostelligera (Hustedt) Houk & Klee were the most abundant diatoms (>20%). The results suggest that some epiphytic diatoms (e.g., A. minutissimum, C. microcephala) can use other plankton algae as substrate; this is a strategy that allows them to remain within the euphotic zone in deep, eutrophic, turbid lakes. The lakes with the lowest nutrient levels had the best preserved terrestrial vegetation cover on the crater walls. This suggests that deforestation has been a factor that has accelerated the process of lake eutrophication during recent decades in this region.

Description of a new marine diatom, Cocconeis caulerpacola sp. nov. (Bacillariophyceae), epiphytic on invasive Caulerpa species

A new species of Cocconeis has been found growing on thalli of the invasive green alga Caulerpa taxifolia collected from the Croatian Adriatic Sea (Bay of Stari Grad, the Island of Hvar, Central Adriatic, Croatia), the coasts of the Mediterranean (Saint Raphaël, west of Cannes, France) and the eastern coast of Australia (Moreton Bay, southeast Queensland). Additionally, it was observed on samples of Caulerpa racemosa, another invasive alga in the Mediterranean. Preserved thalli of Caulerpa and cleaned material of the new diatom were studied by light and electron microscopy (SEM and TEM). The morphology and fine structure of the new marine epiphytic diatom, for which we propose the name Cocconeis caulerpacola Witkowski, Car & Dobosz, was determined, including the internal and external structure of the raphe and sternum valve, and the cingulum. Comparison between the new species and three closely related species, C. borbonica, C. diruptoides and C. pseudodiruptoides, was made using material from our samples, as well as material from Vis (Adriatic Sea) obtained from the Hustedt collection. Surprisingly, C. caulerpacola is able to colonize Caulerpa taxifolia in very high abundance, but its occurrence seems to be strongly patchy. Indeed, it seems that C. taxifolia is a suitable host for epiphytic diatoms, in particular this tiny Cocconeis, despite its reputation as a ‘killer seaweed’. Cocconeis caulerpacola was observed on Caulerpa species in varying abundance over a wide geographical range.

Late Holocene thermokarst variability inferred from diatoms in a lake sediment record from the Lena Delta, Siberian Arctic

Thermokarst lakes in the Siberian Arctic contain sediment archives that can be used for paleoenvironmental inference. Until now, however, there has been no study from the inner Lena River Delta with a focus on diatoms. The objective of this study was to investigate how the diatom community in a thermokarst lake responded to past limnogeological changes and what specific factors drove variations in the diatom assemblage. We analysed fossil diatom species, organic content, grain-size distribution and elemental composition in a sediment core retrieved in 2009 from a shallow thermokarst lake in the Arga Complex, western Lena River Delta. The core contains a 3,000-year record of sediment accumulation. Shifts in the predominantly benthic and epiphytic diatom species composition parallel changes in sediment characteristics. Paleoenvironmental and limnogeological development, inferred from multiple biological and sedimentological variables, are discussed in the context of four diatom zones, and indicate a strong relation between changes in the diatom assemblage and thermokarst processes. We conclude that limnogeological and thermokarst processes such as lake drainage, rather than direct climate forcing, were the main factors that altered the aquatic ecosystem by influencing, for example, habitat availability, hydrochemistry, and water level.

Epiphytic diatoms and their relation to moisture and moss composition in two montane mires, Northeast China

Moss-inhabiting diatom communities were studied in two montane mires on the western flank of the Changbai Mountains (Northeast China). A total of 119 taxa belonging to 44 genera were found in 33 samples. Height above the water table (HWT), pH and Sphagnum-species were found to be three significant factors controlling diatom distribution using redundancy analysis. HWT was found to be the primary factor influencing diatom distribution. Eunotia paludosa var. paludosa, Luticola mutica var. mutica and Hantzschia amphioxys were typical species of dry hummocks. Samples from wet hollows were characterized by Eunotia steineckii, Microcostatus mac­ eria, Adlafia bryophila and Eunotia mucophila. In addition, a small but significant portion of the variance in diatom data can be explained by the sole effect of pH or bryophyte coverage. Chamaepinnularia hassiaca was observed in samples with relatively high pH values and abundant Sphagnum palustre. Furthermore, significant correlations were observed between the valve length of E. paludosa var. paludosa and moisture parameters (i.e. water content and HWT), indicating that diatoms would acclimate to moisture limitation by decreasing cell size. This study revealed that both moss-inhabiting diatom communities and diatom cell sizes were sensitive to moisture changes and both of them should be useful proxies for paleohydrological reconstruction in mires.

Cadmium effects on total individual numbers, species diversity and pigments content of epiphytic diatoms growing on Myriophyllum spicatum L.

This study investigated the total organism change, species richness, chlorophyll- a and carotenoid changes in the floral composition, which are developed by epiphytic diatom species growing on Myriophyllum spicatum L., collected from the Gokpinar Lake (Sivas), against the effects of cadmium. The concentrations of the cadmium (0, 2, 4, 6, 8, and 16 mg/L) were used and they were exposed to heavy metal for 96 hours. The experimental set up was already assembled before the epiphytic diatoms separated from the M. spicatum . While the total organism decreased in all cadmium concentrations compared to the control group, the viability rate within the total organism decreased from 90 to %60. Cocconeis placentula var. euglypta (Ehr.) Grun. and Gomphonema minutum Ag. were found to be dominant species in the flora. The index of diversity was determined as 0.82 in the control group. Depending on the increase in cadmium concentration, this value decreased to 0.38. Chlorophyll- a and carotenoid quantities decreased with the increase of cadmium concentration.

Growth form analysis of epiphytic diatom communities of Terra Nova Bay (Ross Sea, Antarctica)

Diatoms have been long collected from the Southern Ocean but almost no data exist for epiphytic communities, despite their high ecological significance as an important food source in Antarctic coastal food chains. Here, we present a first growth form analysis of diatoms associated with rhodophyte hosts from Terra Nova Bay, Ross Sea, Antarctica. We performed this study to gather baseline information on the species composition of epiphytic diatom communities, determine the influence of some environmental variables on the diatom distribution patterns, and assess the caveats that must be taken into account in terms of sampling design. Macroalgal material was collected during the Italian Antarctic expeditions between 1990 and 2004. Epiphytic diatoms were studied by means of scanning electron microscopy. In terms of growth forms, there were no significant differences between the diatom communities on the different macroalgal host species. Motile (mainly small-celled Navicula perminuta and other Navicula spp.) and adnate (Cocconeis spp.) diatoms dominated the community throughout the study period. Many of the macroalgal blades examined were also covered by epiphytic animals (calcareous bryozoans, hydroids) over most of their surface, with a significant effect on the associated diatom community structure. Our findings suggest that the bio-physicochemical characteristics of each sampling site affected the epiphytic diatom communities more than the substrate type provided by the macroalgal host or the sampling depth.

MORPHOLOGICAL STUDIES OF SOME MARINE MASTOGLOIA (BACILLARIOPHYCEAE) BELONGING TO SECTION SULCATAE, INCLUDING THE DESCRIPTION OF NEW SPECIES.

Epiphytic diatoms on seagrass and seaweed were collected from tropical (e.g., Siladen Island, Celebes Sea, Indonesia and Phú Bài, China Sea, Vietnam), subtropical (e.g., Sharm el-Sheikh, Red Sea, Egypt), and temperate regions (e.g., Patmos Island, Greece) in 2000, 2005, and 2006. Eight species of Mastogloia, belonging to the section Sulcatae, are described mainly through scanning electron microscopy, including two new species to science, M. oculoides and M. sergiana. These species show a differently shaped median depression on the external valve face between the raphe-sternum and the valve margin. Moreover, they lack a developed conopeum or pseudoconopeum, which covers the median depression in other species of the section Sulcatae. This study gives new insights on the ultrastructure of the Mastogloia's valves and provides an update of their current geographical distribution.

Effects of small‐scale environmental variation on metaphyton condition and community composition

Summary1. Metaphyton mats typically originate as benthic algal biofilms that experience higher solar radiation and temperatures, and reduced access to nutrients, once they reach the water surface, but the impacts of these physicochemical changes on metaphyton condition and community composition have received little attention.2. Using microprobes positioned at 0, 2, 4 and 6 cm depth, we recorded small‐scale differences in water chemistry within metaphyton mats constrained in floating nets during an 8‐week period. Concurrent weekly samples of filamentous algae and their diatom epiphytes were collected from the top, middle and bottom of the mats and were analysed for changes in ash‐free dry mass (AFDM) and chlorophyll‐a, nutrient (N, P, C, Si) content and taxonomic composition.3. Light intensity, temperature and dissolved oxygen declined both with increasing depth in the mat and over the study period. The autotrophic index (=AFDM/chlorophyll‐a) was greatest at the top of the mats and increased over time; samples also had higher C/P and C/N ratios than samples deeper within the mat.4. Pithophora was consistently the dominant algal filament throughout the study (representing 85% of all filaments averaged over time and depth); epiphytic diatom cover on Pithophora (calculated as epiphyte area index) declined over time, particularly at the top of the mat.5. Densities of the diatom epiphytes Gomphonema, Cocconeis and Fragilaria increased with increasing depth within the mat, whereas Cymbella/Encyonema was more common in surface samples.6. Our results indicate that metaphyton mats are highly dynamic communities, spatially organised in part by small‐scale environmental variation and subject to changes in taxonomic composition following their arrival at the water surface.

Alpine freshwater ecosystems in a protected area: a source of diatom diversity

The “All Taxa Biodiversity Inventories” (ATBIs) project coordinated by the European Distributed Institute of Taxonomy (EDIT) aims to achieve a baseline biodiversity assessment of flora and fauna in those regions characterized by a lack of knowledge and a high potential for biodiversity. Within the framework of the ATBIs, the aim of this study was to analyse the diatom flora and ecology of a complex of freshwater ecosystems in the Maritime Alps Natural Park (Italy), designated as a Special area of Conservation under the European Habitat Directive. We sampled epilithic and epiphytic diatoms in different habitats in 24 sites: shallow lake, springs, and streams. Our analysis resulted in a list of 138 diatom taxa, highlighting the great biodiversity and the complex structure of the investigated diatom communities. The taxa list included a wide range of uncommon species, including some recorded for the first time in North-Western Italy. Among the different habitats the highest level of diversity was found in the more lentic waters, in particular in limnocrene springs. These results show that the diatom communities of pristine and undisturbed high-mountain environments are rich and complex, despite the severe environmental conditions.

Factors correlating with deterioration of giant kelp Macrocystis pyrifera (Laminariales, Heterokontophyta) in an aquarium setting

Survival of the giant kelp, Macrocystis pyrifera (Linnaeus) C. Agardh, in its natural habitat is governed by abiotic and biotic factors such as temperature, light, nutrients, current velocity, and predators. Factors affecting the survival of the alga in an aquarium setting, however, have not been investigated. The National Museum of Marine Biology and Aquarium (NMMBA), in subtropical Taiwan, is the only aquarium in the world that displays giant kelp that does not have naturally occurring specimens in nearby waters. Giant kelp displayed in aquaria often deteriorates within a 3-month period, yet the cause of this mortality is unknown. We investigated abiotic and biotic parameters affecting survival of giant kelp in aquaria over a 3-month period. The results indicated that temperature, salinity, pH, light, and nutrient concentrations did not affect giant kelp survival. However, the massive proliferation of epiphytic diatoms on kelp blades (from 7 × 102 cells cm−2 initially to 3 × 104 cells cm−2 after 1 month) was identified as being the most likely candidate affecting survival of giant kelp in an aquarium setting. Potential factors that may stimulate epiphyte proliferation include lack of epiphytic algae control via predation, high nutrient concentrations, a weak current, and a generally stable environment.

Interaction between a river and its wetland: evidence from the Murray River for spatial variability in diatom and radioisotope records

Sinclair Flat is small wetland, located within the gorge section of the Murray River floodplain. situated near Blanchetown, South Australia, the wetland is closely linked to the River and, since regulation, has become permanently inundated. High summer evaporation rates deplete the volume of water within the wetland. However, this is compensated by perennial inflow via a permanent inlet from the River. This site provides an opportunity to explore the relative contribution of river and wetland diatom flora to the sediment record, and the fluvial and aerial contribution of radiometric isotopes to the system. The geochronological and biostratigraphic data provide an insight into the history of the water quality of Sinclair Flat. Evidence exists for the River being a source of sediments and isotopes and of diatom species typical of the main river channel. Prior to 1950, Sinclair Flat was an oligotrophic, oligosaline, clear-water wetland. The wetland shifted gradually to an environment that favoured clear-water benthic species, most likely as a consequence of changes following river regulation in the 1920s, although the capacity to date these sediments is limited. During the 1950s, the wetland became plankton dominated. Peaks in epiphytic diatoms during the 1960s suggest increased emergent macrophyte cover. The contemporary condition is of a connected, turbid, eutrophic and mesosaline lagoon. The ecological condition of Sinclair Flat has diverged considerably from its historical range of condition. This record supports evidence from upstream of widespread state switches in the Murray–Darling Basin floodplain wetlands. This record also lends considerable weight to modern studies attesting to the degraded state of the waterways of the Murray–Darling Basin and the impact of river regulation practices on the water quality of these ecosystems.

Substrate-dependent differences between the structures of epiphytic and epilithic diatom assemblages off the southwestern coast of the Gulf of California

To test the postulate that diatom assemblage structures differ between those growing on macroalgae and those on rocks used as substratum by macroalgae, we analyzed 29 samples of macroalgae and 15 of rocky substrata from localities in Bahía de La Paz, México. Assemblage structure was measured as relative abundance, species diversity, dominance, and equitability. To measure similarity between assemblages, we used the Bray-Curtis index, similarity profiles (SIMPROF), and a nonmetric multidimensional scaling (NMDS) ordination. Overall, 271 diatom taxa were identified. Species richness, diversity, and equitability values were higher in epilithic assemblages than in epiphytic assemblages. The NMDS technique based on species composition and relative abundances discriminated between epilithic and epiphytic assemblages in all localities. This differentiation was corroborated by SIMPROF analysis, which showed significant differences between epilithic and epiphytic diatom assemblages in all localities. In Calerita and El Caimancito, epiphytic diatom assemblages were discriminated by phylum, genus, and species of macroalgae. NMDS discriminated epilithic diatom assemblages among localities. However, in epiphytic assemblages, such discrimination was influenced at phylum level by the macroalgal host. Based on the observed differences, we conclude that the structure of epilithic and epiphytic diatom assemblages are independent from one another, and that species composition is determined by particular substrata.