Recent investigations at previously unexplored localities on the Mexican coast have confirmed the high taxonomic potential of benthic marine diatom assemblages (BMDA) in the region. An exploratory study of epiphytic diatoms of macroalgae in the Revillagigedo Archipelago (RA) suggested that further studies would yield many more taxa, prompting the hypothesis that diversity measurements, based on ecological indices, would be among the highest in pristine environments. Thus, the aim of this research was to enrich the record of epiphytic diatom floristics of the RA, and to estimate species diversity based on information theory (H′). Floristically, 167 identified taxa are added here to the BMDA species list of the RA, bringing the total to 397 taxa overall, including 52 taxa that are potentially new records for the Mexican Pacific coast. Among the most conspicuous genera are Mastogloia with five new taxa and it remains the most diverse genus with 55 taxa overall, followed by Cocconeis (27), Nitzschia (24), Amphora (23), Navicula (19), Diploneis (17) and Grammatophora (15). As expected for a pristine environment, the computed species diversity values for the BMDA were high, ranging from H′ = 3.92–5.2, depicting stability. Future surveys are expected to further increase the species richness of BMDA for the RA.

The environmental contingency caused by the recent massive arrivals of pelagic sargasso (Sargassum natans and S. fluitans) on Mexico’s Caribbean coasts have given rise to several areas of scientific research. Our work proposed identifying the diatom flora adhered to the thalli of these two sargasso species collected on the coasts of Cancun, Isla Mujeres, and Puerto Morelos. We recorded 184 diatom taxa (all illustrated) from 68 genera. Taxa from the genera Mastogloia (37), Cocconeis (11), Nitzschia (10), Diploneis (8), and Amphora (9) represented 41% of the total, while 44 of these genera were represented by a single species. In the total floristic count, 41 taxa occurred exclusively on S. fluitans, 53 exclusively on S. natans (22 and 29%, respectively) and 90 (49%) were found on both. Species of Navicula were scarce, and Navicula barbara var. densestriata was here transferred to the genus Lyrella (Lyrella barbara var. densestriata (Foged) López-Fuerte & Siqueiros Beltrones comb. nov.). Overall, 17 (9%) of the identified taxa were new recordings for Mexico’s coasts. Supporting the hypothesis proposed, the 37 Mastogloia taxa suggested a tropical affinity, while the high species richness denoted that the surfaces of both sargasso species constituted favorable substrata for the growth of diatom assemblages.

The ecological significance of benthic diatom taxocoenoses in marine ecosystems renders them a suitable reference when making decisions on conservation, as well as ecological and biogeographical issues. This study focuses on floristics of epiphytic diatoms of macroalgae from the central region of the Gulf of California and their biogeographical affinities. Based on current floristics for benthic diatoms of the Mexican NW, it was hypothesized that species richness would be higher for the central gulf (transitional) than for the tropical Revillagigedo Archipelago. Additionally, it was hypothesized that it would be composed mainly of common taxa of the region, with tropical and temperate components. Likewise, it was hypothesized that Mastogloia (tropical) would include less taxa and Lyrella would include more taxa in the transitional central gulf. The diatom flora yielded 333 taxa and 101 genera, and out of which the best represented were Navicula with 29 taxa, Amphora (27), Cocconeis (25), Nitzschia (24), Diploneis (19), Achnanthes (13), Halamphora (14), Fallacia (10), Lyrella (9), and Mastogloia (8), adding up to 53% of the total. The inspected diatom taxocoenosis included mostly taxa commonly distributed in the Mexican NW, albeit with 16 new records. This, plus the high species richness, as well as the proportions of selected taxa depicting this diatom taxocoenosis as being of transitional biogeographical affinity, back the posed hypotheses.

Studies on marine benthic diatoms in environments contaminated by metals are scarce. The typical structure of benthic diatom assemblages (species richness, diversity, dominance, dominant taxa) from undisturbed environments may be used as reference for contrasting with contaminated environments in order to observe how said assemblages respond to such disturbance. Thus, the Ho that the structure of benthic diatom associations and morphology of their frustules under contamination by metals would be normal, as in unpolluted environments was tested. To do this, concentrations of 24 metals were surveyed in a coastal zone impacted by mining residues, and the structure of benthic local diatom assemblages was described. Metal concentrations measurements for 15 metals surpassed the normal values of the upper earth cortex, seven were under the low range effect, and three (Cd, Cu, Zn) surpassed the medium range effect values. At a control site no element concentration was above the reference values for low range effect (LRE) or medium range effect (MRE) standards. There, diatom species richness (S) was high, particularly on seaweeds; where, 397 diatom taxa were recorded. In contrast, at the contaminated area 217 diatom taxa were recorded, but diversity (H’) ranged from 2.4 to 4.3. Relative high frequencies of deformed diatom valves mainly of Achnanthes spp. were recorded in contaminated sediments. In general, diatom taxocenoses presented a typical structure for non-contaminated environments. However, scarceness of specimens, lower S, and frequency of deformed valves suggest responses to metal contamination. For marine environments, the latter values corresponding to A. longipes may be considered a reliable reference to the response of benthic diatoms to metal contamination.

For the Cuban littorals in the Caribbean Sea, information on benthic diatoms is scarce, and hitherto non-existent for natural protected areas. Thus, to describe the structure of benthic diatom associations (BDAs) from a marine protected area in Cuba, sediment samples were collected in Playa Las Gaviotas (Refugio de Vida Silvestre Cayo Santa María) during dry (November) and rainy (July) seasons. Diatoms were separated, mounted in synthetic resin and identified under light microscopy. Species diversity of the BDA was estimated using Shannon (H′), Simpson (1 − λ) and Pielou evenness (J′) indices. We identified 354 taxa including 200 new records for Cuba; the nomenclature of 45 previously recorded taxa was updated. The currently recorded species richness of marine benthic diatoms for Cuba is 595. The most abundant taxa were Amphora cf. copulata, A. proteus, Diploneis smith var. pumila, Halamphora turgida, H. coffeiformis, Navicula zostereti, Nitzschia marginulata var. didyma and Psammodictyon panduriforme. The best-represented genus was Mastogloia with 70 taxa. Similarity values indicated homogeneous distribution of epipelic diatom taxa along the sandy bottom, suggesting a single diatom association. Mean values of H′ ranged from 4.91 bit/taxon in November to 4.95 bit/taxon in July. Structure analysis suggests a stable BDA with high species diversity characteristic of productive, pristine environments.

Certain diatom species, mainly those that are conspicuous within an assemblage, may constitute useful references while addressing various ecological and biogeographical issues. This may well be the case with Mastogloia species which are frequently the most diverse diatom taxa in benthic marine assemblages from warm seas. Twenty three new records of Mastogloia taxa collected from several thalli of red and brown macroalgae are here added to the epiphytic diatom floristics of the Revillagigedo Archipelago, summing a total of 51 Mastogloia taxa. Most conspicuous forms were frequent, and several taxa seemed to dominate the assemblages, e.g., Mastogloia binotata, M. cocconeiformis, M. cuneata, M. fimbriata, M. inaequalis, M. ovum-paschale, and M. punctatissima, as well as others recorded earlier. Of the new records, M. asperuloides, M. graciloides, M. peragalloi, and M. mauritiana also appeared frequently. The list includes 8 new records for Mexican waters, although no new taxa for this genus were found. However, because of the few macroalgae thalli inspected, it is expected that further inspection of surfaces of other species of macroalgae, and other substrates, will increase the species richness of Mastogloia recorded for the archipelago.