The first floristic list of benthic marine diatoms from Mexico (^{López-Fuerte & Siqueiros-Beltrones, 2016}) showed that most of the taxa from Mexican littorals have been recorded for the Mexican northwestern region. Few, however, correspond to the Gulf of California coasts. Thus, there is reason to assume that a considerable number of taxa have yet to be recorded in this area.

A reliable taxonomic record of benthic diatoms for any given locality is recommended due to their ecological significance (^{Siqueiros-Beltrones, 2002}). They are an essential food-source for many benthic animals, including those of commercial importance, and their assemblages could be used as a reference to assess natural or anthropogenic environmental impact (^{Siqueiros-Beltrones et al., 2014}). It is thus imperative to account for new records that derive from studies in surveyed and un-surveyed areas. This is especially true in Mexican littorals where new records of benthic diatom species are expected to be made in upcoming surveys, as is the case of a recent floristic survey in a lagoon located within a protected area (^{Siqueiros-Beltrones & Argumedo-Hernández, 2017}). Accordingly, in this paper, new records are derived from a floristic survey from an allegedly contaminated beach of Santa Rosalía, Baja California Sur (Gulf of California).

Sediment and rock samples were collected during May 2015, January 2016, and March 2016 from a beach in Santa Rosalía. At this location, nine sites were chosen: five in the port area, where samples were collected at wadding depth, and two sites to the north (Punta Tanques), and two south (Cuevas) of Santa Rosalía, where samples were collected by scuba diving (Fig. 1). Sediments were collected using petri dishes 75 mm in diameter, and two rocks approximately 20 x 10 cm, devoid of macroalgae. Epilithic diatoms were collected by brushing an area of 100 cm2 from the sampled rocks. The brushed-off material and collected sediments were oxidized, following ^{Siqueiros-Beltrones (2002)} for diatom cleaning and mounting. Mounted diatoms were examined under an Olympus CH-2 microscope at 630× and 1000×. Species references and identification followed ^{Cleve-Euler (1955)}, ^{Hartley (1996)}, ^{Péragallo and Péragallo (1908)}, ^{Hein et al. (2008)}, ^{Siqueiros-Beltrones (2002)}, and ^{Stidolph et al. (2012)}.

Figure 1 Location of study area and sampling sites at the Gulf of California, Mexico. 

The taxonomic status of the taxa was updated according to ^{Guiry & Guiry (2017)}; http://www.algaebase.org/search/?genus=genusname&species=speciesname, and the Catalogue of Diatom Names of the California Academy of Sciences (CAS) web site by ^{Fourtanier & Kociolek (2017)}, http://researcharchive.calacademy.org/research/diatoms/names/index.asp. Also, the International Code of Nomenclature for algae, fungi, and plants (Melbourne code, ^{McNeill et al., 2012}) was consulted. Morphometric data for the new records are provided. Micrographs were taken with a CMOS Konus digital lens.

Three taxa are here depicted which are recorded for the first time in the Mexican northwestern region, including a new combination.

Caloneis liber var. bicuneata (Grunow) Cleve (Figs 2A-C)

Dimensions: Length = 50-70 μm; width = 8-12 μm; 18 striae in 10 μm (n = 5).

Reference: ^{Cleve-Euler (1955)}, Fig. 1124b.

Occurrence: Common, found in most samples.

Synonymy: None.

Basionym: Navicula bicuneata Grunow.

Life form: Epilithic and epipsammic.

Figures 2A-L A-C) Caloneis liber var. bicuneata (Grunow) Cleve; D-G) Entomoneis pulchra var. pulchella (Peragallo) Siqueiros-Beltrones et Martínez comb. nov.; H-L) Gyrosigma naja (Meister) Sterrenburg. A-K = 1000x. L = 630x. Bars = 10 μm. 

Other distribution: Seems to be distributed worldwide, Ceylon, Red Sea (^{Desikachary, 1988}); Northern Sea (Sweden and Finland; ^{Cleve-Euler, 1955}); English coasts (^{Hartley, 1996}); Galapagos Islands (^{Stidolph et al., 2012}).

Remarks: ^{Witkowski et al. (2000)} contend that Caloneis liber var. bicuneata is a synonym of Caloneis bicuneata (Grunow) Wolle. The form they depict, however, differs from ours in having poorly lanceolate ends, a lower number of striae, and being larger. Their specimen coincides with the image in ^{Stidolph et al. (2012)}, whereas ours resembles the one in ^{Cleve-Euler (1955)}, Fig. 1124b. A suggestive variation for C. bicuneata is shown in ^{Desikachary (1988)}, p. 5; Pl. 478, Figs. 3-6, 9.

Entomoneis pulchra var. pulchella (Peragallo) Siqueiros-Beltrones et Martínez comb. nov. (Figs 2D-G).

Dimensions: Length = 60-75 μm width =12 -15 μm; 16 striae in 10 μm (n =2).

Reference: ^{Péragallo & Péragallo (1908)}, p. 183, Pl. 37, Figs. 4, 5.

Occurrence: Rare, few specimens from a single site and date.

Synonymy: None.

Basionym: Amphiprora pulchra var. pulchella Péragallo. ^{Péragallo & Péragallo (1908)}, p. 183, Pl. 37, Figs. 4, 5.

Life form: Epipsammic.

Other distribution: Laguna Figueroa, B.C., México as Entomoneis alata (^{Siqueiros Beltrones, 2002}); England and French north coast (^{Péragallo & Péragallo, 1908}).

Remarks: This taxon has half the length and half the number of striae in 10 μm than Entomoneis pulchra (Bailey) Reimer, which has been recorded solely for the Gulf of Mexico (^{Licea et al., 2016}). Although this new combination can be found earlier, (http://fcelter.fiu.edu/data/database/diatom/index.htm?species=4161), its formal transference to Entomoneis is here proposed.

Slide enero 16-S4. SR Puerto (Puerto Santa Rosalía, BCS, Mexico) in the Benthic diatom scientific collection housed in CICIMAR-IPN is designated as a reference.

Gyrosigma naja (Meister) Sterrenburg (Figs 2H-L)

Dimensions: Length = 230 μm; width = 13 μm; 28 transverse and 20 long striae in 10 μm (n = 1).

Reference: ^{Sterrenburg (2000)}, p. 302, Figs. 5-10; ^{Hein et al. (2008)}, p. 57, Pl. 31, Fig. 3.

Occurrence: Rare, both in the above reference and in the present report.

Synonymy: Gyrosigma spathulatum Ricard.

Basionym: Pleurosigma naja Meister.

Life form: Epipsammic, planktonic.

Other distribution: Indonesia (^{Sterrenburg, 2000}), Bahamas (^{Hein et al., 2008}).

Remarks: Contrary to previous descriptions, our specimen shows conspicuous longitudinal striae; the transverse striae pattern was al most non-discernible under LM.

No studies on benthic diatoms had hitherto been carried out for the northern eastern coasts of Baja California Sur (Gulf of California). The fact that Santa Rosalia’s beach is contaminated with toxic elements, such as Cu, Zn, Co, Pb, and Cd (^{Jonathan et al., 2016}), derived from mining activities makes it imperative to generate reliable taxonomic inventories of benthic diatoms that allow precise detection of floristic changes in order to monitor the environmental impact.

These three taxa were recorded for the first time in the Mexican northwestern region. Two of them, Entomoneis pulchra var. pulchella and Gyrosigma naja had a rare occurrence in sediments from a single sampling locality and date (in the port in January 2016) out of 18 rock samples and nine sediment samples. The former taxon was transferred to the genus Entomoneis from Amphiprora, following ^{Péragallo & Péragallo (1908)}, where A. pulchra Bailey and A. pulchra var. pulchella are depicted. The latter (G. naja), was described by ^{Sterrenburg (2000)} who transferred it from the genus Pleurosigma to Gyrosigma, while conside ring it a rare taxon. In contrast, Caloneis liber var. bicuneata was found on rocks and in sediments; it occurred commonly in all samplings. Whether these new records and their scarcity could be related to any environmental condition of the study area will be determined in the near future; the fact is that these three taxa are reported for the first time for the Mexican northwestern region.