Epiphytic Diatoms from the Central Region of the Gulf of California: Floristics and Biogeographic Remarks

Abstract

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.

1. Introduction

Conservationist, ecological, and biogeographical perspectives in general for any given area require formal scientific knowledge of all living resources within. The first step of the unavoidable analysis required should be the identification of said elements as being either faunistic or floristic. In marine ecosystems, floristic elements include benthic diatom taxocoenoses whose ecological significance has distinguished them as a suitable reference when making decisions on conservation issues and ecological relations, particularly for protected areas [1].

After constructing reliable diatom floristics and estimating relative or proportional abundances (inventories) of the recorded taxa, they can be used for estimating ecological parameters that better describe the assemblages. Said ecological studies based on analyses of classical parameters can reflect the ecological status in pristine environments or various types of impact [2]. Thus, unlike species richness alone, the analysis of species diversity depicts a structure in benthic diatom assemblages based on species richness, diversity, dominance, etc., that can be used to assess environmental conditions in protected areas [3]. However, although floristics still constitutes the main basis for achieving this, in some parts of the biosphere where diatoms are abundant, very little is known about them. Moreover, the relevance of taxonomic issues such as the classification, determination, and identification of benthic diatoms in marine environments has shown the benefits of focusing on remote protected areas and unexplored environments worldwide [4]. According to this, not knowing enough about the distribution of benthic diatom taxa precludes building biogeographical distributional models.

In spite of the above, the publication of floristic studies carried out in the northwestern region of Mexico have been occasionally banned in certain journals under administrative policies without considering their scientific structure [5], undermining the eventual support for conservation issues. Notwithstanding, floristic studies of benthic diatoms along Mexican littorals have hitherto documented new records since the last published general taxonomic list [6] which included 1162 taxa, whilst the first one [7] comprised 493 taxa for the Baja California Peninsula (BCP); in both lists, a recurrence of many taxa is observed for several localities in the Mexican NW region. Therefore, it can be expected that the current floristic data for the Mexican NW region may allow for the detection of biogeographic affinities. Moreover, taxa recorded recently both from the central region of the Gulf of California (CRGC) and the Revillagigedo Archipelago (RA), such as Achnanthes apiculata (Greville) Riaux-Gobin, Compère, Hinz and Ector, Caloneis egena (Schmidt) Cleve, Coronia ambigua (Greville) Ruck and Guiry, Grammatophora gallopagensis Grunow, Gyrosigma parvulum Hustedt, Psammodictyon pustulatum (Voigt ex Meister) C.S. Lobban, Psammodictyon roridum (Giffen) D.G. Mann, Seminavis basilica Danielidis, Trachyneis aspera var. oblonga (Bailey) Cleve, and Tryblionella nicobarica (Grunow) D.G. Mann, among others [1,4,8,9], could be indicating a biogeographical connectivity between these regions, where warm conditions determine the presence of taxa with a tropical affinity. Additionally, the diatom assemblages may also be examined relying on the shared taxa belonging to a particular genus, such as Mastogloia, referring to its promised tropical affinity, or Lyrella, for which the highest number of taxa for any locality in NW Mexico was recorded for a temperate coastal lagoon on the western coast of the BCP, where transitional conditions prevail [1].

The generated floristic theory is expected to constitute a reliable basis for other hypothesis-driven studies, albeit dealing with ecological diversity measurements, both alpha and beta, that comprehend more extensive areas, revealing biogeographic patterns and requiring further complex (numerical) techniques. According to this, the following questions have been addressed: Which and how many epiphytic diatom taxa are to be found living on macroalgae in a locality in the central region of the Gulf of California? How are the Mastogloia and Lyrella taxa represented proportionally in these diatom assemblages? Additionally, can any biogeographical evidence be observed? The corresponding hypothesis stated that the epiphytic diatom assemblages living on macroalgae in the CRGC would be mainly composed of common taxa that are homogeneously distributed in the NW region of Mexico on both coasts of the BCP, albeit including conspicuous tropical and temperate components that define a biogeographical transition zone. Likewise, a hypothesis stated that the species richness (S) would be higher for the diatom taxocoenoses from the CRGC due to its transitional nature than for the diatom taxocoenoses from a tropical region such as in the RA. Additionally, it was expected that the S for Mastogloia would be much lower than for the RA taxocoenosis, while the number of Lyrella species would be much higher for CRGC.

2. Materials and Methods

The sampling site, Santa Maria, is located in the central part of the Baja California Peninsula’s eastern coast, facing the Gulf of California (GC), 8 km north of the well-known mining town Santa Rosalia (27°20′ N, 112°16′ W) (Figure 1) [10]. Annual precipitation in the region is very low (<100 mm) and occurs mainly in association with tropical storms [11]. The area is influenced by GC currents which are linked to the intensification of the Mexican coastal current that flows S–N and enters along the eastern coast with alternating fluxes and exiting (N–S) along the peninsula coast [12,13]. These conditions are determinant for avoiding contamination risk by metals from the Santa Rosalia mining activities further south [14]. The assumed pristine conditions in Santa Maria ensure that the benthic diatom assemblages used as reference hold a high species richness and a species composition that can be compared to other benthic diatom surveys in the gulf region.

Six thalli of various macroalgae species were collected by hand according to their availability at two sites in Santa Maria (Figure 1) during March 2016. The collection of a single specimen of each macroalgae species was carried out using semi-autonomous diving equipment (hookah), at a depth of approximately 8 m. In the laboratory, sampled thalli were identified via their genus level according to [15] as follows: Codium sp., Dictyota sp., Sargassum sp. 1, Sargassum sp. 2, and Gracilaria sp. One specimen was just identified as being of the class Phaeophyceae. Epiphytic diatoms were scraped off from the macroalgae thalli while rinsing with purified water, and the total scraped-off material from each macroalgal taxon was poured into a test tube as a compound sample. Brushed-off diatom samples were placed in 60 mL test tubes and treated following [2] for oxidizing all organic matter inside and outside the diatom frustules in a mixture of sample, commercial alcohol, and nitric acid at a ratio of 1:1:5. The samples were rinsed repeatedly afterwards with purified water until reaching a pH > 6. Cleaned diatoms were mounted in duplicate on permanent slides in Pleurax^® (RI = 1.7) (Bill Daily, University of Pennsylvania, Philadelphia, PA, USA). All slides were inspected thoroughly by doing a systematic sweep for all diatom taxa present. The diatom taxa were identified morphologically at 1000× under an Olympus compound microscope (Olympus, Tokyo, Japan) with planachromatic lenses, and a photographic ocular lens. Micrographs were taken of specimens of representative diatom taxa, and others were selected mainly according to the particular objectives of the study. Taxa identification followed that of [1,2,4,8,9,16,17,18,19,20,21,22,23,24,25,26,27] and was updated according to the Algaebase electronic platform [28].

3. Results

The inspection of epiphytic diatom flora from the macroalgae specimens collected at Santa Maria (CRGC) yielded a total of 333 taxa on the species and infra-species (SS) level, comprised within 101 genera (

Table A1. Floristic list of epiphytic diatoms of macroalgae from the Central Gulf of the California region on the Baja California Peninsula. SS = species and infra-species. NR = new record for the Mexican NW region.

Achnanthes apiculata (Greville) Riaux-Gobin, Compère, Hinz and Ector	(Figure A1a–d)
Achnanthes brevipes var. brevipes C. Agardh	(Figure A2k,l)
Achnanthes brevipes var. intermedia (Kützing) Cleve	(Figure A2j)
Achnanthes brockmannii Hustedt	(Figure A1g)
Achnanthes curvirostrum Brun	(Figure A1s)
Achnanthes fimbriata (Grunow) R. Ross	(Figure A1q)
Achnanthes longipes C. Agardh	(Figure A2s–u)
Achnanthes manifera Brun
Achnanthes groenlandica var. phinneyi McIntire and Reimer	(Figure A1m,q,r)
Achnanthes septata Clever-Euler	(Figure A2i)
Achnanthes sp. 1	(Figure A1j)
Achnanthes sp. 2	(Figure A2a–g)
Achnanthes yaquinensis McIntire and Reimer	(Figure A2h)
Achnanthidium sp. 1	(Figure A2n,o)
Achnanthidium sp. 2 NR	(Figure A15j,k)
Actinocyclus curvatulus Janisch	(Figure A46b and Figure A47a,d)
Actinocyclus octonarius var. crassus (Smith) Hendey	(Figure A44a)
Actinocyclus octonarius var. octonarius Ehrenberg	(Figure A44b,c, Figure A45b,c, and Figure A46a)
Actinocyclus octonarius var. tenellus (Brébisson) Hendey	(Figure A43b and Figure A47b)
Actinocyclus roperi (Brébisson) Grunow	(Figure A46d)
Actinocyclus subtilis (Gregory) Ralfs	(Figure A45a)
Actinoptychus aster Brun	(Figure A42a,b)
Actinoptychus concentricus A.W.F. Schmidt	(Figure A41a–c,f)
Actinoptychus minutus Greville
Amphitetras antediluviana Ehrenberg	(Figure A49d)
Amphora americana Wachnicka and Gaiser	(Figure A21b,c,q)
Amphora angusta Gregory	(Figure A13e,f)
Amphora arcuata A.W.F. Schmidt	(Figure A23c,d)
Amphora arenaria Donkin	(Figure A26a)
Amphora aspera Petit	(Figure A23h)
Amphora bigibba var. bigibba Grunow	(Figure A23f)
Amphora cf. amoena Hustedt	(Figure A24j)
Amphora cf. angustissima (Van Heurck) A. Mann
Amphora cf. crassa W. Gregory	(Figure A25b,c)
Amphora cf. janischii A.W.F. Schmidt	(Figure A24g–i)
Amphora cf. obtusa var. rectangulata H. Peragallo and M. Peragallo	(Figure A25d)
Amphora cf. zebrina (Schmidt) Cleve	(Figure A21n)
Amphora cingulata Cleve	(Figure A22i,j)
Amphora crassa var. punctata Grunow	(Figure A24a,b)
Amphora elegantula Hustedt	(Figure A21j)
Amphora graeffeana Hendey	(Figure A23g)
Amphora helenensis Giffen	(Figure A21k)
Amphora holsaticoides T. Nagumo and H. Kobayasi	(Figure A22d,e)
Amphora laevis Gregory	(Figure A22k,l,n)
Amphora maletracta var. constricta (Heiden) Simonsen	(Figure A21s)
Amphora ostrearia var. vitrea Cleve	(Figure A22a and Figure A24c,d)
Amphora proteus var. contigua Cleve
Amphora proteus var. proteus Gregory	(Figure A21d–f,l)
Amphora sp. 1	(Figure A24k,l and Figure A25h,i)
Amphora sp. 2	(Figure A2a–g and Figure A19h)
Amphora sp. 3	(Figure A25f)
Amphora spectabilis W. Gregory	(Figure A21g and Figure A23k)
Ardissonea formosa (Hantzsch) Grunow	(Figure A39b,c)
Ardissonea fulgens (Grevillei) Grunow	(Figure A38d)
Ardissonea robusta (Ralfs) De Notaris	(Figure A40b)
Astartiella glacialis Witkowski, Lange-Bertalot, and Metzeltin	(Figure A1r)
Astartiella punctifera (Hustedt) Witkowski and Lange-Bertalot	(Figure A1w,x)
Asteromphalus flabellatus (Brébisson) Greville	(Figure A47e)
Auricula complexa (Gregory) Cleve	(Figure A23i)
Auricula pulchra (Greville) Cleve	(Figure A20b)
Azpeitia nodulifera (Schmidt) G.A. Fryxell and P.A. Sims
Bacillaria socialis (Gregory) Ralfs	(Figure A27l)
Biddulphia alternans (Bailey) Van Heurck	(Figure A49e)
Biddulphia biddulphiana (Smith) Boyer	(Figure A52a,b)
Biddulphia tridens (Ehrenberg) Ehrenberg	(Figure A48e)
Biddulphia tuomeyii (Bailey) Roper	(Figure A51a,b)
Biddulphiopsis membranacea (Cleve) von Stosch and Simonsen	(Figure A54b)
Biremis ambigua (Cleve) D.G. Mann	(Figure A26b,c)
Biremis sp.	(Figure A26e)
Brachysira sp. NR	(Figure A2v and Figure A26d)
Caloneis bicuneata (Grunow) Boyer	(Figure A8i,k,l)
Caloneis linearis (Grunow) Boyer	(Figure A8g,h,j)
Campylodiscus cordatus R. Hagelstein	(Figure A30f and Figure A31d)
Campylodiscus giffenii Lobban and J.S. Park	(Figure A29f)
Campylodiscus latus Shadboldt NR	(Figure A31a,c)
Campylodiscus neofastuosus Ruck and Nakov	(Figure A29a,b and Figure A30e)
Campylodiscus ralfsii W. Smith
Campylodiscus simulans Gregory
Campylodiscus sp.
Campylodiscus thuretii Brébisson	(Figure A30c,d,g)
Campylopyxis garkeana (Grunow) Medlin
Catenula adhaerens (Mereschkowsky) Mereschkowsky	(Figure A22m)
Climacosphenia moniligera Ehrenberg	(Figure A38a–c)
Cocconeiopsis sp.
Cocconeis contermina A.W.F. Schmidt	(Figure A3j,k)
Cocconeis convexa Giffen	(Figure A3u–w,bb)
Cocconeis dirupta var. dirupta Gregory	(Figure A3a–e)
Cocconeis dirupta var. flexella (Janish and Rabenhorst) Grunow	(Figure A3f–h,o,p)
Cocconeis diruptoides Hustedt	(Figure A3ee,ff)
Cocconeis disculoides (Hustedt) Stefano and Marino	(Figure A3q,r)
Cocconeis guttata Hustedt and Aleem	(Figure A3l–n)
Cocconeis heteroidea Hantzsch	(Figure A3a–d)
Cocconeis krammeri Lange-Bertalot and Metzeltin	(Figure A4l)
Cocconeis molesta Kützing	(Figure A4o)
Cocconeis neodiminuta Krammer
Cocconeis nugalas M.H. Hohn and J. Hellerman	(Figure A3i)
Cocconeis pediculus Ehrenberg	(Figure A4m,n)
Cocconeis pellucida var. nankoorensis Grunow	(Figure A4e,f,i,j)
Cocconeis peltoides Hustedt	(Figure A4t,u)
Cocconeis pinnata W. Gregory ex Greville	(Figure A4g)
Cocconeis pseudodiruptoides Foged	(Figure A3z,aa)
Cocconeis pseudomarginata W. Gregory	(Figure A4q)
Cocconeis scutellum Ehrenberg	(Figure A4k,p)
Cocconeis sp. 1	(Figure A3cc,dd)
Cocconeis sp. 2 NR	(Figure A3x,y)
Cocconeis sp. 3 NR	(Figure A1o,p, Figure A3s,t and Figure A4v,w)
Cocconeis sp. 4	(Figure A4x,y)
Cocconeis stauroneiformis H. Okuno	(Figure A4r,s)
Cocconeis vetusta A.W.F. Schmidt	(Figure A4h)
Coronia decora (Brébisson) Ruck and Guiry	(Figure A30a,b)
Coscinodiscus radiatus Ehrenberg	(Figure A43a,c)
Cosmioneis grossepunctata (Hustedt) D.G. Mann	(Figure A18c)
Craspedodiscus cf. nankoorensis Grunow NR	(Figure A49f)
Craspedostauros cf. indubitabilis (Lange-Bertalot and Genkal) E.J. Cox NR	(Figure A14b–d)
Craticula halophila (Grunow) D.G. Mann	(Figure A15s,t)
Cyclotella striata (Kützing) Grunow	(Figure A48d and Figure A49g)
Cymatoneis margaritae Witkowski	(Figure A18d,e)
Cymbellonitzchia sp.
Cymbellonitzschia banzuensis Stephanek, Hamscher, Mayama, Jewson, and Kociolek
Delphineis minutissima (Hustedt) Simonsen
Dimeregramma marinum (Gregory) Ralfs
Diploneis bombus (Ehrenberg) Ehrenberg	(Figure A5j)
Diploneis crabro var. crabro (Ehrenberg) Ehrenberg	(Figure A5b)
Diploneis crabro var. dirhombus (Schmidt) Cleve	(Figure A5c–e,k)
Diploneis crabro var. pandura (Brébisson) Cleve	(Figure A5h,i)
Diploneis didyma (Ehrenberg) Ehrenberg	(Figure A6l)
Diploneis fusca var. hyperborea (Grunow) Hustedt NR	(Figure A5n)
Diploneis gorjanovici (Pantocsek) Hustedt NR	(Figure A5f,g)
Diploneis littoralis var. clathrata (Østrup) Cleve	(Figure A6c)
Diploneis muscaeformis (Grunow) Cleve	(Figure A5l,m)
Diploneis obliqua (Brun) Hustedt	(Figure A6k)
Diploneis papula var. constricta Hustedt	(Figure A6b,q)
Diploneis serratula (Grunow) Hustedt	(Figure A6e,j)
Diploneis smithii var. recta Peragallo	(Figure A6d)
Diploneis smithii var. smithii (Brébisson) Cleve	(Figure A6a,e,j)
Diploneis sp. 1 NR	(Figure A6p)
Diploneis splendida Cleve	(Figure A5a)
Diploneis suborbicularis (Gregory) Cleve	(Figure A6f)
Diploneis vacillans var. renitens (Schmidt) Cleve	(Figure A6o)
Diploneis vacillans var. vacillans (Schmidt) Cleve	(Figure A6g,h)
Donkinia carinata (Donkin) Ralfs	(Figure A20c)
Ehrenbergiulva hauckii (Grunow) Witkowski, Lange-Bertalot, and Metzeltin
Entomoneis alata (Ehrenberg) Ehrenberg
Fallacia cf. nicobarica (Grunow) Witkowski, Lange-Bertalot, and Metzeltin
Fallacia diploneoides (Hustedt) D.G. Mann	(Figure A12h,i)
Fallacia forcipata (Greville) Stickle and D.G. Mann	(Figure A12q,t)
Fallacia inscriptura (Hendey) Witkowski, Lange-Bertalot, and Metzeltin	(Figure A12j–l)
Fallacia litoricola (Hustedt) Mann	(Figure A12f,g)
Fallacia minima (Østrup) Witkowski, Lange-Bertalot, and Metzeltin	(Figure A12a–c)
Fallacia ny (Cleve) D.G. Mann	(Figure A12e)
Fallacia nyella (Hustedt) D.G. Mann	(Figure A12d)
Fallacia subforcipata (Hustedt) D.G. Mann	(Figure A12r,s)
Fallacia vittata (Cleve) D.G. Mann	(Figure A12m–p)
Fragilariopsis doliolus (Wallich) Medlin and P.A. Sims	(Figure A35l)
Fragilariopsis pseudonana (Hasle) Hasle	(Figure A35i)
Glyphodesmis rhombica (Cleve) Simonsen	(Figure A35c)
Glyphodesmis williamsonii f. lanceolata (Peragallo and Peragallo) Hustedt	(Figure A35a,b)
Gomphonemopsis sp.	(Figure A15r)
Gomphoseptatum aestuarii (Cleve) Medlin
Grammatophora hamulifera Kützing	(Figure A33b,e)
Grammatophora marina var. gibba Grunow	(Figure A33i,j and Figure A34a,b,d,e)
Grammatophora marina var. marina (Lyngbye) Kützing	(Figure A32a–c,e,g–i, Figure A33a,d,f–h, and Figure A34c,h)
Grammatophora oceanica Ehrenberg	(Figure A32d and Figure A33a,c)
Grammatophora gallopagensis Grunow	(Figure A32f and Figure A34f,g,i,j)
Grunowago bacillaris (Grunow) Lobban and Ashworth
Gyrosigma parvulum Hustedt	(Figure A20d)
Gyrosigma peisonis (Grunow) Hustedt	(Figure A20a)
Gyrosigma tenuissimum (Smith) J.W. Griffith and Henfrey
Halamphora coffeiformis (Agardh) Mereschkowsky	(Figure A23e)
Halamphora costata (Smith) Levkov	(Figure A25a,e)
Halamphora cymbifera (Gregory) Levkov	(Figure A23a,b)
Halamphora dusenii (Brun) Levkov	(Figure A22f)
Halamphora eunotia (Cleve) Levkov
Halamphora holsatica (Hustedt) Levkov	(Figure A22b,c)
Halamphora hybrida (Grunow) Levkov	(Figure A23j)
Halamphora kolbei (Aleem) Álvarez-Blanco and S. Blanco	(Figure A23l)
Halamphora pseudohyalina (Simonsen) J.G. Stepanek and Kociolek	(Figure A21m)
Halamphora staurophora (Juhlin-Dannfelt) Álvarez-Blanco and S. Blanco	(Figure A21o)
Halamphora subangularis (Gregory) Levkov	(Figure A21r)
Halamphora terroris (Ehrenberg) P. Wang
Halamphora turgida (Gregory) Levkov	(Figure A23m)
Halamphora wisei (Salah) I. Álvarez-Blanco and S. Blanco	(Figure A21i)
Hantzschia sp.
Karayevia clevei (Grunow) Bukhtiyarova	(Figure A1k,l)
Lampriscus shadboltianum (Greville) H. Peragallo and M. Peragallo	(Figure A53a and Figure A54a)
Libellus grevillei (Agardh) Cleve
Licmophora abbreviata C. Agardh	(Figure A36e)
Licmophora debilis (Kützing) Grunow	(Figure A36h)
Licmophora flabellata (Greville) C. Agardh	(Figure A37a–d)
Licmophora gracilis (Ehrenberg) Grunow	(Figure A36a)
Licmophora juergensii C. Agardh	(Figure A36b,c)
Licmophora paradoxa (Lyngbye) C. Agardh	(Figure A36d)
Lioloma delicatulum (Cupp) Hasle	(Figure A36f)
Lyrella abruptoides (Hustedt) D.G. Mann	(Figure A11c)
Lyrella approximatoides (Hustedt) D.G. Mann	(Figure A10e and Figure A11d)
Lyrella clavata var. caribbaea (Cleve) Siqueiros Beltrones	(Figure A9a,b)
Lyrella clavata var. indica (Greville) J.L. Moreno-Ruíz	(Figure A9d)
Lyrella excavata (Hustedt) D.G. Mann	(Figure A10b–d)
Lyrella exsul (Schmidt) D.G. Mann	(Figure A9c)
Lyrella impercepta (Hustedt) J.L. Moreno-Ruíz
Lyrella irrorata (Greville) D.G. Mann	(Figure A11a,b)
Lyrella lyra (Ehrenberg) Karayeva	(Figure A10a)
Mastogloia binotata (Grunow) Cleve	(Figure A7j)
Mastogloia borneensis Hustedt	(Figure A8f)
Mastogloia ciskeiensis M. H. Giffen	(Figure A8a,b)
Mastogloia crucicula (Grunow) Cleve	(Figure A7i)
Mastogloia elegans Lewis	(Figure A7h)
Mastogloia fimbriata (Brightwell) Grunow	(Figure A7a)
Mastogloia marginulata Grunow	(Figure A8c–e)
Mastogloia subaffirmata Hustedt	(Figure A7b–g)
Meridion sp.
Navicula abunda Hustedt	(Figure A15c,d,f)
Navicula ammophila Grunow	(Figure A15n,o,x,y)
Navicula beaufortiana Hustedt
Navicula bipustulata Mann
Navicula cancellata Donkin	(Figure A15i and Figure A18b)
Navicula carinifera Grunow	(Figure A18a)
Navicula cf. consentanea Hustedt NR
Navicula cincta (Ehrenberg) Ralfs	(Figure A15a,b,u)
Navicula directa (Smith) Ralfs	(Figure A15g,h,m)
Navicula diversistriata Hustedt	(Figure A15e,p)
Navicula johanrossi Giffen	(Figure A15l)
Navicula lanceolata Ehrenberg	(Figure A18g)
Navicula leptoloba Meister	(Figure A15w and Figure A16j,k)
Navicula libellus Gregory	(Figure A18h)
Navicula longa var. irregularis Hustedt	(Figure A16b)
Navicula longa var. longa Gregory	(Figure A16a)
Navicula microdigitoradiata Lange-Bertalot	(Figure A15v)
Navicula palpebralis Brébisson ex W. Smith	(Figure A16e)
Navicula pavillardii Hustedt	(Figure A16c)
Navicula pennata A.W.F. Schmidt	(Figure A16d)
Navicula platyventris Meister	(Figure A15q)
Navicula salinarum Grunow	(Figure A16f,g)
Navicula sp. 1	(Figure A16h,i)
Navicula sp. 2	(Figure A17a,b,e,f,i,j,n,o)
Navicula sp. 3	(Figure A17k–m)
Navicula sp. 4	(Figure A17p,q)
Navicula sp. 5	(Figure A17c,d,g,h)
Navicula subagnita Proshkina-Lavrenko	(Figure A17r,s)
Navicula zosteretii Grunow
Neosynedra provincialis (Grunow) Williams and Round	(Figure A37f)
Nitzschia agnita Hustedt	(Figure A27h)
Nitzschia angularis W. Smith	(Figure A27e,m)
Nitzschia behrei Hustedt
Nitzschia bicapitata Cleve	(Figure A27o,p)
Nitzschia bombiformis Grunow	(Figure A27b)
Nitzschia dissipata (Kützing) Grunow
Nitzschia distans Gregory	(Figure A28d)
Nitzschia fluminensis Grunow	(Figure A28e)
Nitzschia frustulum var. perminuta Grunow
Nitzschia grossestriata Hustedt	(Figure A27i)
Nitzschia hybrida Grunow
Nitzschia incrustans Grunow
Nitzschia insignis W. Gregory	(Figure A28a,b)
Nitzschia jelineckii Grunow	(Figure A27g)
Nitzschia laevis Frenguelli	(Figure A27a)
Nitzschia longissima var. costata Hustedt
Nitzschia longissima var. longissima (Brébisson) Ralfs
Nitzschia macilenta W. Gregory
Nitzschia microcephala var. bicapitellata Cleve-Euler	(Figure A27n)
Nitzschia scalpelliformis Grunow	(Figure A28c)
Nitzschia sicula (Castracane) Hustedt
Nitzschia sigma (Kützing) W. Smith	(Figure A28f,g)
Nitzschia sigma var. sigmatella Grunow
Nitzschia spathulata Brébisson ex W. Smith
Odontella aurita (Lyngbye) C. Agardh	(Figure A49c)
Odontella obtusa Kützing	(Figure A49h, Figure A50a,b, and Figure A53a)
Oestrupia powelli (Lewis) Heiden	(Figure A6n)
Opephora marina (Gregory) Petit
Opephora schwartzii (Grunow) Petit	(Figure A35e)
Parlibellus delognei (Van Heurck) E.J. Cox	(Figure A19a)
Parlibellus hammulifer (Grunow) E.J. Cox	(Figure A19b)
Parlibellus rhombicula (Hustedt) Witkowski	(Figure A19c)
Parlibellus sp. 1	(Figure A19d)
Petrodictyon gemma (Ehrenberg) D.G. Mann
Pinnularia cf. splendida Hustedt
Plagiogramma interruptum (Gregory) Ralfs	(Figure A35f–h)
Plagiogramma minus (Gregory) Chunlian Li, Ashworth and Witkowski	(Figure A35d)
Plagiogrammopsis sp.	(Figure A35k)
Plagiotropis pusilla (Gregory) Kuntze
Planothidium delicatulum (Kützing) Round and Bukhtiyarova	(Figure A1m,n)
Planothidium hauckianum (Grunow) Bukhtiyarova	(Figure A1t–v)
Planothidium polare (Østrup) Witkowski, Lang-Bertalot and Metzeltin	(Figure A1h,i)
Pleurosigma intermedium W. Smith
Pleurosigma nicobaricum Grunow NR
Podocystis spathulata (Shadbolt) Van Heurck	(Figure A40a)
Podosira montagnei Kützing	(Figure A43e)
Podosira stelligera (Bailey) A. Mann	(Figure A41l)
Progonoia musca (Gregory) H.J. Schrader	(Figure A6m)
Proshkinia sp.
Protoraphis hustedtiana Simonsen NR	(Figure A37e)
Psammodictyon constrictum (Gregory) D.G. Mann	(Figure A27d)
Psammodictyon panduriforme (Gregory) D.G. Mann
Psammodictyon panduriforme var. latum (OWitt) M.A. Harper	(Figure A27k)
Psammodiscus nitidus (Gregory) Round and Mann	(Figure A41j,k)
Pseudictyota dubia (Brightwell) Sims and Williams	(Figure A48f,g and Figure A49a,b)
Raphoneis sp.
Rhabdonema adriaticum Kützing	(Figure A40c)
Rhoicosphenia sp.	(Figure A2p)
Rhoikoneis bolleana Grunow
Rhopalodia gibberula (Ehrenberg) O. Müller
Rhopalodia sterrenburgii Krammer	(Figure A13i,j)
Roperia tesselata (Roper) Grunow ex Pelletan	(Figure A43d and Figure A46c)
Schizostauron trachyderma (Meister) Górecka, Riaux-Gobin and Witkowski	(Figure A1e,f)
Seminavis eulensteinii (Grunow) Danieledis, Ford and Kennett NR	(Figure A13g,h)
Seminavis obtusiuscula (Grunow) Danieledis and D.G. Mann	(Figure A21a)
Seminavis strigosa (Hustedt) Danieledis and Economou-Amilli
Shionodiscus oestrupii (Ostenfeld) Alverson, Kang and Theriot	(Figure A42f,g)
Staurophora sp. 1	(Figure A14a)
Staurophora sp. 2	(Figure A19e,f)
Stictodiscus cf. californicus Greville NR	(Figure A47c)
Surirella recedens A.W.F. Schmidt	(Figure A29a,b and Figure A31b)
Synedra commutata Grunow	(Figure A36k)
Synedrosphenia cuneata (Grunow) Azpeitia
Tabularia affinis var. acuminata (Grunow) Aboal	(Figure A36i)
Tabularia tabulata (Agardh) Snoeijs	(Figure A36l)
Tetramphora rhombica (Kitton) Stepanek and Kociolek
Tetramphora securicula (Peragallo and Peragallo) Stepanek and Kociolek	(Figure A21p, Figure A22g,h, and Figure A24e,f)
Thalassionema nitzschioides (Grunow) Mereschkowsky	(Figure A36j)
Thalassiosira eccentrica (Ehrenberg) Cleve	(Figure A42c)
Thalassiosira simonsenii Hasle and G. Fryxell	(Figure A42e)
Thalassiosira sp.	(Figure A42d)
Toxarium undulatum Bailey	(Figure A39d)
Trachyneis aspera var. aspera (Ehrenberg) Cleve	(Figure A8m and Figure A14h)
Trachyneis aspera var. elliptica Hendey NR	(Figure A14e,f)
Trachyneis velata A. Schmidt	(Figure A8n and Figure A14g,i)
Triceratium pentacrinus var. quadrata Tempère and Peragallo	(Figure A48a–c)
Tropidoneis sp.
Tryblionella acuminata W. Smith	(Figure A27f)
Tryblionella apiculata W. Gregory
Tryblionella coarctata (Grunow) D.G. Mann	(Figure A27c,j)
Tryblionella marginulata var. didyma (Grunow) Haworth and Kelly
Zygoceros ehrenbergii E.A. Sar	(Figure A49i)

, Appendix A), and 283 of these taxa are represented in the iconographic catalog (Appendix B). The best represented genera were Navicula with 29 SS, Amphora (27), Cocconeis (25), Nitzschia (24), Diploneis (19), Achnanthes (12), Halamphora (14), and Fallacia (10). The number of SS by genus varied from 1 to 29, while 58 of the identified genera were represented by a single SS (singletons), i.e., 56%, and 13 SS were identified only to genus level. Out of the total taxa, 16 constituted new records for the region (Table A1). Concerning reference to selected taxa for ascribing biogeographical affinity, the Lyrella were represented by 9 SS, while Mastogloia included 8 SS and Grammatophora included 5 SS.

4. Discussion

The assumed pristine conditions for the study area corresponded with a high species richness (S) reaching 333 species and infra-species (SS) of the analyzed benthic diatom taxocoenosis (BDT), and this is similar to those recorded in other non-impacted productive environments from the NW Mexican region, either from the W coast of the Baja California Peninsula (BCP) or the Gulf of California. However, the recorded S for the Revillagigedo Archipelago (RA), that is currently 395 SS, actually resulted from two separate studies [4]. Notwithstanding, on the basis of information theory (H′), the estimated diversity in these localities reached very high values, 4.8 for CRGC and 5.2 for RA, using a relative abundance of species [29]. However different, when using the genus-to-species ratio as data for computing H’, the former reached 5.48 while the latter was 5.47. From this perspective, the authors attributed this to the displacement of equitability toward the rare species side of the distribution, whereby the percentage (57%) of the identified genera in the CGCR represented by a single SS (59) or singletons is somewhat higher than that of Revillagigedo BDT (52%) with 53 genera [29]. On the other hand, while the floristics from other studies allocated in the central region of the Gulf of California (CRGC) were not included here, a floristic study of epiphytic diatoms on Phyllodictyon pulcherrimum (Chlorophyceae) in the CRGC was carried out earlier [9]. Although said work did not include a diversity measurement based on the relative abundance of the taxa, it yielded a high species richness (S = 244 taxa) and 86 genera, whereby the best represented of which were Nitzschia (25 SS), Amphora (17), Diploneis (15), Navicula (13), Cocconeis (12), and Achnanthes (7), whilst in RA these were Cocconeis (27), Nitzschia (24), Amphora (23), Navicula (19), Diploneis (17), and Grammatophora (15). In contrast, the recorded genera containing more SS (Amphora, Cocconeis, Navicula, Nitzschia, and Diploneis) within the BDT from CRGC exhibited similar proportional numbers as in RA and several other localities [29].

In view of the biogeographical connotations of this study, it has to be made clear that the above comparisons did not consider that samplings were carried out in different seasons, while in this study, springtime conditions prevailed, conditions in the RA corresponded to Winter, and the P. pulcherrimum specimen was collected during summer. Although the tropical characteristics of these regions could suggest similar stable conditions, seasonal current circulation patterns comprising the interaction with the open Pacific Ocean and the Gulf of California water mass [30] are likely to influence the seasonal development of benthic diatom assemblages according to the more temperate or tropical affinity of the specific taxon. Further studies should be designed on this scenario in order to model the distribution of benthic diatom taxa and their biogeographical affinities.

In the present work, Amphora (27 spp.) stood out as being distinctively more diverse than in the other two studies. However, the main genera are the same and, although their S also differ, in all cases this could be explained by multifactorial causes, mainly the host macroalgal taxon and the season of sampling [25]. Thus, although the determining taxa in BDT at the genus level are much the same as in most of the surveyed environments in the Mexican marine coasts, the number of species within each genus differs, and so do the floristic assemblages. Thus, an ex profeso study is in hand to tackle this problem.

Concerning the distinction of taxa within the Lyrella and Mastogloia genera and their proportionality in relation to biogeographic affinity, the inspected locality (CGCR) yielded nine SS for the former, and eight for the latter, plus five Grammatophora SS, in contrast with those of RA which had fourteen, fifty-six, and fifteen SS, respectively. These comparisons indicate a likely biogeographical difference in which the Mastogloia and Grammatophora incline toward the tropical affinity, whilst the Lyrella seem to depart from it, inasmuch that a record number of SS for this genus was reported for the western coast of the Baja California Peninsula [1] from an ecosystem influenced by the cold waters of the California current. On the other hand, the majority of identified taxa (85%) were distributed commonly in the Mexican NW region [2,6,26], which also backs the posed hypothesis.

Since 2018, several new records have been added to the diatom flora of the CRCG and NW Mexico, 3 by [31] and 38 by [9], including a new taxon [32]. This, along with the 16 new records and the 13 unidentified taxa in this study, together with the estimated diversity using both relative abundances and the genus-to-species ratio [29], evidence the diversity potential of the region and show the need for continuous floristic efforts in the Gulf of California littorals. These types of simple but valuable floristic studies with basic design are useful for constructing the taxonomic bases of further hypothesis-based, ecological, and biogeographical research, and are still common worldwide, e.g., [33]. Likewise, further studies would conveniently be followed by ecological diversity assessment to establish scales of reference between protected pristine and non-protected pristine areas, using established approaches of beta diversity or by proposing new methodological perspectives, as has been achieved in the present case [29]. This will aid in supporting better ecosystem management, either under conservation perspectives, sustained exploitation, or biogeography issues for which other considerations are at hand. From this perspective, the feasibility of a close connection between the CGCR diatom flora and that from the Revillagigedo Archipelago should be explored by comparing various community parameters between epiphytic diatom samples from both localities.

5. Conclusions

In general, the Mastogloia may be considered to be a genus indicating tropical affinity, and to a lesser degree, the same goes for Grammatophora and Caloneis, while the Lyrella, Fallacia, and Achnanthes (13) correspond more to temperate latitudes. The high species richness, and proportions of indicator taxa, i.e., few Mastogloia and Grammatophora, and more Lyrella, depict BDT from the CGCR of transitional affinity, backing the posed hypothesis. The new records and the unidentified taxa in this study suggest a greater floristic reserve for the RCGC, and indicate the need for continuing the efforts toward the taxonomic identification or determination of benthic diatoms in the Gulf of California. Likewise, as with the distribution of diatom taxa either in marine, brackish, or fresh water environments which are commonly transgressed in various floristics reports, the distribution of genera in multiple environments and latitudes is also a common phenomenon. Thus, establishing distributional boundaries for benthic diatom taxa would require much more floristic research. Additionally, surveying various substrates such as rare macroalgae species as hosts for epiphytic diatoms may provide opportunities to seek new records of diatom taxa, or even new taxa, in regions around the world. Finally, based on the still incipient status of floristic studies along Mexican littorals and the sampling strategies used, i.e., spatially or temporarily discrete, it is inferred that new records of taxa for the region are to be found, whose absence from the overall species list is explained by discontinuous seasonal sampling or the omission of substrates.