Checklist of Diatoms (Bacillariophyceae) from the Southern Gulf of Mexico: Data-Base (1979-2010) and New Records

The objective of this study was to compile a coded checklist of 430 taxa of diatoms collected over a span of 30 years (1979-2010) from water and net-tow samples in the southern Gulf of Mexico. The checklist is based on a long-term survey involving the 20 oceanographic cruises. The material for this study comprises water and net samples collected from 647 sites. Most species were identified in water mounts and permanent slides, and in a few cases a transmission or scanning electron microscope was used. The most diverse genera in both water and the net samples were Chaetoceros (44 spp.), Thalassiosira (23 spp.), Nitzschia (25 spp.), Amphora (16 spp.), Diploneis (16 spp.), Rhizosolenia (14 spp.) and Coscinodiscus (13 spp.). The most frequent species in net and water samples were, Actinoptychus senarius, Asteromphalus heptactis, Bacteriastrum delicatulum, Cerataulina pelagica, Chaetoceros didymus, C. diversus, C. lorenzianus, C. pelagicus, C. pseudocurvisetus, Coscinodiscus radiatus, Cyclotella striata, Diploneis bombus, Guinardia flaccida, Hemiaulus sinensis, Leptocylindrus danicus, Odontella aurita, O. mobiliensis, Paralia sulcata, Proboscia alata, Pseudo-nitzschia pseudodelicatissima, P. pungens, Skeletonema costatum, Thalassiosira eccentrica, T. partheneia, Thalassionema nitzschioides, and Thalassiothrix longissima. Ninety three taxa were new records for this region. Checklist of Diatoms (Bacillariophyceae) from the Southern Gulf of Mexico: Data-Base (1979-2010) and New Records


Introduction
Diatoms are the most important primary producers of both marine and freshwater environments. Its role in regulating the ocean's silicon cycle is considerable [1]. For taxonomists and ecologists these microalgae are useful for monitoring past and present environmental conditions. They are commonly used in studies of water quality, because they are sensitive to many environmental conditions related to water acidification, eutrophication and climate changes. Diatoms have also been used as valuable indicators in historical assessments of water quality [2][3][4][5][6].
Diatoms (Division Bacillariophyta, Class Bacillariophyceae) have been studied since the early nineteenth century when they were popular among microscopists. In the late nineteenth century many European workers produced hand illustrated monographs and descriptions of species which are still serve as valuable references reviewed in Werner [7]. Hasle and Syvertsen [8] analyzed the new taxonomic information on diatom morphology and presented a revision of the classical identification literature (atlases, floras and handbooks). Krayesky et al. [9] listed 850 diatom species for the entire Gulf of Mexico; however many of them were insufficiently known or, doubtful entities. Besides, several of the species in this list, have other accepted names according to Guiry and Guiry [10], an on-line resource available since September 2004. As a result, only 575 taxa have valid names.
Diatom studies in Mexican waters were recorded by Schmidt et al. [11] in the "Atlas der Diatomacenkunde" with illustrations of 313 species from locations in the Bay of Campeche in the southern Gulf of Mexico. Of these, 108 species were assigned to the genus Navicula, 36 to Amphora, 28 to Campylodiscus and 26 to Triceratium. Additionally, they included some infra-specific taxa that were considered as doubtful cases. Today, many of these names are no longer valid or their taxonomic status has changed. Through the 60's until the 80's, Soviet and Soviet-Cuban expeditions in the southern Gulf [12] found out that diatoms were the dominant group. Recently, many researchers reported the taxonomy and distribution of diatoms in this region [13][14][15][16][17][18][19][20]. However, they did not report many of the small and rare species.
This study is part of a large-scale phytoplankton relational database for the southern part of the Gulf of Mexico. The diatom section of this program comprises 434 diatoms with a total of 14801 entries between June 1979 and December 2010. The objective of this study was to compile a checklist of diatom species from the southern Gulf of Mexico and to provide an update of valid taxonomic names for each one.

The study area
The study area in the southern Gulf of Mexico (herein referred to as SGM) lies between 24°38´ and 18°15´ N and between 86°15´ and 98°12´ W ( Figure 1). The hydrographic conditions in this area are greatly influenced by the Loop Current and the detachment of anticyclonic eddies that migrate westward around the Gulf. These eddies tend to move northwards or southwards, depending on the expulsion of water masses. The south region of Campeche Bay has a predominantly cyclonic circulation mainly associated with the Yucatan Channel waters [21]. The presence of cold winds between October and April causes the formation of cold fronts. The dominant cyclonic circulation and fronts of the rivers create a dynamic system which provides the region with a unique environment. The Coatzacoalcos and the Grijalva-Usumacinta rivers represent approximately 11% of all fluvial discharges into the Gulf of Mexico. There is a notable presence of a permanent cyclonic eddy in the central region and beyond the continental shelf, as well as several lagoons that contribute to coastal outwelling [22][23][24]. More information concerning this area may be found in Yañez-Arancibia et al. [25]. The Yucatan Shelf is also greatly influenced by an upwelling in the north of Cape Catoche [26,27]. Consequently, while one portion of this water flows towards the west, the other part moves towards the east [28].
This region is a highly productive fishing area, and profitable for the oil industry. Both activities have turned this region into an economically important area, but potentially critical as well due to continuous oil spills and the presence of toxic substances in untreated water that affect this region [29]. It is important to recognize marine diatoms as indicators of modern changes in oceanographic conditions [5], and likewise for oil and gas exploration [30].

Sampling strategy and laboratory analyses
This diatom checklist was compiled from 20 oceanographic cruises between July 1979 and May 2010. Most surveys were done on board the R/V "Justo Sierra". During this period 647 sites were sampled (sometimes more than once) and the sampling stations covered the entire southern Gulf. Discrete water samples were taken by a CTD Neil Brown with a rosette of Niskin bottles and were preserved with acidified Lugol's solution. Vertical net samples were collected using 20 µm and 35 µm mesh-sized plankton nets within 5 m from the bottom to the surface were carried out at each sampling site and the samples were preserved with 2% neutralized formaldehyde. Most species were identified in water mounts or on an inverted light microscope. In addition, acid-cleaned samples were mounted in Naphrax [31]. In some cases, transmission or scanning electron microscope allowed us the identification of difficult species of the genera Amphora, Pseudonitzschia, Thalassiosira, Psammodictyon and many small taxa. The reported species composition is based upon the database sponsored by The National Council for the Study and Conservation of Biodiversity (CONABIO) [32,33].

Species identification
The identification of some species was achieved using classic books [8,11,[34][35][36][37][38][39][40][41][42], but specialized literature was needed for specific taxa. In addition, some diatom databases on web sites were also consulted [10,43,44]. Images in several databases illustrated intraspecific variability. Light and eventually electron microscopes were commonly used for routine analyses. In addition databases were used to review valid names to improve their spelling, and to standardize authorized names. Besides, there are pertinent links to obtain additional information, unless they are specifically unauthorized.
This checklist comes from a phytoplankton database that is at a medium stage of development. It contains mainly light micrograph images from 25 oceanographic cruises carried out at the SGM, also electron microscope photographs, light microscope digital images, information about samples, a short description of each species as well as a collection of permanent mounted slides from most net samples. This collection is known as the MEX-UNAM Diatom Collection and is stored at the Instituto de Ciencias del Mar y Limnología from the Universidad Nacional Autónoma de México under curation of the corresponding author.

Collection and identification techniques
This is the first account of diatoms from the SGM. A total of 430 taxa are recorded and no synonyms are listed in Table 1. The great majority of species are tropical, subtropical or cosmopolitan. Some species have been recorded as brackish-water, fresh water and benthic. Around 45% of the diatoms were identified during analysis of water samples with the inverted microscope (Carl Zeizz ICM405) to identify forms and structures of colonies. The standard phase contrast Carl Zeizz photomicroscope was useful to observe other structures of taxonomic importance (striation, rimoportulae and fultoportulae etc.).
The abundance of suspended sediments in the coastal zone hinders species identification, especially of "small centrics and pennates", and even medium-sized species of Thalassiosira, Diploneis, Fragilaria Lyrella, Navicula and Nitzschia. However some 90% of the recorded species can be identified on permanent slides under a light microscopy with a magnification up to X1200 (oil immersion, phase contrast and Nomarski optics). Scanning and transmission electron microscopes were required to identify certain species such as Pseudonitzschia, Minidiscus and many other small-celled taxa. There are several sampling techniques to study diatom species because there is a considerable variation in size, from approximately 3-5 µm to 1000 µm. Besides, it must be taken into account that rare species are usually low represented in water samples and net tows are selective.

Identification problems
It was difficult to distinguish closely related species even on permanent mounts since some descriptions were insufficient or inadequate. An example is the characterization of valves; they could be 'rectangular', 'capitate', 'small', 'neat' or 'lanceolate'. Their morphological variation can be very broad, even if the striation is measured. An electron microscopy is necessary to solve this problem. Diatoms are extremely diverse and there are many species that have not been described yet, the species delimitation is still controversial.

The use of databases
Online databases have been very useful, especially those that provide additional bibliographic information and species distributions. Some online diatom collections and databases offer open access with descriptions, images, environmental and bibliographic data, and in some cases, the use of software for various purposes. It is important to recognize that there is a continuous need to update taxonomy and it requires a careful surveillance. We were able to review all species listed on Table 1 with specialized literature and with on line databases since they provided us with the necessary information to take a decision.
A correct identification of every species in a given region is of great importance, since every taxon plays a role in the ecosystem.
In particular, small species (3-8 µm) are difficult to identify and an electron microscope is frequently required. On occasions, some of these species may become very abundant and widely distributed; they may even have blooms that affect the ecosystem. In the studied region we found several species with these characteristics: Minidiscus trioculatus, Cyclotella litoralis, Delphineis minutissima, Leptocylindrus minimus, Nitzschia bifurcata, Thalassiosira allenii, and Pseudonitzschia among others. The contribution of river flow and the discharge of several coastal lagoons into the region could explain the presence of fresh and brackish water species along the coast. Examples of these are many species of the genera Amphora, Achnanthes, Cyclotella, Diploneis, Navicula, and Nitzschia.

The potential use of validated diatoms checklists
In the past there was a tendency to erect taxa on the basis of tiny, subtle differences in morphology, sometimes in individual specimens without attempting to establish their stability. On the other hand, in the last 40 years, new genera or living diatoms have been discovered.
Validated and updated checklists are essential for ecological studies involving monitoring assessments or changes in the species composition in a given site. Without these checklists it would be impossible to detect changes in the structure of a community and the use of indicator species would be limited. There is a long-standing debate regarding the ecology of phytoplankton and this has been the controversy underlying a non-uniform distribution of species. A possible answer for the disagreements is that it depends on the spatial scale. With a scale of hundred kilometers, the differences between diatom communities are utterly related to geographic or hydrographic features. We were able to establish four regions in the southern Gulf of Mexico by using species association as part of this checklist [16]. Other authors have identified similar regions but using other organisms as a reference [45]. Another potential use for a diatom database is the detection of non-native species introduced via ship ballast water [30].
There is great risk that human activities could cause a loss in diatom biodiversity, it is fundamental to know about changes in species composition and its implications for ecosystem function. There is an urgent need to understand the marine ecosystems and other problems derive from natural and anthropogenic sources.
This checklist needs to be constantly updated, which is useful as a reference for ecological work considering that diatoms are good indicators of environmental changes. A good knowledge of the species composition is important to be competent in the recognition of changes in the paleo-environment.
Diatom species could indicate substantial differences and the degree of endemism in different regions, since they are indicators of the discharge of rivers and coastal lagoons [4,5]. Furthermore, diatoms are also useful for historical water quality assessments, bio monitoring and climate change., and additionally a checklist is important for the detection of non-native species, introduced by ballast water.