Phytoplankton of the northern coastal and shelf waters of the Yucatan Peninsula , southeastern Gulf of Mexico , Mexico

stations beyond the 200-m isobath were considered. Coastal samples were collected at 17 sites along the coast line between Chuburná and Dzilam de Bravo (Figure 1). Sea water was taken at 0.5-1.0 m depth, 10-20 m from the beach; the water temperature ranged from 20.3 to 35.9oC and salinity ranged from 27.0 to 39.4 (down to 15.0 in the marina of Dzilam de Bravo). The samples were analyzed quantitatively using the Utermöhl method (Hasle 1978). Carl Zeiss Axiovert 100 and Olympus CK2 inverted microscopes were used. Identifications were principally made at the species level when possible; otherwise, identification was at the generic level. Tychoplanktonic species were also included. Information about the main habitat for some species of cyanobacteria, chlorophytes and pennate diatoms such as Haslea spp. remain unknown. AlgaeBase was consulted to verify currently accepted taxonomic names (Guiry and Guiry 2012). Abbreviations of authors of scientific names are used according to Brummit and Powell (1992) unless they were not listed in the book. The major eukaryotic groups are ordered in the list according to Adl et al. (2012). For diatoms, the division into three classes by Round et al. (1990) was followed, taking into account the most recently published catalogue of diatom genera (Fourtanier and Kociolek 1999).


Introduction
While there are checklists of both diatoms and dinoflagellates found in the Gulf of Mexico as a whole (Krayevsky et al. 2009, Steidinger et al. 2009) and a checklist of dinoflagellates of the southern Gulf of Mexico (Licea et al. 2004), information on planktonic algae of the Yucatan waters is scarce.Some data can be found in recently published literature (Licea et al. 2004;Troccoli Ghinaglia et al. 2004;Álvarez-Góngora and Herrera-Silveira 2006;Herrera-Silveira and Morales-Ojeda 2009;Álvarez-Góngora et al. 2012) and in a number of unpublished bachelors, masters and doctoral theses; the latter are considered gray literature and are not cited here.
To provide a preliminary list of both planktonic and tychoplanktonic microalgae found in the coastal waters of the northern Yucatan Peninsula, based on original waterbottle samples, was the main purpose of this study.

Materials and Methods
A dry warm climate characterizes the study area.Three seasons can be distinguished: a dry season from March to early June, a rainy season from June to October, and the "nortes" (northerly winds) season with short periods of storms and strong winds coming from the north, from November to February (Herrera-Silveira 1993).
Based on recently published literature on diatoms and dinoflagellates (Okolodkov et al. 2007;2011a;Okolodkov, 2008;2010;Krayevsky et al. 2009;Steidinger et al. 2009;Aké et al. 2012) and Wood (1968), new records for the Gulf of Mexico were revealed (Table 2).Based on the list of diatom species by Krayevsky et al. (2009), five diatom species are new records for the Gulf of Mexico, although we prefer to consider them new records for the southern (Mexican) Gulf of Mexico (Table 3) because we are unaware of the most recent advances in the diatom floristics in the northern/U.S. territorial waters of the Gulf of Mexico.Out of five new records of diatom species, four are tychoplanktonic.The dinoflagellates Oxytoxym constrictum and O. tesselatum are absent in the list of Steidinger et al. (2009); however, they were reported from the Straits of Florida and the Caribbean Sea (Wood 1968).Thus they can be considered new records for the southern Gulf of Mexico.The highest occurrences were shown by Neostreptotheca subindica and Oxytoxum tesselatum; four species were found exclusively during coastal surveys (Table 3).Scrippsiella spinifera, originally described from the Mediterranean, was also observed in the northern Yucatan waters as a relatively common species accompanying a bloom caused by S. trochoidea   Troccoli Ghinaglia et al. 2004), although it is not included in a checklist of dinoflagellates for the Gulf of Mexico published more recently (Steidinger et al. 2009).Therefore, we report it here for the first time for the state of Yucatan.
Most of the tychoplanktonic diatoms (57 species out of a total of 152 diatoms, or 37.5%) were also observed principally from coastal samplings.The portion of freshwater species (mainly cyanobacteria and chlorophyceans) was pronounced (21 species, or 6.8%); however, they were never observed as abundant or even frequent in terms of the number of cells.The small number of benthic dinoflagellates found in plankton samples, compared to diatoms, can be explained by the rather low species richness of benthic dinoflagellates in general: at the northern Yucatan coast, 20 epiphytic dinoflagellate species from 12 genera were encountered (Okolodkov et al. 2011b).It is widely known that benthic diatoms outnumber benthic dinoflagellates in the number of species both locally and globally.
Due to the optical limitations of inverted microscopes, we failed to identify Pseudo-nitzschia species.Nevertheless, our findings in coastal waters of the State of Veracruz (Parsons et al. 2012) imply the occurrence of at least five toxic species that can cause amnesic shellfish poisoning.In contrast, among dinoflagellates the potentially toxic species were easier to identify.Most of these species were rare in the samples.Only Prorocentrum minimum was observed in high abundances in marinas (particularly in Chuburná, Yucalpetén, Telchac y Dzilam de Bravo; stations 1, 4, 9 and 17 of the coastal surveys; (Figure 1), up to 7.1x10 7 cells l -1 (Merino-Virgilio et al. 2011a).Another species, Pyrodinium bahamense var.bahamense, considered potentially toxic only due to a single report on the toxicity of var.bahamense in the coastal waters of East Florida (Landsberg et al. 2006), reached the population density of 1.8x10 5 cells l -1 (Merino-Virgilio et al. 2011b).In total, 18 potentially toxic species (16 dinoflagellates and two cyanobacteria) were found.
The presented list can be considered preliminary.Among planktonic diatoms, we expect to identify more Thalassiosira Cleve and Coscinodiscus Ehrenb.species, as well as even more tychoplanktonic diatoms, above all, from the genera Licmophora C. Agardh, Amphora Ehrenb.ex Kütz., Nitzschia Hassall, Navicula Bory, Pleurosigma W. Sm., Cocconeis Ehrenb.and some others that escaped our attention during routine counts and that need a thorough examination of frustule morphology.As regarding flagellates, the species diversity, first of all, of naked dinoflagellates from the order Gymnodiniales and nanophytoflagellates was underestimated due to methodological problems.

Figure 2 .
Figure 2. Distribution of major taxonomic groups of phytoplankton in the coastal and shelf waters of the northern part of the Yucatan Peninsula, southeastern Gulf of Mexico.

Table 1 .
Hydrological (at 10 m depth) and bathymetrical data on oceanographic cruises around the northern Yucatan Peninsula.

Table 3 .
New records of microalgae for the southern Gulf of Mexico (CS -coastal surveys).