Epiphytic diatoms associated with red mangrove ( Rhizophora mangle ) prop roots in Bahía Magdalena , Baja California Sur , Mexico

The first floristic inventory of benthic diatoms is provided for the Bahía Magdalena-Bahía Almejas lagoon system. Samplings were carried out during November of 1999. The oxydized samples were mounted permanently. Eighty six diatom taxa were identified, out of which 59 are new records for the Bahía Magdalena area, and 12 taxa are new for the Baja California peninsula. Taxa recorded previously as rare in other substrata are common or abundant on the epiphytic macroalgae of mangrove prop roots. Other species are mainly epipelic forms, while 24 are commonly found as tychoplankton in the area. Certain taxa appear to be characteristic of mangrove systems in general. Rev. Biol. Trop. 54(2): 287-297. Epub 2006 Jun 01.

Much of the epiphytic flora growing on marine macrophytes are diatoms (Kita andHarada 1962, Jacobs andNoten 1980).These may contribute significantly to the primary production of mangrove systems where diatoms from different substrata are the main microalgae forms (Siqueiros Beltrones 2002).Epiphytes enter the second level of the food chain when they are grazed by invertebrates (Navarro 1987).However, information regarding other host-epiphyte relations in mangroves is lacking, only a few floristic inventories exist, with some remarks on possible interactions (Navarro 1988), while fewer yet intent to define the structure of the epibiosis (Inclán-Rivadeneyra 1989).
Biogeographycally the Bahía Magdalena system is located within a transitional zone where tropical and temperate elements coexist (Castro Aguirre and Torres Orozco 1993).Mangroves are characteristic of the Baja California Sur coasts and B. Magdalena represents the northern limit of their distribution in the north-eastern Pacific (Blasco 1984).Mangroves constitute one of the most conspicuous tropical components of the system, and along with the ichthyological component it may have persisted since the peninsula separated from the Mexican continent six to seven m.y.ago (Castro Aguirre and Torres Orozco 1993).
Currently this area is of great importance for the fishing industry of the state.Although ichthyological studies for Bahía Magdalena are available, which support some of the most important fisheries, research on the benthic microflora that relate to other resources is lacking.Recently the microflora related to abalone fishing sites just outside the lagoon system has been undertaken (Siqueiros Beltrones 2000), but for the lagoon itself only indirect observations of tychoplanktonic diatoms have been made (Gárate Lizárraga and Siqueiros Beltrones 1998) albeit lacking taxonomic precision.
In this study we show the first results of the taxonomic survey of diatoms found on red mangrove (Rhizophora mangle L.) prop roots in order to determine which diatom species may determine the structure of the epiphytic assemblage on mangrove prop roots.

MATERIALS AND METHODS
The sampling was carried out in Bahía Magdalena during November 1999.Twelve 30 cm 2 segments were peeled off from submerged and exposed prop roots of red mangrove plants.The epiphytic macroalgae found on submerged portions of the roots were separated under a dissecting microscope.The root segments were brushed using a toothbrush rinsing with water to obtain a concentrate of sediments, organic matter and diatoms that were observed in fresh mounts to determine the proportion of live cells.The diatom frustules from the roots and macroalgae were cleaned by oxydizing the organic matter using nitric acid and alcohol.Permanent preparations were mounted by triplicate using Meltmount (R.I. = 1.7) (Siqueiros Beltrones 2000).
Drawings were made of the observed taxa on curatorial cards which include information on location, habitat, frequency, and meristic and morphometric data.These were included along with the slides in the diatom collection (Diatomario) of the Museo de Historia Natural of the Universidad Autónoma de Baja California Sur.Microphotographs of representative taxa were taken under an Olympus CH-2 microscope with phase contrast illumination.
Study Area.Bahía Magdalena is a shallow lagoon located on the western coast of Baja California Sur, between 24 o 15' -25 o 20' N and 111 o 30' -112 o 12' W (Fig. 1).It is part of the Bahía Magdalena-Bahía Almejas lagoon system (Gárate Lizárraga and Siqueiros Beltrones 1998).The middle part of the Bahía Magdalena littoral where the sampling point was located (San Carlos docking area) is densely populated by mangroves mainly Rhizophora mangle and Avicennia germinans (black mangrove) bordering the coastline.

RESULTS
A total of 86 diatom taxa were identified as epiphytes of Rhizophora mangle prop roots (Table 1).59 taxa are new records for the system.
Most of the taxa in this study were better represented on the macroalgae (Ulva sp., 1 species of the gelidiales, and a Polysiphonia species) separated from the root segments which appeared heavily epiphytized.In vivo observations of macroalgal thalli showed the presence of multispecific colonial growths in mucilaginous films (Amphora bigibba, Amphora proteus, Amphora ventricosa, Caloneis elongata, Cocconeis dirupta, Cocconeis scutellum,  Taxa such as Navicula contenta, Navicula platyventris, Navicula pennata and Navicula salinarum and most of the Achnanthes species had been recorded elsewhere as epiphytes (Table 1) either from macroalgae or mangrove prop roots (Navarro 1987).Caloneis elongata, Navicula contenta, Navicula platyventris, Psammodyction constricta, Synedra gaillonii, and Trachyneis velata were abundant in our samples, although they are recorded for the first time for the peninsula, along with 6 other taxa (Table 1).
A representative array of the diatom taxocoenosis in the study site is presented in Figs.3-6.Notes: * New records for the area.E = Previously recorded as epiphytic on mangrove prop roots or as abundant in this study; T = recorded also as tychoplankton; O = also collected offshore.

DISCUSSION
Previous studies on benthic diatoms in the system comprising only the rocky environment just outside Bahía Magdalena account for the remaining 27 taxa recorded in the system (Siqueiros Beltrones 2000, Siqueiros Beltrones and Valenzuela Romero 2001).
On the other hand, otherwise rare taxa from previous records in the region such as Achnanthes yaquinensis, Amphora bigibba, Cyclotella stylorum, and Melosira nummuloides (Siqueiros Beltrones 2002), were either common or abundant.Such observations have been made also on Macrocystis pyrifera in undergoing studies (Siqueiros Beltrones et al. 2002).All this indicates that host-epiphyte relations are important for the growth of certain benthic diatoms and some may be specific, mainly of the epiphytic macroalgae, but apparently not for the mangrove roots.However, the specificity of the macroalgae-prop root relation should be analyzed in order to determine if an indirect association exists between diatoms and mangrove roots.
Several species, although not abundant occur frequently enough to be considered epiphytes (e.g.Amphora proteus, Amphora ocellata, Caloneis elongata, Caloneis linearis) but further observations are required to confirm this.Other taxa, such as Diploneis obliqua, Lyrella exsul, Petroneis granulata, Surirella fastuosa and Surirella fastuosa var.cuneata occur principally in sediments of mangrove systems (Siqueiros- Beltrones and Sánchez-Castrejón 1999).These species may be opportunistic colonizers from the bottom sediments that migrate upward on the sediments covering the prop roots.
The epiphytic or epipelic taxa from the B.C.S. mangrove systems are somewhat similar to those from mangroves in different parts of the world (Foged 1975, 1984, Navarro 1987) and Sinaloa, México (Moreno et al. 1996, Siqueiros Beltrones and Martínez López unpublished.).The current inventory of benthic diatoms from mangrove systems, including this paper, has yielded 181 new records altogether for the Baja California peninsula since 1999 (Siqueiros-Beltrones and Sánchez Castrejón 1999, Siqueiros-Beltrones and Morzaria Luna 1999).Although the northern limit for most of them is not known certain taxa appear to be characteristic of the mangrove environment and have not been observed in other localities along the peninsula, especially on the northern part (Siqueiros Beltrones 2002).Twenty four taxa from our inventory (Table 1) have been collected previously in phytoplankton tows (tychoplankton) representing an important part of the estimated biomass (Gárate Lizárraga and Siqueiros Beltrones 1998).Although the species were not then identified, we observed them recently in samples collected by the above authors at the northern mouth of the Bahía Magdalena system, and can be considered as common tychoplankton.Also, 21 of the benthic diatom taxa in this study (Table 1) have been collected in deep water traps deployed 300 m offshore north-west of Bahía Magdalena.The occurrence of benthic diatoms in the traps and in the samples from phytoplankton tows suggests that organic material is exported offshore through the northern opening of the system (Martínez López et al. 2004).
Tidal mixing has been identified as an important factor causing the presence of abundant benthic diatoms in the water column (Gárate Lizárraga and Siqueiros Beltrones 1998).However, much work has to be done to complete the floristic inventory and analyze the association structure of benthic diatoms from the Bahía Magdalena-Bahía Almejas lagoon system.This information may provide important tools to explain oceanographic, ecological, and biogeographical phenomena in the area.

ACKNOWLEDGMENTS
The sampling for this study was carried out during field trips for the Microalgas Marinas course of the Departamento de Biología Marina of the Universidad Autónoma de Baja California Sur.The Museo de Historia Natural of the Universidad Autónoma de Baja California Sur financed laboratory material for processing the samples.Elisa Serviere from the Centro de Investigaciones Biológicas del Noroeste aided in the identification of the macroalgae.

Fig. 1 .
Fig. 1.Location of the Bahía Magdalena mangrove system and sampling site.