Rotifers are widely recognized as being important components of freshwater ecosystems, and whether this assessment is based on numbers or biomass, their contribution to trophic dynamics in these waters is striking. In some instances their importance even exceeds that of the microcrustaceans: cladocerans and copepods 1. In estuarine and marine habitats, rotifers are generally thought to play a minor role in community dynamics 2345. Therefore brackish and marine rotifers, with the notable exception of the Brachionus plicatilis species complex, have received little attention worldwide. Because of its value in aquaculture 678, this species complex has received special attention and this intense study has yielded valuable insights into evolutionary processes such as cryptic speciation and molecular phylogenetics 9101112 and genomics 13. In addition, rotifer species inhabiting saline and subsaline lakes in northern Canada possessed greater haplotype diversity than their freshwater counterparts 14.

While quite a bit is known about zooplankton present in México (e.g., copepods 1516; cladocerans 17, few reports have been published on brackish and marine rotifers 31819. Although some of these studies have focused on species diversity and community dynamics 4, none of them have included the Chihuahuan Desert.

With increased exploitation of aquifers for agriculture, cattle, industry, and drinking water, we can expect an increase in the salinization of existing watersheds and water sources particularly in arid areas. These changes will negatively impact ecosystem processes 20. Here we examine selected inland saline waters in the Chihuahuan Desert of México, a region renowned for its high biodiversity in terrestrial and aquatic systems 21222324. We also present an initial species list of the rotifers, group sites by water chemistry, conduct pair-wise comparisons of rotifer community diversity between sites, and investigate ecological correlates of rotifer presence/absence.

While zooplankton inhabiting saline aquatic habitats have received some attention worldwide (e.g., China 5, Spain 26272829, Canada 1430, Western US 3132, Africa 33343536, Japan 37, Australia 38394041, Arabia 42), there is a genuine need for additional studies of rotifers in saline and marine environments of México 3. A recent study noted that 74% of rotifer species in México (n = 42) were cosmopolitan, 5% were restricted to North America, 10% were tropical, and 4% were shared with Europe-Asia-Africa 43. Most work on rotifers in saline habitats in México has been done by Sarma and his colleagues. For example, Sarma & Elías-Gutiérrez 44 found 31 rotifer species in their survey of an estuarine lagoon; 11 of which were found in our study (Platyias quadricornis, Tripleuchlanis plicata, Colurella uncinata, Lepadella ovalis, Lecane bulla, L. closterocerca, L. hornemanni, L. luna, L. obtusa, L. pyriformis, and L. thalera). In addition, Sarma et al. 3 reported 37 species of rotifers in Mecoacan, a brackish (5–35 ‰) lagoon located in Tabasco. Our survey of 48 waterbodies in the Mexican Chihuahuan desert shared few of these species (Anuraeopsis fissa, Ascomorpha saltans, Brachionus angularis, Euchlanis dilatata, and Lecane bulla) all of which have reportedly cosmopolitan distributions 45. Another recent study 19 reported 128 taxa from 36 aquatic sites in southeastern México including some brackish habitats. Of these species, 23 (A. fissa, B. bidentatus, C.obtusa, C. uncinata, E. dilatata, K. americana, L. arcula, L. bulla, L. closterocerca, L. cornuta, L. crepida, L. hornemanni, L. leontina, L. luna, L. lunaris, L. obtusa, L. spinulifera, L. thalera, L. triptera, Platyias quadricornis, Polyarthra dolichoptera, Scaridium bostjani and Tripleuchlanis plicata) also were found in our survey.

In a larger study of freshwater habitats in the Méxican Chihuahuan Desert that includes the sites reported here, Wallace et al. 25 note that many species occurred as singletons or doubletons, and species inhabiting particular sites are quite unique. Here we further address community similarity among high salinity habitats using the SAI, and again the uniqueness of communities is apparent (Table 2). While our study adds substantially to the characterization of rotifer communities, clearly much more research needs to be accomplished if we are to develop a good understanding of the biogeography of rotifers in saline waters in North America.

The saline systems in the Mexican part of the Chihuahuan Desert have less total dissolved solids and lower conductivity than some of the waters of the Northern Chihuahuan Desert in the United States, notably those at White Sands National Monument, New Mexico and the Bottomless Lakes near Roswell, New Mexico. Only few species typical for saline waters were found over a wide range of aquatic habitats in the Chihuahuan Desert, such as Proales similis. However, a redundancy analysis performed on the data of all the saline systems that we have sampled in the Chihuahuan Desert, showed that the presence or absence of macrophytes is an important variable in determining species composition in all of these systems (unpublished data).

High salinity and conductivity levels can have major impacts on zooplankton community structure. A study in coastal lakes found that salinity level had significant impacts on zooplankton 46, leading the authors to predict that relatively small increases in salinity levels will cause reduced biodiversity of freshwater ecosystems. In a mesocosm experiment manipulating salinity, Hart et al. 31 found dramatic shifts in zooplankton community structure and shifts in the abundance of many species. As salinity increased, densities of the dominant rotifer species decreased and at the highest salinities 2 species were reduced to very low numbers. Shiel and his colleagues found that salinity was a significant, but site-specific, factor in determining rotifer community composition in rivers in the Lake Eyre Basin 41. Saline systems had reduced species richness compared to their freshwater analogs (0–4 versus 0–31). In the Mexican saline systems studied here, as salinity increases the number of species found decreases substantially (Table 1; Fig 1, 2, 3). Chloride is a significant factor in determining the occurrence of rotifers. Some species are positively correlated with chloride content and others negatively associated (see Results, Fig 2a, 3a). It appears in our systems that typical planktonic freshwater species (e.g., Asplanchna brightwellii, A. priodonta, K. americana, K. cochlearis, and Synchaeta pectinata) are replaced by salinity tolerant species such as Hexarthra oxyuris and Notholca acuminata.

Inland saline systems often harbor diverse and unique community assemblages. Unfortunately, human exploitation can be extremely disruptive to ecosystem processes and services provided by these important water sources. During our sampling efforts, several historical springs near Janos, México were dry. In addition, Ojo de la Casa has recently dried and Ojo en de Medio has dried and re-surfaced in the past year, probably due demands of a geothermal electrical plant for cooling water and agricultural and domestic uses. Increasing human population size and global climate change will only make this scenario more prevalent. Thus, it is imperative that governmental agencies establish policies that protect these fragile ecosystems 47.

The authors declare that they have no competing interests.

EJW, TS, RLW, JVRA, and RRM all participated in collecting trips and in species identifications. EJW drafted the manuscript. TS ran the statistical analyses (three-way plots of water chemistry and Redundancy Analyses). RLW calculated the Jaccard's and Simpson's Asymmetric Indices. All authors read and approved the final manuscript.