Introduction

The genus Ruppia (L. 1737) (family Ruppiaceae) belongs to a taxonomic group of aquatic vascular plants that lives in poikilosaline environments, which means that it tolerates hypersaline, brackish, and freshwater conditions (Setchell 1924; Kuo and den Hartog 2001; Den Hartog and Kuo 2006). There has been some taxonomic disagreement among specialists concerning the number of species within this genus, which has resulted in many proposed species, most of them being morphotypes of Ruppia maritima L. (1753) (Setchell 1946; Phillips 1958; Richardson 1980; Den Hartog 1981; Kantrud 1991; Zhao and Wu 2008). Ruppia maritima (also known as wigeongrass) is the most representative species of the genus because of its wide distribution in temperate and tropical latitudes in both hemispheres (Oliveira et al. 1983; Orth and Moore 1988; Reyes and Merino 1991; Malea et al. 2004; Keates and Osher 2007). It is a narrow-leaved species, characterized by a fast growing rate and low underground and aboveground biomasses. These characteristics give R. maritima the possibility to be an excellent competitor, able to colonize large areas in short periods of time after a disturbance or when unfavorable environmental conditions for climax seagrass species occur (temperature, light, salinity). Sea temperature fluctuations produced by global warming or El Niño Southern Oscillations (ENSO) are responsible for major reductions of seagrass populations around the world (Johnson et al. 2003; Ehlers et al. 2008). In these conditions, it is possible that climax seagrass species, like Thalassia testudinum (Koenig) (Robblee et al. 1991) or Zostera marina (L.) (Orth and Moore 1988), could be partially or completely replaced by opportunistic species (i.e., seaweeds or R. maritima), producing a radical change in the trophic structure of the community (Cho et al. 2009).

There are few studies that focus on the distribution or abundance of R. maritima along the littoral zone of México. One possible reason for this is the common knowledge that this species thrive in brackish–freshwater habitats (Oliveira et al. 1983; Kuo and den Hartog 2001). Therefore, botanists tend to overlook this species in the marine environment, perhaps misidentifying Ruppia sp. as Halodule sp. Wigeongrass is distributed along the Pacific and Atlantic coasts of México (Dawson 1962; Felger et al. 1980; Flores-Verdugo et al. 1988; Reyes and Merino 1991). At the Gulf of California and the west coast of the Baja California peninsula, R. maritima has been documented as a component of the seagrass community in some wetlands (Meling-López and Ibarra-Obando 1999; Santamaría-Gallegos et al. 2001, 2007; Riosmena-Rodríguez et al. 2010). However, information regarding R. maritima is not complete as records referring its presence do not give any information on its geographic location within these wetlands or its abundance. This fact could indicate that the presence of R. maritima was too low compared to the presence of the seagrasses Z. marina (also known as eelgrass), Phyllospadix torreyi (S. Watson,) and Phyllospadix scouleri (Hooker) at the time of the surveys.

Dawson (1962) was one of the first to report R. maritima in northwest México, specifically at Bahía San Quintín (Fig. 1). Oliva-Martínez and Ortega (1983) studied the submerged vegetation in two wetlands in the state of Sinaloa and found that R. maritima was the only vascular plant present. This is one of the few works that illustrates the spatial location of R. maritima and includes a qualitative estimation of its spatial and temporal distribution. Felger et al. (1980) and Felger and Moser (1985) reported the presence of R. maritima at the Gulf of California, in the water body between Tiburon Island and the state of Sonora, a region known as Canal de Infiernillo (Fig. 1, site 18). Ramírez-García and Lot (1994) published the most extensive description of R. maritima for the Gulf of California; they report its presence at 11 sites, ranging from Angel de La Guarda Island in the north, to Cabo San Lucas at the southern end of Baja California Sur (Fig. 1, sites 16 and 7, respectively). In the present study, we include data from recent surveys made at the northern end of the Gulf of California, Canal de Infiernillo, and along both coasts of the southern Baja California peninsula (Fig. 1, underlined localities), expanding our knowledge on the variability of R. maritima meadows.

Fig. 1
figure 1

Location sites for Ruppia maritima in northwest México. Underlined numbers are localities sampled for this work. 1 Bahía San Quintín (Dawson 1962; Ward et al. 2004); 2 Scammon’s Lagoon (Santamaría-Gallegos et al. 2001); 3 Punta Abreojos; 4 Laguna San Ignacio (Santamaría-Gallegos et al. 2001); 5 Bahía Magdalena (Santamaría-Gallegos et al. 2007; Riosmena-Rodríguez et al. 2010); 6 Bahía Almejas (Santamaría-Gallegos et al. 2007); 7 Cabo San Lucas (Ramírez-García and Lot 1994); 8 Bahía San José del Cabo (Ramírez-García and Lot 1994); 9 Bahía de La Paz (Ramírez-García and Lot 1994); 10 Bahía Balandra; 11 Bahía San Gabriel at Espiritu Santo Island (Ramírez-García and Lot 1994); 12 Loreto (Ramírez-García and Lot 1994); 13 Bahía Concepción (Ramírez-García and Lot 1994); 14 Mulege (Ramírez-García and Lot 1994); 15–16 Angel de La Guarda Island (Ramírez-García and Lot 1994); 17 Hardy River and Santa Clara Wetland; 18 Canal de Infiernillo (Felger et al. 1980; Ramírez-García and Lot 1994; Torre 2002); 19 Bahía Kino (Meling-López, unpublished data); 20 El Siuti (Meling-López, unpublished data); 21 Bahía Lobos (Meling-López, unpublished data); 22 El Tobari (Meling-López, unpublished data); 23 Isla Siari (Meling-López, unpublished data); 24 Santa Barbara (Meling-López, unpublished data); 25 Yavaros-Moroncarit (Meling-López, unpublished data); 26 Agiabampo (Meling-López, unpublished data); 27 Bahía Topolobampo (Ramírez-García and Lot 1994); 28 Laguna El Verde (Flores-Verdugo et al. 1988); 29 Laguna Caimanero-Huizache (Oliva-Martínez and Ortega 1983)

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Recently, studies have been done on the vascular vegetation in three major coastal lagoons on the Pacific side of Baja California Sur: Laguna Ojo de Liebre (Scammon’s Lagoon), Laguna San Ignacio, and Bahía Magdalena (Fig. 1). Santamaría-Gallegos et al. (2001) studied Laguna San Ignacio and Laguna Ojo de Liebre during February and June 1998 and reported the presence of R. maritima and Z. marina for both lagoons in February and the absence of intertidal meadows in June. Santamaría-Gallegos et al. (2007) focused on the ecology of Z. marina at Bahía Magdalena; they mentioned the presence of P. scouleri and P. torreyi in the high-energy outer shore of the bay, and that R. maritima was found with relatively low abundance inside the bay (the authors did not mention specific geographic coordinates). Bizarro (2008) published a review of the physical and biological characteristics at Bahía Magdalena but the only vascular plants he refers to as present in the bay are three species of mangrove (Avicenia germinans L.; Laguncularia racemosa L.; Rhizophora mangle L.) and two species of seagrass (Z. marina and P. torreyi).

The purpose of this study was to document the increasing importance of the presence of R. maritima in three wetlands of Baja California Sur México (Punta Abreojos, Laguna San Ignacio, and Bahía Magdalena), based on three main aspects: the increase of its biomass, the increase of its spatial distribution, and its key trophic role in some of these wetlands. We discuss the implications of this increase for the seagrass management strategies of marine protected and non-protected areas.

Materials and methods

During the course of extensive evaluations of seagrass populations (Fig. 1, sites 3, 4, 5, 7, 8, 9, 10, 13, 17, 18) and feeding habits of the east Pacific green turtle (also known as black turtle, Chelonia mydas agassizii Cornelius) in northwest México, we have found noticeable amounts of Ruppia maritima growing close to Z. marina meadows in areas and densities unknown for the northwest of México (Fig. 1; sites 3, 4, 5, 9, 10, 13, 17, 18), and in other areas we noted the absence of this species (Fig. 1, sites 7 and 8). At Laguna San Ignacio, we extensively sampled the bed size and shoot density and collected specimens from the subtidal zone (1–3 m depth) in a locality known as El Mapache (26°50’8.88”N, 113°15’30.3”W; site 4, Fig. 1) in September 2008, December 2008, and April 2009 (Fig. 2; Table 1). We obtained several samples of stomach contents from dead green turtles found along the shore, as a result of incidental bycatch during June 2008 (Table 2); during May 2009, we collected oesophagic contents from nine live turtles at a locality called La Piedrita (26°56’15.71”N, 113°9’34.51”W).

Fig. 2
figure 2

Shoot density for Ruppia maritima (solid bars) and Zostera marina (dashed bars) at two sites: Bahía Magdalena (solid light-gray and horizontally-dashed bars) (BMA; site CFE) and Laguna San Ignacio (solid dark-gray and diagonally-dashed bars) (LSI; site El Mapache)

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Table 1 Size (in ha) of meadows of Ruppia maritima and Zostera marina (in parenthesis) in two localities: Bahía Magdalena (site CFE) and Laguna San Ignacio (site El Mapache), in winter 2008 and spring 2009
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Table 2 Percentages of primary food sources for green turtle (Chelonia mydas agassizii) of Laguna San Ignacio (LSI; sites NID and La Piedrita), Bahía Magdalena (BMA; site Estero San Buto), and Punta Abreojos (PAO; site Estero El Coyote) in summer 2008 and spring 2009
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At Bahía Magdalena, we extensively sampled the bed size and shoot density and collected specimens from the subtidal zone (1–3 m depth) in a locality adjacent to the Comisión Federal de Electricidad (24°48’44.88”N, 112°5’58.85”W; site CFE, Table 1) in November 2008, December 2008, February 2009, and May 2009 (Table 1). All bed-size estimations were done following the meadows’ outer contour from a 7-m-long outboard motorboat using a GPS unit (Trimble Juno ST®). We also sampled oesophagic contents from several live green turtles inside Estero Banderitas (24°54’53.01”N, 112°7’4.03”W), Estero San Buto (24°49’55.39”N, 112°2’51.93”W) during August 2008 (both estuaries are part of Bahía Magdalena), and Punta Abreojos (26°49’10.05”N, 113°27’17.41”W) during March 2009 (Fig. 1, sites 5 and 3). Oesophagic contents of green turtles were sampled and analyzed as described by Forbes and Limpus (1993). We present the percentage of consumption of R. maritima (Table 2) by captured green turtles in relation to the main constituents of their diet (Z. marina and seaweed).

We were able to perform additional seagrass surveys at Bahía Balandra (site 10; Fig. 1) and at Bahía de La Paz (site 9) in May 2009. We surveyed El Requeson beach at Bahía Concepción (26°38’23.53”N, 111°49’56.12”W; Fig. 1, site 13) in December 2008 and June 2009. Visits to Bahía San Jose del Cabo and Cabo San Lucas (Fig. 1, sites 8 and 7) were done in December 2008 and March 2009. In the case of Canal de Infiernillo (Fig. 1, site 18), we visited the area in March 2009, October 2009, March 2010, and April 2010, and found populations of R. maritima in the last three samplings. At the northern end of the Gulf of California, near the Colorado River Delta (Fig. 1, site 17), we found specimens of R. maritima growing abundantly in two habitats: the agricultural drains near the Hardy River (32°13’57.24”N, 115°18’22.07”W; 32°9’3.70”N, 115°16’53.11”W) and at the border of the Santa Clara Wetland (32°2’0.39”N, 114°51’22.27”W); during February, May, June, and October 2005.

Extensive collections of R. maritima specimens were done in both described sites (El Mapache and CFE). Detailed morphological evaluations of the sampled populations at Laguna San Ignacio and Bahía Magdalena were done using all the plants collected in 3 replicate sediment cores (around 100 plants per core).

Results

Based on the literature survey and our sampling efforts, we found 29 sites with records for Ruppia maritima (Fig. 1). Out of the total, in five we were able to confirm the presence of the species (sites 4, 5, 9, 13, and 18), three were new records (sites 3, 10, and 17) and in two, we were not able to find the species again and the record remains unconfirmed (sites 7 and 8). Shoot densities were high in autumn (1,600 shoots m−2) with constant decrements in both localities (Laguna San Ignacio and Bahía Magdalena), down to a minimum of 200 shoots m-2 in spring (Fig. 2). At Bahía Concepción (site 13), shoot densities were higher in June 2009 (1,900 ± 100 shoots m−2) than in December 2008 (1,500 ± 50 shoots m−2) (± 1SD). At Bahía Concepción, we have been performing periodic botanical surveys since 1989, and before 2006 we have not found evidence of R. maritima shoots. Something similar occurred for other wetlands from Baja California Sur. We have visited Bahía Balandra (site 10) at least twice a year since 1986, and it was not until 2009 that we found the first specimens of R. maritima. At Laguna San Ignacio, members of the Programa de Botánica Marina made surveys in the months of January and June 1997, 1998, 2000, 2001, 2003, and 2004 without any report of R. maritima; it was not until September 2008 that we made the first observations for the species. Non-systematic visits to Bahía Magdalena were done from 1989 to 1995 and then again from 2004 to 2007; nevertheless, the first reports of R. maritima for this lagoon came from 2006 and then again in 2008.

We found significant seasonal differences (p < 0.05) but no significant spatial differences for R. maritima shoot densities between Laguna San Ignacio (site El Mapache) and Bahía Magdalena (site CFE), with the highest variability in December 2008 for both lagoons (Fig. 2). A similar trend was observed for the seasonality of bed size at both lagoons (Table 1). R. maritima bed size at Bahía Magdalena was high in late fall and early winter (Table 1) with lowest values in late winter (February 2009) and spring (May 2009); likewise at Laguna San Ignacio, R. maritima bed size was highest in December 2008 (6.70 ha) and lowest in April 2009 (0.08 ha). Presence of Zostera marina for both sites was low. At Bahía Magdalena, patches of Z. marina were absent in November 2008, appeared in December with an average shoot density of only 400 shoots m−2 and disappeared again in February 2009 (Fig. 2; Table 1). Eelgrass meadow appeared again at the CFE site in May with its maximum bed size and shoot density (1.5 ha; 520 shoots m−2), while wigeongrass had almost disappeared (0.01 ha; 368 shoots m−2). At Laguna San Ignacio, R. maritima winter bed size was twice as large (6.7 ha) as at Bahía Magdalena (3.4 ha); likewise, winter eelgrass bed size was larger at Laguna San Ignacio (17.49 ha) than at Bahía Magdalena (0.24 ha), significantly decreasing from December (17.49 ha) to April (0.89 ha).

Six morphological vegetative characteristics for the collected R. maritima specimens were compared: distance between rhizome nodes, leaf length, leaf width, sheath length, leaf apex, and presence of a central vein in the leaf. All leaves showed serrated leaf apexes and uninervated leaves. There were no significant differences (p > 0.05) between sites (CFE and El Mapache) in the distance between rhizome nodes (0.82 ± 0.28 cm; 0.40 ± 0.20 cm), leaf length (4.50 ± 1.42 cm; 3.41 ± 0.74 cm), leaf width (0.55 ± 0.17 mm; 0.40 ± 0.08 mm), and sheath length (1.18 ± 0.22 cm; 1.31 ± 0.51 cm).

According to oesophagic contents, the largest constituent of green turtle diet was between Gracilaria textorii (43%, Laguna San Ignacio), Z. marina (44%, Laguna San Ignacio), Gracilaria vermiculophylla (58%, Bahía Magdalena), and R. maritima (24%, Punta Abreojos). Wigeongrass constituted between 0.40 and 24.33% of the diet of green turtles at these three sites, while eelgrass values were between 3.33 and 44.21% (Table 2).

Discussion

There is an outstanding expansion of Ruppia maritima’s distribution in northwest México compared to its historical known distribution, based on our current sampling efforts and the periodic measurements performed for over 20 years (since 1986) as part of the research conducted by the Programa de Botánica Marina in several wetlands of northwest México (Riosmena-Rodríguez and Sánchez-Lizaso 1996; Sánchez-Lizaso and Riosmena-Rodríguez 1997; Meling-López and Ibarra-Obando 1999; Santamaría-Gallegos et al. 2001, 2007; Riosmena-Rodríguez et al. 2010; Rodríguez-Salinas et al. 2010). In 4 of the 10 sampled sites for this work (Bahía Magdalena, Bahía Balandra, Laguna San Ignacio, Bahía Concepción), it seems that the appearance or greater abundance of R. maritima begun around 2007.

Morphological measurements of our specimens (apex serrated, leaf uninervated, leaf width and sheath length) did not show significant differences (p > 0.05) with reported measurements (Oliveira et al. 1983), with the exception of leaf length and distance between rhizome nodes that were significantly smaller (at both sites). These differences could be a phenotypic response of R. maritima to environmental conditions. Sampled sites at Laguna San Ignacio (El Mapache) and Bahía Magdalena (CFE) are low energy subtidal environments with an average depth of 2 m; sediment texture was a mixture of fine-sand and silt, and sediment color was black just a few millimeters below the surface indicating an important content of organic matter (sediment odor was also indicative of this). Temperature during sampling was around 20°C (December 2008). Thus, seagrasses at both sites experience roughly the same temperature, light availability and sediment characteristics, all of them contributing to the observed phenotype.

Wigeongrass meadows of these bed sizes and densities were not observed a few years ago in northwest México. First reports available on the presence of R. maritima from the 1980s and 1990s (Ramírez-García and Lot 1994; Santamaría-Gallegos et al. 2001) perhaps correspond to small populations inhabiting protected shallow-muddy areas inside some estuaries. So, the large populations currently located at Laguna San Ignacio (El Mapache) and Bahía Magdalena (CFE) could have two origins (or a combination of both). They could come from local R. maritima populations which experienced an anomalous expansion triggered by a change in environmental conditions (global warming, ENSO, eutrophication) or from alloctonous populations brought by migratory species (waterfowl) traveling each year to northwest México which transport wigeongrass seeds or shoots attached to their bodies or within their digestive tracts (Charalambidou et al. 2003), and ship traffic coming from around the world to the Baja California peninsula, bringing material attached to the ship’s hull or trapped in the bilge water. A phylogenetic analysis of these invasive populations will be an excellent method to determine their biogeographic origin.

There is ongoing competition for substrate between R. maritima and Z. marina at Laguna San Ignacio and Bahía Magdalena. Former monospecific eelgrass populations at El Mapache and CFE now show, during autumn and winter, a dominant or large component of wigeongrass (Table 1, Fig. 2) while eelgrass now seems to occupy the space during winter and spring. This is more clearly observed at CFE site where a larger and denser eelgrass meadow occurred during May, when the wigeongrass meadow was at its minimum. An eelgrass population was not present at the CFE site during November and February (Table 1), possibly because of inadequate environmental conditions or competition for space with the R. maritima population. The increasing abundance of wigeongrass in habitats previously restricted to eelgrass or other climax species has been reported elsewhere (Cho et al. 2009) and is partially a consequence of global warming and ENSO phenomena. Temperature is a fundamental factor for the survival of seagrasses. It has been reported that R. maritima is a species with high tolerance to temperature (10–30°C) and salinity (0–35) variations (Phillips 1960; Lazar and Dawes 1991; Cho et al. 2009) and that the thermal (10–20°C) and saline (10–25) tolerances for Z. marina are narrower (Nejrup and Pedersen 2008). Therefore, R. maritima has an advantage over Z. marina to occupy shallow areas that experience temperature or salinity increases. Water temperature inside Bahía Magdalena is highly dominated by temperature variability of the neighboring ocean; moreover, the impact of interannual phenomena like El Niño/La Niña tends to be equal or larger inside than outside the lagoon (Lluch-Belda et al. 2000). It has been reported that the anomaly of sea-surface temperature in this area has been increasing since the first half of the twentieth century and that the increase is steeper since the middle 1970s (Lluch-Belda et al. 2000). So, the declining of Z. marina meadows from El Mapache and CFE sites could be a consequence of this ocean warming. It is possible that something similar is happening at Laguna San Ignacio. It is worth mentioning that Bahía Magdalena is the southern limit for Z. marina in the eastern North Pacific (Riosmena-Rodríguez and Sánchez-Lizaso 1996).

The loss of eelgrass meadows in recent years at the lagoons of northwest México already has consequences on the biodiversity and trophic structure of these wetlands. The Pacific black brant (Branta bernicla nigricans Lawrence) is a goose that migrates each winter from Alaska and Canada to the wetlands of southern California and northwest México to feed on eelgrass meadows (Ward et al. 2005). The B. bernicla population has been steadily declining since 1990 (Sedinger et al. 2007) and is changing its wintering grounds from Baja California peninsula to more northward regions (California, Washington, Oregon) due to less availability and abundance of Z. marina in México (Ward et al. 2005, 2009). Although there are some studies that report wigeongrass as part of B. bernicla’s diet (Ward 1983; Lindberg et al. 2007), most published works agree that eelgrass is the preferred food source for the Pacific black brant population (Nienhius and Ierland 1978; Ganter 2000; Ward et al. 2005).

R. maritima and its ecological role in the wetlands of northwest México have increased. It is known that Z. marina constitutes the main seagrass food source in the diet of immature green turtles (Chelonia mydas) that use several northwest Mexican wetlands as nurseries (Lopez-Mendilaharsu et al. 2002, 2005, 2008). We were able to confirm this for two of our sampled wetlands: Laguna San Ignacio and Bahía Magdalena. However, at Punta Abreojos, this did not hold true: Z marina was replaced by R. maritima as the major seagrass food source for this species. Even more, wigeongrass was the largest food source for the juveniles of C. mydas living at Punta Abreojos and the third largest food source at Laguna San Ignacio and Bahía Magdalena (Table 2). At Bahía Magdalena, we also sampled in the locality Estero Banderitas and found R. maritima; however, it did not appear in the oesophagic contents of the turtles captured in this estuary. Nevertheless, Talavera-Saenz et al. (2007) found that wigeongrass can constitute 43% of the stomach contents for immature green turtles at Estero Banderitas. Punta Abreojos is an outstanding record, first because there are no previous reports of wigeongrass for this lagoon and second because, at least during spring, wigeongrass is displacing eelgrass as the primary food source for immature green turtles (Table 2). This fact has important management implications, considering that C. mydas is an endangered species included in the IUCN Red List (International Union for Conservation of Nature 2009) and that Punta Abreojos is an important feeding ground for juvenile turtles (López-Castro et al. 2010). Therefore, feeding grounds for immature individuals of this species must be considered as high priority areas for conservation: Z. marina meadows in Laguna San Ignacio and Bahía Magdalena and, in the case of Punta Abreojos, R. maritima meadows.

Mexican environmental legislation and management plans for marine protected areas must include seagrass communities as a key coastal ecosystem in order to secure marine biodiversity. Special areas must be designated where no fishing or urban activity is allowed, and fishing regulation should encourage the use of fishing gear and techniques non-destructive of the ocean floor and its inhabitants. In wetlands like Bahía Magdalena located outside a marine protected area, more caution should be taken, otherwise the environmental degradation could be severe in the upcoming years. This work intends to be a baseline for the urgent need of awareness and conservation efforts of one of the key coastal ecosystems along the northwest coast of México such as seagrasses, outlining new challenges spawned by the outstanding expansion of R. maritima meadows. Wigeongrass now represents a key species for the region because its spatial and temporal evolution can be used as an indirect measurement of the effects of global warming at a regional scale.