Wolffiella oblonga, (Phil.) Hegelm.

Blanco, Mario A. & Jiménez, José Esteban, 2019, Duckweeds (Araceae: Lemnoideae) growing on wet, vertical rocks behind a waterfall in Costa Rica, with a new country record of Wolffiella oblonga (Phil.) Hegelm., Adansonia (3) 41 (15), pp. 193-200 : 195-198

RESULTS

Several patches of duckweeds were found growing on the vertical wet wall of the rock shelter behind the waterfall during all four visits, mostly located c. 0.5-1.5 m away from the free-falling water curtain. The patches were irregular in shape and variable in size ( Fig. 1B ); the largest one seen was c. 2 m 2, located 1-2.5 m above the rocky bottom of the shelter. Possibly more duckweeds occurred higher up (both within the shelter and on the wet rock face of the waterfall not in direct contact with the falling water), but the constant dripping makes it very hard to look up inside the shelter, and it is almost impossible to distinguish the tiny duckweeds from other plants and green algae that grow on the rock face higher up on the waterfall from more than 2 m away. The estimated visible area of rock wall covered by duckweeds inside the rock shelter during our first three visits was 7-11 m 2. During our fourth visit (after the partial collapses of the rock shelter) it was much reduced, c. 3-4 m 2.

The duckweeds were loosely adhered to the rock surfaces only by means of the permanent, thin film of water, never physically anchored ( Fig. 1C, D, F ). A splash of water could easily wash the duckweed colonies off the rock wall, but we never perceived such an event during several hours of cumulative observation.

The duckweed colonies were composed almost exclusively of Wolffiella oblonga (Phil.) Hegelm. (voucher: Blanco et al. 4300), the plants of which accumulated in dense mats of several layers in some places ( Fig. 1C, D, E ). Growing on top of the W. oblonga plants (and occasionally forming small pure patches directly on the wet rock) were far fewer, scattered plants of Lemna valdiviana Phil. (voucher: Blanco et al. 4301, Fig. 1D, F ), and even fewer plants of L. aequinoctialis Welw. (mixed with L. valdiviana in Blanco et al. 4301). At first sight, these duckweeds (especially W. oblonga ) were difficult to recognize as such, as they resemble green algal mats, thalloid liverworts, or even fern gametophytes. No flowers nor fruits were detected on several dozen duckweed plants of either species that were examined under a dissecting scope.

Small groups of Lemna aequinoctialis were seen stranded among aquatic plants and floating debris c. 150-250 m downstream from the waterfall, but none above.

Both Marathrum foeniculaceum Bonpl. (voucher: Blanco et al. 4296) and Tristicha trifaria (Bory ex Willd.) Spreng. (voucher: Blanco et al. 4303) (both Podostemaceae ) grew abundantly on the boulders at the bottom of the waterfall during our first three visits, but not in the rock shelter. Marathrum foeniculaceum was much less abundant at the bottom of the waterfall during our fourth visit, apparently because many of the plants were buried by the recently collapsed rocks. This species also grows on the rocky river bed immediately above the waterfall and on the vertical, constantly wet rock wall of the waterfall (visible with binoculars).

DISCUSSION

The Llanos de Cortés waterfall is the only place in the world where duckweeds have been documented growing on wet rocks behind a waterfall. This phenomenon is even more remarkable because of the large size of the duckweed community found there, at least before the recent partial collapse of the rock shelter.

Duckweeds have been documented growing on wet rock walls or in the drip of waterfalls on very few occasions ( Landolt 1986: 141-142; 1998b; 2000). Lemna aequinoctialis has also been collected growing on a permanently wet cement wall in Costa Rica (Hammel et al. 20017, CR, MO). For duckweeds to grow successfully in a hygropetric or madicolous environment, they require the presence of a permanent, thin film of water on the substrate, not being exposed to rain or water overflow (which could easily wash the tiny plants off as they cannot secure themselves to a solid substrate), and having sufficient illumination for the plants to survive. The rock shelter behind the Llanos de Cortés waterfall stays permanently wet because of the constant spray produced by the falling water. During periods of unusually high precipitation, the water comes out with such force that most of it loses contact with the rock from the upper edge of the waterfall. Thus, even when waterfalls are lotic habitats par excellence, this rock shelter behaves more as a static aquatic microhabitat, although very different from typical lentic habitats.

According to Landolt (1986: 142) only duckweeds with roots are capable of growing on wet, vertical rocks (heretofore, a few species of Lemna are the only duckweeds that have been documented growing hygropetrically; Landolt 1986, 1998b). However, duckweed roots lack a mechanism to anchor themselves to a substrate (much less solid rock), and thus roots do not represent an actual requisite for duckweeds to grow hygropetrically. The case presented here evidently shows that the rootless genus Wolffiella is also capable of growing in the same manner, and can form large colonies under the appropriate conditions.

Some duckweed patches in the rock shelter grew on parts with relatively low illumination. Wolffiella oblonga , by far the most abundant species in the rock shelter, belongs to a genus that commonly thrives in relatively dim light, submerged just under the surface in dense colonies, usually under the cover of other floating plants (including other duckweed genera; Hillman 1961; Armstrong 1992). The other two duckweed species present in the rock shelter ( Lemna aequinoctialis and L. valdiviana ), which normally grow in more exposed situations, are much less abundant and probably grow there at a relatively slow rate. They manage to stay on top of the dense colonies of W. oblonga thanks to their hydrophobic upper frond surface.

The dense colonies of Wolffiella oblonga in the rock shelter apparently have grown vegetatively, spreading on the rock surface as new fronds emerge from old ones and push each other, and by occasional short-distance dispersal by water splashing. Conceivably, short distance dispersal (i.e., within different parts of the rock shelter) can also occur when individual duckweeds temporarily adhere to moving snails, crabs, aquatic insects, or other small invertebrates of the hygropetric microhabitat. Possibly some displacement of the two species of Lemna occurs on the surface of the water film (the upper frond surface of these species is hydrophobic).

Duong & Tiedje (1985) showed that part of the nutritional needs of duckweeds growing in ponds is provided by their epiphytic nitrogen-fixing cyanobacteria. A similar situation likely occurs on the Llanos de Cortés rock shelter, where cyanobacteria grow on virtually every surface of the hygropetric habitat (including on the duckweeds themselves). The algal and invertebrate communities of the Llanos de Cortés waterfall are currently being studied by researchers at the Centro de Investigación en Ciencias del Mar y Limnología (CIMAR, Universidad de Costa Rica).

It is unknown how these duckweeds first arrived at this unusual place, but conceivably they could have been carried by the river from suitable upstream habitats, or they could have been transported from other areas by aquatic birds seeking food or temporary refuge in the rock shelter. In any case, it is evident that the presence of duckweeds on the wet rock wall of the shelter is permanent and not an accidental, temporary situation.

The present report also constitutes the first record of Wolffiella oblonga in Costa Rica. This species has a wide latitudinal and elevational distribution in the tropics and subtropics of the New World (from California, Texas, Louisiana and Florida to Argentina and Chile, and from 3600 m down to sea level; Landolt 1986); thus, its occurrence in Costa Rica was expected ( Grayum 2003).

Wolffiella oblonga can be difficult to recognize from W. lingulata (Hegelm.) Hegelm. (also expected in Costa Rica, Grayum 2003); the morphology of both taxa is somewhat variable and overlapping ( Landolt 1986: 538; Crawford et al. 1997; Crawford et al. 2006; Armstrong 2011, 2012). However, W. oblonga can be distinguished from W. lingulata by its generally narrower (3-5 vs 4-10 mm) and flatter (vs recurved) fronds, a narrower budding pouch (angle 40-70° vs 80-120°), and a tract of elongated cells running along or close to the lower edge of the pouch (vs running halfway between the median line and the lower edge of the pouch) ( Landolt 1986; Armstrong 2011, 2012). The plants from the Llanos de Cortés waterfall fit well the above-mentioned description of W. oblonga ( Fig. 1E ). It has been suggested, on the basis of morphological, molecular and geographical evidence (both taxa have largely overlapping distributions and frequently occur in sympatry), that W. oblonga and W. lingulata may not constitute distinct species after all ( Kimball et al. 2003). In any case, W. oblonga has nomenclatural priority over W. lingulata .

It is curious that W. oblonga was first found in Costa Rica growing in such an atypical situation, and that neither W. oblonga or Lemna valdiviana have been found in the extensive seasonal marsh of Palo Verde National Park (PVNP), located c. 21 km to the SW of the Llanos de Cortés waterfall at near sea-level, in the adjacent Tempisque River basin. The aquatic macrophytes of the PVNP marsh have been studied in detail (e.g., Crow & Rivera 1986; Hernández-Esquivel 1990; Hernández-Esquivel & Gómez-Laurito 1993; Crow 2002), and four duckweed species have been recorded among them: Lemna aequinoctialis (voucher: Blanco et al. 1958, misidentified as L. gibba L. in Crow & Rivera 1986 and in Hernández-Esquivel & Gómez-Laurito 1993), Spirodela intermedia W. Koch (voucher: Blanco et al. 1959, newly reported here for the site), S. polyrhiza (L.) Schleid. (reported by Crow 2002, without citing a voucher) and W. welwitschii (Hegelm.) Monod (voucher: Blanco et al. 1957). In Costa Rica, L. valdiviana is rarely encountered at elevations below 600 m ( Grayum 2003); the cool environment of the rock shelter probably provides the appropriate conditions for this species to occur at this relatively low elevation.

The abundance of two species of riverweeds ( Marathrum foeniculaceum and Tristicha trifaria , of the Podostemaceae , a family of submerged rheophytic plants specialized on river rapids) indicates that the river is not contaminated or very little so ( Philbrick & Novelo 1995). Furthermore, although the tolerance to pollutants of species of Wolffiella has been scarcely tested ( Ziegler et al. 2016), there is anecdotic evidence that they are much less tolerant to turbid water, sewage and detergents than other duckweeds ( Armstrong 1992). This is somewhat surprising as agricultural and cattle fields partially flank the Potrero River upstream from the waterfall, although recent aerial images show that substantial portions of its riparian forest still remain in place.

Janzen (1976) described the Llanos de Cortés waterfall as “perhaps the best remaining example of a moist canyon effect [caused by the blast of cool, hyper-humid air from the waterfall] in Guanacaste province ”, and remarked that “its organisms are unique” (see Zartman & Pittillo 1998 for a discussion of the highly specific plant communities of spray cliffs around waterfalls). The existence of a large, permanent duckweed community in the rock shelter of the waterfall underscores the uniqueness of this site and the need for its protection. Janzen (1976) mentioned that “the increasing agricultural exploitation of the Bagaces–COMELCO region, and specifically the area immediately above the waterfall, is very likely to destroy both the water flow and the actual riparian forest itself [at this site]” (COMELCO stands for “Compañía Ganadera El Cortés, S.A.”). Fortunately, more than 40 years after Janzen´s dire predictions, the waters of the Rio Potrero still flow all year round and do not show visible signs of contamination at the site of the waterfall. The current prohibition for visitors to enter the rock shelter is a positive measure in favor of the conservation of this unusual duckweed colony and its delicate associated biotic community. The surrounding riparian vegetation already shows some signs of degradation due to the intensive visitation for recreational use that this site experiences.

Construction of a visitor complex adjacent to the waterfall by the Bagaces county municipality is planned for the immediate future (Sánchez-Herrera 2016). The project proposal aims to minimize its environmental impact, preserving much of the surrounding riparian forest. We suggest that measures be taken to protect and minimize potential damage to the unique rock shelter duckweed colony.

Figures

FIG. 1. — Llanos de Cortés waterfall and its duckweed community:A, front view of the Llanos de Cortés waterfall on July 2015,before the partial collapse of its rock shelter; the contour of the latter is partially visible behind the water curtain; B, permanently wet, vertical rock wall of the shelter behind the waterfall, covered by duckweeds (green); C, portion of duckweed mat with several layers of individual plants, mostly of Wolffiella oblonga (Phil.) Hegelm.; D, detail of duckeeds (mostly W. oblonga) clinging to the rock wall only by means of the permanent water film, but notice plants of Lemna valdiviana Phil. near both fingernails; E, four plants of W.oblonga as seen under the dissecting microscope;F, small colony of L. valdiviana, clinging to the wet rock wall.Scale bars:B,1 m; C,5 cm; D, F,1 cm;E, 5 mm.