Wolffiella lingulata

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 : 198

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.