Tropical Diatoms of Costa Rica
Background and Location

La Selva Biological Station, Costa Rica

La Selva Biological Station is located in the wet tropical lowlands on the Caribbean Slope of Costa Rica (Fig. 1). La Selva is part of an unbroken altitudinal transect (from 35 to 2906 m.a.s.l. at Volcan Barva summit) entirely protected in Braulio Carrillo National Park and the adjoining La Selva Biological Station. La Selva Biological Station is owned by the Organization for Tropical Studies (OTS), a nonprofit consortium of 63 universities and colleges. The 1600-hectare property is 62% primary forest with the remaining area is secondary forest, reclaimed coffee plantation, and pasture (McDade et al. 1994). Headwater streams and second to fourth order streams at La Selva drain into the Rio Puerto Viejo and Rio Sarapiquí.

Fig. 1: Topographic map of Costa Rica, La Selva Biological Station: 35-100 m.a.s.l.

Geothermal groundwater and biological effects

Solute-rich groundwater moves laterally from the dormant, but geologically active, Volcan Barva and enters streams at La Selva at the gradient break, where the foothills of the volcanic mountains converge with the coastal plain (Figs. 2 and 3). This solute-rich groundwater is at ambient temperature (~25 ºC) and has elevated levels of solutes such as phosphorus, sodium, calcium, potassium, and magnesium. This topography results in solute-poor streams above the gradient break (e.g.,<20 µg soluble reactive phosphorus [SRP] L ^-1 ) and solute-rich streams below the break (e.g., >250 µg SRP L^-1 ) (Pringle and Triska 1991, Pringle et al. 1993).

Fig. 2: Diagram of process of groundwater solute enrichment. Magmatic vapors percolate upward and are absorbed into the groundwater which moves lateral to old lava flows. Solute-rich groundwater emerges at the gradient break (at La Selva Biological Station) where the volcanic range meets the coastal plain

Sampling Locations

Periphyton samples have been collected from nineteen sites in nine streams (both high and low solute)(Fig. 3). Streams have been selected to represent the published range of SRP concentrations ranging from 4.7-351.5 µg SRP L^-1 (Table 1). Many of these sites have also been used in prior research examining leaf decomposition processes along a phosphorus gradient (Ramírez 2001, Rosemond et al. 2002)(Fig. 3). Samples have been collected at these sites from rocks (when possible) and sediment in light gaps during the wet (July), dry (March), and transitional (November) seasons. Water samples have been collected as part of a long-term stream chemistry dataset for the STREAMS Project.

Fig. 3: Map of La Selva streams with gradient break indicated. Yellow shaded areas = low solutes; unshaded areas = may receive high solute inputs. Red circles indicate sampling sites.

Table 1: Published soluble reactive phosphorus (µg SRP L^-1) values for La Selva streams (Ramírez et al. 2001, Rosemond et al. 2002); n/a = site not sampled.