Revisiting the limnology of Lake Río Cuarto, Costa Rica, thirty-five years later

Abstract

Lake Río Cuarto is a meromictic lake at low elevation in the North of Costa Rica. It offers an opportunity to compare its present state with the condition it had when first studied in the late 1970’s and occasional samplings since then. This comparison expects to identify changes that could be attributed to incipient effects of global climate change. We studied the limnology and conditions of its drainage area for three years (2013-2016) to compare with previous data. Vertical profiles of temperature, dissolved oxygen, conductivity, pH, chlorophyll a, dissolved H2S were performed several times per year, for a total of 22 samplings. Aerial photographs taken since 1950 were analyzed to describe land use changes. The lake had a shallow Secchi depth (< 5 m) at all times. It was stratified on all occasions, with a thermocline that fluctuated between 10 and 20 m. It has a monimolimnion, with a chemocline at 14 to 22 m. Below the chemocline it was always anoxic, and during annual partial mixing events in the mixolimnion, oxygen levels decreased compared to stratified periods. There was a continuous presence of H2S from 20 m downwards, with annual fluctuations, being lower during partial mixing events. A peak in chlorophyll was detected on all occasions just below the thermocline. Land use around the lake hasn’t changed much since 1952, when only a rim of tree cover was left around the steep margins of the lake. The lake has maintained its limnological characteristics, with the only exception that it didn’t cooled down to historical levels. This limited response could be the result of the high relative depth and steep margins of the lake, which prevent the downward distribution of heat and keeps the lake in a meromictic state, preventing its mixing for long periods of time.

Plankton variation in Río Cuarto Lake, Costa Rica (2013-2017)

“…However, in this study, species composition has changed again, and it was dominated by Cyanobacteria again, but mainly picocyanobacterial of the genus Synechoccocus and Aphanocapsa, and later with the dominance of small coccoid green algae. These changes reflect a change in the conditions of the lake, which was stratified at more shallow depth than in previous years but showed a deepening of the thermocline and chemocline during the course of the study period (Umaña-Villalobos & Farah-Pérez, 2018). The changes were not cyclical, but showed a monotonic tendency of decline, mainly due to the decline of Synechoccocus sp.…”

“…increased with time. Concomitant with these changes, there was also a shift in the nutrient concentrations in the lakes, with a tendency to decrease with time (Umaña-Villalobos & Farah-Pérez, 2018). What these data reveal is that year to year variation can be more important in some tropical lakes, even in lakes with very low seasonality and great limnological stability such a Lake Rio Cuarto.…”

“…The changes were not cyclical, but showed a monotonic tendency of decline, mainly due to the decline of Synechoccocus sp. by 2016, which seems to be related to a more deepmixing of the lake that took place at the beginning of that year, which reached more than 20 m by then (Umaña-Villalobos & Farah-Pérez, 2018). At the same time some green coccoid algae such as Oocystis lacustris, Discostella stelligera and Amphidinium sp.…”

“…Forest cover still remains in the steep shoreline that encircles the lake, but the rest of its drainage basin was already deforested by 1950's. According to previous studies the lake has remained between a mesotrophic to eutrophic state during the past 30 years (Umaña-Villalobos & Farah-Pérez, 2018).…”

“…Physical and chemical parameters were also sampled in each visit. The parameters and the sampling methodology are described in Umaña-Villalobos and Farah-Pérez (2018). For the purposes of this study, of the mentioned parameters we will use Secchi disc depth, thermocline depth and nutrient level (ammonim, nitrite, nitrate and phosphate); since, these parameters could be related to the abundance and diversity of plankton.…”