Between May 2004 and May 2005, we sampled fish in 19 sites, grouped in four elevations, ranging from the river mouth to 650 m.a.s.l. in the Río Pacuare, Caribbean versant of Costa Rica. Changes in the distribution and composition of the fish fauna, as well as patterns of alpha and beta diversity along an elevational gradient were assessed. Additional analyses of habitat preferences, trophic guilds, functional groups and general ecology for the most abundant species are included. All fish captured were classified into 22 families, 43 genera and 53 species. The most abundant family was Characidae, followed by Gobiidae, Mugilidae, Poeciliidae and Heptateridae, which together comprise 87.9 % of all sampled individuals. Elevation shows an inverse effect on species diversity, we observed a monotonic decrease in species richness with increasing elevation (p < 0.05), as reported in other tropical rivers. According to our results, in the Río Pacuare the total fish fauna diversity is found within the first 500 m.a.s.l. Species turnover increases with elevation, while nestedness decreases. Turnover was dominated by the loss of species rather than gain; the higher species loss was registered between the river mouth and the lower river reach (< 100 m.a.s.l.). Seven species can be classified as typical or core species (Astyanax aeneus, Sicydium altum, Agonostomus monticola, Poecilia gillii, Brycon costaricensis, Rhamdia laticauda and Joturus pichardi) along the elevation gradient. The habitat availability and the integration of ecomorphological, feeding and reproductive traits help to explain better the elevation distribution of the complete set of species observed. Although it is possible to identify groups of species characteristic of each reach of river, this does not mean that they are isolated from each other. Natural drift and movement along the river of some species during their life cycle, especially S. altum, A. monticola and J. pichardi, are key processes linking the whole watershed. The present study constitutes a first step in documenting and understanding the distribution and composition of fish assemblages in a watershed that is relatively intact and well-conserved in the Caribbean versant of Costa Rica.

Distribution of Agonostomus monticola and Brycon behreae in the Río Grande de Térraba, Costa Rica and relations with water flowThiago Cotta Ribeiro and Gerardo Umaña VillalobosFish monthly samples were made in three tributaries and in the main stream in order to study population dynamics and the influence of flow on abundance of Agonostomus monticola (Mugilidae) and Brycon behreae (Characidae) in the basin of Térraba River Basin (Costa Rica). Flow was seasonal in these rivers, with peak flow in October. Recruitment was different among the species, B. behreae recruited from January to June and A. monticola all year round with a peak in June-July and October-November. Juveniles were more abundant in smaller streams. Abundance was higher in general during low flow and low turbidity periods. The results suggest that the streams have an important role in the maintenance of both species in the ecosystem, where juveniles can use the streams as nursing habitats during their development, allowing these small streams to be considered as essential fish habitats. Also, it was determined that the increased volume and turbidity of water can significantly affect the proportion of adults and juveniles between the main river and its tributaries.Com o objetivo de estudar a biologia de Agonostomus monticola (Mugilidae) e Brycon behreae (Characidae) da bacia do rio Térraba (Costa Rica), coletas mensais destas duas espécies foram feitas em diferentes pontos da bacia. A dinâmica populacional entre as categorias de tamanho dos peixes entre três tributários e rio Térraba foram analisados, bem como a variação da abundância dos peixes de acordo com as variações do nível das águas ao longo do ano. A flutuação do nível das águas é sazonal nestes rios, com um pico máximo em outubro. O recrutamento foi diferente entre as espécies, B. behreae apresentou o recrutamento de janeiro a junho e A. monticola durante todo o ano com um pico nos meses de junho-julho e outubronovembro. Juvenis foram mais abundantes nos riachos menores. A abundância foi, em geral, maior nos períodos de baixo nível de água e baixa turbidez. Os resultados sugerem que os córregos têm um papel importante na manutenção de ambas as espécies neste sistema, onde os indivíduos jovens podem usar os córregos como habitats de refúgio e proteção durante o seu desenvolvimento, o que faz estes pequenos tributários serem considerados como um habitat essencial para essas espécies. Verificamos também que o aumento do volume e da turbidez da água pode afetar significantemente a dinâmica da proporção de adultos e juvenis entre o rio principal e seus tributários.

Zooplankton in small tropical lakes has been little studied and its variation during the year may respond to different factors. In this study, the zooplankton of a small neotropical lake was studied at different times during one year and compared with changes in the lake conditions and in phytoplankton composition. The lake stratifies from March until September, and mixes during the hemispherical winter, from October until February. Besides the lake show a seasonal fluctuation in water level according to the seasonality of rains. Zooplankton was composed of at least 13 species, four Cladocerans, three Copepods, four Rotifers and other groups such as an Ostracod and the larvae of Chaoborus. The most abundant were Thermocyclops sp., Daphnia sp. and Keratella tropica. Their abundance fluctuated along the year, probably in response to changes in lake level and changes in hydrological conditions, increasing during dry season when the main outlet of the lake dried up. Changes in phytoplankton composition do not seem to be as relevant for zooplankton variation in this small lake.

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.

Abstract:The temporal variation in lake's phytoplankton is important to understand its general biodiversity. For tropical lakes, it has been hypothesized that they follow a similar pattern as temperate ones, on a much accelerated pace; nevertheless, few case studies have tried to elucidate this. Most studies in Costa Rica have used a monthly sampling scheme and failed in showing the expected changes. In this study, the phytoplankton of the small Barvas's crater lake was followed for more than three years, first with monthly and later with weekly samplings, that covered almost two years. Additional information on temperature and oxygen vertical profiles was obtained on a monthly basis, and surface temperature was measured during weekly samplings around noon. Results showed that in spite of its shallow condition (max. depth: 7m) and low surface temperature (11 to 19°C), the lake stratifies at least for brief periods. The phytoplankton showed both, rapid change periods, and prolonged ones of relative stasis. The plankton composition fluctuated between three main phases, one characterized by the abundance of small sized desmids (Staurastrum paradoxum, Cosmarium asphaerosporum), a second phase dominated by equally small cryptomonads (Chryptochrysis minor, Chroomonas sp.) and a third phase dominated by the green alga Eutetramorus tetrasporus. Although data evidenced that monthly sampling could miss short term events, the temporal variation did not follow the typical dry and rainy seasons of the region, or any particular annual pattern. Year to year variation was high. As this small lake is located at the summit of Barva Volcano and receives the influence from both the Caribbean and the Pacific weather, seasonality at the lake is not clearly defined as in the rest of the country and short term variations in the local weather might have a stronger effect than broad seasonal trends. The occurrence of this short term changes in the phytoplankton of small tropical lakes in response to weather variations needs to be further explored in other lakes. Rev. Biol. Trop. 58 (4): 1405-1419. Epub 2010 December 01.