Temporal variation of phytoplankton in a small tropical crater lake, Costa Rica

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.

RESUMEN

Se ha especulado que la variación temporal del fitoplancton en lagos tropicales sigue un patrón similar al de lagos templados de forma más acelerada, pero pocos estudios de caso han abordado el tema. La mayoría de los estudios en lagos de Costa Rica se han basado en muestreos mensuales y no han logrado mostrar los cambios señalados. En este trabajo se estudió el fitoplancton del lago del volcán Barva por un periodo de más de tres años, al principio con un muestreo mensual y al final con un muestreo semanal por casi dos años. Mensualmente se hicieron perfiles verticales de temperatura y durante el periodo de muestreo semanal se registró la temperatura superficial cerca del medio día. A pesar de su condición somera (7m) y baja temperatura superficial, el lago se estratifica por periodos breves. El fitoplancton mostró tanto periodos de cambio rápido como periodos de estabilidad relativa. La composición fluctuó entre tres condiciones principales, la primera fase se caracteriza por la abundancia de desmidios (Staurastrum paradoxum, Cosmarium asphaerosporum), una segunda dominada por cryptomonadaceas (Chryptochrysis minor, Chroomonas sp.) y la tercera por Eutetramorus tetrasporus. Aunque fue evidente que los muestreos mensuales pueden obviar eventos de corta duración, la variación temporal no se ajustó a las estaciones de la zona. Dado que el lago se localiza en la cima del volcán Barva y recibe la influencia del clima tanto del Caribe como del Pacífico, la estacionalidad no está bien marcada en el sitio. Como resultado, las variaciones en el tiempo atmosférico local de corto plazo pueden tener un mayor efecto que los patrones estacionales del clima regional.