Sunday, April 18, 2010

Phytoplankton: A Scientific Discussion

Jiří Komárek, University of South Bohemia, is the world’s leading specialist in the taxonomy, biology, and ecology of blue green algae, including the cyanobacteria that is of concern at Lago de Atitlan.

Jana Veselá, University of South Bohemia, is a graduate student working with Dr. Komerek. She studies diatoms, a common type of algae.

Photo: Dr. Komárek and Jana Veselá on the boat

Phytoplankton in the whole lake has more or less the same species composition, which corresponds to normal state of the large mesotrophic, similar reservoirs. The dominant species are Ceratium cf. hirundinella and Fragilaria crotonensis, one species of Dinobryon is codominant. These species are regularly distributed approximately to the depth 20-30 m, in lower layers their quantity decreases and in sediments are only the empty scales of diatoms and Dinobryon.

In the upper layer above 20 m occurs also a slightly developed community (comp. transparency) of other microalgae, especially green algae (Eudorina sp., Planktosphaeria gelatinosa, Oocystis parva, Oocystis sp., Elakatothrix sp.), diatoms (several morphotypes of the planktic Aulacoseira), one small species of Peridinium and (not “water-bloom”-forming) cyanobacterium Aphanothece (Anathece) microscopica. This community is almost completely missing below 20-30 m (light conditions, and evidently after the predation pressure of zooplankton, which is common in lower layers).

The presence of the green alga Botryococcus cf. braunii (from Trebouxiophyceae), which can potentially cause a small “water bloom”, is remarkable. The same composition of phytoplankton is developed in the bays, but sometimes with higher quantity of some species from the same community, e.g. Ceratium and Fragilaria in the vicinity of San Juan, or Aulacoseira (as well in various modifications) near Santiago Atitlán. Solitary cells of littoral diatoms also sometimes occur here.

The vegetation of potential “water bloom” forming cyanobacteria is particularly important. Lyngbya robusta occurs very sporadically in the whole reservoir, including the lower layers in the centre of the lake. The filaments are in a good physiological state. L. robusta was found very rarely also in both bays, but only sporadically. Not any filament or hormogonium of this species was found in the bottom sediment in the bay near San Juan. However, numerous hormogonia (not vegetative filaments) of this species were found in the samples from the bottom near the shore at Hotel Atitlán. More attention to this fact is recommended.

Solitary colonies of Microcystis occur in the whole lake. It seems to be a special, morphologically distinct population from the vicinity of M. aeruginosa – M. botrys. In contrast to Lyngbya, it was evidently found in higher quantity in the bay of San Juan, where its population formed small “water bloom” in the samples. We will try to transfer the both species into cultures for further experiments.

Photos: cultures and samples ready for transport