Steady state of phytoplankton assemblage in the tropical Lake Catemaco (Mexico)

Phytoplankton of the tropical lake Catemaco (Veracruz, Mexico) showed similar species composition during samplings from 1993 to 1995. There were two small dominant cyanobacterial species Cylindrospermopsis catemaco Kom.-Legn. et Tavera and Cylindrospermopsis philippinensis (Taylor) Kom., and a group of larger algae and cyanobacteria that were always present, however in smaller numbers: Aulacoseira granulata (Ehr.) Simons morphotype curvata, A. cf. italica (Ehr.) Simons mf. curvata, Fragilaria construens (Her.) Grun., Achnanthes minutissima Kütz., Planktolyngbya circumcreta (G.S.West) Anagn. et Kom., Chroococcus microscopicus Kom.-Legn. et Cronberg. Moreover we found several other scarcely present species. The percentage of total biomass of the two dominant species of Cylindrospermopsis varied between 34 and 81%, but they accounted for 80 to 95% of abundance. Apart from geomorphological features and climate conditions, biological variables played an important role. Fish-stock was formed by filter-feeding native herbivorous species of fish Dorosoma petenense (Günther), Bramocharax caballeroi (Contreras et Rivera), Astyanax mexicanus (Filippi), and an introduced, also herbivore Oreochromis niloticus (L.). Feeding activity of fish removed large species of algae and cyanobacteria as well as detrital remnants and zooplankton from the water. Smaller, inedible cyanobacteria remained in the water and formed the stable portion of the phytoplankton, dominant both in biomass and abundance. CANOCO analysis of samples and species variability demonstrated results of competition between two species of Cylindrospermopsis: steady state during the dominance of C. catemaco lasting probably for the whole year 1993 (one dry and one wet season) and steady state during the dominance of C. philippinensis in 1994 and 1995. According to the functional classification of phytoplankton suggested by Reynolds et al. (2002), Catemaco dominant assemblage would belong to the functional group SN.