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The diatom flora of Unare and Píritu Lagoons, Venezuela, comprises freshwater, estuarine and marine organisms. In this paper, 15 species of diatoms belonging to the subclass Bacillariophyceae D.G. Mann are described. Water samples from Unare and Píritu lagoons (Venezuela) were collected from November 1988 to July 1989. Samples were fixed with neutral formaldehyde (10%) and were examined under phase contrast microscopy. Species under study were Dictyoneis marginata (Lewis) Cleve, Cymbella affinis (Kützing) Cleve, Cymbella sp., Achnanthes brevipes var. intermedia (Kützing) Cleve, Lyrella irroratoides (Hustedt) Mann, Navicula carinifera (Grunow) Peragallo, Navicula liber (W. Smith) Peragallo, Navicula vacillans (Schmidt) Peragallo, Navicula sp, Amphora crassa var. campechiana Grunow, Amphora decussata Grunow, Caloneis powelli (Lewis) Cholnoky, Tryblionella acuta (Cleve) Mann in Round et al., Tryblionella apiculata Gregory, Surirella febigerii Lewis.

When an ecosystem is disturbed, it reverts to an early successional state, characterized by changes in community structure and function, that can be anticipated as response to stress. The purpose of this paper was to check the effect of a disturbance caused by application of chemical compound (Copper Sulfate, to 100 micrograms L-1 granular of 92%) on the phytoplankton community in a pond located in Guaicaipuro, Edo. Miranda (Venezuela), and compare the diversity and composition of the community before and after the chemical application. An additional purpose was, to test the Odum's hypothesis about the community response to chemical stress. The chemical compound was apply in different random points to assure homogeneous distribution in the water body. Only one application of the chemical was used 6 hours after the first sample was taken. Dissolved oxygen in surface and depth were measured. In order to estimate the diversity and composition of phytoplankton community, samples were taken in 5 random points of the pond, until a 2 liter sample was completed in an intermediary basis, during 30 days in the beginning of rainy season, between june and july of 1994. Oxygen showed a maximum value at surface of 9.3 mg O2/L and minimal value at depth (2.3 mg O2/L). Species richness varied between 4 and 18. Phytoplankton community varied between 7.1 x 10(4) and 7.8 x 10(6) cel/L, and diversity index between 0.12 and 1.39. Oxygen, species richness and phytoplankton minimal values were noticed at the fifth day after chemical application. Before chemical application, phytoplankton community was dominated by filamentous microalgae, especially Chlorophyta. After the fifth day, this group decreased in abundance and species number. On the seventh day small-size Cyanobacteria became dominant, particularly Chroococcoid forms. The community reverted to an early stage of ecological succession. Since the ninth day, community showed the following abundance sequence: Chrysophyta, Pyrrophyta, Bacillariophyta, and Euglenophyta, with an increased species richness. At the end of the study period, filamentous Chlorophyta species appeared. These results support Odum's hypothesis.