Background:

Phytoplankton is considered an important indicator of natural and anthropogenic processes that take place in basins and coastal environments. An example of these systems is the formed by the El Carmen, Pajonal and La Machona lagoons, in which there is intense fishing and oyster farming activity, as well as agricultural activities, oil extraction and forestry in their surroundings.

Goals:

Identifying the phytoplankton species, with emphasis on the potentially harmful and / or toxic ones, their distribution and differences between the samplings, north winds (2015) and rainy (2016), as well as reporting the physicochemical variables was the aimed of this work.

Methods:

Net samples with a mesh size of 20 µm were taken and physicochemical variables were measured with a water quality sonde. 300 organisms were counted and the specific richness, relative abundances and diversity index were calculated. To identify differences between the two seasons sampled the Mann-Whitney U test was performed. In addition, the most important physicochemical variables were identified to explore their relationship with the species through a distance-based redundancy analysis (dbRDA).

Results:

A total of 86 species were identified, of which ten are new records and 15 are potentially harmful and / or toxic ones. The taxonomic group with the most species in both seasons was Bacillaryophyta (43 Northeast and 22 Rains). Specific richness was significantly different between seasons. In the north winds season, the most abundant species were from the Cyanoprokaryota and Bacillaryophyta groups, while for rainy season they were from the Dinoflagellata.

Conclusions:

Water temperature and salinity were the physicochemical variables that defined the phytoplankton composition in the CPM lagoons; the presence of potentially toxic species is an alert for the lagoon system because it reflects a latent situation that could trigger blooms at any time.