Background:

Lake Pátzcuaro has been exposed in the recent past to environmental deterioration with increasing eutrophication as a result of deforestation, indiscriminate use of agrochemicals, overfishing, unplanned urban growth, and sewage dumping.

Goals:

In order to contribute to the phycofloristic inventory of the Lake, we present the taxonomic description of the periphytic diatoms found, including morphometric information, meristic data, figures, as well as ecological and distributional notes.

Methods:

Sampling was carried out in four sites during two periods: after the rainy season (October 2004) and the dry season (May 2005); samples were gathered by scraping rooted and floating vegetation. Diatoms´ thecae were cleared with hydrochloric acid and hydrogen peroxide; permanent slides were made with Naphrax® resin and analyzed through optic microscopy.

Results:

A total of 83 periphytic diatom species were recorded, including eight varieties and one form; 20 species (24%) represent the first records for Mexico and 57 (68.6%) are new to the study area. Sixty-two species (74.7%) have cosmopolitan distribution and six (7.2%) have tropical affinity. Species richness was slightly higher after the rainy season (October 2004) with a total of 69 taxa, compared to 65 species recorded during the dry season (May 2005). Among the sampling sites, the island of Janitzio had the highest diatom diversity (46 taxa during the dry season).

Conclusions:

Deterioration of Lake Pátzcuaro in the previous decades has taken a toll on the health of its ecological system. Diatoms identified in this study indicate a clear tendency of eutrophication of the waterbody.