![]() The results obtained on the progress of the black spot phenomenon led us to hypothesize that: (i) S. After early September, the black spot completely disappeared, and the causative organism was detected at low concentration only in the southern basin of the lake. During this period, high chlorophyll a values were obtained in water samples collected from the euphotic zone due to the presence of the endosymbiont Chlorella. amethystinus occurred in the euphotic zone. amethystinus bloom took place during the first 20 days of August, and the highest density of S. Continuous monitoring of the northeast of Lake Garda showed that the apex of the S. Using morphological and molecular analysis, we characterized the protozoan responsible for the bloom as Stentor amethystinus and its symbiotic algae as a Chlorella sp. Ciliates represent a small percentage of the total zooplanktonic community of Lake Garda and have never produced bloom episodes. A first analysis by optical microscopy revealed that the origin of the black spot was a ciliated protozoan. A few days later, this phenomenon spread throughout the lake. At the end of July 2004, a “black-spot” appeared in the western portion of Lake Garda, an oligomictic lake classified as meso-oligotrophic. ![]()
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