Phytoplanktonic associations as indicators of zinc pollution
In order to study the response of native phytoplanktonic communities to different zinc concentrations (2.5, 10, 20 - 25 and 40 - 50 mg.l-1), four bioassays in vitro were carried out. They were conducted in autumn, winter, spring and summer under controlled conditions during a month and the samplings were performed each 2-3 days. The algal responses were variable according to the season and zinc-concentration, species, inoculum density and temperature dependent. 2.5 and 10 mg.l-1 of Zinc in the test medium caused a stimulatory effect on the growth rate of certain diatoms; with 20-25 mg.l-1 of Zinc the development of Chlorella vulgaris (Chlorophyceae) was stimulated, and with 40-50 mg.l-1 of Zinc it was generally found important mortality rates except for Chlorella vulgaris and for some tolerant diatom species: Cyclotella meneghiniana, Gomphonema parvulum, Navicula sp., Nitzschia palea, Nitzschia sp., Pinnularia biceps, Synedra acus and Synedra ulna var. amphirrhynchus. The Cyanophyceae, Euglenophyceae, Tribophyceae, Chrysophyceae, Zygophyceae and Dinophyceae were particularly sensitive to Zinc. In presence of abundant inoculums, the sensivity to Zinc was observed up to 25 mg.l-1 (autumn-20°C, spring-20°C and summer-25°C) while in presence of smaller inoculums the sensivity manifested up to 10 mg.l-1 (winter-15°C). In general, as Zinc concentration increased, Chlorella vulgaris gradually became the dominant taxon and thus, a decrease in the specific diversity of the assayed algal system made of Zinc-tolerant species was observed.