engleski

Biocenotical structure of phytoplankton communities in deep karstic lakes (Plitvice Lakes NP)

Plitvice Lakes are a specific hydrogeological phenomenon, made of 16 cascading lakes in the central part of Croatia. The study was conducted at Lakes Prošće (636 m asl, area 0.68 km2, depth 37 m) and Kozjak (534 m asl, depth 46 m, area 0.83 km2). Samples for water analysis and phytoplankton determination were taken monthly during 2009 and 2010 at the deepest point. Biomass was estimated from the product of the population and the mean cell volumes. In Lake Kozjak a total of 80 taxa were identified, distributed in seven taxonomic divisions: Bacillariophyta (27), Chlorophyta (26), Chrysophyta (8), Cyanobacteria (8), Dinophyta (7), Cryptophyta (2), Euglenophyta (2). In terms of total phytoplankton biomass, the most representative group was Bacillariophyta (avg 0.53 mg l-1), with Chrysophyta as a subdominant group (avg 0.17 mg l-1). Several centric (Cyclotella spp. (Kützing) Brébisson, Discostella stelligera (Cleve & Grunow) Houk & Klee) and pennate diatoms (Synedra sp. Ehrenberg, Ulnaria ulna var. acus (Kützing) Lange-Bertalot), together with Dinobryon divergens O.E.Imhof were dominant, thus characterizing the community. Dinophyceans (Peridinium spp. Ehrenberg) were less numerous but contributed considerably to the biomass, while colonial chrysophytes (Dinobryon sociale (Ehrenberg) Ehrenberg, D. cylindricum O.E.Imhof) were well represented in spring and winter. Planktonic (Asterionella formosa Hassall) and benthic diatoms (Achnanthidium sp. Kützing) along with unicellular (Closteriopsis acicularis (Chodat) J.H.Belcher & Swale) and colonial chlorophyceans (Elakatothrix gelatinosa Wille) were frequently present throughout the seasons, but with small biomass contribution. Species composition of Lake Prošće revealed 52 taxa distributed in seven taxonomic divisions: Bacillariophyta (18), Chlorophyta (16), Chrysophyta (7), Cyanobacteria (5), Dinophyta (4), Cryptophyta (1), Euglenophyta (1). Total phytoplankton biomass was dominated by Chrysophyceans (avg 1.66 mg l-1), while Dinophyta (avg 0.21 mg l-1) and Bacillariophyta (avg 0.20 mg l-1) were subdominant. During the spring the phytoplankton community was almost completely dominated by the bloom of Dinobryon sociale. Some chrysophyceans (D. divergens), flagellate cryptophyceans (Cryptomonas sp. Ehrenberg), and planktonic diatoms (Stephanodiscus sp. Ehrenberg, Fragilaria crotonensis Kitton) were abundant and contributed considerably to the biomass in all samples, therefore characterizing the community. Dinophyceans (Peridinium inconspicuum Lemmermann, P. volzii Huitfeldt-Kaas) were numerically abundant in spring and winter, showing high contribution to the overall biomass. Several diatom (Asterionella formosa, Cyclotella sp., Achnanthidium sp., Navicula sp. Bory de Saint- Vincent) and chlorophycean species (Oocystis pusilla Hansgirg, Ankyra judayi (G.M.Smith) Fott, Elakatothrix gelatinosa) along with Bitrichia chodatii (Reverdin) Chodat, showed a relatively high frequency of occurrence, but with low biomass contribution. Several species occurred with low frequency, but with a notable biomass: Coenococcus planctonicus Korshikov, Cymbella sp. C.Agardh, Ulnaria ulna var. acus, Dinobryon sertularia Ehrenberg and D. cylindricum.

phytoplankton; karstic; lakes

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