engleski

Succession of phytoplankton in the extreme euxinic environment (Rogoznica Lake, eastern Adriatic coast)

Marine Lake Rogoznica is a small, karstic naturally eutrophic, hypoxic and periodically euxinic lake. The phytoplankton composition in the lake is characterised by low species diversity and high single species abundance (up to 107 cells L-1). Phytoplankton development is determined by mixing of the water column and characterized by the dominance (abundance >2 x 105 cells L-1 and frequency of appearance >30%) of the diatom Chaetoceros curvisetus, the dinoflagellate Prorocentrum arcuatum and the heterotrophic microflagellate Hermesiunum adriaticum. Succession of phytoplankton was investigated on the bases of 12 samplings in the period during 2011-2012. Microphytoplankton abundance dominated over nanophytoplankton indicating nutrient sufficient environment. The microplankton community (max abundance 3.2 x 106 cells L-1) was composed of 17 taxa, mainly diatoms (94.94 %), dinoflagellates (2.98 %) and ebridian flagellate (heterotrophic microflagellate H. adriaticum 2.91%). Dominant diatoms Thalassionema nitzschioides (abundance 1.6 x 106 cells L-1, frequency of appearance 46%), Cyclotella choctawhatcheeana (abundance 2.8 x 106 cells L-1, frequency of appearance 46%), Dactyliosolen fragilissimus (abundance 2.5 x 106 cells L-1, frequency of appearance 57%) and C. curvisetus (abundance 2.3 x 105 cells L-1, frequency of appearance 52%) alternated in domination in the water column over the research period. The spring bloom in 2011 did not develop. Phytoplankton succession was abruptly terminated after bloom of T. nitzschioides in March. In May we found the phytoplantkton community composed of C. choctawhatcheeana/ C. curvisetus/ D. fragilissimus). In summer, phytoplankton abundance was extremely low, probably due to nutrient depletion. In autumn , development of phytoplankton assemblage occurred with dominant T. nitzschioides. In October 2011 an anoxic event occurred in the whole water column ; however phytoplankton recovered fast after this event. Consequently, in March 2012 spring bloom of diatoms C. choctawhatcheeana and D. fragilissimus started and typical phytoplankton development. In general, domination of diatoms T. nitzschioides, D. fragilissimus and C. curvisetus in 2011 was replaced by C. choctawhatcheeana and D. fragilissimus in 2012.

Succession; phytoplankton; euxinic lake

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