engleski

Northern Adriatic mesocosm experiment Rovinj 2003: pico- and nanoplankton dynamics

Background and Purpose: The microbial food web is a highly dynamic system involved in carbon flix in the aquatic environment. Experimental enclosures with natural plankton communities from the northern Adriatic were used to follow the response of planktonic microbial food web in warying perturbations induced by inorganic nutrients addition. Two different enrichment regimes, P-deficient and nutrient-balanced, were investigated. Materials and Methods: The abundance of pico- and nanoplankton assemblages was estimated using epifluorescence microscopy. The primary and bacterial production rates were assessed by uptake of sodium 14-C-bicarbonate and 3-H-leucine, respectively. Results and Conclusions: The additions of all nutrients had a most profound effect on diatoms increase in quantity with the same temporal pattern for the initial bloom observed in both treatments. The relative nutrient availability affected autotrophic pico- and nanoplankton. Enlargement of the autotrophic picoflagellate population in the P-deficient treatment followed the diatom peak, while cyanobcteria were less succesful. Meanwhile, in the nutrient balanced treatment, at the lowest phosphate concentration, elongated morphotype of cyanobacteria cells outnumbered the picoflagellates. Autotrophic nanoflagellates, in both treatments, experienced a steady increase in abundance and a greater contribution to total phytoplankton abundance in the later phases of the experiment. The succesion patterns of autotrophic plankton indicate different competitive capacities of those groups under phosphate deficiency. The quality of DOM released by diatoms, under phosphorus deficient conditions, regulated the level of bacterial growth, production of heterotrophic biomass, and flow of carbon through the microbial food web. The bacterial processing (bacterial production) of organic matter was high during the course of the bloom in nutrient balanced conditions, while a substantial decrease in bacterial production occured during a collapse of the bloom in the P-deficient treatment. Although bacteria substantially contributed to DOM cosumption in nutrient balanced conditions, net DOC accumulation suggests that excess algal production surpassed the physiological and/or structural capacity of the bacterial population.

picoplankton; nanoplankton; bacterial production; mesocosm experiment; northern Adriatic

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