engleski

Effects of nutrient addition on microbial plankton and dissolved organic matter variability. Part 1: productivity aspect.

We followed the effect of nutrient additions on the development of phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNAN), and dissolved organic matter by incubating natural plankton assemblage in situ. The plankton assamblage was collected from the Gulf of Trieste (Northern Adriatic) in early summer (June 1994 - PALEX 1 (Paloma Experiment 1)) and early spring (April 1995 - PALEX 2 (Paloma Experiment 2)). Four artificial nutrient enrichments were applied (single additions of nitrogen, phosphorus, silicate and a mixture of all nutrients in an approximate Redfield ratio); rain water and river water at 10-15% v/v were also used as nutrient sources. In both PALEX 1 and PALEX 2 the initial plankton assamblages were P-limited (inorganic N/P nutrient ratios in field > 40), but their biomass was low (chlorophyll a < 0,6 microg/l), while bacterial (>15 microg/l) and HNAN (> 1,5 microg/l) carbon and DOC (~200 microM) were rather high. PALEX 2 experiment was started with a plankton assamblage that was dominated by diatoms, had higher chlorophyll a (> 1,5 microg/l), while heterotrophic biomass (~10 microg/l and ~1 microg/l, bacterial and HNAN C, and DOC ~100 microM) was lover. In both experiments the treatment with phosphate and mixed nuitrients had the most significant effects on production and biomass of autotrophs and heterotrophs. Size-fractionated primary production measurements showed the largest response of 2-10 microm fractions in PALEX 1 and PALEX 2, respectively; Cyanobacteria were outcompeted in both experiments. Phytoplankton pigment biomarkers showed a particularly strong increase of 19'-hexanoyloxyfucoxanthin and chlorophyll b biomass in PALEX 1, and fucoxanthin in PALEX 2. In both experiments bacterial carbon was significantly (negatively) correlated with DOC but not with chlorophyll a biomass.

Adriatic; plankton incubation; dissolved organic matter

nije evidentirano