Relación entre actividad enzimática y biomasa de ensambles fitoplanctónicos en el sistema pelágico/Relationship between enzymatic activity and biomass of phytoplankton assemblages in the pelagic system

In environments with drastically fluctuating conditions, physiological and biochemical adaptations can be important for the structuration of phytoplankton assemblages. The objective of this study was to establish whether the enzymatic activity of the phytoplankton, an indicator of its dominant internal metabolism, is related to the autotrophic biomass. The geographic study area covered the South Shetland Islands (62°S), Mejillones Bay (22°S), and Seno de Reloncaví (41°30'S). Samples were collected in the field at the selected sites for enzymatic analyses of the intermediary nitrogen metabolism (nitrate reductase, glutamine synthetase) and autotrophic biomass, as well as dissolved inorganic nutrient analyses. In terms of the size-fractionated enzymatic activity, nitrate reductase (bS Reloncaví = -0.93; bB Mejillones = -0.65) and glutamine synthetase (bS Reloncaví = -0,79) activities decreased as the body size of the phytoplankton increased. These slopes, less than -1.0, indicate the relative importance of diatom-dominated phytoplanktonic assemblages in coastal waters rich in inorganic nitrogenated nutrients. Elevated glutamine synthetase activity (GS) was detected during a bloom of a dinoflagellate species, suggesting that certain dinoflagellate species prefer reduced nitrogen forms (ammonium and urea) over nitrate or nitrite. The estimate, via enzymatic activity, of the f ratio with values > 0.5 and < 0.5 suggests, respectively, an important diatom contribution to new production (based on nitrate) as compared with the flagellate contribution, which is important to regenerated production (based on ammonium). [PUBLICATION ABSTRACT]