CARBON AND NITROGEN DYNAMICS DURING GROWTH AND DEGRADATION OF PHYTOPLANKTON UNDER NATURAL SURFACE IRRADIANCE

Under conditions of natural irradiance, the development and decline of a flagellate-dominated phytoplankton population was followed in a coastal North Atlantic pond over a 3 d period in summer 1986. Irradiance negatively affected phytoplankton biomass estimated as chlorophyll a, which decreased during the day at photosynthetically available radiation (PAR) levels above 600 to 1000-mu-mol m-2 s-1; chlorophyll a increased at PAR values below this threshold. In addition, an inverse relationship was found between changes in chlorophyll a and changes in dissolved inorganic nitrogen, indicating synthesis of nitrogenous biomass mainly at night and degradation mainly during the day, with intense exchanges of material between the particulate and dissolved nitrogen fractions. The natural abundance of C-13 in particulate matter increased initially, and then remained constant, and was controlled mainly by the ratio beta-carboxylases activity : ribulose biphosphate carboxylase activity. The hypothesis that the latter enzyme is broken down under high irradiance and is partly responsible for increases in external dissolved nitrogen was rejected.