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 #tool m-z 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 13C in particulate matter increased initially, and then remained constant, and was controlled mainly by the ratio/?-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.
ulate carbon, we decided to confirm this by using both enzymatic (carboxylase activities) and isotopic (13C: 12C) measurements of the particulate matter.
As ribulose biphosphate carboxylase is a major storage protein in plants, associated with chloroplasts (Huffaker and Peterson 1974, Ellis 1979), and is more easily mobilized than the rest of the protein pool during environmental changes (Ekman et al. 1989), it was hypothesized to act as a nitrogen reservoir and to be responsible for the observed decrease in particulate nitrogen during the day. In addition, carboxylase activities give an instantaneous estimate of carbon assimilation, while the isotopic carbon measurements yield an integrated view of all photosynthetic processes associated with inorganic carbon assimilation as well as gas-exchange processes occurring simultaneously (Descolas-Gros and Fontugne 1985, 1988). We report here on these phenomena, employing a sampling frequency which allows a good resolution of the processes involved in the carbon and nitrogen cycle of such environments.