THE DYNAMIC CHANGES OF STABLE ISOTOPIC-RATIOS OF CARBON AND NITROGEN IN SUSPENDED AND SEDIMENTED PARTICULATE ORGANIC-MATTER DURING A PHYTOPLANKTON BLOOM

The dynamic changes of carbon and nitrogen stable isotopic ratios in suspended and sedimented particulate matter were observed together with many other chemical and biological properties during a phytoplankton bloom induced by nutrient addition in a controlled ecosystem enclosure (CEE, about 70 m3) in Saanich Inlet, British Columbia, Canada. Both of the stable isotopic ratios of carbon (delta-C-13) and nitrogen (delta-N-15) in suspended particulate organic matter showed characteristic patterns of variations in surface water during the bloom. The delta-C-13 of suspended particulate matter increased with the growth of phytoplankton population and decreased gradually after the depletion of NO3- and NO2-. The delta-N-15 of suspended particulate matter was very low soon after the beginning of phytoplankton bloom, but the value increased rapidly with thc decrease in NO3- and NO2-, and reached maximal value following nutrient depletion, after which the delta-N-15 remained high until thc end of thc experiment. In order to understand such variations of delta-C-13 and delta-N-15, we made the mass and isotopic balance models of carbon and nitrogen for the upper layer of the CEE, and simulated thc temporal variations of delta-C-13 and delta-N-15 of particulate organic matter using them in connection with several hypotheses on the isotope fractionations associated with the uptake of inorganic substrates by phytoplankton. While neither change in the dissolved inorganic carbon (i.e., its isotope ratio and/or molecular CO2 concentration) nor the phytoplankton species compositions can well explain the variation of delta-C-13, this variation can be well simulated considering the effect of change in the specific production rate of particulate organic carbon. On the other hand, the variations of delta-N-15 can be clearly understood by a first-order isotope fractionation model under the assumption of large isotopic fractionation during the assimilation of NO3-and NO2- by phytoplankton. The particulate organic matter produced in the nutrient controlled phytoplankton bloom can be classified into three phases from an isotopic viewpoint: (I) thc early stage of the phytoplankton bloom when NO3- plus NO2- were still in excess in sea water (high delta-C-13 but low delta-N-15), (II) the late stage of the bloom when NO3- plus NO2- had just been depleted (high delta-C-13 and high delta-N-15) and (III) the steady state phase, a few days after the depletion of NO3-plus NO2- (low delta-C-13 but high delta-N-15). Thc cooperative variation of delta-C-13 and delta-N-15 in the suspended and sedimented particulate organic matter was also demonstrated.