Abstract

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 (δ13C) and nitrogen (δ15N) in suspended particulate organic matter showed characteristic patterns of variations in surface water during the bloom. The δ13C of suspended particulate matter increased with the growth of phytoplankton population and decreased gradually after the depletion of NO3− and NO2−. The δ15N of suspended particulate matter was very low soon after the beginning of phytoplankton bloom, but the value increased rapidly with the decrease in NO3− and NO2−, and reached maximal value following nutrient depletion, after which the δ15N remained high until the end of the experiment. In order to understand such variations of δ13C and δ15N, we made the mass and isotopic balance models of carbon and nitrogen for the upper layer of the CEE, and simulated the temporal variations of δ13C and δ15N 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 δ13C, 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 δ15N 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) the early stage of the phytoplankton bloom when NO3− plus NO2− were still in excess in sea water (high δ13C but low δ15N), (II) the late stage of the bloom when NO3− plus NO2− had just been depicted (high δ13C and high δ15N) and (III) the steady state phase, a few days after the depletion of NO3− plus NO2− (low δ13C but high δ15N). The cooperative variation of δ13C and δ15N in the suspended and sedimented particulate organic matter was also demonstrated.

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