Relationship of carbon availability in estuanne phytoplankton to isotopic composition

The carbon isotope ratio of particulate carbon in an estuary can vary by at least 10 %O throughout the year. In this study. 2 periods of high primary productivity are compared from data collected in both spring and summer of 1987 and 1988 from the Delaware Estuary (USA). In spring. particulate carbon isotopic compositions (6I3C) were the most positive (up to -17 %0), whereas in summer the values were the most negative (to -32 %o). Equations calculating the CO, concentrations within algal cells were used to show that growth of phytoplankton, especially in spring, may be limited by the availability of dissolved carbon dioxide, C02(d)-An alternative hypothesis for the enrichment of I3c in diatoms from spring blooms includes the possibility that a mechanism for active bicarbonate accumulation is induced during high primary productivity. Similarly, a model is developed for phytoplankton growth during summer with C02(d), rather than bicarbonate, being the species of dissolved inorganic carbon (DIC) transported across the membrane. The influence of respired CO2 on the isotopic composition of total DIC in summer is also calculated to explain the differences in isotopic compositions of particulate carbon. The demand for during periods of high primary productivity limits its availability to phytoplankton. This demand could easily be met by the induction of an active transport system for concentrating DIC, which could explain some of the variability in 6'" of partlculate carbon In estuaries and oceans.