SEASONAL VARIABILITY OF STABLE CARBON AND NITROGEN ISOTOPE RATIOS OF ORGANISMS IN A NORTH PACIFIC BAY

A survey of δ13C and δ15N of organic carbon and nitrogen in net phytoplankton, zooplankton, juvenile demersal fish and benthic fauna and sediments was made during the spring production cycle in Auke Bay and Fritz Cove marine embayments located in southeastern Alaska. The δ13C and δ15N values of net phytoplankton (>73 μm) increased during the spring bloom from -20·6±0·1%. (δ13C) and + 3·3±0·6%. (δ15N) during the prebloom stage to -18·1±0·2%. (δ13C) and +6·9±0·1%. (δ15N) following the bloom peak. Phytoplankton δ13C and δ15N values were significantly correlated with each other and increased with tempearture [+0·71%. per °C (δ13C) and +0·58%. per °C (δ15N)]. Phytoplankton and δ15N values were also correlated with a decline in ambient nitrate concentration suggesting that nitrate was the primary nitrogen source fueling the spring bloom. Net macrozooplankton (>505 μm) δ13C and δ15N values varied during the spring production cycle in a manner similar to phytoplankton, from -21·2%. (δ13C) and +7·2%. (δ15N) during the prebloom stage to -18·2%. (δ13C) and +8·2%. (δ15N) following the bloom peak. The δ13C values of macrozooplankton (> 165 μm) were, on average, 0·5%. (Fritz Cove) to 0·7%. (Auke Bay) heavier than their presumed phytoplankton food. Macrozooplankton were also on average +3·2%. enriched with 15N over phytoplankton. Values of δ13C for juvenile Auke Bay flathead sole (80-100) mm) averaged -17·2±0·2%. and for juvenile pollock (120-130 mm) averaged -17·8±0·2%., which were 3·1%. and 2·5%., respectively, more positive than phytoplankton and 2·4%. and 1·8%., respectively, more positive than macrozooplankton (> 165 μm). Sole δ15N values averaged +14·9±1·5%. and for pollock +13·8±1·9%., 7·1%. and 6·0%., respectively, more positive than zooplankton. The δ13C values of Auke Bay sedimentary carbon (-20·8±0·6%.) were slightly depleted in 13C compared to Auke Bay net phytoplankton and δ15N values of sedimentary nitrogen (+5·9±1·0%.) were slightly enriched in 15N over phytoplankton suggesting that phytoplankton supply the major portion of both carbon and nitrogen to Auke Bay sediments. Benthic invertebrates exhibited a wide range of δ13C (-20·8 to -14·3%.) and δ15N (+7·5 to +12·8%.) values. Much of the variability is explained by the trophic positions of the animals. The δ13C and δ15N values of adult Auke Bay Macoma nasuta exhibited significant seasonal shifts in δ13C and δ15N which were negative and opposite to the positive seasonal shifts in δ13C and δ15N of phytoplankton and zooplankton. Meiofauna (>98% nematodes) did not exhibit a seasonal shift in δ13C and δ15N as did phytoplankton, zooplankton and Macoma nasuta. © 1990.