ABSTRACT: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.

We used stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope analysis to investigate the trophic ecology of 8 small cetacean species of the southwestern South Atlantic Ocean: 6 delphinids (Grampus griseus, Lagenorhynchus cruciger, L. australis, Lissodelphis peronii, Pseudorca crassidens, and Cephalorhynchus commersonii ) and 2 phocoenids (Phocoena dioptrica and P. spinipinnis). We also analyzed samples of possible prey collected from oceanic and coastal habitats adjacent to Tierra del Fuego. Cetacean bone-collagen δ 13 C and δ 15 N data revealed information on both habitat and prey preferences. We observed an isotopic continuum in which coastal species had the highest values of δ 13 C and δ 15 N (L. australis), while oceanic and southern species had the lowest values (L. cruciger and P. dioptrica), indicative of offshore foraging in cold oceanic waters near the Antarctic Convergence. Overlap in mean isotope values between C. commersonii and P. spinipinnis suggests that these species may have similar habitat and/or prey preferences. Isotope results for L. peronii, P. crassidens, and G. griseus suggest that at these latitudes (~54°S) they forage on the outer continental shelf. G. griseus show bimodal isotopic patterns, suggesting that 2 ecotypes that forage in different habitats and/or consume different prey items occur in this region of the southwestern Atlantic Ocean. The isotopic data presented here provide insight into the ecology of these cetaceans, with relevant implications for their successful management and conservation.KEY WORDS: Stable isotopes · δ 13 C · δ 15 N · Small cetaceans · Foraging areas · Food/prey · South Atlantic Ocean · Southern Ocean Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 418: [235][236][237][238][239][240][241][242][243][244][245][246][247][248] 2010 At present, foraging data for small cetaceans of the southwestern South Atlantic is based mainly on the analysis of stomach contents from stranded or incidentally captured animals (Goodall & Galeazzi 1985, Goodall & Schiavini 1995. These studies provide crucial data on general foraging preferences, but they produce only a snapshot of prey preferences and the trophic niche of each species. While some studies based on data from several animals provide somewhat complete information (Bastida et al. 1988, Schiavini et al. 1997, Koen Alonso et al. 1999, other studies are based on data from only a few individuals that stranded far from their normal distribution (Fernández et al. 2003), which may not represent an accurate characterization of the foraging habits of the species. Few studies were conducted by direct observation of live animals from ship or shore (Viddi & Lescrauwaet 2005).Furthermore, there are numerous difficulties in describing marine mammal diet via stomach content or scat analysis (Pierce et al. 2004, Sheppard & Harwood 2005. Specifically, variation in digestion rates and the identifiability of prey remains can create biased estimates of diet composition (Sheffie...