Hydrobiologia 431: 225–241, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 225 Research note Tracing mangrove carbon in suspended matter and aquatic fauna of the Gautami–Godavari Delta, Bay of Bengal (India) F. Dehairs 1,∗ , R. G. Rao 1,∗∗ , P. Chandra Mohan 2 , A.V. Raman 2 , S. Marguillier 1 & L. Hellings 1 1 Analytical Chemistry Dept., Vrije Universiteit Brussel, Brussels, Belgium E-mail: fdehairs@vub.ac.be ∗∗ Present address: Land Environment Management Division, National Environmental Engineering Research Insti- tute, Nehru Marg, Nagpur 440020, India 2 Division of Marine Biology, Zoology Dept., Andhra University, Waltair, India Received 14 December 1998; in revised form 1 March 2000; accepted 21 April 2000 Key words: Stable isotope tracers, carbon, nitrogen, mangroves, litterfall, suspended matter, zooplankton, macrobenthos Abstract Stable carbon isotopic composition and C/N ratio were used to trace the input of carbon associated with mangrove litter into the estuary of the Godavari–Gautami delta system and Kakinada bay (Andhra Pradesh, India). Suspended organic matter in the mangrove channels was more depleted in 13 C (average δ 13 C= −24.5‰) than in Kakinada bay which showed δ 13 C values for suspended matter (average δ 13 C= −22.7‰) closer to those expected for marine phytoplankton. Suspended organic matter from mangrove channels was enriched in nitrogen (average C/N atom ratio ≤ 12.7) and 13 C (average δ 13 C= −24.5‰) relative to mangrove leaf litter, which had a C/N ratio of 75 and a δ 13 C value of −28‰. Lowest C/N ratios for suspended matter were observed during southwest monsoon when rainfall was highest. Although in general, mangrove litter fall was also lower during this period, no clear correlation was observed between litter fall and C/N ratio of suspended matter. In general, the composition of suspended matter pointed towards phytoplankton as a major component. Isotopic composition of zooplankton suggested selective feeding on 13 C-enriched, marine phytoplankton in open Kakinada bay and on 13 C-depleted organic matter, such as estuarine phytoplankton and mangrove litter, in the mangrove channels. From the δ 13 C signature, it appeared that mangrove carbon was present to some extent in zooplankton and macrofauna from the mangrove mudflats and channels, but the signal rapidly decreased in Kakinada bay. Nitrogen isotopic composition of zooplankton and macrofauna indicated a progressive enrichment of 15 N away from the mangrove forest towards the northern part of Kakinada bay, in approach of Kakinada city. This is thought to reflect input of anthropogenic nitrogen enriched in 15 N and subsequent uptake of this enriched nitrogen into the aquatic food chain. Introduction It is a widespread view that mangrove forests represent an important carbon and nutrient source to the adja- cent lagoonal and coastal systems (Odum & Heald, 1972, 1975; Twilley, 1988; Watayakorn et al., 1990; Robertson et al., 1992). Stable isotope studies have confirmed that mangrove carbon finds its way into ∗ Author for correspondence. organisms living in the mangrove swamps, mudflats, coastal inlets and even coastal waters (see Rodelli et al., 1984). While such observations are confirmed by recent studies showing mangrove carbon is indeed in- gested by organisms living within tidal creeks, there is also evidence that this mangrove signal rapidly de- creases with increasing distance from the mangroves. Newell et al. (1995) and Chandra-Mohan et al. (1997a) found evidence for uptake of mangrove carbon by ju- venile penaeid prawns living within tidal creeks, but