Tracing mangrove carbon in suspended matter and aquatic fauna of the Gautami-Godavari Delta, Bay of Bengal (India)

Stable carbon isotopic composition and C/N ratio were used to trace the inp ut of carbon associated with mangrove litter into the estuary of the Godava ri-Gautami delta system and Kakinada bay (Andhra Pradesh, India). Suspended organic matter in the mangrove channels was more depleted in C-13 (average delta(13)C = -24.5 parts per thousand) than in Kakinada bay which showed d elta(13)C values for suspended matter (average delta(13)C = -22.7 parts per thousand) closer to those expected for marine phytoplankton. Suspended org anic matter from mangrove channels was enriched in nitrogen (average C/N at om ratio less than or equal to 12.7) and C-13 (average delta(13)C = -24.5 p arts per thousand) relative to mangrove leaf litter, which had a C/N ratio of 75 and a delta(13)C value of -28 parts per thousand. Lowest C/N ratios f or suspended matter were observed during southwest monsoon when rainfall wa s 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 C-13-enriched, marine phytoplan kton in open Kakinada bay and on C-13-depleted organic matter, such as estu arine phytoplankton and mangrove litter, in the mangrove channels. From the delta(13)C signature, it appeared that mangrove carbon was present to some extent in zooplankton and macrofauna from the mangrove mudflats and channe ls, but the signal rapidly decreased in Kakinada bay. Nitrogen isotopic com position of zooplankton and macrofauna indicated a progressive enrichment o f N-15 away from the mangrove forest towards the northern part of Kakinada bay, in approach of Kakinada city. This is thought to reflect input of anth ropogenic nitrogen enriched in N-15 and subsequent uptake of this enriched nitrogen into the aquatic food chain.