HR: 16:45h
AN: B44A-06    [Abstracts]
TI: Use of a ¹⁵N tracer to determine linkages between a mangrove and an upland freshwater swamp
AU: * MacKenzie, R A
EM: rmackenzie@fs.fed.us
AF: USDA Institute of Pacific Islands Forestry, 1151 Punchbowl St. Rm 323, Honolulu, HI 96813 United States
AU: Cormier, N
EM: ncormier@fs.fed.us
AF: USDA Institute of Pacific Islands Forestry, 1151 Punchbowl St. Rm 323, Honolulu, HI 96813 United States
AB: Mangrove forests and adjacent upland freshwater swamps are important components of subsistence-based economies of Pacific islands. Mangroves provide valuable firewood (Rhizophora apiculata) and mangrove crabs (Scylla serrata); intact freshwater swamps are often used for agroforestry (e.g., taro cultivation). While these two systems are connected hydrologically via groundwater and surface flows, little information is available on how they may be biogeochemically or ecologically linked. For example, mangrove leaf litter was once thought to be an important food source for resident and transient nekton and invertebrates, but this value may have been overestimated. Instead, nutrients or allochthonous material (e.g., phytoplankton, detritus) delivered via groundwater or surface water from upland freshwater swamps may play a larger role in mangrove food webs. Understanding the linkages between these two ecologically and culturally important ecosystems will help us to understand the potential impacts of hydrological alterations that occur when roads or bridges are constructed through them. We conducted a ¹⁵N tracer study in the Yela watershed on the island of Kosrae, Federated States of Micronesia. K¹⁵NO₃ was continually added at trace levels for 4 weeks to the Yela River in an upland freshwater swamp adjacent to a mangrove forest. Nitrate and ammonium pools, major primary producers, macroinvertebrates, and fish were sampled from stations 5 m upstream (freshwater swamp) and 138, 188, 213, and 313 m downstream (mangrove) from the tracer addition. Samples were collected once a week prior to, during, and after the ¹⁵N addition for a total of 6 weeks. Preliminary results revealed no significant enrichment (< 1 ‰) in the ¹⁵N isotope composition of either resident shrimp (Macrobrachium sp.) or mudskipper fish (Periophthalmus sp.). However, the ¹⁵N signature of ammonium pools was enriched 10-60 ‰ by the end of the third week. These results suggest that the tracer was present in the mangrove but was either unavailable to higher organisms or was incorporated into organic matter not utilized by shrimp or mudskippers.
DE: 4235 Estuarine processes
DE: 4804 Benthic processes/benthos
DE: 4815 Ecosystems, structure and dynamics
DE: 4817 Food chains
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2005 Joint Assembly