Marine food web dynamics in Nha Phu estuary, Vietnam

Start date: 11 February, 2012 End date: 20 June, 2012 Project type: Master's Thesis (prior to 2018) Project code: A16535 Countries: Vietnam Institutions: University of Southern Denmark (SDU), Denmark Grant recipient: Sandra Schmidt Total grant: 14,000 DKK



Estuaries are very productive systems because of the rich supply of nutrients carried by rivers. Thereby dense populations of phytoplankton, seagrasses and mangroves develop. They function as important nursery grounds for young fish, and host a large number of crustaceans, mollusks, shorebirds and water birds, but estuaries are among the most threatened and modified ecosystems. Nha Phu estuary is one of these anthropogenic-disturbed estuaries because of wastewater influx, mangrove deforestation, aquaculture, fisheries, and climate change. To provide proper management strategies and future projections there is a need for ecological studies on ecosystems, which this thesis contributes to.
The present thesis is based on field work conducted in Nha Phu estuary and a feeding laboratory experiment at the Institute of Oceanography in Nha Trang, Vietnam. The field work investigated differences between the inner and outer part of the estuary using stomach content and stable isotope analysis. The laboratory experiment was a controlled diet-shift experiment of the clownfish Amphiprion frenatus where food items were changed to calculate fractionation and turnover time of nitrogen and carbon, in muscle and liver tissues, through stable isotope analysis.
The present study found differences between the inner and outer part of Nha Phu estuary, with fish being larger and having higher trophic levels at the outer part. Plankton and fish species had different isotopic signatures from the inner to the outer part. The feeding experiment revealed differences in isotopic signatures between liver and muscle tissues, indicating differences in assimilation between tissues and that the isotopic turnover rates were consistently faster for liver than muscle tissues.