Analysis of primary food sources and trophic relationships of aquatic animals in a mangrove-fringed estuary, Khung Krabaen Bay (Thailand) using dual stable isotope techniques

Abstract

Stable carbon (δ13C) and nitrogen (δ15N) isotopes were used to elucidate primary food sources and trophic relationships of organisms in Khung Krabaen Bay and adjacent offshore waters. The three separate sampling sites were mangroves, inner bay and offshore. The δ13C values of mangrove leaves were −28.2 to −29.4‰, seagrass −10.5‰, macroalgae −14.9 to −18.2‰, plankton −20.0 to −21.8‰, benthic detritus −15.1 to −26.3‰, invertebrates −16.5 to −26.0‰, and fishes −13.4 to −26.3‰. The δ15N values of mangrove leaves were 4.3 to 5.7‰, seagrass 4.3‰, macroalgae 2.2 to 4.4‰, plankton 5.7 to 6.4 ‰, benthic detritus 5.1 to 5.3‰, invertebrates 7.2 to 12.2 ‰, and fishes 6.3 to 15.9‰. The primary producers had distinct δ13C values. The δ13C values of animals collected from mangroves were more negative than those of animals collected far from shore. The primary carbon sources that support food webs clearly depended on location. The contribution of mangroves to food webs was confined only to mangroves, but a mixture of macroalgae and plankton was a major carbon source for organisms in the inner bay area. Offshore organisms clearly derived their carbon through the planktonic food web. The δ15N values of consumers were enriched by 3–4‰ relative to their diets. The δ15N data suggests that some of aquatic animals had capacity to change their feeding habits according to places and availability of foods and as a result, individuals of the same species could be assigned to different trophic levels at different places.