Application of fatty acid profiles in field- and laboratory -based investigations of trophic relationships in Hong Kong wetland

Abstract



This study primarily aimed to evaluate the usefulness of fatty acids (FAs) in revealing trophic relationships in Hong Kong wetlands, through a combination of field studies and laboratory experiments.

A field-based study in Mai Po mangroves involved FA profiling of basal food sources (i.e., leaf litter from three mangrove species, diatoms and macroalgae, and sediments) and consumers (particularly crabs). FA composition of all mangroves was similar, and lacked some polyunsaturated FAs present in diatoms and macroalgae. Uca and Sesarma crabs, with different feeding mechanisms, had divergent FA profiles: Uca arcuata FAs reflected a diet of macroalgae and diatoms, while FAs of Sesarma spp. were typical of mangrove leaves. Temporal changes in consumer FA profiles between 2001 and 2007 appeared attributable to increased sedimentation at Mai Po and shifts in organic content of the substratum.

A second field-based study was conducted at Luk Keng marsh where a salinity gradient (0 to 30?) allowed investigation of the effects of salinity changes in FA profiles and stable isotope (carbon and nitrogen) signatures of the consumers and their foods. Basal food sources were leaf litter, including a fungal biomarker of decomposition (ergosterol), fine particulate organic matter (FPOM) and periphyton. Both FPOM and periphyton (but not leaf litter) contained 20:4 and 20:5 FAs, but their concentrations were affected by salinity. FA 20:4 occurred at higher levels in samples from fresh water, whilst FA 20:5 exhibited the opposite pattern and was more abundant under saline conditions, and thus the ratio of FA 20:4 to FA 20:5 decreased with increasing salinity. Combined application of FA biomarkers and isotopic signatures were able to elucidate trophic relationships between consumers and their food at Luk Keng confirming that FA 20:4 as a useful biomarker in the freshwater portion and FA 20:5 in the more saline area. FA 20:4 was particularly associated with predatory freshwater insects that had high δ15N values, but was scarce in primary consumers (snails, detritivorous beetles) with low δ15N values.

Two laboratory experiments were undertaken to investigate: 1) the effect of diet on FA profiles in the apple snail, Pomacea canaliculata, and 2) interacting effects of diet and salinity on FA profiles of the Indian medaka fish, Oryzias melastigma. The results of the apple snail study showed that dietary-mediated changes in FA profiles were only reflected in the snail tissues after at least three months, and FA profiles of digestive tissues and neutral lipids were first to respond to the dietary change. The results of the medaka study demonstrated that the ratio of FA 20:4 to FA 20:5 was affected by both diet and salinity, reflecting a similar finding in the Luk Keng field study, although diet had a stronger effect on this ratio.

The results of both field studies supported the use of FA profiles as food web tracers in wetlands and were complemented by laboratory results that yielded insights which will allow refinement of FA biomarker applications in food-web studies.