The source and fate of organic matter and the significance of detrital pathways in a tropical coastal ecosystem

Abstract

Stable isotope analysis (SIA) and fatty acid profiling were used to elucidate the supply and fate of energy in a tropical coastal ecosystem in Hong Kong, southern China. To investigate seasonal changes in source supply on the diets of different trophic guilds, measurements were taken in three rocky bays before and after the onset of the summer monsoon, when supply of both marine macroalgal and streamborne terrestrial detritus increases. Particulate organic matter, comprising a mixture of marine and terrestrial sources, was the ultimate food source (.60%) for suspension feeders, which were the major prey items of a predatory gastropod (Thais). Increased levels of bacterial fatty acid biomarkers (BaFA) were recorded after the onset of the summer monsoon, indicating amplified dependence on detrital materials in both primary and secondary consumers. The considerable increase in the detritus fraction of sedimentary organic matter at the onset of the summer monsoon was reflected by enhanced levels of BaFA, possibly due to the degeneration of macroalgae. Significant contributions of this marine algal detritus to deposit feeders (Holothuria, 36%) and of terrestrial detritus to an echinoid grazer (Salmacis, 14%) were revealed by SIA mixing models as well as elevated BaFA concentrations. These results indicate a higher dependence on heterotrophic food chains based on decomposing marine algae and terrestrial detritus after the onset of the summer monsoon. Such seasonal variation in the importance of detrital energy sources is, therefore, likely to be important to coastal ecosystem functioning in the monsoonal tropics.

A groundwater-fed coastal inlet as habitat for the Caribbean queen conch Lobatus gigas—an acoustic telemetry and space use analysis

“…This suggests that queen conches can deplete much of their food resources even at low densities. While food sources were not quantified in the Xel-Há inlet, seasonal cycles may impact their quality/ availability (Medina-Gómez & Herrera-Silveira 2006, Wai et al 2008). Further investigations would be required to determine in detail how variations in the food quality/availability may impact the movement of queen conches.…”

Fatty acids as dietary tracers in benthic food webs

“…MDS then uses the ranks of similarities in an iterative process to find the spatial arrangement of samples that best represents the relationships among them (Clarke & Green 1988, Hair et al 2006. MDS is an exploratory analysis that does not take into account any a priori groups and so can be used to classify consumers with unknown diets (Howell et al 2003, Guest et al 2008 as well as to look for spatial (Hughes et al 2005, Fernandez-Jover et al 2007) and temporal (Wai et al 2008) differences in overall consumer FA composition. It has also been used to analyze the results of controlled feeding studies, both for comparing overall FA composition of animals on different diets (Kelly et al 2008(Kelly et al , 2009) and for tracking changes to FA composition over time (Hall et al 2006).…”

Spatial, seasonal, and ontogenetic variations in the significance of detrital pathways and terrestrial carbon for a benthic shark, Chiloscyllium plagiosum (Hemiscylliidae), in a tropical estuary

“…However, FAP and FA-specific d 13 C revealed that spatial, seasonal, and ontogenetic variations in shark diets were mainly attributable to changes in sources of detrital carbon, especially allochthonous sources. These detrital pathways could not be detected by SIA alone because its resolution is limited by the variety of detrital sources with similar stable-isotope sigantures in this complex environment (Kharlamenko et al 2001;Darnaude 2005;Wai et al 2008).…”

“…Subsets of sharks (n 5 10) and prey samples (n 5 10) were lipid-extracted and the relationship between changes in d 13 C, % lipid (see below) and C : N ratio established, and used to correct the lipid effect on d 13 C using modeling methods (Post et al 2007). Stable isotope values for detritus from terrestrial, macroalgal (Sargassum hemiphyllum), and phytoplankton (i.e., particulate organic matter , 125 mm) sources were obtained from parallel studies (Wai et al 2008;T. C. Wai unpubl.…”

“…Studies of the importance of various carbon sources to invertebrate and fish communities in Hong Kong coastal waters have shown that consumers at lower trophic levels generally depend on autotrophic carbon sources (Lee 2000;Chen et al 2008;Wai et al 2008), but switch to greater dependence on detrital pathways during the wet, summer monsoon, when the availability of terrestrial-, benthic algal-, and phytoplankton-detritus peaks (Wai et al 2008). While mass-balance models suggest that detritus from various sources is important in supporting food webs in the Pearl River estuary and adjacent waters (Li and Lee 1998;Duan et al 2009), little is known about the relative importance of different detrital sources and their contribution to sustaining consumers at high trophic levels such as sharks and marine mammals.…”