Sources of particulate organic matter at the ecosystem scale: a stable isotope and trace element study in a tropical coral reef

Abstract

Spatial and seasonal variability of sources of particulate organic matter (POM) were evaluated in a coral reef ecosystem. Reef water POM was sampled monthly along a south-north gradient. The passage of tropical cyclone Gamede, which crossed Reunion Island in February 2007, provided an ideal opportunity to examine the impact of high river discharge. Stable isotope composition (δ 13 C, δ 15 N) was determined in oceanic, riverine and reef water POM, sedimentary organic matter (SOM), benthic primary producers and detritus. Trace elements (Ti, Fe, Cr, Ni, Cu, Zn) measured in SOM were used as additional terrestrial tracers. Stable isotope analysis of reef water showed that POM was affected by anthropogenic non-point nutrient sources, water circula- tion patterns and residence time. During the cold and dry season, the southern end of the reef received organic matter input derived from ocean primary production, while the middle and northern reef water POM originated primarily from SOM, reflecting an important bentho-pelagic coupling process. During the hot and wet season, reef water was enriched in benthic detritus due to higher autochthonous production. We found important spatial variability in POM sources, high- lighting the importance of small-scale (hundreds of meters) studies when investigating ecosystem functioning. Although coastal tidal currents transported some river discharge material to the south of the reef, riverine POM input was limited, even after cyclone passage. We hypothesized that the major effect of the cyclone was the export of SOM and benthic detritus out of the system and that persistent disturbances such as groundwater discharges can be of greater importance than a cyclone in long-term degradation of ecosystems.