Isotopic ratios and elemental contents as indicators of seagrass C processing and sewage influence in a tropical macrotidal ecosystem (Madagascar, Mozambique Channel)

摘要

Isotopic ratios and elemental concentrations of carbon and nitrogen were measured in seven seagrass species colonising different tidal flats near Toliara (SW Madagascar) in order to determine the potential use of these parameters for assessing C processing and sewage use by tropical seagrasses. Nitrogen concentrations measured in upper intertidal seagrasses near Toliara were almost twice those measured on the tidal flat near a healthy mangrove situated 20 km away from Toliara town. At Toliara Beach,delta ~(15)N values were correlated with the N concentrations of Halodule sp., one of the dominant species on the tidal flat. This correlation did not exist for Halophila ovalis, the other dominant species. An increase in N concentrations and delta ~(15)N values demonstrates the influence of sewage coming directly onto Toliara Beach on the N cycles of intertidal seagrasses. Nevertheless, this influence seems restricted to the upper littoral zone and was not the main cause of seagrass die-off. On the other hand, at the mangrove site, delta ~(15)N values were not correlated with the N concentrations of Halodule sp. or Thalassia hemprichii, showing that natural delta ~(15)N variability is driven by other factors than the delta ~(15) N of N sources. Moreover, inter-individual variability of delta ~(15) N values was greater than inter-specific or inter-site variability, making the delta ~(15) N difficult to interpret in the context of human-disturbance influence on the N cycle of tropical seagrasses. ~(13) C values were close to -9%_c, indicating the use of HCO_3- as an inorganic darbon source by the seven investigated species. Contrary to our hypothesis, variation between sites and location on the tidal flat was limited, suggesting limited impact on delta ~(13)C values of sewage, emersion duration and mechanisms for HCO_3- incorporation.