Algal food processing by Stegastes nigricans, an herbivorous damselfish: differences between an undisturbed and a disturbed coral reef site (La Réunion, Indian Ocean)

Abstract

There has, as yet, been little study of nutrient disturbance and its consequences on the functional biology of primary consumers in coral reefs. The processing of epilithic algal communities by the damselfish Stegastes nigricans was investigated at La Réunion, at two sites of a fringing reef, oligotrophic for one and eutrophic for the other. Differences between sites were analyzed in quantifying organic matter, organic carbon, nitrogen, and organic ‘nutrients’ (carbohydrate, lipid and protein) in the algal food, the digestive contents and the faeces of S. nigricans. In the disturbed site, algal turfs and digestive contents presented significantly higher organic matter, organic carbon, nitrogen and organic nutrient values. Absorption efficiency of total matter was higher in fish from this site (26.8 %) than from the undisturbed site (16.3 %), as well for organic matter, organic carbon, nitrogen and organic nutrients. A larger mean total length was also measured for fish sampled in the disturbed site (11.4 cm versus 10.9 cm). The reinjection of organic nutrients by this herbivorous fish appeared to be enhanced in this site. It is suggested that eutrophication is the main disturbance factor accounting for the recorded differences.