Pressão alimentar dos peixes sobre a comunidade bentônica em diferentes profundidades de costões rochosos na enseada de cagarras em Fernando de Noronha

Abstract

Interactions among organisms and abiotic conditions may structure ecosystems by directly affecting species composition and indirectly affecting trophic interactions. In reef systems fish interactions with the benthic community (top-down) are critical for structuring and functioning of these ecosystems. In these environments, benthic and nektonic communities and their trophic interactions vary according to depth due to changes in abiotic conditions (e.g. light) and the availability of nutrients or food sources. We evaluated fish feeding pressure on the benthos (trophic interactions) and its relation with benthic cover along different depths in the inlet of Cagarras at Fernando de Noronha, NE Brazil. We used remote video recordings in which 2 m² reef areas were recorded for 10 minutes at three depths strata: shallow (4-6m), medium (8-9m) and deep (14-15m), totalizing 15 samples per depth. In each video we recorded the species, functional group, size and number of bites on the substrate of each fish. We also evaluated the benthic cover within the recorded areas through 5 photoquadrats within each recorded area, 75 in each depth stratum, totalizing 225 photos analysed using the Photoquad software. Total feeding pressure decreased with depth, being approximately three times greater in the shallow than in the deep stratum. Most of this pressure was exerted by territorial herbivores (Pomacentridae) and scrapers (Labridae: Scarini), which also showed declining abundances with increasing depth. Both fed more frequently on the matrix of epilithic algal matrix (turf, ~ 80%) and foliose algae (~ 20%), dominant components of the benthic community at all depths. The cover of turf, foliose algae, corticated algae, scleractinean corals and substrate (sand and rubble) varied significantly among depths, where foliose algae were more common in the shallow stratum compared to the deep stratum, as opposed to scleractinean corals and the substrate (sand and rubble) which were more common in the deep stratum than in the shallow stratum. There was no relationship between herbivory and benthic cover, indicating that other abiotic forces may be responsible for the observed pattern in the cover of foliose algae and the decrease of herbivory with depth. Understanding the relative importance of biological interactions and abiotic conditions for community structuring is critical for understanding ecosystem functioning