Benthic source dominance in temperate rocky reefs revealed by stable isotopes

Abstract

Rocky reefs are heavily impacted by anthropogenic activity and since they are important breeding, nursery and feeding areas, their protection is imperative. To improve conservation methods, it is necessary to understand the structure and processes of rocky reef communities. Trophic relationships are one of the examples of species connection, with food webs representing the flow of matter and energy from producers to consumers and from prey to predators. Stable isotope analysis (δ13C and δ15N) was used to characterize the food web of the rocky reefs of Arrábida Marine Protected Area (Portugal) and to explore habitat complexity – niche diversification relationships. Results showed a complex food web with high trophic richness supported by a wide range of basal resources. As a result of the wide range of basal resources and primary consumers the food web was characterized by a high degree of omnivory which led to a relatively short food web. Furthermore, consumers displayed diverse diets and feeding strategies, exploiting different sources of organic matter, of both benthic (macroalgae and benthic POM) and pelagic origin (phytoplankton and pelagic POM). However, the benthic pathway was more important with more than half of the diet of most secondary and tertiary consumers following this route, hinting at the importance held by macroalgae and benthic production in this ecosystem. Overall, the high habitat complexity allied to the shallow and sheltered location of the reefs increased niche diversification (wide range of basal resources) supporting a diversity of omnivore species which improved the connectance among taxa and thus the complexity of the food web. This study is an essential step to better understand shallow temperate rocky reefs and, as such, provide additional support for conservation measures and management decisions.