Competição bentônica em ecossistemas recifais: resultados, mecanismos e efeitos de estressores ambientais

Abstract

Competition among organisms is one of the strongest ecological interactions structuring communities. In reef ecosystems, sessile organisms compete for the limited hard substrate available to settle and grow, and they use a range of mechanisms to maintain their space and outcompete organisms nearby. Calcifying corals (hard corals or scleractinians and hydrocorals) are one of the main organisms that build reefs, however, climate change and local disturbances are causing declines in their abundance. This is shifting the benthic communities toward dominance by non-calcifying taxa like macroalgae, zoantharians, and soft corals, that can outcompete corals for substrate. However, the outcomes of competition can vary with the environmental factors that influence the competitive ability of organisms. This thesis aimed to investigate the outcomes of competition between calcifying corals and macroalgae, soft corals, and zoantharians from Southwestern Atlantic and Indo-Pacific reefs, the mechanisms of competition, and how present and future environmental stressors affect the outcome of these interactions, through laboratory experiments. In the first two data chapters I investigated the mechanisms and outcomes of competition involving calcifying corals from the Southwest Atlantic (Siderastrea sp. and Millepora alcicornis) and the Indo-Pacific (branching Porites sp. and Acropora cf. kenti), competing with common local competitors: a macroalga (Dictyopteris delicatula) and a zoantharian (Palythoa caribaeorum) in the Southwest Atlantic, and a soft coral (Clavularia sp.) in the Indo-Pacific. In Chapter 2, both competitors only harmed the calcifying corals upon physical contact, with the zoantharian being a stronger competitor than the macroalga. In Chapter 3, the soft coral negatively affected the weakest hard coral (branching Porites sp.) through physical damage, and its allelochemicals did not impact any hard coral. Both chapters demonstrated that the susceptibility to damage from competition in calcifying corals is species dependent: Mi. alcicornis suffered more damage than Siderastrea sp. when competing with the macroalga but both corals were highly affected when contacted by the zoantharian, and A. cf. kenti did not suffer damage by the soft coral while branching Porites was negatively affected. In Chapter 4, I assessed the effects of iron enrichment on physical competition between three calcifying corals (Siderastrea sp., Mi. alcicornis, and Mussismilia harttii) and a macroalga (Lobophora variegata) and a zoantharian (P. caribaeorum), considering the potential effects of mining dam rupture disasters that occurred on the Brazilian coast. Competition via direct contact was deleterious for all calcifying corals irrespective of iron, but high iron enrichment aggravated the negative impacts of competition on the most vulnerable coral, Mu. harttii. Finally, in Chapter 5, I investigated separate and combined effects of nitrate enrichment and ocean acidification on competition between hard and soft corals (Stylophora pistillata and Xenia spp., respectively) from the Indo-Pacific. While competition negatively affected the hard coral, the combination of acidification and nitrate enrichment mitigated the negative effects on the responses of its endosymbionts, but acidification reduced the calcification rates of the hard coral. The responses of the endosymbionts of the soft coral were favored by nitrate enrichment, and acidification had no impacts on the soft coral. Overall, calcifying corals suffered detrimental effects when in physical contact with the competitors, while most of the organisms interacting with the calcifying corals did not exhibit detectable damage from competition. This thesis demonstrates that: the severity of competition effects on calcifying corals and the mechanisms used by competitors depend on the species involved and the environmental conditions. Specifically, environmental stressors related to pollution present more risks to calcifying corals when they are competing with other benthic organisms, and scenarios with increasing anthropogenic stressors, either related to climate (global scale) or pollution (local scale), tend to have a lesser impact on non-calcifying competitors. These results increase our understanding of the specificities of competitive interactions on benthic reef ecosystems and enable more accurate forecasts of the consequences of increasing anthropogenic stressors in reef ecosystems.