Entre rochas e homens: ecologia e conservação de bivalves em ambientes costeiros antropizados

Abstract

Anthropogenic impacts are changes to natural ecosystems through human activities. Bivalve molluscs, in general, have socioeconomic importance for many families in coastal regions in Brazil. Although some smaller groups are not used in food due to the little meat available, such as Brachidontes exustus, they have ecological relevance as bioengineering organisms in coastal systems. They are considered key species in these ecosystems, as they regulate the availability of locations and microhabitats for the associated macrofauna, attracting a variety of benthic organisms, including Polychaetes, Flatworms, Nematodes, Cirripedes, etc. The main objective of this work is to verify how anthropization influences bivalve beds and their associated macrofauna on rocky beaches, as well. We hope to 1) analyze the effects of impacts caused by sewage on bivalves in tropical coastal ecosystems; 2) evaluate the effects of anthropic influence on the cover of bivalves and other benthic groups, identify the associated fauna and 3) the effect of anthropization on the abundance of bivalves and associated fauna on urban beaches in Salvador, Bahia. The thesis was divided into three chapters: one theoretical and two empirical chapters. The first was a systematic review of the literature, where we found that chemical methods were prioritized more than ecological methods, at least in the tropical region. The second chapter analyzes the effect of anthropization on the percentage of bivalve coverage and the presence of associated species, using bathing and urbanization as anthropic measures. We identified 13 species associated with the Brachidontes exustus and Isognomon bicolor banks, the bivalve cover had a positive effect on three of them (Chthamalus sp. (Cirripedia), Balanus sp. (Cirripedia). and Echinolittorina lineolata (Gastropoda)) and negative on another three Echinometra lucunter (Echinoidea), Fissurella sp. (Gastropoda) and Zoanthus sociatus (Hexacorallia). The total coverage of bivalves and the presence of B. exustus were greater in more anthropic environments, being negatively related to poor bathing conditions and positively to urbanization. The third chapter identified the taxa found in association with bivalves and analyzed the influence of anthropization on the total density of bivalves and associated taxa. Anthropization was assessed through model selection using Generalized Additive Mixed Models (GAMM), while the effect of B. exustus on the abundance of associated fauna was analyzed using Generalized Linear Models (GLM). We found a total abundance of 78,028 organisms and a total richness of 30 taxa in the bivalve banks, with emphasis on polychaetes and gastropods. The best fitting models were the null models, indicating a limited explanation of anthropogenic variables in the abundance of macrofauna associated with B. exustus. However, the GLM models showed that the abundance of B. exustus has a significant positive effect for most taxa, highlighting the influence of these bivalves as a biological substrate. We highlight the complexity of isolating the anthropic effects of natural stressors, as the coast is a naturally disturbed environment. It is crucial to investigate the variation in biodiversity patterns over time, analyzing changes in bivalve distribution and anthropogenic pressures compared to non-urbanized areas.