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STREAM INTEGRITY IN OCEANIC ISLANDS – A FUNCTIONAL APPROACH USING ORGANIC MATTER DECOMPOSITION
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Abstract(s)
Esta tese apresenta uma avaliação integrada do funcionamento dos ecossistemas ribeirinhos nos Açores, focando-se no efeito das alterações no uso do solo e enriquecimento natural em metais na decomposição da matéria orgânica, um processo essencial determinante para a integridade dos ecossistemas de água doce. Através de experiências de campo e ensaios em laboratório, este trabalho demonstra que os fatores de stress, tanto antrópicos quanto naturais, interagem para alterar o ambiente físico e a estrutura das comunidades bióticas dos ribeiros insulares, resultando em alterações nas dinâmicas de decomposição.
Para avaliar o impacto das modificações na vegetação ripária (plantações de criptoméria e pastagens) na decomposição da matéria orgânica nos ribeiros, realizou-se uma experiência de campo para comparar estes ambientes alterados com ribeiros rodeados por vegetação nativa. Neste estudo usou-se madeira de Ochromona pyramidale e folhas de Clethra arborea dentro de sacos de malha fina e grossa para diferenciar a decomposição exclusivamente microbiana da realizada conjuntamente por microrganismos e macroinvertebrados, respetivamente. As mudanças no uso do solo, especialmente a conversão de florestas nativas de Laurissilva em pastagens e plantações de criptoméria, influenciaram significativamente a taxa de decomposição. Enquanto a decomposição mediada por microrganismos manteve-se semelhante em ribeiros em floresta nativa e plantações comerciais, sugerindo uma possível redundância funcional das comunidades decompositoras, os ribeiros em áreas de pastagem apresentaram uma decomposição mais acelerada, relacionada com temperatura da água e concentração de nutrientes mais elevadas. A contribuição dos detritívoros, especialmente da espécie endémica Limnephilus atlanticus, foi específica de cada ribeiro e fortemente influenciada pelas características do habitat e altitude.
O enriquecimento dos ribeiros em metais, resultante de manifestações de vulcanismo, constitui uma pressão ambiental de origem natural que influencia o processo de decomposição da matéria orgânica. Para avaliar o impacto desse enriquecimento natural em metais sobre a decomposição mediada por microrganismos e detritívoros foram realizadas duas experiências em microcosmos. Na primeira experiência avaliou-se a preferência alimentar, taxa de consumo, crescimento e sobrevivência do detritívoro endémico Limnephilus atlanticus alimentado com três tipos de folhada, Alnus glutinosa, espécie exótica de elevada qualidade (controlo) e Ilex perado e Laurus azorica, duas espécies nativas mais recalcitrantes - previamente submersa num ribeiro naturalmente enriquecido em metais e num ribeiro com baixa concentração de metais. Numa segunda experiência, focada na resposta da decomposição microbiana, folhada de Clethra arborea foi submersa em dois ribeiros com concentrações de metais contrastantes (baixa e elevada) e depois encubada em seis tratamentos representativos de um gradiente de concentração de metais: 100% (água do ribeiro naturalmente enriquecido em metais), 75%, 50%, 25%, 10% e 0% (água do ribeiro de referência). A decomposição microbiana foi avaliada através da perda de massa da folhada, e taxa de esporulação, riqueza específica e estrutura das comunidades de hifomicetes aquáticos. A deposição de metais sobre a superfície foliar reduziu a sua palatabilidade para os detritívoros e alterou a colonização microbiana, levando à diminuição da esporulação dos fungos aquáticos e da eficiência do processo de decomposição. Esses efeitos foram mais acentuados sob altas concentrações de metais e foram influenciados por características intrínsecas dos substratos, como dureza e teor de compostos secundários. Evidências de compensação nas taxas de alimentação e respostas do tipo hormese nos decompositores demostraram a complexidade das respostas bióticas ao stress por metais.
Conjuntamente, estes resultados reforçam o valor da decomposição da matéria orgânica como um indicador funcional da integridade dos ecossistemas. No entanto, também evidenciam a necessidade de interpretações contextualizadas, especialmente em ambientes naturalmente enriquecidos em metais. Embora este trabalho contribua para um entendimento mais profundo dos processos ecológicos em ilhas oceânicas, são necessários mais estudos in situ para calibrar essas ferramentas em cenários ambientais naturais. Finalmente, esta tese ressalta a importância de incorporar indicadores funcionais juntamente com medidas estruturais na monitorização dos ecossistemas aquáticos insulares a fim de melhorar as estratégias para a sua conservação.
ABSTRACT: This thesis presents an integrative evaluation of stream ecosystem functioning in the Azores, focusing on how land-use changes and natural metal enrichment derived from volcanic activity affect organic matter decomposition, an essential process that determines freshwater ecosystem integrity. Through field experiments and laboratory trials, this work demonstrates that both anthropogenic and natural stressors interact to alter the physical environment and biotic community structure of island streams, leading to variations in decomposition dynamics. To evaluate the impact of riparian forest modifications (cryptomeria plantations and pastures) on organic matter decomposition in streams, a field experiment was conducted to compare these altered environments with non-impacted streams dominated by native laurel vegetation. The study used Ochroma pyramidale wood and Clethra arborea leaves as substrates, enclosed in fine and coarse mesh bags, to differentiate between microbial-driven decomposition and decomposition mediated by both microbes and macroinvertebrates, respectively. Land-use changes, particularly the conversion of native laurel forests to pastures and cryptomeria plantations, significantly influenced decomposition rates. While microbial-driven decomposition remained stable across native and commercial plantation streams, suggesting potential functional redundancy of decomposer communities, pasture streams exhibited enhanced decomposition related to higher water temperatures and nutrient content. Shredder contributions, particularly by endemic taxa such as Limnephilus atlanticus, were stream-specific and strongly shaped by habitat features and elevation. Metal enrichment of streams due to volcanic activity constitutes a specific environmental pressure from natural origin that influences the organic matter decomposition process. To evaluate the impact of natural metal enrichment on shredders and microbial decomposition, two microcosm experiments were conducted. The first experiment assessed the feeding preferences, consumption rates, growth, and survival of the endemic Azorean shredder caddisfly Limnephilus atlanticus. We used three types of leaf litter: the high-quality exotic Alnus glutinosa (as control) and the more recalcitrant native species Ilex perado and Laurus azorica. These were submerged in a metal-enriched stream and a reference stream with low metal concentrations. The second experiment focused on microbial decomposer responses. Leaf litter from Clethra arborea was initially submerged in two streams with contrasting metal concentrations (low and high). Subsequently, the litter was incubated in six water treatments, representing a gradient of metal concentrations: 100% (only metal-enriched stream water), 75%, 50%, 25%, 10%, and 0% (only reference stream water). Microbial decomposition was assessed through leaf litter mass loss and aquatic hyphomycete responses, including sporulation rate, taxa richness, and community structure. Metal deposition on leaf litter surfaces reduced palatability for shredders and altered microbial colonization, leading to diminished fungal sporulation and decomposition efficiency. These effects were more pronounced under high metal concentrations and were influenced by leaf litter traits, such as toughness and secondary compounds content. Evidence of compensatory feeding and hormesis responses in decomposers underscored the complexity of biotic responses to metal stress. Together, these findings reinforced the value of organic matter decomposition as a functional indicator of ecosystem integrity. However, they also highlighted the necessity of context-specific interpretation, especially in metal-enriched environments. While this work contributes to a deeper understanding of ecosystem processes on oceanic islands, further in situ studies are needed to calibrate these tools under natural environmental variability. Ultimately, this thesis emphasizes the importance of incorporating functional indicators alongside structural metrics in freshwater bioassessment frameworks to enhance conservation strategies in island ecosystems.
ABSTRACT: This thesis presents an integrative evaluation of stream ecosystem functioning in the Azores, focusing on how land-use changes and natural metal enrichment derived from volcanic activity affect organic matter decomposition, an essential process that determines freshwater ecosystem integrity. Through field experiments and laboratory trials, this work demonstrates that both anthropogenic and natural stressors interact to alter the physical environment and biotic community structure of island streams, leading to variations in decomposition dynamics. To evaluate the impact of riparian forest modifications (cryptomeria plantations and pastures) on organic matter decomposition in streams, a field experiment was conducted to compare these altered environments with non-impacted streams dominated by native laurel vegetation. The study used Ochroma pyramidale wood and Clethra arborea leaves as substrates, enclosed in fine and coarse mesh bags, to differentiate between microbial-driven decomposition and decomposition mediated by both microbes and macroinvertebrates, respectively. Land-use changes, particularly the conversion of native laurel forests to pastures and cryptomeria plantations, significantly influenced decomposition rates. While microbial-driven decomposition remained stable across native and commercial plantation streams, suggesting potential functional redundancy of decomposer communities, pasture streams exhibited enhanced decomposition related to higher water temperatures and nutrient content. Shredder contributions, particularly by endemic taxa such as Limnephilus atlanticus, were stream-specific and strongly shaped by habitat features and elevation. Metal enrichment of streams due to volcanic activity constitutes a specific environmental pressure from natural origin that influences the organic matter decomposition process. To evaluate the impact of natural metal enrichment on shredders and microbial decomposition, two microcosm experiments were conducted. The first experiment assessed the feeding preferences, consumption rates, growth, and survival of the endemic Azorean shredder caddisfly Limnephilus atlanticus. We used three types of leaf litter: the high-quality exotic Alnus glutinosa (as control) and the more recalcitrant native species Ilex perado and Laurus azorica. These were submerged in a metal-enriched stream and a reference stream with low metal concentrations. The second experiment focused on microbial decomposer responses. Leaf litter from Clethra arborea was initially submerged in two streams with contrasting metal concentrations (low and high). Subsequently, the litter was incubated in six water treatments, representing a gradient of metal concentrations: 100% (only metal-enriched stream water), 75%, 50%, 25%, 10%, and 0% (only reference stream water). Microbial decomposition was assessed through leaf litter mass loss and aquatic hyphomycete responses, including sporulation rate, taxa richness, and community structure. Metal deposition on leaf litter surfaces reduced palatability for shredders and altered microbial colonization, leading to diminished fungal sporulation and decomposition efficiency. These effects were more pronounced under high metal concentrations and were influenced by leaf litter traits, such as toughness and secondary compounds content. Evidence of compensatory feeding and hormesis responses in decomposers underscored the complexity of biotic responses to metal stress. Together, these findings reinforced the value of organic matter decomposition as a functional indicator of ecosystem integrity. However, they also highlighted the necessity of context-specific interpretation, especially in metal-enriched environments. While this work contributes to a deeper understanding of ecosystem processes on oceanic islands, further in situ studies are needed to calibrate these tools under natural environmental variability. Ultimately, this thesis emphasizes the importance of incorporating functional indicators alongside structural metrics in freshwater bioassessment frameworks to enhance conservation strategies in island ecosystems.
Description
Tese de Doutoramento, Biologia, 28 de outubro de 2025, Universidade dos Açores.
Keywords
Pedagogical Context
Citation
ESCOBAR, Ana Balibrea. (2025). "STREAM INTEGRITY IN OCEANIC ISLANDS – A FUNCTIONAL APPROACH USING ORGANIC MATTER DECOMPOSITION". Ponta Delgada: Universidade dos Açores, 2025. 202 p. Tese de Doutoramento em Biologia. Disponível em http://hdl.handle.net/10400.3/8745