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Decomposição dos detritos vegetais em rios - Um indicador do efeito do uso do solo na integridade funcional do ecossistema
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A decomposição da matéria orgânica é um processo vital para o funcionamento ecológico dos ecossistemas lóticos nos quais os detritos provenientes da vegetação ripícola constituem a principal fonte de energia para as teias alimentares aquáticas. As atividades humanas, através da alteração da vegetação ripícola, da carga de nutrientes ou de poluentes na água, podem afetar este processo, diminuindo a disponibilidade de energia para as comunidades aquáticas. Este trabalho, desenvolvido no âmbito do projeto LandComp, avaliou como quatro usos do solo (floresta nativa, agricultura, plantações de eucalipto e urbano) influenciam o processo de decomposição de folhada em ribeiros do Centro de Portugal. Em cada um dos quatro locais, as amostras de Alnus glutinosa (L.) Gaertn, Alnus acuminata Humboldt, Bonpland & Kunth e Quercus rubra L. foram expostas em sacos de rede de malha grossa e de malha fina durante 49 dias para avaliar a perda de massa total, a perda de massa por decomposição microbiana, a perda de massa por fragmentação e a colonização por macroinvertebrados. A decomposição foi mais rápida em A. glutinosa, rica em nutrientes, pobre em compostos secundários e macia, que também teve maior perda de peso por decomposição microbiana e por fragmentação biótica e abiótica. As outras duas espécies, com maior teor de compostos secundários, decompuseram-se mais lentamente. A. acuminata teve a menor perda de peso por decomposição microbiana enquanto Q. rubra, mais resistente à abrasão física, teve a menor perda de peso por fragmentação. A decomposição foi mais rápida nos locais de eucalipto e de floresta nativa, onde a ação conjunta dos macroinvertebrados e da corrente provocou a maior perda de peso por fragmentação. A decomposição foi mais lenta no local urbano, onde a concentração de azoto inorgânico dissolvido era elevada e onde ocorreu a menor perda de peso por fragmentação. Apesar de no local de agricultura ter ocorrido a maior colonização por macroinvertebrados fragmentadores, a decomposição total foi semelhante ao local urbano e a perda de peso por fragmentação foi intermédia. A razão entre a perda de peso nos sacos de malha grossa e nos sacos de malha fina, utilizada frequentemente para avaliar a integridade funcional dos cursos de água, separou claramente o local de floresta nativa do local urbano, mostrando uma diminuição progressiva da contribuição da fragmentação, maior no local de floresta nativa, intermédia nos locais de eucalipto e de agricultura e menor no local urbano. Em conclusão, a decomposição dos detritos diferiu entre os locais sujeitos a diferentes usos do solo, diminuindo com a redução da presença de vegetação ripícola, essas diferenças deveram-se principalmente à fragmentação, e a fragmentação terá sido em grande parte devida à abrasão física dos regimes hidrológicos torrenciais que ocorreram devido aos fenómenos de precipitação. O uso de sacos de malha grossa e de malha fina demonstrou ser um método útil para avaliar o efeito das alterações dos usos do solo na integridade funcional dos ecossistemas ribeirinhos baseados em detritos.
The decomposition of organic matter is a vital process for the ecological functioning of lotic ecosystems when detritus from the riparian vegetation constitutes the main source of energy for the aquatic food webs. Human activities, by altering riparian vegetation, increasing the load of nutrients or of pollutants, may affect this process, decreasing the availability of energy for the aquatic communities. This work, developed in the framework of the LandComp project, evaluated how four land uses (native forest, agriculture, eucalypt plantations, and urban) influence the process of leaf litter decomposition in streams of Central Portugal. In each of the four sites, samples of Alnus glutinosa (L.) Gaertn, Alnus acuminata Humboldt, Bonpland & Kunth and Quercus rubra L. were exposed in coarse and in fine mesh bags for 49 days to assess total mass loss, mass loss by microbial decomposition, mass loss by fragmentation, and macroinvertebrate colonization. Decomposition was fastest in A. glutinosa, rich in nutrients, poor in secondary compounds, and soft, which also lost more mass by microbial decomposition, biotic and abiotic fragmentation. The other two species, with higher contents of secondary compounds, decomposed slower. A. acuminata had the lowest mass loss by microbial decomposition while Q. rubra, more resistant to physical abrasion, had the lowest mass loss by fragmentation. Decomposition was fastest in eucalypt and native forest sites, where the joint action of the macroinvertebrates and current promoted the higher mass loss by fragmentation. Mass loss was slowest in the urban site, where the concentration of dissolved inorganic nitrogen was highest and where the mass lost by fragmentation was lowest. Although the highest shredder macroinvertebrate colonization occurred in the agriculture site, total decomposition was similar to the urban site, and mass loss by fragmentation was intermediate, The ratio between mass loss in coarse mesh bags and in fine mesh bags, often used to evaluate functional integrity of streams, clearly separated the native forest from the urban site, showing a decrease in the contribution of fragmentation to decomposition, highest in the native forest site, intermediate in the eucalypt and agriculture sites, and lowest in the urban site. In conclusion, leaf litter decomposition differed among the sites, decreasing with the decrease in riparian vegetation cover, those differences were mainly due to fragmentation, and fragmentation was mainly due to current abrasion promoted by the torrential discharge after heavy rain events. The use of coarse and fine mesh bags was an useful method to evaluate the effect of land use in detritus-based streams.
The decomposition of organic matter is a vital process for the ecological functioning of lotic ecosystems when detritus from the riparian vegetation constitutes the main source of energy for the aquatic food webs. Human activities, by altering riparian vegetation, increasing the load of nutrients or of pollutants, may affect this process, decreasing the availability of energy for the aquatic communities. This work, developed in the framework of the LandComp project, evaluated how four land uses (native forest, agriculture, eucalypt plantations, and urban) influence the process of leaf litter decomposition in streams of Central Portugal. In each of the four sites, samples of Alnus glutinosa (L.) Gaertn, Alnus acuminata Humboldt, Bonpland & Kunth and Quercus rubra L. were exposed in coarse and in fine mesh bags for 49 days to assess total mass loss, mass loss by microbial decomposition, mass loss by fragmentation, and macroinvertebrate colonization. Decomposition was fastest in A. glutinosa, rich in nutrients, poor in secondary compounds, and soft, which also lost more mass by microbial decomposition, biotic and abiotic fragmentation. The other two species, with higher contents of secondary compounds, decomposed slower. A. acuminata had the lowest mass loss by microbial decomposition while Q. rubra, more resistant to physical abrasion, had the lowest mass loss by fragmentation. Decomposition was fastest in eucalypt and native forest sites, where the joint action of the macroinvertebrates and current promoted the higher mass loss by fragmentation. Mass loss was slowest in the urban site, where the concentration of dissolved inorganic nitrogen was highest and where the mass lost by fragmentation was lowest. Although the highest shredder macroinvertebrate colonization occurred in the agriculture site, total decomposition was similar to the urban site, and mass loss by fragmentation was intermediate, The ratio between mass loss in coarse mesh bags and in fine mesh bags, often used to evaluate functional integrity of streams, clearly separated the native forest from the urban site, showing a decrease in the contribution of fragmentation to decomposition, highest in the native forest site, intermediate in the eucalypt and agriculture sites, and lowest in the urban site. In conclusion, leaf litter decomposition differed among the sites, decreasing with the decrease in riparian vegetation cover, those differences were mainly due to fragmentation, and fragmentation was mainly due to current abrasion promoted by the torrential discharge after heavy rain events. The use of coarse and fine mesh bags was an useful method to evaluate the effect of land use in detritus-based streams.
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Keywords
Decomposição microbiana Detritos foliares Macroinvertebrados Projeto LandComp Ribeiros