A caracterização espacio-temporal permitiu confirmar a ocorrência dos cinco estratos definidos a priori, tipificados pela ocorrência de determinadas espécies cuja abundância, densidade e biomassa variou temporal e espacialmente. A existência de um gradiente ambiental foi acompanhada pela ocorrência de um gradiente biológico, ou zonação onde foi possível observar a distribuição diferenciada das espécies nos diferentes estratos. A área de estudo enquadra-se no modelo de zonação universal descrito por McLachlan e Jaramillo (1995) com a primeira zona correspondente a OS a ser tipificada pelo anfípode
Talorchestia deshayesii, a segunda zona correspondente a ES a ser tipificada pelo isópode cirolanideo Eurydice pulchra e a terceira zona correspondente aos três estratos inferiores e a serem tipificadas por uma comunidade variada de poliquetas e bivalves como
Scrobicularia plana ou Cerastoderma edule. A EM, com as suas características intermédias, quer biológicas quer ambientais, constitui uma transição entre os estratos superiores e inferiores.
A caracterização espacio-temporal permitiu determinar o anfípode Talorchestia deshayesii, o gastrópode Hydrobia ulvae, o bivalve Scrobicularia plana e o poliqueta
Nereis diversicolor como espécies-chave para o funcionamento da comunidade e que são responsáveis pela maior parte da produção secundária.
As relações tróficas que se estabelecem na comunidade são complexas, sendo possível observar dois tipos de ecossistema: um ecossistema de interface presente nos estratos superiores e dependente de detritos e nutrientes alóctones, onde os sub- sistemas intersticial e decompositor são extremamente importantes e um ecossistema auto-sustentado presente nos três estratos inferiores onde o "loop" microbiano embora não abrangido pelo presente estudo deverá desempenhar um papel crucial na remineralização dos nutrientes que são utilizados pelos produtores primários e secundários, abundantes nestes estratos. Embora presente, o sub-sistema herbívoro terá um papel secundário numa comunidade francamente baseada no sub-sistema decompositor. Deste modo, os macroinvertebrados bentónicos são um elo fundamental na transferência de matéria orgânica e energia para níveis tróficos superiores. A elevada produção secundária nos estratos inferiores, deverá ser maioritariamente exportada uma vez que as aves e peixes que constituem os níveis tróficos superiores desta cadeia alimentam-se nestas zonas mas não são residentes.
CONSIDERAÇÕES FINAIS
Fig. 19 – Esquema global das fontes de alimento e produção secundária dos macroinvertebrados e das principais relações tróficas que se estabelecem entre os vários grupos de organismos.
OS: orla supra-litoral: ES: zona eulitoral superior; EM: zona eulitoral média; EI a: zona eulitoral inferior com algas e EI: zona eulitoral inferior sem algas.
A sobreposição das assinaturas isotópicas δ15N dos organismos estudados não
permite uma separação clara em diferentes níveis tróficos para algumas das espécies analisadas e é indicadora dos hábitos alimentares generalistas e dietas flexíveis típicos dos organismos estuarinos. Para mais foram registadas taxas isotópicas δ15N elevadas
para os produtores primários, que se pensa serem resultado da introdução de azoto no sistema estuarino. O aumento das taxas isotópicas nos produtores primários evidencia a influência de agentes antropogénicos no ecossistema.
Os ecossistemas estuarinos dispõem de uma grande variedade de fontes de carbono, cuja importância não foi avaliada neste estudo. Para um melhor conhecimento do funcionamento destes ecossistemas é necessário que futuros estudos incluam determinações isotópicas de δ13C para matéria orgânica dissolvida, matéria orgânica
particulada, detritos vegetais, microalgas bentónicas, entre outros. A contribuição do presente estudo pode ser resumida no modelo conceptual da estrutura trófica da comunidade apresentado na figura 19 onde são indicadas as relações tróficas das espécies mais importantes e definidos os sistemas que actuam nas diferentes zonas numa praia estuarina tipificada pelo local de estudo.
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