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Dinâmica dos nutrientes na Ria Formosa: efeitos da interacçäo da laguna com as suas interfaces na reciclagem do azoto, fósforo e sílica
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Abstract(s)
A Ria Formosa é uma laguna costeira pouco profunda e mesotidal, com uma área
húmida de cerca de 10 500 ha, situada na costa sul de Portugal. Este sistema lagunar funciona
como nursery relativamente a grande número das espécies ictiológicas costeiras e as suas áreas
intertidais são usadas na cultura extensiva de amêijoa e na produção piscícola. Pelo facto de
ser uma laguna pouco proíúnda, as interacções entre o sedimento e a coluna de água
reflectem-se intensamente na química da água. Nesta laguna o metabolismo bentónico envolve
não só remineralização heterotrófica, mas também produção de matéria orgânica e consumo de
nutrientes. Estas características, associadas a temperaturas elevadas de verão e a uma
considerável pressão antropogénica, podem gerar situações pontuais de crescente stress
ambiental e contribuem para uma grande variabilidade dos processos físicos, químicos e
biológicos internos ao ecossistema, que condicionam a reciclagem dos nutrientes e a
produtividade primária da coluna de água.
Procedeu-se a um programa de amostragens na laguna e na zona costeira adjacente,
para conhecimento dos ciclos biogeoquímicos dos nutrientes, tendo em conta as trocas
sedimento-água, laguna-mar e efeitos de escorrências costeiras de ordem antropogénica e
fluvial. As amostragens foram realizadas na coluna de água da laguna; na zona costeira
adjacente e na água intersticial dos sedimentos superficiais para determinação de compostos
dissolvidos de azoto, fósforo e sílica e no material particulado e sedimento superficial, para a
determinação de compostos de carbono, azoto e fósforo. Estes estudos processaram-se em
diferentes escalas de tempo; minutos, correspondentes ao início da inundação dos sedimentos
intertidais; semi-diúrnas e quinzenais, correspondentes a ciclos de maré; mensais,
correspondentes à produtividade primária do ecossistema.
Os resultados obtidos mostraram globalmente que:
(i) A variabilidade dos nutrientes na laguna é controlada sazonalmente e pelas trocas de massas
de água com a zona costeira. O ciclo sazonal dos nutrientes é inverso do ciclo sazonal do
fitoplâncton e, nas épocas do ano em que o consumo de nutrientes é excessivo estes são
importados da zona costeira; (ii) Os inpuís antropogénicos de carbono, azoto e fósforo reflectem-se intensamente na camada
superficial dos sedimentos intertidais, contrariamente ao que se verifica nas partículas em
suspensão;
(iii) A maré tem um efeito dinamizador na mistura de nutrientes dentro da laguna, nas trocas
laguna-mar e nas trocas sedimento-água das zonas intertidais;
(iv) As trocas sedimento-água nesta laguna, dependem notoriamente da temperatura, da
qualidade do sedimento e da fauna bentónica;
(v) As zonas intertidais são a principal fonte de enriquecimento nutritivo da coluna de água,
contribuindo para a regulação dos ciclos de nutrientes na laguna.
The Ria Formosa is a shallow mesotidal lagoon located at the south ot Portugal with a wet area of 10 500 ha. This ecosystem, is considered a nursery for a large number of coastal species, and its extensive intertidal areas being used as growth bank of clams and aquaculture ponds. Benthic-pelagic coupling is of particular importance because the water volume that overlies a given area of the bottom is relatively small. The influence of benthic processes on water chemistry is, thus correspondingly large. These characteristics, associated to high temperatures at summer and local anthropogenetic effects, lead to occasional environmental stress and contribute to a large variability of physical, chemical and biological processes. Nutrient remineralization and primary productivity in water column may be greatly influenced by that processes. A sampling program was carried out in the lagoon and adjacent coastal waters with the aim to determine the biogeochemical cycling of nutrients associated with primary productivity in the water column and relating these to the sediment-water exchanges, lagoon water-coastal water exchanges and externai inputs (anthropogenetic and fluvial). Several surveys were designed for the; (i) water column of lagoon; (ii) adjacent coastal waters and (iii) interstitial water of superficial sediments in order to determine dissolved and particulate nitrogen, phosphorus, carbon and silicates in the several compartments of the system. Field studies were carried out in different time scales: minutes, corresponding to the beginning of intertidal sediments inundation; semi-diumally and fortnightly associated to tidal cycles, monthly associated to primary productivity of ecosystem. Results obtained in this studies showed that: (i) Nutrients variability in the lagoon water is controlled both seasonally (associated to the phytoplancton biomass) and tidally (associated to the water volume imported/exported); (ii) Anthropogenic inputs of carbon, nitrogen and phosphorus are widely reflected in superficial layer of intertidal sediments while no evidence was found ot this discharge in the suspended particles; (iii) The mixing of nutrients inside lagoon, the exchanges lagoon water-coastal water and the sediment-water exchanges in intertidal zones are notoriously tidally dependent; (iv) Sediment-water exchanges of nutrients vary with temperature, sediment porosity and benthic macrofauna; (v) The intertidal areas are the main source of nutritive ennchment ot water column, contributing to the regulation of nutrient cycles in lagoon
The Ria Formosa is a shallow mesotidal lagoon located at the south ot Portugal with a wet area of 10 500 ha. This ecosystem, is considered a nursery for a large number of coastal species, and its extensive intertidal areas being used as growth bank of clams and aquaculture ponds. Benthic-pelagic coupling is of particular importance because the water volume that overlies a given area of the bottom is relatively small. The influence of benthic processes on water chemistry is, thus correspondingly large. These characteristics, associated to high temperatures at summer and local anthropogenetic effects, lead to occasional environmental stress and contribute to a large variability of physical, chemical and biological processes. Nutrient remineralization and primary productivity in water column may be greatly influenced by that processes. A sampling program was carried out in the lagoon and adjacent coastal waters with the aim to determine the biogeochemical cycling of nutrients associated with primary productivity in the water column and relating these to the sediment-water exchanges, lagoon water-coastal water exchanges and externai inputs (anthropogenetic and fluvial). Several surveys were designed for the; (i) water column of lagoon; (ii) adjacent coastal waters and (iii) interstitial water of superficial sediments in order to determine dissolved and particulate nitrogen, phosphorus, carbon and silicates in the several compartments of the system. Field studies were carried out in different time scales: minutes, corresponding to the beginning of intertidal sediments inundation; semi-diumally and fortnightly associated to tidal cycles, monthly associated to primary productivity of ecosystem. Results obtained in this studies showed that: (i) Nutrients variability in the lagoon water is controlled both seasonally (associated to the phytoplancton biomass) and tidally (associated to the water volume imported/exported); (ii) Anthropogenic inputs of carbon, nitrogen and phosphorus are widely reflected in superficial layer of intertidal sediments while no evidence was found ot this discharge in the suspended particles; (iii) The mixing of nutrients inside lagoon, the exchanges lagoon water-coastal water and the sediment-water exchanges in intertidal zones are notoriously tidally dependent; (iv) Sediment-water exchanges of nutrients vary with temperature, sediment porosity and benthic macrofauna; (v) The intertidal areas are the main source of nutritive ennchment ot water column, contributing to the regulation of nutrient cycles in lagoon
Description
Tese de dout. em Ciências do Mar, Instituto de Investigação das Pescas e do Mar, Univ. do Algarve, 1996
Keywords
Sistema lagunar Nursery Espécies ictiológicas Nutrientes Zonas intertidais