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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Tracing the flow of organic matter from primary producers to filter feeders in Ria Formosa lagoon, southern Portugal
Auteurs :	R. Machas, Auteur ; R. Santos, Auteur ; B. Peterson, Auteur
Type de document :	Article
Dans :	Estuaries (26(4A) AUG 2003)
Article en page(s) :	846-856
Résumé :	The flow of organic matter along the main navigation channel of Ria Formosa, Portugal, was assessed using determinations of suspended particulate matter (SPM), particulate organic matter (POM), and chlorophyll a (chl a) concentrations in conjunction with stable isotope values of primary producers, particulate matter, and two filter feeders. SPM in the lagoon is dominated by inorganic particles comprising 80% of total weight with organic matter averaging about 20%. The algal component of the POM averaged about 5% with the remainder comprised of detritus. The delta(13)C values of primary producers ranged from -9.1parts per thousand in the intertidal seagrass; Zostera noltii to -30.7parts per thousand in the red seaweed Bostrychia scorpioides revealing underlying differences in the mechanisms of carbon uptake. The delta(13)C value of B. scorpioides, which develops entangled on the salt marsh species Spartina maritima, suggests that its main source of inorganic carbon is atmospheric CO(2). The delta(13)C Values of the high marsh macrophyte Sarcocornia perennis significantly increased with distance from the ocean while delta(13)C values of Z. noltii decreased, probably because higher decomposition of organic matter at inner stations lowers the delta(13)CO(2) value in the water. The delta(15)N values of Ulvales, seagrasses, and marsh plants significantly increased from outer stations to inner stations. This increase may be due either to recycling of nitrogen (N) within the marsh (with loss of light N(2) or NH(4)) or to inputs of isotopically heavy N from sewage. The delta(15)N values of particulate matter showed an opposite trend, which indicates higher microbial degradation of organic matter at the inner lagoon. The data demonstrate that the seston in the lagoon is a mixture of detritus from lagoon primary producers with a minor contribution of microalgae. The filter feeders are most likely assimilating a mixture of phytoplankton and microphytobenthos. Digestion of lagoon seston is selective. The delta(15)N values of both muscle and digestive gland of filter feeders showed the opposite gradient of particulate matter indicating that the depleted delta(15)N of SPM at inner stations was not assimilated or even ingested. Stable isotopes values did not differ between the filter-feeders-the mussel Mytilus galloprovincialis collected on buoys and the clam Tapes decussatus collected in the sediment-suggesting a considerable mixture of benthic-pelagic organic matter throughout the water column. Assessment of the changes in isotopic decomposition of detritus as it decays is required to refine our understanding of organic matter transfers in detrital food webs.

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