SPATIOTEMPORAL DISTRIBUTION OF THE MICROPHYTOBENTHIC BIOMASS IN INTERTIDAL FLATS OF TAGUS ESTUARY (PORTUGAL)

A study on spatio-temporal distribution of microphytobenthos in intertidal zones of Tagus Estuary was carried out from 1990 to 1992. Near Lisbon, Portugal, Tagus Estuary is a shallow mesotidal estuary, covering an area of 320 km(2). The intertidal area ranges from 20 to 40% of the total area and it is constituted mainly by mudflats. Intertidal flats are richly populated by microalgae, diatoms being the most important and ubiquitous group. Spatial variation of microphytobenthos was studied in spring 1990, 21 different sites were sampled. Microphytobenthos biomass was evaluated as chlorophyll a content of the surface centimeter, ranging from 10 to 240 mg m(-2). A Principal Component Analysis showed that 62% of the total variability found in intertidal flats of Tagus estuary could be attributed to two major factors: sediment type and tidal height. A hierarchical grouping defined 3 major groups of similar stations, each one representing a different strata of the ecosystem. One station from each group was chosen for the study of the temporal variation. A sampling program took place from April 1991 to April 1992, with fortnightly sampling, the Chl a ranged from 20-300 mg m(-2). No clear seasonal variation was found, and our results indicated that tidal height of sampled site played an essential role in temporal biomass evolution, thus upper littoral sites were influenced by climatic parameters, whereas in lower sites action of tides mainly controlled microphytic biomass.