Seasonality of microphytobenthos revealed by remote-sensing in a South European estuary

Abstract

The spatio-temporal variation of microphytobenthos (MPB) at the scale of a large estuary (Tagus estuary, Portugal) was studied using a combination of field and satellite remote sensing data during 2003. This is the first attempt to use remote sensing to study MPB in an ecosystem with a Mediterranean-like climate. Satellite pour l'Observation de la Terre (SPOT) and Medium Resolution Imaging Spectrometer (MERIS) images were used to map benthic microalgae through the application of a Normalized Difference Vegetation index (NDVI). A significant relationship between in-situ benthic chlorophyll a measurements and SPOT NDVI values was used to derive a map for biomass spatial distribution.At the scale of the whole intertidal area, NDVI time-series from 2003 revealed that MPB showed clear temporal variations, with lower values observed in summer compared to winter. This seasonal trend was found both in the SPOT and MERIS images and may be the result of extreme high temperatures that inhibit MPB growth. The main MPB biofilms were spatially stable through time at a large scale. Maximum NDVI values during the winter were found in the high shore with decreasing NDVI values towards the low shore. MPB light limitation at the lowest bathymetries is likely to occur in winter due to the high turbidity of Tagus estuary.The biomass spatial distribution map, obtained for January 2003, indicated low values ranging from 0 to 20mgChl am−2 for the lower shores, while in the upper shore biomass varied between 60 and 80mg Chlam−2. This study suggests striking differences in MPB seasonal patterns between the northern and southern European estuaries and stresses the need for ecophysiological approaches to investigate the role of thermo- and photo-inhibition as structuring factors for MPB biomass distribution.