In this project we studied the effects of temperature and dissolved inorganic carbon (DIC) availability in two MPB communities of the Tagus estuary, in central Portugal, applying new methods of remote sensing, namely laser induced fluorescence (LIF) and modulated imaging fluorescence (Imaging-PAM). The LIF methodology (chapter 2) was used to trace migratory rhythms of benthic epipelic microalgae typical of mud sediments. Indeed, it was confirmed that, during the diurnal low tide these microalgae migrate to the surface of sediment to drive photosynthesis and then migrate in depth before the arrival of the tide.

Furthermore, epipelic diatoms show photophobic migration when exposed to high light levels, a process known as behavioral photoprotection. This technique proved to be adequate to the study of MPB communities, both for the determination of surface biomass and the study of migratory rhythms. The possibility of using LIF for middle-range remote sensing under natural light conditions opens good perspectives for its use in environmental monitoring of estuarine systems.

Important hardware differences exist between conventional and imaging systems and therefore a comparative study of the results obtained with these technologies in MPB was required (chapter 3). In fact, we observed significant differences between the results obtained with conventional and imaging systems, especially in what refers to the analysis of rapid fluorescence curves as a function of irradiance (RLCs, Rapid Light Curves). Lower values of alpha (initial slope of the rETR vs. E curve), ETRmax (maximum relative ETR), Ek (light saturation parameter) and Fv/Fm (maximum quantum efficiency of photosystem II of dark-adapted samples) were obtained using the Imaging-PAM system. The discrepancy between instruments was dependent on sample type, being more pronounced in mud than in sandy MPB sediments. The differences may be largely explained by the interplay between the different depth-integration of the fluorescence signal, dependent on the thickness of the photosynthetic layer, and the different attenuation coefficients of the light sources used by the two types of instrument. It is therefore important to take into account the specificity of each fluorescence system when interpreting and comparing chlorophyll fluorescence data of MPB communities, taking full advantage of the application of imaging fluorescence to the MPB communities, in particular in what refers to its unique ability to study the spatial heterogeneity of their photosynthetic activity.

The in-depth understanding of the primary production processes of MPB is a prerequisite to assess the impacts of climate change on estuarine systems. Among the global change variables, increased temperature and atmospheric CO2 (and concomitantly dissolved inorganic carbon, DIC) are expected to have the most significant impact over the primary productivity in estuaries. Therefore, we studied their effect, as well as their interaction, on the photosynthesis of MPB biofilms. The short-term effect of increasing temperature in the photosynthetic activity of two communities of MPB (Alcochete and Trancao) was studied (chapter 4).

We also studied the effect of dissolved inorganic carbon availability (DIC) in photosynthesis, on the two intertidal MPB communities, taking as reference the model diatom species Phaeodactylum tricornutum, using fluorescence imaging (on intact MPB communities) and polarographic oxygen measurement (in cell suspensions) (chapter 5). (Abstract shortened by ProQuest.).