NONDESTRUCTIVE TRACING OF MIGRATORY RHYTHMS OF INTERTIDAL BENTHIC MICROALGAE USING IN-VIVO CHLOROPHYLL-A FLUORESCENCE

In vivo chlorophyll (Chi) a fluorescence was measured in undisturbed i ntertidal sediments with the purpose of tracing the vertical migratory rhythms of benthic microalgae. A pulse amplitude fluorometer, an inst rument which does not require physical contact with the sample, was us ed, thus allowing successive measurements to be taken on the same samp le without causing any type of disturbance to the sediment structure. The basis of the method is the possibility to detect changes in the Ch i a concentration near the sediment surface caused by the vertical mov ement of the microalgae. This requires the verification of two conditi ons: the possibility to follow changes in the sediment Chi a content f rom fluorescence intensity, and a sediment photic depth smaller than t he vertical distances covered by the moving microalgae. Both condition s were experimentally verified in intertidal muddy sediments of the Ta gus estuary, Portugal. In vivo fluorescence was shown to vary linearly with the sediment Chi a content, and the sediment photic depth was es timated to reach 0.27 mm, a value clearly smaller than the reported de pths for microalgal migrations. Sediment samples kept under in situ co nditions exhibited large hourly variations (over 400%) in the Chi a fl uorescence intensity, which were closely synchronized with the daytime periods of emersion. The rhythmic fluctuations in Chi a fluorescence were confirmed further to represent microalgal migration by (1) its en dogenous nature (fluorescence continued to follow diurnal and tidal cy cles after removal of environmental stimuli), (2) its dependence on th e vertical distribution of the microalgal population within the sedime nt (vertically homogenized samples failed to display fluorescence vari ations), and (3) the lack of significant temperature and light effects on the fluorescence emission under in situ conditions (tested in thre e species representative of the main groups found in the studied micro phytobenthic communities-the diatom Phaeodactylum tricornutum (Bohlin) , the cyanobacterium Spirulina maxima (Setch. et Gard.), and the eugle nophyte Euglena granulata (Klebs) Lemm.). The results obtained indicat e that, in spite of the potential concurrent effects of factors other than the Chi a concentration on the fluorescence intensity, in vivo Ch i a fluorescence can be used to trace nondestructively the migratory b ehavior of benthic microalgae.