Use of in vivo chlorophyll a fluorescence to quantify short-term variations in the productive biomass of intertidal microphytobenthos

This study investigates the ability to use in vivo chlorophyll a fluorescen ce to quantify the productive biomass of undisturbed microphytobenthic comm unities, defined as the photosynthetic biomass present in the photic zone o f the sediment and actually contributing to measurable photosynthesis. The purposes of defining and quantifying productive biomass are (1) to evaluate the effect of the migratory rhythms on the variability of microphytobenthi c photosynthesis and (2) to characterise the community photophysiological r esponse independently of the migratory stage, through the estimation of bio mass-specific, community-level photosynthetic rates. The possibility of usi ng chl a fluorescence, as measured non-destructively at the sediment surfac e, to trace variations in the productive biomass of microphytobenthos was c onfirmed by testing (1) the variability of the relationship between fluores cence emission and chl a concentration under varying temperature and irradi ance levels and (2) the effects of natural variability in the vertical prof ile of chl a within the photic zone of the sediment on the depth-integrated fluorescence signal, F, measurable at the surface. Dark-level fluorescence , F-0, was found to allow for tracing variations in chl a concentration und er the range of temperature and irradiance variability found in situ. Depth -integration of fluorescence emission resulted in only a fraction of total productive biomass being detectable at the surface. However, this fraction was found to be sufficiently constant to allow for the use of F-0 to propor tionally follow variations in community productive biomass. On average, mea surements of productive biomass on natural samples overestimated by a facto r of 1.30 during low tide and underestimated by a factor of 0.66 during hig h tide, mostly due to variations in the chl a profile in the photic zone as sociated with vertical migrations. The method was applied to intertidal mic rophytobenthic communities of the Tagus estuary, Portugal, by non-destructi vely measuring F-0 (using pulse amplitude modulated fluorometry) and photos ynthesis (using oxygen microelectrodes) on the same samples under in situ c onditions, The results showed that a significant proportion of the hourly a nd fortnightly variability in the community photosynthetic light response w as explained solely by variations in-Fc, associated with movements of micro algae, identifying migratory rhythms as the main cause for short-term varia bility in intertidal benthic primary productivity.