In situ temporal studies of microphytobenthos using pulse-modulated fluorescence

The use of remote sensing techniques for the measurement of the photosynthetic activity of microscopic primary producers on intertidal soft sediments has been advancing rapidly in recent years. Optical measurements from algal pigments at the sediment surface can be taken rapidly and are non-destructive, and this facilitates short-term studies previously impossible. The technique used in this study was the measurement of chlorophyll a fluorescence using a pulse-modulated fluorometer. The sediment surface was monitored during periods of low tide on six consecutive days. The coefficients of variation of 15 replicate measurements were used to determine the patch size of microphytobenthos distribution which was < 1.2 cm(2). Three replicates, 10 cm apart would be sufficient to determine the surface distribution of microphytobenthos at this site.

Microphytobenthos accumulated at the sediment surface on exposed sediment and remained there prior to tidal flood. Some cells were found at the surface of the sediment as the tide ebbed during daylight. The initial biomass of microphytobenthos at the sediment surface increased as the tidal ebb became later on subsequent days. Thus a significant biomass of microphytobenthos was present at the sediment surface during both tidal ebb and flood during daylight. This has implications for re-suspension of cells during daylight immersion. Biomass was always lower at tidal ebb than flood; thus cells, once immersed, must migrate away from the surface, be removed or grazed. The percentage rate of increase of microphytobenthos at the surface was remarkably similar among days of varying time of low tide despite differing physical influences (time of tidal cycle, irradiance, temperature).