MICROBIOLOGICAL MEDIATION OF SPECTRAL REFLECTANCE FROM INTERTIDAL COHESIVE SEDIMENTS

Analysis of upwelling radiation (spectral reflectance) by remote sensi ng may provide valuable information on the nature and distribution of the primary producers, the microphytobenthos, on intertidal mudflat sy stems. Spatial and temporal variation in the spectral reflectance sign al (400-900 nm) from the surface of an exposed intertidal mudflat was investigated in relation to the density and vertical position of micro phytobenthos assemblages within the sediment. Spatial measurements wer e obtained from areas of sediment colonized by different assemblages o f microphytobenthos (mainly mixtures of diatoms and euglenids). In add ition, a station was selected to examine temporal changes in spectral reflectance (corrected for variations in ambient light) over an exposu re period. It was shown that the spectral reflectance signal varied, d epending on the pigment compliment and biomass of the surface assembla ges of microphytobenthos. The qualitative variation in composition of the assemblages visualized by low-temperature scanning electron micros copy was confirmed by analysis of species composition (light microscop y) and by pigment fingerprinting (diode array high-performance liquid chromatography [HPLC]). Time-series analysis showed that the migration of cells to the surface of the sediment during a daytime emersion per iod rapidly changed the optical properties of the sediment surface. An alysis of sediment pigment content was conducted using standard surfac e scrapes (0.5-cm depth) and a high resolution sectioning technique (2 00-mu m layers). The migration influence was only detected by fine-sca le analysis of pigments. This was demonstrated by a step-wise eliminat ion analysis, which showed that correlations between the absorbance ch aracteristics of the sediment and pigment content improved as premigra tion data were excluded. Maximum light penetration into the natural se diment was of the order of 2 mm under light conditions comparable with ambient levels. Two main conclusions were drawn from this study: (1) interpretation of spectral reflectance data must be consistent with kn owledge of the ecology and behavior (cyclic migration patterns) of int ertidal microphytobenthos, and (2) ground truthing of pigment-related signals from intertidal Bats should be conducted on a scale relevant t o the process of spectral reflectance from sediments. The implications of these findings are discussed.