Spatial variation of chlorophyll on estuarine mudflats determined by field-based remote sensing

We present an approach to field-based remote sensing of the spatial variation of chlorophyll (as an index of microphytobenthic biomass) in intertidal soft sediments using digital colour-infrared imagery. Variation in amounts of chlorophyll a was quantified from small (cm) to large (100 m) scales within a single tidal cycle, using levels of replication and resolution that would be difficult to achieve using conventional sampling strategies. Eight scales of interest were selected: 100, 50, 2 and 1 m, and 40, 8, 4 and <2 cm. These scales have been associated with variability in chlorophyll in previous studies. The fully hierarchical design allowed spatial variance to be estimated independently at each chosen spatial scale. Data were collected along 2 replicate transects at 2 locations within each of 2 different estuaries. A strong linear relationship was found between measured chlorophyll and image estimates of chlorophyll. Replicate transects in 2 of the 4 locations showed similar patterns of variability across spatial scales. Relatively large amounts of variability were found at the smallest spatial scales (4 and <2 cm) and at the 40 cm scale, but the 8 cm scale was an unimportant source of variation. At some scales, transects gave similar results, providing generality to the conclusions reached. At other scales, transects gave quite different results, drawing attention to problems of inferring ecological processes when there are only single sets of data. Large differences in variability were also found from location to location. These patterns suggest that spatial variability is governed by complex interactions between fauna, microflora and sediment.