Quantitative imaging to measure photosynthetic biomass on an intertidal rock-platform

Conventional methods for measuring amounts of epilithic photosynthetic biomass in intertidal habitats by estimating amounts of chlorophyll in rock samples are imprecise, laborious and destructive. An alternative remote sensing method for quantifying chlorophyll amounts at small spatial scales using field-based digital colour-infrared (CIR) imagery is presented. CIR images were obtained from 4 areas of the emersed rock-platform. Experimental plots were initially scraped to remove any macro- or micro-algae. After different periods of recolonization of micro-algae, images of the plots were acquired under full sunlight or under artificial shade, before and after the rocks were uniformly wetted. Samples of rock were taken for laboratory determination of amount of chlorophyll. Relative absorption by chlorophyll was estimated from CIR data using a ratio of near-infrared (NIR) and red bands. The image ratios were validated by comparing them to data from a field spectrometer. Measurements of the amount of chlorophyll extracted from rock samples were linearly related to estimates from the NIR:red ratio. Ratios derived from images acquired under full sunlight, before and after wetting the surface, had the strongest relationship with chlorophyll (r² = 0.84 and 0.83, respectively). Artificial shading of the rock reduced the strength of the relationship between chlorophyll and the NIR:red ratio before and after wetting (r² = 0.79 and 0.63, respectively). Micro-algae in scraped areas and micro- and macro-algal mixtures could be estimated using the same equation. The technique enables rapid contiguousin situmeasurements of chlorophyll to be made without the microflora being destroyed and will facilitate more comprehensive studies of competitive interactions among intertidal grazing animals.