In situ measurement of gross photosynthesis using a microsensor‐based light‐shade shift method

We present a measuring procedure that allows the quantification of gross photosynthesis at the ambient light intensity, P(I), from light transition measurements similar to those employed in the light-dark shift method but without the necessity of achieving complete darkness. The method is thus more readily applicable in situ, where, during daylight, complete sample darkening is very difficult or even impossible to achieve. The procedure involves rapid microsensor-based monitoring of oxygen in the sample during a series of light transitions from ambient light intensity to a few intermediate levels, and determination of the initial rates of oxygen decrease during each partial darkening (shading) period. P(I) is then recovered by fitting the measured rates with a function derived from the model describing the change in photosynthesis rate with light intensity (P-I curve) and extrapolating it to the full light-to-dark transition. We validated this approach in the lab on coral and microbial mat samples and found that a satisfactory estimate of P(I) can be obtained with as few as 4–5 shade levels. We also applied the procedure in situ and showed that the gross photosynthesis rate at the ambient sunlight in the microbial mat and in the coral reached, respectively, approximately 50% to 65% and 94% to 97% of the saturated rate.