Validation of the in situ slurry method for measuring primary production of microphytobenthos in tidal flats as compared to culture experiments and mooring systems

Microphytobenthos (MPB) are important primary producers in tidal flats that supply various ecosystem services. However, high turbidity water can limit the amount of photosynthetically active radiation (PAR) that reaches the seafloor in estuaries. Most studies assessing MPB production have been conducted using culture bottles placed in situ for short periods (e.g., one day). However, the in situ method does not take into account changes in weather conditions and/or tidal cycles, and errors may occur when obtaining representative values over a longer period (monthly to yearly). Here, we measured, with high resolution, primary production (PP) of MPB in the Midori River tidal flat facing the Ariake Bay, Japan, by combining photosynthesis–irradiance curves from laboratory experiments and continuous PAR measurements from a mooring system above the seafloor. We compared the median PP and in situ PP (PPin situ, assumed to be PP during the spring tides) for each season. We found that the PPin situ values were 0.887 to 1.92 times the values obtained from the laboratory experiment and the mooring system. The PPin situ values, which varied greatly and were uniformly distributed, had maximum values that were 2.8 times higher than the representative values, except in the spring (1.29 times higher); and the minimum values ranged from 0.00994 to 0.654 times the representative values. Our results suggest that the monthly PP values obtained from the in situ slurry method are erroneous.