Effect of photoinhibition during bottle incubations on the measurement of seasonal primary production in a shallow coastal water

Primary productivity in Vineyard Sound, Massachusetts (USA), a shallow well-mixed coastal water, was determined at monthly intervals during May-October, 1981. Over the course of each study day fresh samples first were obtained at 3 h intervals and then incubated until sunset with timecourse measurements of ${\mathrm{H}}^{14}{\mathrm{C}\mathrm{O}}_{3}^{-}$ uptake made at frequent intervals. Two seasonal patterns of ¹⁴C uptake were apparent. First, rapid initial uptake of ¹⁴C over each incubation during the spring and fall was followed by reduced uptake during periods of strong sunlight. The duration over which rapid uptake was sustained was a function of the prevailing light intensity at the start of the incubation. Under these conditions estimates of near surface primary productivity based on summing total production over the first 3 h of each back to back incubation were higher by 50 to 130 % than estimates based on single end-point measurements from full-day incubations. During summer, however, uptake of ¹⁴C was linear throughout each incubation when both total daily irradiation and temperature were maximal so that the 2 techniques for estimating productivity led to comparable results. The reasons for these seasonal differences in the pattern of ¹⁴C uptake are unclear, but may be related to changes in temperature or phytoplankton species, or both. Our results for a shallow well-mixed water are in general accord with the conclusion of Harris (1980) that when vertical mixing is sufficient to prevent prolonged exposure of phytoplankton to photoinhibiting light intensities at the surface, photoinhibition could be far more pronounced during a bottle incubation than is occurring in the water column.