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The hypothesis that variation in the initial slope of the photosyiithesis-irradiance curve due to variations in wavelengths of light could be responsible for lower rar es of estimated photosynthesis in the open ocean is tested by direct measurement of photosynthetic action spectra (12-X bands, 25-nm bandpass, eight intensities per band) for natural populatio 1s from the well mixed waters containing the spring plankton bloom in the Sargasso Sea. We conclude that this possibility is unlikely, at least at this time of year. The spectral shape of a was similar to that of laboratory diatom or dinoflagellate populations, with a peak at 450 nm, a valley between 500 and 600 nm, and a slight rise at 675 nm. No evidence was found for absorption or photosynthesis by phycobiliproteins and there was little variation with depth. The action spectra covaried with the absorption spectra (determined on glass-fiber filters) except at 400 nm, where 1 ighcr absorption and relatively little photosynthetic response was observed. Apparent quantum requirements estimated from absorption and action spectra were spectrally invariant (except 400-nm band) and ranged from 60 to 200 moles quanta per mole carbon reduced. It is concluded that estimates of open ocean production from chlorophyll-light models with parameters estimated from laboratory algal cultures are likely to be too high because of a large component of absorptiotl that, while covarying spatially and spectrally with photosynthetic pigment, is itself photosynthetically inactive.The traditional view of the open ocean as a biologically unproductive sea (e.g. Ryther 1969) has been challenged recently (Gieskes et al. 1979; Johnson et al. 198 1; Shulenberger and Reid 198 1;Jenkins 1982), and, as a result, rates of photosynthesis in these areas are currently uncertain to within at least an order of magnitude (see Kerr 198 3). A preliminary analysis of the upper bounds for estimates of primary production in the open sea based on conservation of energy and mass indicated that some of the recently reported rates of primary production exceeded the limits imposed by generally accepted views on the photosynthetic physiology of marine algae. Some of the estimated high rates may be in error bccausc, of spatial and temporal aliasing of the sampling (see Platt 1984), but the possibility cannot be dismissed that processes were ignored in the calculations by Platt et al.( 1 984) that would have a large influence on the magnitude of marine primary production.Here . we first compare, by means of formal sensitivity analysis, the errors introduced into calculations of areal photosynthesis as a rl:sult of errors in the determination of t'le initial slope (CU) and the maximum p lotosynthetic rate (P,) of photosynthesis-i radiance curves. We show that under most c ceanographic conditions, errors in the initial slope will have a greater effect on estimate!; of areal photosynthesis than errors in the maximum rate. We also demonstrate that inaccuracies introduced into estimates of areal photosynthes...

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The photosynthetic response, defined as the initial slope of the photosynthesis‐irradiance curve, was determined spectrally (every 25 nm from 400 to 675 nm; 25 nm half‐maximum bandpass) for natural phytoplankton populations from High Arctic, Grand Banks and Sargasso Sea waters, as well as for populations living in the lower margin of sea ice off Newfoundland, All spectra were similar in shape with a maximum at 425–450 nm, a broad shoulder to 550 nm, a valley from 600 to 650 nm and a rise at 675 nm. The error resulting from the use of spectrally averaged initial slope to predict photosynthesis under different optical and fluid dynamical conditions at sea is discussed.

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