Abstract
Computing the vertical structure of primary production in ocean ecosystem models requires information about the vertical distribution of available light, chlorophyll concentration and photosynthesis response parameters. Conversely, given information on vertical structure of chlorophyll and light, we can extract photosynthesis parameters from vertical profiles of primary production measured at sea, as we illustrate here for the Bermuda Atlantic Time-Series Study. The procedure is based on a model of the production profile, which itself depends on the underwater light field. To model the light field, attenuation coefficients were estimated from measured optical profiles using a simple model of exponential decay of photosynthetically-available irradiance with depth, which accounted for 97% of the variance in the measured optical data. With the underwater light climate known, an analytical solution for the production profile was employed to recover photosynthesis parameters by minimizing the residual model error. The recovered parameters were used to model normalized production profiles and normalized watercolumn production. The model explained 95% of the variance in the measured normalized production at depth and 97% of the variance in measured normalized watercolumn production. A shifted Gaussian function was used to model biomass profiles and accounted for 93% of the variance in measured biomass at depth. An analytical solution for watercolumn production with the shifted Gaussian biomass was also tested. With the recovered photosynthesis parameters, maximum instantaneous growth rates were estimated by using a literature value for the carbon-to-chlorophyll ratio in this region of the Atlantic. An exact relationship between the maximum instantaneous growth rate and the daily growth rate in the ocean was derived. It was shown that calculating the growth rate by dividing the production by the carbon-to-chlorophyll ratio is equivalent to calculating it from the ratio of the final to the initial biomass, even when production is time dependent. Finally, the seasonal cycle of the recovered assimilation number at the Bermuda Station was constructed and analysed. The presented approach enables the estimation of photosynthesis parameters and growth rates from measured production profiles with only a few model assumptions, and increases the utility of in situ primary production measurements. The retrieved parameters have direct applications in satellite-based estimates of primary production from ocean-colour data, of which we give an example.
Highlights
Strict experimental quantification of phytoplankton primary production was initiated with the introduction of the 14C method in 1952 by Steemann Nielsen [1]
We extend the application of the method to the Bermuda Atlantic Time-Series Study data set
The presented work dealt with the estimation of photosynthesis parameters from primary production data collected at Bermuda Atlantic Time-Series Study (BATS) over the past 30 years
Summary
Strict experimental quantification of phytoplankton primary production was initiated with the introduction of the 14C method in 1952 by Steemann Nielsen [1]. We find ourselves with global scale maps of phytoplankton production represented by daily carbon uptake at our disposal [4,5,6,7]. Such maps are made possible by combining satellite measurements of chlorophyll concentration with primary production models forced by photosynthetically-available surface radiation [8,9]. Complementary estimates based on satellite phytoplankton carbon and growth rates have been developed [10] At their core, chlorophyll-based models are founded on knowledge of the photosynthesis–light relationship, which enables the calculation of primary production
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