Predicting the Electron Requirement for Carbon Fixation in Seas and Oceans

Marine phytoplankton account for about 50% of all global net primary productivity (NPP). Active fluorometry, mainly Fast Repetition Rate fluorometry (FRRf), has been advocated as means of providing high resolution estimates of NPP. However, not measuring CO<inf>2</inf>-fixation directly, FRRf instead provides photosynthetic quantum efficiency estimates from which electron transfer rates (ETR) and ultimately CO<inf>2</inf>-fixation rates can be derived. Consequently, conversions of ETRs to CO<inf>2</inf>-fixation requires knowledge of the electron requirement for carbon fixation (Φ<inf>e,C</inf>, ETR/CO<inf>2</inf> uptake rate) and its dependence on environmental gradients. Such knowledge is critical for large scale implementation of active fluorescence to better characterise CO<inf>2</inf>-uptake. Here we examine the variability of experimentally determined Φ<inf>e,C</inf> values in relation to key environmental variables with the aim of developing new working algorithms for the calculation of Φ<inf>e,C</inf> from environmental variables. Coincident FRRf and ¹⁴C-uptake and environmental data from 14 studies covering 12 marine regions were analysed via a meta-analytical, non-parametric, multivariate approach. Combining all studies, Φ<inf>e,C</inf> varied between 1.15 and 54.2 mol e^- (mol C)^-1 with a mean of 10.9±6.91 mol e^- mol C)^-1. Although variability of Φ<inf>e,C</inf> was related to environmental gradients at global scales, region-specific analyses provided far improved predictive capability. However, use of regional Φ<inf>e,C</inf> algorithms requires objective means of defining regions of interest, which remains challenging. Considering individual studies and specific small-scale regions, temperature, nutrient and light availability were correlated with Φ<inf>e,C</inf> albeit to varying degrees and depending on the study/region and the composition of the extant phytoplankton community. At the level of large biogeographic regions and distinct water masses, Φ<inf>e,C</inf> was related to nutrient availability, chlorophyll, as well as temperature and/or salinity in most regions, while light availability was also important in Baltic Sea and shelf waters. The novel Φ<inf>e,C</inf> algorithms provide a major step forward for widespread fluorometry-based NPP estimates and highlight the need for further studying the natural variability of Φ<inf>e,C</inf> to verify and develop algorithms with improved accuracy. © 2013 Lawrenz et al.