Predicting the electron requirement for carbon fixation in seas and oceans

Abstract

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 CO2-fixation directly, FRRf instead provides photosynthetic quantum efficiency estimates from which electron transfer rates (ETR) and ultimately CO2-fixation rates can be derived. Consequently, conversions of ETRs to CO2-fixation requires knowledge of the electron requirement for carbon fixation (Φe,C, ETR/CO2 uptake rate) and its dependence on environmental gradients. Such knowledge is critical for large scale implementation of active fluorescence to better characterise CO2-uptake. Here we examine the variability of experimentally determined Φe,C values in relation to key environmental variables with the aim of developing new working algorithms for the calculation of Φe,C from environmental variables. Coincident FRRf and (14)C-uptake and environmental data from 14 studies covering 12 marine regions were analysed via a meta-analytical, non-parametric, multivariate approach. Combining all studies, Φe,C 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 Φe,C was related to environmental gradients at global scales, region-specific analyses provided far improved predictive capability. However, use of regional Φ e,C 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 e,C 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 e,C 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 e,C algorithms provide a major step forward for widespread fluorometry-based NPP estimates and highlight the need for further studying the natural variability of Φe,C to verify and develop algorithms with improved accuracy.

Figure 1. Data sets used in the meta-analysis.

Gulf of Finland includes both the SUPREMO2011 and the Raateoja et al. study (See Table 1 for details).

Figure 2. Variability in the electron requirement of carbon fixation (Φ_e,C) derived from corresponding FRRf-ETR and ¹⁴C-primary productivity measurements.

Boxes represent the median, 0.25 and 0.75 quartile, whiskers are the 1.5 interquartile range. Outliers are indicated by open circles. The dashed grey line is the theoretical reference ratio of 4 mol e⁻ [mol C]⁻¹. AMT  =  Atlantic Meridional Transects, CS  =  Celtic Sea, NS  =  North Sea, GoF  =  Gulf of Finland 2000 (time series), UK-OA  =  UK Ocean Acidification cruise, MA  =  Massachusetts Bay (time series). SYNTAX2010 is a Baltic Sea cruise, GoF2011 is the SUPREMO2011 study and the Pacific Ocean study is the BIOSOPE- cruise to the Southeast Pacific (see Table 1, Figure 1 for details).

Figure 3. Changes in the electron requirement of carbon fixation (Φ_e,C) and NO₃ ⁻ across 4 cruise transects.

(A) and (E) Pacific Ocean (BIOSOPE), (B) and (F) AMT 6, (C) and (G) AMT 11, and (D) and (H) AMT 15. Units of Φ_e,C (left panels) are in mol e⁻ (mol C)⁻¹), NO₃ ⁻ concentrations (right-hand panels) are in µmol L⁻¹. Dashed lines denote transitions between different biogeographical provinces, where SPG is the South Pacific Gyre, HNLC High Nutrient Low Chlorophyll areas, UPW denotes upwelling regions, AB is the Angola Basin, SAG South Atlantic, NAG the North Atlantic Gyre, EM the Eastern Margin off Western Europe, and CZ is the Suptropical Convergence zone. Note, difference in scales on both axes and the colour contours.

Figure 4. Three-dimensional non-metric Multidimensional Scaling (nMDS) ordination of the environmental conditions of all field campaigns.

Panels (A), (B) and (C) are the x-y, x-z and y-z plane of the nMDS 3D ordination, respectively. (D) is the x-y plane where the symbols denote the various clusters as determined by cluster analysis in combination with similarity profile (SIMPER) tests. Clusters were significantly different from another (SIMPROF, p<0.005). For an overview of sample groupings according to these clusters see details in text and Table 4.

Figure 5. Variability in the electron requirement for carbon fixation (Φ _e,C) and environmental conditions within different clusters of samples.

Clusters were generated by cluster analysis combined with a SIMPROF test. For samples contained in each cluster see also Table 4. Values are means and error bars are standard deviations (with n of 2–67, see Table 4) of Φ _e,C (mol e⁻ mol C⁻¹), temperature (°C), salinity, chlorophyll a (Chl a, mg m⁻³), nitrate and phosphate (µmol L⁻¹), the vertical attenuation coefficient of photosynthetically available radiation (K_d, m⁻¹), optical depth ξ (dimensionless) and sampling depth (z, in meters).

Figure 6. Mean electron requirement for carbon fixation (Φ_e,C) across different biogeographical provinces for four cruises.

Φ_e,C versus depth (left panels) and NO₃ ⁻ (right panels) are shown for the Pacific Ocean (A and E), AMT 6 (B and F), AMT 11 (C and G) and AMT 15 (D and H). Error bars are standard deviations with n = 3 to 14. SML denotes surface mixed layer, DCM the deep chlorophyll maximum, SPG is the South Pacific Gyre, HNLC High Nutrient Low Chlorophyll areas, UPW upwelling regions, SA South Africa, AB Angola Basin, Eq equator, NAG North Atlantic Gyre, EM Eastern Margin in the North East Atlantic, and STCS the Subtropical Convergence Zone. Note change in x- and y-axis scale.