Photosynthesis versus irradiance relationships in the Atlantic sector of the Southern Ocean
Creators: name: S Thomalla
name: E Kean
name: M Lucas
name: M-J Gibberd
name: R Barlow
Publisher: name: Taylor & Francis
Year: 2017
Subjects: subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: Ecology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: Inorganic Chemistry
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Plant Biology

description: Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO<sup>–</sup><sub>3</sub>), regenerated production (ρNH<sup>+</sup><sub>4</sub>), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (<i>P</i> vs <i>E</i>) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (<i>P</i><sup>B</sup><sub>max</sub>) and low quantum efficiency of photosynthesis (α<sup>B</sup>) for ρNO<sub>3</sub>, but high <i>P</i><sup>B</sup><sub>max</sub> and α<sup>B</sup> for ρNH<sub>4</sub>, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of <i>P</i><sup>B</sup><sub>max</sub> and α<sup>B</sup> for ρNO<sub>3</sub> and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on <i>P</i> vs <i>E</i> parameters, using surface temperature, chlorophyll-<i>a</i> concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO<sub>3</sub>) and α<sup>B</sup> for ρNO<sub>3</sub> suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO<sub>3</sub> reservoir. However, the observed relationships between community structure and the <i>P</i> vs <i>E</i> parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-<i>a</i> and α<sup>B</sup>, between <i>P</i><sup>B</sup><sub>max</sub> and α<sup>B</sup>, and between temperature and α<sup>B</sup> may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO<sub>3</sub> and α<sup>B</sup> for ρNO<sub>3</sub>, and between the light-saturation parameter <i>E</i><sub>k</sub> for ρNO<sub>3</sub> and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO<sub>3</sub> reservoir and that larger-celled communities are more efficient at fixing NO<sub>3</sub> in low light conditions. Although the mean <i>P</i><sup>B</sup><sub>max</sub> results for ρC were consistent with estimates of global production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.