Photosynthesis versus irradiance relationships in the Atlantic sector of the Southern Ocean

• Main Authors: S Thomalla, E Kean, M Lucas, M-J Gibberd, R Barlow
• Format: Text
• Language: unknown
• Published: Taylor & Francis 2017
• Subjects: 59999 Environmental Sciences not elsewhere classified; FOS Earth and related environmental sciences; Ecology; FOS Biological sciences; Inorganic Chemistry; FOS Chemical sciences; Plant Biology; Southern Ocean
• Online Access: https://dx.doi.org/10.6084/m9.figshare.5202784.v1; https://tandf.figshare.com/articles/journal_contribution/Photosynthesis_versus_irradiance_relationships_in_the_Atlantic_sector_of_the_Southern_Ocean/5202784/1

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO – 3 ), regenerated production (ρNH + 4 ), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance ( P vs E ) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity ( P B max ) and low quantum efficiency of photosynthesis (α B ) for ρNO 3 , but high P B max and α B for ρNH 4 , 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 P B max and α B for ρNO 3 and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll- a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO 3 ) and α B for ρNO 3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO 3 reservoir. However, the observed relationships between community structure and the P vs E 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- a and α B , between P B max and α B , and between temperature and α B 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 3 and α B for ρNO 3 , and between the light-saturation parameter E k for ρNO 3 and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO 3 reservoir and that larger-celled communities are more efficient at fixing NO 3 in low light conditions. Although the mean P B max 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.