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

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (rho NO3-), regenerated production (rho NH4+), and total carbon production (rho C) 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-max(B)) and low quantum efficiency of photosynthesis (alpha(B)) for rho NO3, but high P-max(B) and alpha(B) for rho NH4, 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-max(B) and alpha(B) for rho NO3 and rho C. 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 (NO3) and alpha(B) for rho NO3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NO3 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 alpha(B), between P-max(B) and alpha(B), and between temperature and alpha(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 NO3 and alpha(B) for rho NO3, and between the light-saturation parameter E-k for rho NO3 and the phytoplankton community's size structure, which imply that Fe and light co-limitation drives access to the surplus NO3 reservoir and that larger-celled communities are more efficient at fixing NO3 in low light conditions. Although the mean P-max(B) results for rho C 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.