How sensitive is the growth and elemental composition of the widespread Arctic diatom Chaetoceros gelidus to changing environmental conditions?

By augmenting deliveries of continental nutrients and reinforcing the vertical stratification of the upper Arctic Ocean, increasing river discharge may alter the relative importance of different nitrogen (N) sources for phytoplankton nutrition. In combination with warming and the greater penetration of sunlight resulting from sea-ice loss, these changes can potentially alter the productivity and elemental composition of algal assemblages growing at the surface and in the subsurface chlorophyll maximum (SCM). Algal communities of the SCM can realize a major portion of annual net primary production in the coastal Beaufort Sea, where the cosmopolitan diatom Chaetoceros gelidus often dominates. In order to assess how changing growth conditions may affect biogeochemistry across the transpolar Pacific-Atlantic conduit, the elemental stoichiometry of a C. gelidus strain isolated from the Beaufort Sea and its response to the availability of light, temperature and different N sources were examined in the laboratory. Growth rate, chlorophyll a and the maximum quantum efficiency of PSII were measured along with particulate nitrogen, carbon, phosphorus and biogenic silica using semi-continuous batch cultures of C. gelidus acclimated to two light intensities and a range of temperature with ammonium, nitrate or urea as sole N-source. Despite a clear response of cellular quotas to N source, the growth rate and elemental composition of the cells were relatively unaffected by this treatment. By contrast, elemental ratios responded to varying degrees to light and temperature and these responses were remarkably similar to those observed in prior studies of temperate diatoms. Overall the results suggest that the widespread an ecologically-relevant C. gelidus of today’s Arctic Ocean possess the ability to remain competitive despite ongoing environmental changes and that phytoplankton-mediated biogeochemical fluxes are partially resilient to these changes in the coastal Arctic Ocean