To enhance the accuracy of carbon cycling models as applied to sea ice in the changing Arctic, we analyzed a large data set of particulate organic carbon (POC) and nitrogen (PON) measurements in first-year bottom sea ice (n = 257) from two Arctic shelves, the Canadian Arctic Archipelago and Beaufort Sea shelf, including dark winter and spring seasonal measurements Using the Redfield ratio for POC:PON conversion would provide reasonable estimates only over a limited range of sea-ice biomass concentrations observed in this study. Therefore, our results argue in favour of using variable POC:PON stoichiometry in sea-ice biogeochemical models, supported by the wide range of biomass concentrations in first-year sea ice and the evidence of variability in sea-ice POC:PON ratios at the regional (CAA), shelf (CAA versus BSS), and seasonal (dark winter versus spring) scales. Incorporating this variability into analytical and predictive modelling efforts is essential to achieve a better understanding of the role of first-year sea ice in regional food webs and global biogeochemical cycles. The use of the power function model presented here is recommended, as it reflects that POC:PON ratios do not remain constant over the observed sea-ice POC and PON concentrations. For sea-ice biogeochemical modellers, recommendations include: 1) parameterization using variable POC:PON ratios rather than consistent Redfield or average values, 2) the inclusion of distinct power functions to parameterize dark winter versus spring POC:PON ratios and, 3) Arctic-wide or regionally-based models based on areals Wide ranges of sea-ice POC:PON ratios were observed during both the dark winter (12-46 mol:mol) and spring (3-24 mol:mol) periods. Sea-ice POC:PON ratios and chlorophyll a concentrations were significantly higher in the Archipelago versus the Beaufort Sea shelf (p < 0.01), yet there was a highly significant relationship between sea-ice POC and PON during spring for both shelves (r2 = 0.94). POC:PON ratios were not consistent over the range of measured POC and PON concentrations, justifying the use of a power function model to best describe the relationship between POC and PON. Distinct relationships between POC:PON ratios and chlorophyll-based biomass were observed for the dark winter and the spring: dark winter sea-ice POC:PON ratios decreased with increasing sea-ice biomass whereas spring POC:PON ratios increased with increasing sea-ice biomass. The transition from the dark period to the spring growth period in first-year sea ice represented a distinct stoichiometric shift in POC:PON ratios. Our results demonstrate that the Redfield ratio has limited applicability over the four-order of magnitude range of biomass concentrations observed in first-year sea ice on Arctic shelves. This study emphasizes the need for variable POC:PON stoichiometry in sea-ice biogeochemical models and budget estimates, in particular at high biomass concentrations and when considering seasonality outside of the spring period in first year ice. The use of a power function model for POC:PON relationships in sea ice is also recommended to better constrain carbon estimates in biogeochemical sea-ice models.
Niemi, A., & Michel, C. (2015). Temporal and spatial variability in sea-ice carbon:nitrogen ratios on canadian arctic shelvestemporal and spatial variability in sea-ice carbon: Nitrogen ratios. Elementa, 3. https://doi.org/10.12952/journal.elementa.000078