A note on the C/N and C/P ratio of the biological production in the Nordic seas

Abstract

In this paper, we study the consumption of carbon, nitrogen and phosphate in the upper layer of the Nordic seas. It is found that the biological draw-down of nutrient concentrations and dissolved inorganic carbonate concentration, C_T, during summer cannot be explained in terms of the common Redfield ratio (C:N:P = 106:16:1) for organic material. According to Børsheim and Myklestad, the seasonal variation of dissolved organic carbon, DOC, concentration is of order 20 μmol/kg. Based on these measurements, we argue that production of DOC with a high carbon content (C:N≈15) is the main cause of the high consumption of C_T. Further, there are indications that we need a C:N = 9 ratio for organic material to complete the upper ocean carbon budget at station M. However, the uncertainty in the gas exchange formulation, which is of order 30%, makes this kind of interpretation of data vulnerable. The flux of CO₂ from the atmosphere happens to cancel the influence of DOC on the C_T. Thus, an interpretation of data, without considering the air-sea gas exchange, will falsely reveal that the production follows the Redfield ratio.