New production enhanced by nutrient supply from non-Redfield remineralisation of freshly produced organic material

Abstract

Budgets of dissolved inorganic carbon (DIC), nitrate and phosphate for the euphotic surface layer of the Baltic Sea and the Baltic Intermediate Water (BIW) are estimated for the growing season 1994/1995. These budgets are the bases for the calculation of new production in the euphotic layer based on DIC consumption (net community production) or nitrate and phosphate consumption, respectively. The net community production is on average 1.5 times the new production based on nitrate consumption. It is assumed that the freshly produced particulate organic matter (POM) adheres to the Redfield ratios, but that nitrogen and phosphorus of the POM are preferentially remineralised, enabling primary production to continue when the nutrients are exhausted and thus use DIC to a greater extent than would be estimated by the nitrate consumption alone. Since the BIW is enclosed at the top by a strong thermocline during the summer and at the bottom by the permanent halocline, all changes during the growing season are related to the biology within this layer. The carbon-enriched particulate material of the surface layer is exported to the BIW where it is remineralised resulting in a high DIC surplus during the growing season. In the BIW, the apparent oxygen utilisation (AOU) corresponds to the production of excess DIC, while no corresponding excess in nitrogen or phosphorus is observed. This confirms the export of POM to the BIW, and the preferential remineralisation of nitrogen and phosphorus in the euphotic surface layer. The role of the so-called biological carbon dioxide pump is discussed.