On the seasonal nitrogen dynamics of the Baltic proper biogeochemical reactor

During the last decade it has become increasingly obvious that the turnover of dissolved organic nitrogen DON in marine environments is quite vigorous. This paper quantie es the turnover of DON in the Baltic proper regarded as a biogeochemical reactor. In a nitrogen model for the reactor, dissolved inorganic nitrogen DIN, DON and molecular N, e xed by cyanobacteria, can be used for plant production. The decomposition of particulate organic matter is assumed to produce DON and DIN as end products in the proportions (1 2 h ) to h (0 # h # 1). The model includes two internal sink processes, denitrie cation and sequestering in the bottom sediments and accounts for external sources and sinks by import and export of DIN and DON. The annual net production in the Baltic proper is about 12.8 10 6 tonC (50gC m 2 2 ) requiring about 2.3 10 6 ton N. However only about 1.0 10 6 ton N are available as DIN and the dee cit has to be covered by an uptake of N from DON and/or e xed molecular nitrogen. The results of the model depend on the value of h . With h 5 1 the use of DON for primary production is at a minimum (0.19 10 6 ton N) while there are maxima for nitrogen e xation (1.0 10 6 ton N) and denitrie cation (1.5 10 6 ton N). However, both these values are considered unrealistically large. A more likely value of h is determined from the model in such a way that the annual rate of nitrogen e xation in the Baltic proper is in accordance with a recent estimate from the literature (0.11 10 6 ton N). This gives h 5 0.55 implying that about 0.67 10 6 ton N is denitrie ed, and 1.10 10 6 ton DON is used for net production, and 0.91 10 6 ton DON is produced by decomposition of particulate organic matter and the turnover time for DON is about 4 years. The e nding that there is a vigorous turnover of DON on the reactor level has important consequences. Firstly,earlier estimates of denitrie cation rates were based on budgets for oxygen and DIN and overlooked the DON decomposition pathway, why denitrie cation rates are severely overestimated, often by a factor of 2 or greater. Secondly, the extensive use of DON for primary production in the Baltic proper in combination with abundance of DON, challenge the widely accepted opinion that nitrogen is the production-limiting nutrient on the systems (reactor) level in the Baltic proper.