Abstract

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 quantifies 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, fixed 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- η) to η (0 ≤ η ≤ 1). The model includes two internal sink processes, denitrification 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 12.8 106 ton C (50 gC m-2) requiring about 2.3 106 ton N. However only about 1.0 106 ton N are available as DIN and the deficit has to be covered by an uptake of N from DON and/or fixed molecular nitrogen. The results of the model depend on the value of η. With η = 1 the use of DON for primary production is at a minimum (0.19 106 ton N) while there are maxima for nitrogen fixation (1.0 106 ton N) and denitrification (1.5 106 ton N). However, both these values are considered unrealistically large. A more likely value of η is determined from the model in such a way that the annual rate of nitrogen fixation in the Baltic proper is in accordance with a recent estimate from the literature (0.11 106 ton N). This gives η = 0.55 implying that about 0.67 106 ton N is denitrified, and 1.10 106 ton DON is used for net production, and 0.91 106 ton DON is produced by decomposition of particulate organic matter and the turnover time for DON is about 4 years. The finding that there is a vigorous turnover of DON on the reactor level has important consequences. Firstly, earlier estimates of denitrification rates were based on budgets for oxygen and DIN and overlooked the DON decomposition pathway, why denitrification 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.

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