A nitrogen source in spring in the surface mixed-layer of the Baltic Sea: Evidence from total nitrogen and total phosphorus data

- The total nitrogen budget indicates a nitrogen source in May in the Baltic Proper.
- The nitrogen source is correlated with the consumption of excess phosphate.
- Early ("cold") nitrogen fixation and nitrogen transport by migrating plankton organisms are discussed as the source of nitrogen.

The combined mass balance for total nitrogen (TN: concentrations of organic + inorganic nitrogen compounds) and total phosphorus (TP: concentrations of organic + inorganic phosphorus compounds) at 12 stations in the Baltic Sea for the period 1995-2013 clearly indicated the existence of a significant nitrogen source in the surface mixed layer in May of up to 86 mmol m− 2 month− 1 at the Baltic Proper stations. The analysis is based on monthly changes in TN with reference to the concentrations in April, and taking into account the atmospheric deposition of inorganic nitrogen compounds and the sedimentation of particulate organic nitrogen. The nitrogen source is closely linked to the consumption of excess phosphate and thus to the production of organic matter in the surface mixed-layer. Processes such as early nitrogen fixation and nitrogen transport by migrating plankton organisms are discussed as possible causes for the nitrogen input into the surface mixed layer in May. The long-term mean of the nitrogen source amounted to 55 mmol m− 2 in the Baltic Proper corresponding to a regional nitrogen input of 83 kt. We also obtained data for the mid-summer nitrogen fixation of 141 mmol m− 2 which corresponds to an input of 202 kt.