The flow of nitrogen into the euphotic zone of the Baltic Proper as a result of the vertical migration of phytoplankton: An analysis of the long-term observations and ecohydrodynamic model simulation

• This study has shown a significant role of vertical migrations of phytoplankton.
• The nitrogen flux was estimated at 60 mmol m− 2 month− 1 in spring and 4.7 in summer.
• The total transport of nitrogen to the euphotic zone was estimated at 129 mmol m− 2.
• The estimated transport corresponds to a new primary production of 10.3 gC m− 2.

The effect of the vertical migration (VM) of phytoplankton on the depletion of mineral nitrogen in the water layer intermediate between the thermocline and halocline during the summer thermal stratification of the Baltic Proper was investigated. The flux of nitrogen transported to the mixed layer from the intermediate layer beneath it was estimated on the basis of long-term observations (1970–2000) made in spring and summer at station BY15 in the Gotland Deep (central part of the Baltic Proper). The total nitrogen flux was estimated at ca 60 mmol m− 2 month− 1 in spring and at ca 4.7 mmol m− 2 month− 1 in summer. The total transport of nitrogen to the euphotic zone from April to August was estimated at 129 mmol m− 2 which, assuming the Redfield ratio to hold, corresponds to a new primary production of ca 10.3 gC m− 2. The deep chlorophyll maxima linked to the VM of phytoplankton may not be remotely sensed, which causes the level of primary production calculated on their basis to be somewhat underestimated. The ProDeMo ecohydrodynamic model was modified in such a way as to take account of the VM of dinoflagellates. Comparison of the model results with measured vertical distributions of mineral nitrogen enabled the choice of an appropriate numerical algorithm and confirmed the hypothesis that dinoflagellate VM may be responsible for the depletion of mineral nitrogen beneath the thermocline.