Effect of Langmuir circulation on mixing and carbon dynamics in a shallow lagoon

In fall 2020 and 2021, two field surveys examined the water column dynamics and surface mixing in a shallow lagoon, Szczecin (Stettin) Lagoon, located at the border between Germany and Poland. This was part of a larger experiment, looking into water column and air-sea interactions, and momentum fluxes, but this study is focused on how the presence of proposed Langmuir circulation affects the carbon and oxygen dynamics, and primary production in this shallow lagoon.

Measurements were collected from a station in Szczecin Lagoon, located near the Polish border, with water depth of about 4 meters. Measurements at and around the station were made using mobile FerryBox systems, or Pocket FerryBoxes, which measured almost continuously water temperature, salinity, dissolved oxygen, chlorophyll fluorescence, pH, turbidity, colored dissolved organic matter (CDOM) and in 2021 partial pressure of CO2 (pCO2). In addition, water column measurements of currents (ADCP) and water level were available, as well as surface drifters, and drone aerial measurements.

We found that during low wind conditions, the water column was well-mixed to a depth controlled by expected Langmuir cells, and bottom waters below this depth were quite different in most of the biogeochemical parameters measured. Therefore Langmuir circulation most likely controlled water column structure in large regions of the Szczecin Lagoon, consequently influencing the community, carbon and dissolved gas distributions in this shallow lagoon, and most likely the air-sea gas exchange rate. Only during short storm events, these conditions changed, and the water column structure and concentrations of biogeochemical parameters were altered.