A consistent high primary production and chlorophyll-a maximum in a narrow strait – effects of hydraulic control

4 years long time-series of primary production, Chl-a, salinity, oxygen, and Secchi depth sampled weekly or bi-weekly along a transect in the narrow (~ 1 km) Little Belt and Little Belt region are analysed. Little Belt (LB) is one of the three Danish straits that connect the Baltic Sea and the North Sea.

The time-series were supplemented with Scan Fish transects — a towed CTD, ADCP measurements, and nutrient data. There is a significant maximum in primary production (mg C m− 2 day− 1) in central LB, which is 30% higher than outside the LB region. Chl-a concentrations are 30% higher in central LB where Secchi depth reaches a minimum.

Stratification showed a clear minimum in central LB where extended mixing prevails whereas strong stratification occurred in northern and southern LB. It is shown that mixed conditions in central LB were related to hydraulic control and super-critical flow conditions, as current derived energy for the mixing by comparison was too low.

Nutrient (NO2 + NO3) concentrations remained high (~ 5 μM) in the bottom layer following the spring bloom. It is shown that there is a more or less continuous inflow of nutrient rich bottom water into central LB, which through the strong mixing sustains the high primary production of organic material.

The frequency of the hydraulic control governed mixing is presumably high and with a tidal frequency. It was estimated that the 30% higher primary production equalled an annual export out of Little Belt of 142 tons Chl-a or about 4261 tons C that might add to the observed oxygen depletion in the area.