Bio-optical properties and development of a sub-surface chlorophyll maxima (SCM) in southwest Kattegat, Baltic Sea

The study presents high spatial resolution bio-optical, physical, and chemical data obtained inside and outside a sub-surface chlorophyll maxima (SCM). The peak of the SCM was located just above the pycnocline, which separated nutrient rich bottom water from nutrient depleted surface waters. The SCM was dominated (∼90%) by diatoms, in particular, Nitzschia sp. and Cerataulina pelagica and a few phytoflagellates. Chl a concentrations in the SCM reached 7.5 mg Chl a m−3. The SCM was a genuine maximum in carbon biomass. The 480/665 absorbance ratio in the Chl a samples proved to be a very useful tool regarding the nutritional status of the phytoplankton. Chromomorphic dissolved organic matter (CDOM) showed a clear maximum along with the SCM, which might indicate that in situ organic matter degradation processes were responsible for this relation. The SCM developed because of an inflow of nutrient rich bottom water from the Kattegat, whereby estimated vertical flux was raised from 9.8 × 10−5 to 2.0 × 10−4 mM N m−2 s−1. This vertical flux was estimated to sustain a production of 1368 mg C m−2 day−1, which equals approximately the spring bloom in Århus Bay. The study indicated that the SCM developed in a balance between nutrient availability and scalar irradiance. Estimations showed that the SCM accounted for 17% of the total primary production during that period in the bay.