Diffuse attenuation coefficients Kd(PAR) at the estuarine North Sea–Baltic Sea transition: time-series, partitioning, absorption, and scattering

Abstract

The study is based on a 1 year record ( n=46) of the diffuse attenuation coefficient— K d(PAR) (m −1)—in combination with CTD-casts at one position in Århus Bay, Kattegat, the North Sea–Baltic Sea estuarine transition. Water samples were collected occasionally for determination of suspended particulate matter (SPM) and chromomorphic dissolved organic matter (CDOM). The K d(PAR) varied between 0.152 and 0.557 (m −1) with an average of 0.293 (m −1). The depth of the photic zone (1% light level) varied between 8.3 and 15.7 m at maximum and average K d(PAR). Maximum K d(PAR) values occurred in periods of high chl-a concentrations. The average CDOM absorption coefficient was 0.232 (m −1) whereas the average chl-a and inorganic SPM concentrations equalled 3.3 (mg m −3) and 4.5 (g m −3). Light attenuation partitioning in percent was: water (9), CDOM (17), SPM (42), and phytoplankton (32) at average conditions. Attenuation by phytoplankton reached up to 74% at high chl-a concentrations whereas SPM light attenuation reached 45% at high SPM concentrations. Results show that light attenuation in the bay is governed by phytoplankton and SPM concentrations whereas CDOM is of less significance. Light absorption in the PAR range— a(PAR)—ranged between 0.262 (m −1) at average conditions and 0.49 (m −1) at maximum phytoplankton concentrations. Scattering was governed by SPM at average conditions whereas phytoplankton scattering was higher than SPM scattering at chl-a concentrations >4.5 (mg m −3).