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

The study is based on a 1 year record (n = 46) of the diffuse attenuation coefficient-K-d(PA R) (m(-1))-in combination with CTD-casts at one position in Angstromrhus 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 Kd(PAR). Maximum K-d(PAR) values occurred in periods of high chla 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)). (C) 2004 Elsevier Ltd. All rights reserved.