Pelagic and seabed fluxes of particulate matter and carbon, and C:N ratios resolved by sediment traps during a spring bloom, southwest Kattegat

Vertical particulate matter fluxes were measured by sediment traps at 1.4 m (seabed) and 8 m (pelagic) above the seabed, along with fluorescence (CTD casts), current conditions and salinity, in a semi-enclosed bay in southwest Kattegat covering a close to three-month period. The Kattegat forms the estuarine transitional zone between the high-saline North Sea and low-saline Baltic Sea, as shown by measured maximum and minimum bottom salinities of 32.5 and 17.0. There were no indications of suspended matter transport into the bay that affected the vertical flux. There was a significant and close correlation (r2 = 0.24) between current shear stress and seabed vertical flux. Average vertical flux was 11.8 g m−2 d−1 at the seabed with a wide range (0.4–57.6 g m−2 d−1), which is 6.9 times the average flux of 1.7 g m−2 d−1 at 8 m above the seabed. The flux at this height was generally governed by the production of organic material in the water and showed little variation. The C:N ratio of material collected in the 8 m trap was on average 6.9, close to the Redfield ratio (6.7) and implying flux of fresh material. The average C:N seabed ratio was 8.8. Periods of low C:N seabed ratios close to the Redfield ratio were observed before enhanced organic production in the water column, i.e. spring bloom and sub-surface peak. There was a positive and significant correlation (r2 = 0.13) between seabed fluxes and C:N ratio. This shows that more decomposed material with a higher C:N ratio is resuspended at higher fluxes. Organic carbon content (percentage) in collected material was high with a maximum of 18% following the spring bloom at the 8 m trap, and an average of 11.4%. Average seabed organic carbon was 8.6%. There was a positive correlation (r2 = 0.42) between vertical flux and seabed organic carbon content. This shows that the inorganic part of the collected material increases at higher current shear stress. It was concluded that the seabed flux was not affected by changes in the flux related to a spring bloom in terms of amounts, but that the related changes in C:N ratio and organic carbon were discernible, particularly in periods of low current shear stress.