Turbidity, particle fluxes and mineralisation of carbon and nitrogen in a shallow coastal area

Abstract

During a research programme carried out in a shallow coastal area (úrhus Bight, Denmark), short-term and long-term particle dynamics were measured continuously for a year in the water column and in the seabed. The research area is a homogeneous accumulation area for fine-grained sediment, with a water depth of 15 m.

Fluxes in the water column were measured at nine depths with a time resolution of one to two weeks during one year, using duplicate sediment traps with an aspect ratio of 5. Significant correlation between turbidity and vertical particle fluxes was found.

Continuous measurements close to the bottom gave an indirect measure of the concentration of suspended particles every 5 min. In this way, it was possible to measure resuspension dynamics with a detailed time resolution.

A new method was developed for the separation of downward fluxes of primary and secondary (resuspended) sediment. The net deposition rate of total particulate matter (TPM) found by this method (D_TPM = 1050 g m^-2 year^-1) is in good agreement with the accumulation rate found by ²¹⁰Pb core datings (A_TPM = 890 g m^-2 year^-1).

The accumulation rate of particulate organic carbon (POC) and total particulate nitrogen (TPN) beneath the benthic mixing layer was five times smaller than the net deposition rate; this is assumed to be caused by mineralization mainly in the benthic mixing layer. Resuspension events are believed to play a major role in mineralization of organic matter and exchange of nutrients with the water column.

Resuspension rates were 10 to 100 times greater than the net deposition rates in all measuring periods. Supply of new, easily resuspendable sediment during the spring phytoplankton bloom dramatically increased the possibility of resuspension.