Sedimentation of particulate material in Nordasvannet, a hypertrophic, landlocked fjord in western Norway

Abstract

Sedimentation of particulate material to the sea bottom (90 m) was measured in Nordasvannet, a land-locked, highly stratified, west-Norwegian fjord, receiving fresh water from a small glacier-free watershed and untreated domestic sewage from 30,000 persons equivalents. Five cylindrical\rsediment traps positioned 20, 40, 45, 50, and 65 m below the water surface were deployed from March 1982 to April 1983. Variability of seasonal sedimentation rates was small, especially at greater depths. Sedimentation rates of total particulate mater~al and particulate organic carbon decreased from 594 and 83 at 20 m, to 389 and 58 g m-2 yr-l, respectively, in deeper water. Possible reasons for\rdecreasing sedimentation with increasing depth are mineralization in the water column and lack of resuspension in deeper water. However, sedimentation rates of particulate total phosphorus and chlorophyll a equivalents increased between 20 and 45 m depth. These conflicting results were interpreted as a possible consequence of specific lower catchment efficiencies of the uppermost traps and artificial mineralization inside the traps. No evidence was found for sedimentational responses on time variations of suspended biomass, yet larger parts of the biomass of the spring phytoplankton\rbloom settled to the seafloor of Nordasvannet. Besides fecal pellets few recognizable structures were found in the traps. The POC content of the deposited material was high compared to results given in literature and comprised 10 to 21%. It is hypothesized that a large and continuous allochthonous supply of nutrients to Nordasvannet equalizes both primary production and its sedimentation.