Reconstructing possible causes for the darkening of Skagerrak coastal water in the Hvaler estuary during the past century

Geochemical and micropalaeontological analyses conducted on two dated sediment cores from the Hvaler estuary (SE Norway) indicate a relationship between the darkening phenomenon of Skagerrak coastal waters and an increase of riverine discharge to estuarial waters. The sediments cores are collected from different stations in the Hvaler estuary, and demonstrate varying influences of terrestrially derived organic matter, primarily determined by their proximity to the coastland and the Glomma River. The sediment cores represent a time series from the early 1930s to the present. Increased riverine discharge, accompanied by bottom trawling, and dredging in the area, are suggested to promote the darkening of the Skagerrak coastal water. This is reflected in the majority of analyses performed on the sediment cores; grain size distribution, heavy metal concentrations, TC, TN, TOC, δ13C, δ15N and changes of the benthic foraminiferal assemblages. The heavy metal concentrations show distinct increasing trends to 1963, which partially reflects the previously documented pollution development in the estuary. Organic material present in the sediments is predominantly marine of origin but becomes more terrestrial over the years. The benthic foraminiferal assemblage reflects the increasing TOC and TIC accumulation with a distinct shift in the late 1990s/early 2000s with an increasing abundance of agglutinated foraminifera species (primarily Textularia earlandi and Liebusella goesi). Stainforthia fusiformis, a species tolerant to organic matter enrichment, increases simultaneously as the TOC accumulation rates increase