Environmental status: From "natural" to polluted conditions in the Bunnefjord, inner Oslofjord : A micropaleontological and geochemicalstudy of a sediment core

Abstract

The aim of this study was to assess the temporal development of the environment in Bunnefjord, the innermost part of Oslofjord (SE Norway), define its reference conditions, and link the results to other studies. A 55 cm long sediment core was collected from Bunnefjord (N59.84285, E10.72587), for micropaleontological and geochemical analyses. The core provided chronostratigraphical data for the past 300 years, dating back to the late 1600s. There was analyzed for heavy metals, total organic carbon (TOC), water content, and benthic foraminifera (protists) were picked and counted. Ecological status was determined and risk assessed using the Norwegian Pollution Control Authority’s guides. The natural conditions, around 1700 AD, were found to be of ecological class “Moderate”. Subsequently, the anthropogenic induced increase of organic carbon content and in turn deteriorating oxygen concentrations at the end of the 1800s worsened the ecological status. The pollution and organic-flux maxima were in the 1970’s, and there has been some improvement ever since. However, the ecological status has been kept at “Very Bad”, due the slow remediation. There was established risk of ecological consequences and need of mitigating measures from 1925 until present. The present study displayed worse conditions than others nearby, due to the location within a small subbasin.

Keywords: silled fjord; benthic foraminifera; natural conditions; environmental status; ecotoxicological risk assessment; Oslofjord.