Capturing past eutrophication in coastal sediments – Towards water-quality goals

Abstract

Björnöfjärden is a semi-enclosed brackish bay located in the Stockholm archipelago (Baltic Sea, Sweden). Anthropogenic phosphorus (P) loading to the bay over the past century has overwhelmed the largely unchanged natural supply of elements and compounds that permanently sequester P in sediments. At the same time, eutrophication has shifted surface sediments from oxic to anoxic conditions and reduced their P-retention capacity. Consequently, the release of P from anoxic sediments has become the main P source to the water column. Here we report on a long-term remediation program to reverse eutrophication in Björnöfjärden. After the implementation of measures that reduced the land-based external load to the bay, sediment-P retention was increased by mixing a solution of aluminum (Al) chloride into the anoxic and azoic sediments (>6 m water depth) at a dose of 50 g Al/m2, a first in a brackish environment. As a result, P accumulation in the surface sediment reached 2.0 g P/m2 after 3.4 years, corresponding to 1.6 mg P/m2-day. This is the first time that the P accumulation rate has been determined in aquatic sediments following the addition of P-sequestering material, such as Al. The P that accumulated was dissolved P that mainly migrated from below the layer of P accumulation. The aim of the Al-addition was to sequester legacy P that had accumulated during the past century and to return Björnöfjärden to a low productivity regime, which would allow the surface sediment to become oxic and enable natural P binding by iron.