Articles | Volume 15, issue 9
Research article
17 May 2018
Research article |  | 17 May 2018

A Baltic Sea estuary as a phosphorus source and sink after drastic load reduction: seasonal and long-term mass balances for the Stockholm inner archipelago for 1968–2015

Jakob Walve, Maria Sandberg, Ulf Larsson, and Christer Lännergren

Abstract. Internal phosphorus (P) loading from sediments, controlled by hypoxia, is often assumed to hamper the recovery of lakes and coastal areas from eutrophication. In the early 1970s, the external P load to the inner archipelago of Stockholm, Sweden (Baltic Sea), was drastically reduced by improved sewage treatment, but the internal P loading and its controlling factors have been poorly quantified. We use two slightly different four-layer box models to calculate the area's seasonal and annual P balance (input–export) and the internal P exchange with sediments in 1968–2015. For 10–20 years after the main P load reduction, there was a negative P balance, small in comparison to the external load, and probably due to release from legacy sediment P storage. Later, the stabilized, near-neutral P balance indicates no remaining internal loading from legacy P, but P retention is low, despite improved oxygen conditions. Seasonally, sediments are a P sink in spring and a P source in summer and autumn. Most of the deep-water P release from sediments in summer–autumn appears to be derived from the settled spring bloom and is exported to outer areas during winter. Oxygen consumption and P release in the deep water are generally tightly coupled, indicating limited iron control of P release. However, enhanced P release in years of deep-water hypoxia suggests some contribution from redox-sensitive P pools. Increasing deep-water temperatures that stimulate oxygen consumption rates in early summer have counteracted the effect of lowered organic matter sedimentation on oxygen concentrations. Since the P turnover time is short and legacy P small, measures to bind P in Stockholm inner archipelago sediments would primarily accumulate recent P inputs, imported from the Baltic Sea and from Lake Mälaren.

Short summary
Phosphorus (P) release from sediments can delay recovery of eutrophicated waters. For the Stockholm inner archipelago (Baltic Sea), a P budget shows that legacy P release was small and restricted to the 20 years following the large P load reduction in 1975. Current P release is largely a seasonal P recycling in oxic conditions and the area is still a poor P sink. Short P turnover time in the area means that measures binding P in sediments will not be very effective in improving water quality.
Final-revised paper