Improving oxygen conditions in periodically stagnant basins using sea-based measures – Illustrated by hypothetical applications to the By Fjord, Sweden

To improve the water quality of a habitat or to stop eutrophication driven by internal loading of phosphorus from anoxic bottoms, the deepwater in inshore and offshore sea basins may be oxygenated using sea-based measures. The minimum oxygen concentration DOmin in periodically stagnant basins is controlled by three factors, namely (i) the rate of oxygen depletion dDO/dt. (ii) The residence time Te of the stagnant water. (iii) The oxygen concentration DOstart in the basin water at the beginning of a stagnation period. A theoretical framework is presented, suitable or evaluation of sea-based measures to improve the oxygen conditions in periodically stagnant basins. Five different sea-based measures to improve the oxygen conditions are recognized and discussed. The application of sea-based measures might be particularly attractive for habitat improvements in basins with poor oxygen conditions despite land-based measures are fully implemented. The sea-based measures are illustrated by hypothetical applications to the By Fjord on the west coast of Sweden, a salt-stratified fjord with periodically stagnant anoxic basin water confined behind the shallow Sunninge Strait in the mouth. An interesting result of one of the hypothetical applications is that dredging of the Sunninge Strait in 1958 and 1976 reduced tidal currents by a factor of 4. This reduced the tidal power supply to vertical mixing in the basin water, which led to prolonged residence time Te, from 2-3 years to 3–5 years, and deteriorated oxygen conditions in the basin water.

The paper can be found under the tab Downloads.

The Eutrophication of the Baltic Sea has been Boosted and Perpetuated by a Major Internal Phosphorus Source

The phosphorus (P) concentration c1 in the surface layer of the Baltic proper in winter depends on the land-based P source LPS, and the ocean P source OPS, which are known. It also depends on the internal P source IPS from anoxic bottoms and the sum of internal and external P sinks TPsink, which are estimated in this paper. IPS is parameterized as fs·Aanox, where fs is the specific annual mass flux of P from anoxic sediments and Aanox is the area of anoxic bottoms, and TPsink is parameterized as c1·TRVF, where TRVF is the total removal volume flux. We use a time-dependent P budget model, and 47 years of observational data, and the method of least squares to determine the best estimates of the unknown parameters fs and TRVF. The result is TRVF = 3,000 km3 year−1 and fs = 1.22 tons P km−2 year−1. With these parameter values, the model gives a quite good description of the observed evolution of c1. The observed runaway evolution of c1, with increasing c1 since the 1980s although the land-based supply LPS has been halved, is well-described by the model. It is concluded that the internal P source IPS provides a positive feedback mechanism that has boosted and perpetuated the eutrophication of the Baltic proper and that IPS is the major driver of the Baltic Sea eutrophication since the late 1990s. It is suggested that measures to eliminate IPS should be included in the management strategy to reduce the eutrophication of the Baltic proper.

 The paper can be found under the tab Downloads. 

On the response of the Baltic proper to changes of the total phosphorus supply.

The environmental state of the Baltic proper has changed with the increasing eutrophication due to increasing phosphorus (P) content in the water column. The volume of anoxic water has increased by a factor of about 7 from the period 1965 – 1999 to the period 2000 -2016. The very large inflows of new deepwater from the Kattegat in 2014 and 2015 did not lead to any substantial reduction of the volume of anoxic water in the Baltic proper. The land-based supply of P to the Baltic Sea has been halved since the 1980s. Why doesn’t the P content of the water column of the Baltic proper decrease when the land-based supply decreases?

In a recently published paper, it is shown that the development of the equilibrium P content of the water column of the Baltic proper can be explained if one considers the total P supply, which equals the land-based supply plus the internal supply from anoxic bottoms plus the (small) oceanic supply by inflowing sea water from Kattegat. Present day magnitudes, in tonnes P year-1, of the three sources are about 30000 (land-based, NB! only about 1/3rd of this may be removed), about 100000 (internal) and 10000 (oceanic). In the paper, it is shown that the equilibrium P content of the water column of the Baltic proper is a linear function of the total supply of P! This means that the only way to substantially decrease the P content of the water column of the Baltic proper is to decrease the internal source. This can be done by oxygenation of the at present anoxic bottoms. In the paper, it is shown that it would take 10-15 years for the Baltic proper to reach equilibrium with the total P supply. It is concluded that the Baltic proper in 10-15 years can be restored to a state that is determined by the external land-based source (which at the present is at the same level as in the 1950s) if only the internal source is eliminated by oxygenation of deep bottoms. The paper is written by Anders Stigebrandt and published in Ambio. It can be found under the tab Downloads.



Using a time-dependent phosphorus (P) budget model for the Baltic proper, describing sources and sinks at the external borders of the water column, one may compute the e-folding time T of the adjustment of the winter surface water P concentration c1 to abruptly changed total P supply. The restoration time TR = 3T is introduced as a practical measure of the time it takes to achieve 95% of the change of c1 towards the final, equilibrium, state c1e. The P budget model, including an internal source emanating from deep anoxic bottoms, also shows that c1e is proportional to the total P supply to the water column. About 70% of present time total P supply to the Baltic proper comes from deep anoxic bottoms. If deep bottoms were kept oxygenated, this internal P supply would be turned off and the equilibrium concentration c1e would be reduced by about 70%. This should imply that the Baltic proper may be restored to a state determined by the external P supplies from land-based and oceanic sources. According to the model, restoration would take 10–15 years. Thereafter most of the equipment used for oxygenation may be shut off since also the deepwater oxygen demand by decomposition of fresh organic matter, would have decreased by about 70% implying that the deepwater would be kept oxic by the natural vertical circulation. The model presented in this paper provides a new science-based solution of the eutrophication problem of the Baltic proper, which is of great interest from a management point of view.

Oxygenated deep bottoms beneath a thick hypoxic layer lack potential of benthic colonization

To follow the progress of possible colonization and the progress of chemical oxygenation and reduction processes of the bottom overlain by hypoxic water in the deeper East Gotland Basin, a sediment profile image survey similar to the July 2015 survey of Rosenberg et al. (2016) was undertaken in April 2016. Only one record of a benthic individual, Bylgides (Harmothoe) sarsi, occurred in oxygenated deepwater. At most stations, the superficial oxygenated layers had been thinner from 2015 to 2016. The results of the survey are described and discussed in a recent paper in Ambio by Anders Stigebrandt, Rutger Rosenberg, Marina Magnusson and Torsten Linders. It is concluded that colonization of deep bottoms of oxygenated layers that are covered by a thick layer of anoxia or hypoxia can probably best be performed by horizontal movement of adults or larvae carried by the inflowing new deepwater. The paper can be found under the tab Downloads.

Rapid-re-oxygenation of Baltic Sea sediments following a large inflow event

Sediment profile images obtained along a transect with increasing water depths in July 2015 showed that a thin surface layer of earlier anoxic bottom sediments in the East Gotland Basin (EGB) had been oxidized by contact with new deepwater that arrived to EGB three months earlier, i.e. in March 2015. This is the first time re-oxygenation of surficial sediments after inflows of oxygen-rich water is shown in situ in the Baltic Sea. It was also found that the recently oxygenated bottoms had not been colonized by animals. This is described in a paper by Rutger Rosenberg, Marina Magnusson and Anders Stigebrandt published in Ambio 2016. The paper can be found under the tab Downloads.

Updated publication lists for the BOX and BOX-WIN projects now published

Researchers participating in the BOX Project continues to publish results from the BOX project. More than 20 publications emanating from BOX have now been published and additional papers are in the pipeline. Many aspects of oxygenation are covered by these publications, which will be of great value for a future Environment Impact Analysis (EIA) for the whole Baltic Proper. The publication list for the BOX-WIN project is also published. Both lists can be found under ”Publication list”.

Oxygenation of the Bornholm Basin would promote cod reproduction and feeding

A recently published paper in the scientific journal Ocean Science shows that oxygenation of the Bornholm Basin by pumping down well oxygenated water from 30 m depth into the stagnant deepwater would have a number of positive effects, see the Abstract below. The paper, written by Anders Stigebrandt, Rutger Rosenberg, Love Råman Vinnå and Malin Ödalen, can be found under the tab Downloads (scroll down to “Other publications”). Interested may also want to download the original manuscript of the paper together with reviews and external comments to the manuscript and the authors comments to these, all published together in Ocean Science Discussions.

We develop and use a circulation model to estimate hydrographical and ecological changes in the isolated basin water of the Bornholm Basin. By pumping well oxygenated so-called winter water to the greatest depth, where it is forced to mix with the resident water, the rate of deepwater density reduction increases as well as the frequency of intrusions of new oxygen-rich deepwater. We show that pumping 1000 m3 s-1 should increase the rates of water exchange and oxygen supply by 2.5 and 3 times, respectively. The CRV (cod reproduction volume), the volume of water in the isolated basin meeting the requirements for successful cod reproduction (S > 11, O2 > 2 mL L-1), should every year be greater than 54 km3, which is an immense improvement, since it has been much less in certain years. Anoxic bottoms should no longer occur in the basin, and hypoxic events will become rare. This should permit extensive colonization of fauna on the earlier periodically anoxic bottoms. Increased biomass of benthic fauna should also mean increased food supply to economically valuable demersal fish like cod and flatfish. In addition, re-oxygenation of the sediments should lead to increased phosphorus retention by the sediments.

Sweden now positive to start testing large scale oxygenation of the Baltic Sea

Positive news for oxygenation solutions for the Baltic Sea have been published in Swedish newspaper Sydsvenskan. The original text can be found here (in Swedish):
Sydsvenskan – Ingenjörskonst motar algsörja.

English translation:
The Swedish Ministry of the Environment proposes that Helcom should consider methods in the open sea to decrease eutrophication and stop vast cyanobacteria blooms. It is about trying to speed up the recovery of the Baltic says Senior administrative officer Mr Anders Alm who will present the suggestion at the forthcoming Helcom meeting.

The outlet of phosphorus from sewage treatment plants, industries and agriculture has decreased strongly the last twenty years. In spite of this the number and strength of algal blooms increase. Anders Alm says that it might take fifty or one hundred years for the Baltic to recover if efforts are limited to decrease land sources. The countries around the Baltic do it well in building sewage treatment plants and decreasing the leakage from agriculture but in spite of this the situation has not improved.

The bottoms contain huge amounts of phosphorus that is released under anoxic conditions. According to Professor Anders Stigebrandt is the phosphorus supply from bottoms about three times greater than the supply from water rivers and other sources on land. According to Stigebrandt and his research group is the solution to pump oxygen rich water down into the deepwater to bind the phosphorus. The method has been tested in smaller scale in the By Fjord at Uddevalla and he sees only positive effects. In large scale the method should also be of benefit for cod.

Anders Alm has a small bundle of suggestions to Helsinki that he hopes that Helcom’s experts can evaluate. One method is the Stigebrandt oxygenation pump. Another method aims at chemically bind phosphorus to the bottom sediment. A third method is to dredge the uppermost nutrient rich layer of the bottom sediment. The method sucks the sediment layer without spreading sediment particles Anders Alm says. He don’t consider it obvious that Helcom will welcome thoughts of using engineering methods to decrease algal blooms and bottom death.

Those who want to make scaremongering of this call it geoengineering and says that we manipulate natural systems. But we manipulate in many other ways, for instance through large scale fisheries and our farming landscapes, senior administrative officer Mr Anders Alm says.

Results of ByFjord oxygenation experiment

A paper regarding the recently ended oxygenation experiment in the ByFjord on the Swedish west coast and major geochemical, toxocological and ecological results has just been published in the scientific journal AMBIO. First author of the paper is BOX-WIN project leader Anders Stigebrandt.

The paper can be downloaded from our Downloads-page and is found under the topic “Other publications”.


In a 2.5-year-long environmental engineering experiment in the By Fjord, surface water was pumped into the deepwater where the frequency of deepwater renewals increased by a factor of 10. During the experiment, the deepwater became long-term oxic, and nitrate became the dominating dissolved inorganic nitrogen component. The amount of phosphate in the water column decreased by a factor of 5 due to the increase in flushing and reduction in the leakage of phosphate from the sediments when the sediment surface became oxidized. Oxygenation of the sediments did not increase the leakage of toxic metals and organic pollutants. The bacterial community was the first to show changes after the oxygenation, with aerobic bacteria also thriving in the deepwater. The earlier azoic deepwater bottom sediments were colonized by animals. No structural difference between the phytoplankton communities in the By Fjord and the adjacent Havsten Fjord, with oxygenated deepwater, could be detected during the experiment.