An experiment with forced oxygenation of the deepwater of the anoxic By Fjord, western Sweden.

Anders Stigebrandt,Loreto De Brabandere,Marina Magnusson,Jonatan Hammar,Lars Rahm,Lena Viktorsson,Alexander H Treusch,Michael Forth,Mikhail Kononets,Fredrik Norén,Åke Granmo,Per Hall,Daniel Hansson,Bengt Liljebladh

doi:10.1007/s13280-014-0524-9

Abstract

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-014-0524-9) contains supplementary material, which is available to authorized users.

Highlights

Coastal areas occupied by hypoxic and anoxic waters are increasing globally (Diaz and Rosenberg 2008)
The decrease in the rate of salinity of the deepwater due to dilution by surface water was in the range 0.025–0.04 day-1, meaning that it took 25–40 days to decrease the salinity by
We found indications that the bacterial community below 15 m became more similar to that in the naturally oxic Havsten Fjord

Summary

Introduction

Coastal areas occupied by hypoxic and anoxic waters are increasing globally (Diaz and Rosenberg 2008). Several authors call for caution and discuss various possible negative consequences of man-made oxygenation, e.g., leakage of toxic substances from earlier anoxic bottoms due to deep-digging worms (Conley 2012), and building up large amounts of ironbound phosphorus in oxygenated sediments that may be released when the oxygenation is shut off (Reed et al 2011). An example of the latter can be seen in the Bornholm Basin where the deepest bottoms since the 1960s switch between anoxic and oxic states (Stigebrandt et al 2013)

Methods

Results

Discussion

Conclusion