Briefing brochure HYPER project
BONUS Briefing 11, October 2011
The drastic expansion of hypoxia (oxygen depletion)
from less than 10,000 km² before 1950 to over 60,000
km² in recent years is one of the most profound effects
of eutrophication in the Baltic Sea. Increasing nutrient
input from land and atmosphere is the main cause for
this trend. The expansion of hypoxia has significantly
altered nutrient cycles. Over the years, denitrification,
which is the most important removal process for nitrogen,
has shifted from mainly taking place in the sediment
to the water column. Hypoxia can be reversed,
but only if nutrient inputs are reduced. The recovery
process can, however, be further enhanced by benthic
animals that bio-irrigate the sediments and increase
ecosystem resilience toward perturbations from
hypoxia. Targeting hypoxia as part of the Baltic Sea
Action Plan also addresses other ecological objectives
of HELCOM.KEY RESULTS
Oxygen trends constructed by the project over the last 110
years show that hypoxia was confined to a spatially restricted
area before 1950. Since then the hypoxic area has increased
drastically to a present level around 60,000 km². Trends of
hypoxia are closely linked with nutrient inputs.
Significant amounts of phosphorus are buried in the sediments
in organic forms, when hypoxic conditions prevail. This pool of
phosphorus will be remineralised during oxic conditions and
bound to iron. If the system becomes hypoxic again the ironbound
phosphorus is released to the water, potentially sustaining
large harmful algal blooms. Thus, the Baltic Sea contains a
potential “P-bomb" that can be released with alternating
hypoxic-normoxic conditions.
Denitrification is the most important pathway for removing
nitrogen. Low oxygen levels induce a shift from denitrification
to another process (DNRA), which recycles nitrogen back to the
water column. Moreover, the increase in hypoxia over time has
displaced the zone of denitrification from the sediment to the
water column.
Benthic invertebrates play an important role modulating nutrient
cycles and enhancing ecosystem recovery. Hypoxia can be
reversed, but it requires that nutrient inputs are first reduced
to achieve oxygen levels suitable for these species to colonize.
Bio-irrigating benthic organisms will speed up and maintain
the recovery process after colonizing the area.
WHO NEEDS THE INFORMATION
These results will give more precision to the Baltic Sea Action Plan
revision process, involving stakeholders from environmental ministries,
HELCO and NGOs. HYPER results have advanced our scientific
understanding of processes in the Baltic Sea.
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