Natural variability is a large source of uncertainty in future projections of hypoxia in the Baltic Sea

H E Markus Meier,Christian Dieterich,Matthias Gröger

doi:10.1038/s43247-021-00115-9

Abstract

Coastal seas worldwide suffer from increasing human impact. One of the most severe environmental threats is excessive nutrient pollution from land, which causes oxygen depletion and harmful algal blooms. In 2018, the semi-enclosed Baltic Sea was determined to contain the largest hypoxic area among the world’s coastal seas, with a size equal to the Republic of Ireland. In this study, ensemble modelling was used to investigate whether climate change will intensify hypoxia in the Baltic Sea and whether nutrient load abatement strategies would counteract this scenario. We analysed the largest ensemble of scenario simulations for the Baltic Sea currently available (including different boundary conditions) and estimated the magnitude of various sources of uncertainty. The results showed that natural variability was a larger source of uncertainty than previously considered. The earliest time and appropriate location to detect a trend above the background noise were estimated. A significant decrease in hypoxia can be achieved by further reductions in nutrient loads implemented in combination with existing measures.

Highlights

Among the world’s coastal seas, the Baltic Sea (Fig. 1) has the largest annual maximum hypoxic area (70,000 km2), followed by the northern Gulf of Mexico (23,000 km2), the north-western Black Sea (20,000 km2) and the East China Sea (12,000 km2)[1]
Scenario simulations for the Baltic Sea have suggested an expansion of the hypoxic area in a future climate because of increased nutrient loads owing to enhanced river runoff, reduced oxygen fluxes from the atmosphere to the ocean and the accelerated recycling of organic matter in response to higher water temperatures[21]
Two nutrient load scenarios were applied: the Baltic Sea Action Plan (BSAP) and a business-as-usual or reference scenario (REF) that assumes the same average nutrient loads for the future as during a reference period (2010–2012)[29,37]. Nutrient inputs during the latter were modified by taking into account the projected increase in river discharge (Fig. 3)

Summary

Introduction

Among the world’s coastal seas, the Baltic Sea (Fig. 1) has the largest annual maximum hypoxic area (70,000 km2), followed by the northern Gulf of Mexico (23,000 km2), the north-western Black Sea (20,000 km2) and the East China Sea (12,000 km2)[1]. Nitrogen fixation has increased and earlier and more frequent cyanobacterial (blue–green algal) blooms have reinforced eutrophication[12] This internal nutrient source counteracts the reduction in external loads that have been achieved since the 1980s8. The environmental conditions in the Baltic Sea today are still characterised by (1) deoxygenation and hypoxia both in the deep offshore waters[2,14] and in coastal zones[15,16], (2) intensified cyanobacterial blooms[17] and (3) reduced water transparency[18]. Scenario simulations for the Baltic Sea have suggested an expansion of the hypoxic area in a future climate because of increased nutrient loads owing to enhanced river runoff, reduced oxygen fluxes from the atmosphere to the ocean and the accelerated recycling of organic matter in response to higher water temperatures[21]. Restoration plans similar to the BSAP have been implemented for Chesapeake Bay

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Conclusion