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Discussion
The two most important factors leading to the development of hypoxia are biological processes that determine the amount of organic matter available to be degraded and physical factors creating stratification. Stratification in most areas of the coastal zone of the Baltic Sea is due to seasonal temperature changes, although stratification by occasional inflowing saltier water does occur in some estuaries.(14) Hypoxia is common in estuaries located in the Danish Straits due to the large load of nutrients sustaining algal production and the strong stratification caused by large differences in surface and bottom water salinity.(5) Hypoxia in the Swedish and Finnish archipelagos are influenced by phytoplankton growth stimulated by nutrient loads from urban and agricultural sources, but also by restricted water circulation. The Finnish Archipelago Sea is impacted by drifting algal mats that consume oxygen during their decomposition when they sink to the bottom.(15) By contrast, hypoxia is rare in the northern Baltic Sea estuaries located in the Bothnian Sea coastal zone where nutrient loads are lower. Hypoxia is uncommon along the eastern shore from Estonia to Poland due to enhanced circulation of water in open areas along the coastline (Figure 1).
There are very limited data available to examine how far back in time hypoxia has occurred. A key question in this ecosystem affected daily by over 85 million people could be "Is hypoxia a natural phenomenon?".(16) The large numbers of enclosed areas, in many cases with shallow sills that restrict the exchange of bottom water and often in combination with large irregular changes in salinity, might mean that hypoxia has always been present in the coastal zone. However, sediment laminations from Stockholm Archipelago, an indicator of past hypoxia,(17) have been shown to be a recent phenomenon with very few areas with laminated sediments prior to 1900.17,18 The long-term millennial trends in coastal hypoxia remain unknown, although hypoxia has occurred intermittently throughout the last 8000 years in offshore deep waters of the Baltic Sea.(10) Further sediment studies are necessary, similar to those carried out in the open Baltic Sea,(10) to ascertain how far back in time hypoxia was observed in the coastal zone. Key uncertainties remain including how have the driving forces for hypoxia, e.g., climate(19) and nutrient loading, changed through time in the Baltic Sea coastal zone, and do they vary during the same time periods that hypoxia varies in the open waters?(16)
An important consideration in determining the number of ecosystems experiencing hypoxia is the size of the assessment unit. We combined numerous monitoring points and sites to form a coherent coastal unit when bottom waters were physically connected to each other. This means that assessment units considered hypoxic might occur in the same geographical region and could be in close proximately to each other, if their bottom waters were physically disconnected. The Baltic Sea archipelagos have a complex topography with many basins disjoined by shallow sills resulting in many distinct sites, although they are all affected by similar mechanisms. In fact, the Stockholm and Finnish archipelagos contributed 21% of the total assessment units, a relatively large share compared to the total Baltic Sea coastal zone, but >40% of the hypoxic sites. The coastal stretches of the Western Gotland Basin and Gulf of Finland that have a substantial number of hypoxic sites are also archipelagos, which are more prone to hypoxia because of the complex topography, proximity to development, and consequently more sensitive to enhanced nutrient inputs from land.
Surprisingly only 4.0% of all sites were classified as seasonally hypoxic according to our definition when >50% and <80% of profiles during the stratified period have oxygen concentrations less than 2 mg L-1. A statistically rigorous definition of what constitutes seasonal hypoxia does not currently exist and in fact should be linked to ecological consequences for it to be ecologically relevant and not just connected to absolute concentrations.(3) Currently coastal marine ecosystems that experience hypoxia for a period of days to months every year are often considered to be seasonally hypoxic, although we have categorized them here as being episodic, because we required the monitoring units to be hypoxic for most of the stratified period. However, in Limfjorden, Denmark, bottom water oxygen concentrations can vary greatly between weekly samplings (SI Figure S6), although nearly every year hypoxic conditions are observed somewhere in the Limfjorden. Our statistical estimation is that Limfjorden is episodically hypoxic. Diaz and Rosenberg(1) would classify Limfjorden as periodically hypoxic, although we have found it difficult to create a statistically robust indicator that would classify systems as periodically hypoxic. Periodic hypoxia is problematic because it never allows for re-establishment of benthic communities.(20)
Currently, there are 416 areas in the world with reported coastal hypoxia with about 30 previously reported sites in the Baltic Sea region, which includes both coastal zone hypoxia and deep water sites in the Baltic Sea.(1) We have identified an additional 96 sites that have experienced hypoxia, increasing the global total to nearly 500 sites. Of all the known sites around the world, around 20% of the sites are found in the Baltic Sea region. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia.(20) The large number of sites partially reflects the fact that the Baltic Sea is one of the most data rich regions of the world with no tidal mixing and complex bottom topography creating conditions favorable for hypoxia, although these are only the "monitored areas" and many more hypoxic sites probably exist in the Baltic Sea coastal zone. In contrast, there was no evidence of hypoxia at 95 stations in estuarine and coastal waters around Ireland due to sufficient tidal mixing.(21) If such detailed data existed for other coastal regions around the world, it is likely that the number of areas would increase globally.
Oxygen is an important parameter in water quality assessments since sufficient oxygen is essential to aquatic life and is necessary to support healthy biological communities.(3) For example, oxygen concentration is a supporting element to the biological quality elements in the European Water Framework Directive,(21) is one of the ecological objectives in the Helsinki Commission's (HELCOM) eutrophication assessment of the Baltic Sea, and is included in the European Marine Strategy Framework Directive as an effect parameter of eutrophication. Accurate data for the concentration of dissolved oxygen are essential for documenting environmental changes in water resources resulting from natural phenomena and human activities.
Reports of hypoxia globally are increasing1,22 both due to the recognition of the seriousness of hypoxia on ecosystem functioning and as decades-long monitoring records become available in databases. How the large number of newly identified hypoxic areas influences nutrient biogeochemical cycles and ecosystem services in the coastal zone in the Baltic Sea is currently unknown, although the functioning of the coastal filter certainly plays an important role in how adjacent marine systems respond to changes in nutrient loading. Presently about 40% of the world's population lives within 100 km of the coastal zone strongly impacting ocean health,(24) including increasing the occurrence of hypoxic areas.1,2 What is currently lacking is the link to the driving factors of hypoxia including nutrient enrichment, organic carbon loading and climate change,(25) and a determination of appropriate nutrient loading targets for guidance to managers to ameliorate the devastating impact of hypoxia.(24)
