Abstract
AbstractSemi‐enclosed marginal seas like the Baltic Sea are often characterized by permanently anoxic deep layers, and may therefore serve as important model systems to study the causes and consequences of the predicted global expansion of oxygen minimum zones. Here, we focus on the role of lateral intrusions in maintaining the “hypoxic transition zone” (HTZ) of the Baltic Sea, which characterizes the quasi‐permanent hypoxic region located between the oxygenized surface layer and the sulfidic deep‐water region. Based on long‐term deployments of an autonomous profiling system in the central Baltic Sea, we show that oxic mid‐water intrusions are ubiquitous features, providing the most important oxygen source for the HTZ, and largely control the vertical and lateral extent of the hypoxic areas. An oxygen budget for the HTZ suggests that oxygen turnover in the HTZ is, to first order, determined by a long‐term balance between sedimentary oxygen demand and oxygen import by intrusions. The downward mixing of oxygen into the HTZ is generally non‐negligible but unlikely to provide a first‐order contribution to the HTZ oxygen budget. On the long‐term average, mid‐water intrusions were shown to inject 30–60 Gmol of oxygen per year into the deep‐water region below the permanent halocline. This is approximately one order of magnitude larger than the average amount of oxygen imported during the massive deep‐water inflow events (Major Baltic Inflows) that occur on an approximately decadal time scale, highlighting the HTZ as a hotspot for biogeochemical turnover.
Highlights
A number of recent studies have drawn attention to a global decrease in the dissolved oxygen content and a rapid spreading of oxygen minimum zones in the ocean with predicted wide-ranging consequences for biogeochemical cycling and aerobic organisms (Stramma et al 2008; Keeling et al 2010)
Several experimental investigations carried out in the central Baltic Sea found that such Major Baltic Inflows (MBIs) are generally accompanied by lateral intrusions at intermediate depths, strongly impacting on the local water mass properties inside and below the halocline (Zhurbas and Paka 1997, 1999; Wieczorek et al 2008; Holtermann et al 2017)
The role of consumption within the water column is recently discussed as a factor for changing oxygen decay rates between different MBIs (Meier et al 2018)
Summary
A number of recent studies have drawn attention to a global decrease in the dissolved oxygen content and a rapid spreading of oxygen minimum zones in the ocean with predicted wide-ranging consequences for biogeochemical cycling and aerobic organisms (Stramma et al 2008; Keeling et al 2010). Semi-enclosed marginal seas like the Baltic Sea, the Black Sea, and the Cariaco Basin, already characterized by permanently anoxic deep layers, serve as important model systems for understanding the causes and consequences of hypoxia under the extreme conditions predicted for an increasing fraction of the world’s marine systems (Astor et al 2003; Konovalov et al 2005; Carstensen et al 2014). Massive deep intrusions of oxic waters from the North Sea during the socalled Major Baltic Inflows (MBIs), sporadically occurring on a decadal time scale, are currently believed to be the most important physical factor regulating the temporal and spatial distribution of the anoxic regions During such MBIs, oxygenated salty ( dense) waters enter the Western Baltic Sea through the Danish Straits (Fig. 1), and propagate as dense bottom gravity currents through a series of basins and channels until reaching the central Baltic Sea (Stigebrandt 1987; Feistel et al 2008). AB, Arkona Basin; BB, Bornholm Basin; SF, Słupsk Furrow; GB, Gotland Basin; LD, Landsort deep
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
