On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea

Abstract

In December 2014, the third strongest salt water inflow into the Baltic Sea occurred since 1880. It was assumed that the inflow would turn the entire bottom water of the Baltic Sea from anoxic into oxic conditions for an extended period. However, already in late 2015, the central Eastern Baltic Sea had turned back into anoxic conditions. This rapid oxygen decline was in fact surprising since a weaker inflow in 2003 ventilated the Baltic Sea for a longer period of time. With the aid of an ecosystem model of the Baltic Sea, the two inflows in 2003 and 2014 were analyzed in detail. Although the 2014 inflow event was twice as strong as the 2003 inflow event, oxygen transport continued after the latter one, supplying about the same amount of oxygen again. In addition to the major inflow event, a series of smaller inflows in 2003 supplied the extra oxygen transport. Therefore, the strength of a major inflow event alone cannot be used to predict the oxygenation impact. Instead, it is necessary to consider smaller events, in particular those occurring just before and after a major inflow event, as well. An element tagging method showed that the share of oxygen imported across the Danish Straits on the total oxygen arriving at the central Eastern Baltic Sea is between 10% and 20%. Therefore, the oxygen concentration of the inflowing water seems to be of less importance for the oxygenation effect on the central Baltic Sea due to the strong dilution effect.

Deepwater dynamics and mixing processes during a major inflow event in the central Baltic Sea

“…This finding is in line with a recent numerical study by Neumann et al . [], who emphasized the large cumulative effect of the “secondary” oxic intrusions often following MBIs.…”

Platichthys solemdali sp. nov. (Actinopterygii, Pleuronectiformes): A New Flounder Species From the Baltic Sea

“…Furthermore, both flounder species in the Baltic Sea live close to their physiological limits, and salinity and oxygen content, the most important environmental factors driving the distribution of both species, are not stable in time. Short term fluctuations are largely due to the occurrence of large inflows or oxygen rich, high salinity waters from the North Sea, and due to stagnation periods in which salinity and oxygen concentrations decrease (Neumann et al, 2017). In the long term, eutrophication is deoxygenating the Baltic Sea (Carstensen et al, 2014) and climate change is decreasing salinity via increased freshwater runoff (Vuorinen et al, 2015).…”

Large-Scale Nutrient Dynamics in the Baltic Sea, 1970–2016

“…This excess forces dinitrogen into biotic cycling via nitrogen-fixing cyanobacteria, thus increasing primary production, sedimentation and decomposition of organic matter, which, in turn, leads to further expansion of the hypoxic zone with increased denitrification and DIP release. Shrinking of the hypoxic zone after sporadic Major Baltic Inflows of saline oxygen-replete waters causes opposite changes in the N and P pools, but today the improved oxygen conditions do not last long (Neumann et al, 2017). All the major components of this large-scale feedback loop in the biogeochemical cycles of the Baltic Sea have been known throughout history (Richards, 1965;Fonselius, 1969;Grasshoff and Voipio, 1981) and continue being studied and demonstrated in increasing detail, from paleoreconstructions and large-scale considerations to laboratory experiments (e.g., Conley et al, 2002Conley et al, , 2009aVahtera et al, 2007bVahtera et al, , 2010Nausch M. et al, 2008Nausch M. et al, , 2012Savchuk, 2010;Ploug et al, 2011;Raateoja et al, 2011;Funkey et al, 2014;Adam et al, 2016;Olofsson et al, 2016;Motwani et al, 2018).…”