Propagation of Impact of the Recent Major Baltic Inflows From the Eastern Gotland Basin to the Gulf of Finland

Taavi Liblik,Jaan Laanemets,Karin Wesslander,Michael Naumann,Lena Viktorsson,Laura Tuomi,Martin Hansson,Pekka Alenius,Urmas Lips,Günther Nausch

doi:10.3389/fmars.2018.00222

Abstract

Major Baltic Inflows (MBI) have a significant impact on physics, biogeochemistry and marine life in the Baltic Sea. Spreading of the North Sea water from the Danish Straits to the Eastern Gotland Basin has been rigorously studied in recent decades. Investigations of lateral signal propagation using in-situ measurements, which cover the area from the Eastern Gotland Basin to the Gulf of Finland, are missing. Estonian-Swedish-German-Finnish oceanographic data from January 2014 to March 2017 were merged and analyzed to fill the gap. Recent MBIs caused considerable changes in water column properties, and salinity reached the highest values of the last 40–60 years. The arrivals of MBI waters were detected as peaks in the salinity and temperature time-series in the near-bottom layer of the Gotland Deep 4–5 months after the MBI events. Similar peaks were also identified in the Faro Deep, Northern Deep and Kopu West (Northern Baltic Proper) with a further delay of 2–3 months, 3–5 months and 4–6 months, respectively. The first impact of the 2014 December MBI occurred in the Gulf of Finland in nine months as the arrival of the former Northern Baltic Proper deep layer water. Water renewal in the Faro Deep occurred as a gravity current over the sill between Faro and Gotland Deep. Deep layer water in the Northern Baltic Proper and the Gulf of Finland originated from the sub-halocline layer (110–120 m) of the Eastern Gotland Basin. The pre-condition for such mid-layer advection was a denser deep layer in the Gotland and Faro Deep. Fresh oxygen, which arrived in the Gotland Deep in April 2015 and February 2016, was consumed in the near-bottom layer within 3–6 months. Since summer 2016, oxygenated waters occurred in the Gotland Deep in the layer from the halocline to 160 m depth. This oxygen did not reach the area further in the north, except a slight sign of ventilation of the Faro Deep in February 2017. Thus, MBIs did not improve the oxygen conditions in the area north of the Gotland Deep and oxygen conditions rather worsened in the Northern Baltic Proper and the Gulf of Finland.

Highlights

The Baltic Sea is a semi-enclosed, shallow, and brackish sea with limited water exchange with the North Sea
In order to assess propagation and transformation of the Major Baltic Inflows (MBI) water, we evaluated the changes in water characteristics at sills and deep basins: Gotland Deep, Fårö Deep, Northern Deep, Kõpu West, and Kõpu North
High bottom layer salinities and thick hypoxic bottom layer were observed in the Gulf of Finland in 2016. This result is in line with the tendencies after the previous MBIs: stronger stratification and increased hypoxia were observed in the Northeastern Baltic Sea since the mid-1990s (Laine et al, 2007; Liblik and Lips, 2011; Lehtoranta et al, 2017)

Summary

Introduction

The Baltic Sea is a semi-enclosed, shallow, and brackish sea with limited water exchange with the North Sea. Strong barotropic inflows, called Major Baltic Inflows (MBIs), are the main process to ventilate deep water in the Baltic Sea (Matthäus and Franck, 1992). The following inflow event in December 2014 was considered as the strongest since 1951 (Mohrholz et al, 2015) During this MBI event 320 km of salty and oxygenrich water propagated into the western Baltic Sea after a 10year stagnation period. Baroclinic inflows do not carry much oxygen from the North Sea, but they could pick up oxygen by entrainment with the cold intermediate layer at sills in the Baltic Sea and to some extent ventilate deeper basins (Feistel et al, 2006; Mohrholz et al, 2006)

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