28 Jun 2021
28 Jun 2021
Status: a revised version of this preprint is currently under review for the journal BG.

Causes of the extensive hypoxia in the Gulf of Riga in 2018

Stella-Theresa Stoicescu1, Jaan Laanemets1, Taavi Liblik1, Māris Skudra2, Oliver Samlas1, Inga Lips1,3, and Urmas Lips1 Stella-Theresa Stoicescu et al.
  • 1Department of Marine Systems, Tallinn University of Technology, Tallinn, 19086, Estonia
  • 2Latvian Institute of Aquatic Ecology, Riga, LV-1007, Latvia
  • 3EuroGOOS AISBL, Brussels, 1000, Belgium

Abstract. The Gulf of Riga is a relatively shallow bay connected to the deeper central Baltic Sea (Baltic Proper) via straits with sills. The decrease in the near-bottom oxygen levels from spring to autumn is a common feature in the gulf, but in 2018, hypoxia was exceptional. We analyzed temperature, salinity, oxygen, and nutrient data collected in 2018 and historical data available from environmental databases. Forcing data from the study year were compared with their long-term means and variability. The year 2018 was exceptional due to occasionally dominating north-easterly winds supporting the inflow of saltier waters from the Baltic Proper and meteorological conditions causing fast development of thermal stratification in spring. Existing stratification hindered vertical transport between the near-bottom layer (NBL) and the water layers above it. The estimated oxygen consumption rate at the sediment surface in spring-summer 2018 was about 1.7 mmol O2 m−2 h−1 that exceeded the oxygen input to the NBL due to advection and mixing. We suggest that the observed pronounced oxygen depletion was magnified by the prolonged stratified season and haline stratification in the deep layer that maintained a decreased water volume between the seabed and the pycnocline. The observed increase in phosphate concentrations in the NBL in summer 2018 suggests a significant sediment phosphorus release in hypoxic conditions counteracting the mitigation measures to combat eutrophication. We conclude, if similar meteorological conditions as in 2018 could occur more frequently in the future, such extensive hypoxia would be more common in the Gulf of Riga and other coastal basins with similar morphology and human-induced elevated input of nutrients.

Stella-Theresa Stoicescu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-160', Anonymous Referee #1, 12 Jul 2021
    • AC3: 'Reply on RC1', Stella-Theresa Stoicescu, 31 Aug 2021
  • RC2: 'Comment on bg-2021-160', Oleg Savchuk, 19 Jul 2021
    • AC2: 'Reply on RC2', Stella-Theresa Stoicescu, 31 Aug 2021
  • RC3: 'Comment on bg-2021-160', Anonymous Referee #3, 05 Aug 2021
    • AC1: 'Reply on RC3', Stella-Theresa Stoicescu, 30 Aug 2021

Stella-Theresa Stoicescu et al.

Stella-Theresa Stoicescu et al.


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Short summary
Coastal basins with high input of nutrients often suffer from oxygen deficiency. In summer 2018, the extent of oxygen depletion was exceptional in the Gulf of Riga. We analyzed observational data and found that extensive oxygen deficiency appeared since the water layer close to the seabed, where oxygen is consumed, was separated from the surface layer. The problem worsens if similar conditions restricting vertical transport of oxygen occur more frequently in the future.