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

Abstract

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.

Investigating Hypoxic and Euxinic Area Changes Based on Various Datasets From the Baltic Sea

“…The differences between SMHI and ALM datasets of hypoxic area are likely explained by the increased hypoxia in the Gulf of Finland and Gulf of Riga after 2000 (Stoicescu et al, 2019(Stoicescu et al, , 2021.…”

Large-scale effects of benthic fauna on carbon, nitrogen, and phosphorus dynamics in the Baltic Sea

“…In contrast to the Baltic Proper, the Gulf of Riga is relatively well mixed and hypoxia only occurs intermittently under the summer thermocline (Kotta et al, 2008), accompanied by increased release of phosphate from the sediments (Eglite et al, 2014). When occurring more often in the recent decade, the intensity and extension of both sporadic hypoxia and phosphate release have somewhat increased (HELCOM, 2018;Stoicescu et al, 2021). Similarly, the summer cyanobacteria blooms sporadically occurring in the Gulf of Riga before the 2010s (Kahru and Elmgren, 2014), regularly and extensively cover the gulf since 2015 (pers.…”