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, extensive hypoxia was observed. We analyzed temperature, salinity, oxygen, and nutrient data collected in 2018, along with historical data available from environmental databases. Meteorological and hydrological data from the study year were compared with their long-term means and variability. We suggest that pronounced oxygen depletion occurred in 2018 due to a distinct development of vertical stratification. Seasonal stratification developed early and was stronger in spring–summer 2018 than on average due to high heat flux and weak winds. Dominating northeasterly winds in early spring and summer supported the inflow of saltier waters from the Baltic Proper that created an additional deep pycnocline restricting vertical transport between the near-bottom layer (NBL) and the water column above. The estimated oxygen consumption rate in the NBL in spring–summer 2018 was about 1.7 mmolO2m-2h-1, which exceeded the oxygen input to the NBL due to advection and vertical mixing. Such a consumption rate leads to near-bottom hypoxia in all years when vertical mixing in autumn reaches the seabed later than on average according to the long-term (1979–2018) meteorological conditions. 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. Since climate change projections predict that meteorological conditions comparable to those in 2018 will occur more frequently, 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.

Seasonal dynamics and regional distribution patterns of CO2 and CH4 in the north-eastern Baltic Sea

“…Since the GoR is shallower than the GoF and without a permanent halocline, CO2 accumulation and subsequent CO2 flux from the GoR deep layer do not have a similar high potential in autumn-winter as in the GoF. However, the seasonal thermocline was stronger in spring-summer 2018 than on average due to high heat flux and calm wind conditions (Stoicescu et al, 2022).…”

“…In the GoR, sediments have a high organic matter content, and the area undergoes intermittent seasonal hypoxia (Stoicescu et al, 2022). In the shallow areas, likely wind-induced mixing remains relevant for the transport of methane from the sediment,…”

“…Due to the shallowness of the basin, the water column is fully mixed in autumn-winter. Thermal stratification starts to develop in April and decays in October-December, depending on the water depth and yearly variable meteorological conditions (Skudra and Lips, 2017;Stoicescu et al, 2022). Near-bottom seasonal hypoxia can be observed in the deeper areas of the central gulf (Stoicescu et al, 2022).…”

“…The seasonal stratification in the GoR during spring-summer 2018 was stronger than on average. It restricted vertical mixing and led to pronounced near-bottom oxygen depletion(Stoicescu et al, 2022) and probably to relatively low cCH4 in the surface layer of the deeper GoR areas (where the seasonal thermocline existed). Relatively low methane concentrations in October suggest that the autumn mixing had not yet reached the seabed in deep GoR areas by October.…”

Modelling the effects of benthic fauna on carbon, nitrogen and phosphorus dynamics in the Baltic Sea

“…When occurring more often in the most recent decade, the intensity and extent of both sporadic hypoxia and phosphate release have somewhat increased (HELCOM, 2018;Stoicescu et al, 2022). Similarly, the summer cyanobacteria blooms sporadically occurring in the Gulf of Riga before the 2010s (Kahru and Elmgren, 2014) have regularly and extensively covered the gulf since 2015 (Mati Kahru, personal communication, 2021).…”