Depletion of Oxygen in the Bothnian Sea Since the Mid-1950s

Polyakov, Igor V.; Tikka, Kimmo; Haapala, Jari; Alkire, Matthew B.; Alenius, Pekka; Kuosa, Harri

doi:10.3389/fmars.2022.917879

Cited by 3 publications

(1 citation statement)

References 37 publications

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“…It could also be used to study dynamics that affect the state of the Bothnian Sea: for example, at which depths and along which route can nutrient-rich water flow to the Bothnian Sea and when does this kind of water transport occur. Further, we could study if there have been changes in the water exchange dynamics which would in part explain the changes that have been recently observed in the state of the Bothnian Sea (e.g., Kuosa et al, 2017;Polyakov et al, 2022). We could also study the exchange between the Archipelago Sea and the Baltic proper and the Bothnian Sea and how these open sea areas affect the coastal waters, e.g.…”

Section: Discussionmentioning

confidence: 99%

Transport dynamics in a complex coastal archipelago

Miettunen,

Tuomi,

Westerlund

et al. 2024

Ocean Sci.

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Abstract. The Archipelago Sea (in the Baltic Sea) is characterised by thousands of islands of various sizes and steep gradients of the bottom topography. Together with the much deeper Åland Sea, the Archipelago Sea acts as a pathway to the water exchange between the neighbouring basins, Baltic proper and Bothnian Sea. We studied circulation and water transports in the Archipelago Sea using a new configuration of the NEMO 3D hydrodynamic model that covers the Åland Sea–Archipelago Sea region with a horizontal resolution of around 500 m. The results show that currents are steered by the geometry of the islands and straits and the bottom topography. Currents are highest and strongly aligned in the narrow channels in the northern part of the area, with the directions alternating between south and north. In more open areas, the currents are weaker with wider directional distribution. During our study period of 2013–2017, southward currents were more frequent in the surface layer. In the bottom layer, in areas deeper than 25 m, northward currents dominated in the southern part of the Archipelago Sea, while in the northern part southward and northward currents were more evenly represented. Due to the variation in current directions, both northward and southward transports occur. During our study period, the net transport in the upper 20 m layer was southward. Below 20 m depth, the net transport was southward at the northern edge and northward at the southern edge of the Archipelago Sea. There were seasonal and inter-annual variations in the transport volumes and directions in the upper layer. Southward transport was usually largest in spring and summer months, and northward transport was largest in autumn and winter months. The transport dynamics in the Archipelago Sea show different variabilities in the north and south. A single transect cannot describe water transport through the whole area in all cases. Further studies on the water exchange processes between the Baltic proper and the Bothnian Sea through the Archipelago Sea would benefit from using a two-way nested model set-up for the region.

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Section: Discussionmentioning

confidence: 99%

Transport dynamics in a complex coastal archipelago

Miettunen,

Tuomi,

Westerlund

et al. 2024

Ocean Sci.

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Summer heatwaves on the Baltic Sea seabed contribute to oxygen deficiency in shallow areas

Safonova,

Meier,

Gröger

2024

Commun Earth Environ

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Since the early 1980ies, the shallow and brackish Baltic Sea, located in northern Europe, has warmed fastest of all the world’s coastal seas. Hence, the total heat exposure during marine heatwaves increased in frequency and duration, which may have a major impact on the marine ecosystem. We found that marine heatwaves, using two different, commonly used definitions, occurred everywhere on the sea surface since the early 1990s. Periods with sea surface temperatures larger than 20 °C lasting longer than 10 days were particularly numerous in the shallow coastal zone of the southern and eastern Baltic Sea. Furthermore, summer marine heatwaves that reached the seabed mainly occurred in water depths of less than 20 m, where they led to a decrease in oxygen concentration. Since the early 1990s, a positive trend in the expansion of marine heatwaves has been observed on the seabed. The increasing number of marine heatwaves increases the risk of hypoxia events in the coastal zone.

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Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea

Muchowski,

Jakobsson,

Umlauf

et al. 2023

Ocean Sci.

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Abstract. Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source of strong turbulent mixing. Yet, current in situ observations often cannot capture the spatio-temporal development of dynamic and intermittent turbulent mixing related to overflows over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely sampled Southern Quark located in the Åland Sea, connecting the northern Baltic Proper with the Bothnian Sea. Our data were acquired during two 1-week cruises on R/V Electra in February–March 2019 and 2020. We collected high-resolution broadband acoustic observations of turbulent mixing together with in situ microstructure profiler measurements, and current velocities from acoustic Doppler current profilers. We found that a temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. The observed flow reversed during both cruises on timescales of a few days. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and high spatial variability occurred in the surface layer on small scales. Dissipation rates of turbulent kinetic energy, vertical turbulent diffusivities, and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow with average dissipation rates near the sill between 10−7 and 10−6 W kg−1, average vertical diffusivities of 0.001 m2 s−1 in the halocline and up to 0.1 m2 s−1 below the halocline, and average vertical salt flux rates around 0.01 g m−2 s−1 in the halocline and between 0.1 and 1 g m−2 s−1 below the halocline. We suggest, based on acoustic observations and in situ measurements, that the underlying mechanism for the highly increased mixing across the halocline is a combination of shear and topographic lee waves breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea.

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Depletion of Oxygen in the Bothnian Sea Since the Mid-1950s

Cited by 3 publications

References 37 publications

Transport dynamics in a complex coastal archipelago

Transport dynamics in a complex coastal archipelago

Summer heatwaves on the Baltic Sea seabed contribute to oxygen deficiency in shallow areas

Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea

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