The hydrographic-hydrochemical state of the Baltic Sea 2024

Inflow events and salinity

The series of years from 2017 to 2022 which were characterised by weak salt water inflow activity into the Baltic Sea was interrupted in the end of 2023/beginning 2024. During December 2023 intensified inflow activity was recorded, which was classified as midsized Major Baltic Inflow (MBI) event after Mohrholz (2018) and imported a salt mass of 1.7 Gt (salinity >15 g kg^-1) into the deep water of the Arkona Basin. In January a next event of 1.3 Gt salt import followed shortly and during spring 2024 the first events arrived in the eastern Gotland Basin in the upper part of the deep-water layer in 90-140 m water depth. In July, a baroclinic event transported highly saline water of up to 23 g kg^-1 into the Arkona Basin and in December a next barotropic MBI event of weak intensity occurred (1.1 Gt salt import). In general, the inflow activity has restarted since the end of 2023.

At the MARNET monitoring platform “Darss Sill” salinity ranged in mean from 9.01±1.69 g kg^-1 at 7 m depth and 14.15±3.87 g kg^-1 at 19 m throughout the year 2024. A maximum of 23.85 g kg⁻¹ was recorded at these bottom sensor level on 24 July, as signal of baroclinic inflow activity. The deeper sensors capture near-bottom salinity variability, thus serving as sensitive indicators of inflow activity. Unlike the annual means of 2016 (15.6 ± 3.5 g kg^-1) and 2014 (14.9 ± 3.4 g kg^-1), which both experienced strong inflow events, the year 2024 still exhibited above-average mean salinity suggesting moderate to intensified inflow activity. The bottom water of the Arkona Basin (40 m) showed a mean of 14.66±2.17 g kg^-1 and varied a lot during the year. A minimum of 8.01 g kg⁻¹ (12 March) after three weeks of an intensive outflow period and a maximum of 20.99 g kg⁻¹ (4 January) as result of the MBI in December 2023. The 95th percentile at 40 m was ∼17.90 g kg⁻¹, with 1136 h (∼12.9%) exceeding 17 g kg⁻¹, pointing to episodic saline intrusions in winter and late summer to early autumn.

At the MARNET bouy “Arkona Basin” a persistent thermal stratification was established in mid-May and lasted until late September. During the onset, bottom intrusions carried relatively cool, saline water with a temperature signature of about 8 °C (early–late May), consistent with the timing of baroclinic inflow signals further upstream at Darss Sill. Near-surface waters at 7 m experienced marked freshening into early June (minimum salinity ∼6.94 g kg⁻¹ on 7 June), reinforcing the vertical haline gradient. A second phase of baroclinic inflow activity starting in mid-June elevated the bottom temperature towards a quasi-steady ∼12 °C, while near-bottom salinity fluctuated chiefly in the 16– 18 g kg⁻¹ range. Throughout July to September, the water column remained strongly two-layered. The lowest oxygen saturation at 40 m occurred on 11 September (minimum ∼ 10%), reflecting prolonged isolation of the bottom layer under sustained thermal and haline stratification (stagnation period).

Oxygen

The oxygen concentration in surface water is in general controlled by the seasonal changing temperature and primary production. Physical processes like mixing and upwelling can intermediately cause a deviating oxygen concentration.

In 2024, the highest average oxygen concentrations measured during the monitoring campaigns in surface waters of the western Baltic Sea were observed in February, March and May and ranged between about 350-400 µmol l^-1 (about 8-9 ml l^-1) dissolved oxygen. After the summer minimum in August 270-280 µmol l^-1 (about 6 ml l^-1), subsequent cooling and enhanced input of atmospheric oxygen in autumn increased the oxygen concentration of surface water to 306-320 µmol l^-1 (about 7 ml l^-1) oxygen in November. In the bottom water, the Major Baltic Inflow events of December 2023/January 2024 resulted in a consistently good oxygen supply from Kiel Bight to Bornholm Basin in water depths down to 60 m (> 200 µmol l^-1 / 4.5 ml l^-1) up to May 2024. Afterwards depletion started and lowered the oxygen concentrations in the entire region during the summer months. In August, oxygen levels of lower than 200 µmol l^-1 / 4.5 ml l^-1 were detected below 15 m water depth in the Mecklenburg Bight, below 25 m water depth in the Arkona Basin and below 50 m water depth in the Bornholm Basin.

Temperature

The winter of 2023/2024 continued a series of warm winters compared with the 30 years reference period 1991-2020 and was in Warnemünde the tenth warmest since 1948. Thus, the sea surface cooling of the Baltic Sea was weaker compared to the reference period. Considering especially the study area for the biological investigations in the western Baltic Sea, the SST ranged between 4.4 °C in the Kiel Bight and 2.8 °C to 4.0 °C at the Darss Sill to the central Bornholm Basin in February 2024. Thus, the climatological mean of 2.5 °C was exceeded by 0.3 to 2.0 K. No temperature stratification was observed up to 45 m water depth, the bottom water of the Arkona Basin, in February 2024. An exception was the deep water of the Bornholm Basin, below the halocline in about 45 m water depth, which showed deep water temperatures of 6-9 °C. In March 2024 the SST in the Baltic Sea was with 5.6 °C in the Danish straits /Kiel Bight to 4.8-5.0 °C in the Arkona Basin and 4.0-4.4 °C in the Bornholm Basin above February and about 2-3 K above the climatological mean. The upper layer was well mixed down to the bottom from Kiel Bight to Arkona Basin. Warmer bottom water of 6-8 °C was found below the halocline in the Bornholm Basin, like in February. In May 2024 the intensive warming of the SST has started and the seasonal thermocline started to establish throughout the Thalweg transect of the western Baltic Sea in 20-25 m water depth. The sea surface temperatures ranged from 6-9 °C in the Mecklenburg Bight to Arkona Basin at the end of April and increased up to a maximum of 13.3 °C in the Arkona Basin to the mid of May. The long-term mean value (1900-2005) in this subregion is 7.48 °C in May. From the Danish Straits to the Arkona Basin the thermocline depth was at 10 m (Kiel Bight) to 25 m (Arkona Basin). Below this thermocline the cold winter water has had a core temperature of 4-6 °C in the Bornholm Basin. Below the intermediate layer the temperatures increased a bit and the key station Bornholm Deep showed a bottom water temperature of about 6.61 °C at April 26^th in 2024. In August 2024, the surface temperature in the Baltic Sea reached its annual maximum. The surface temperature was exceptional warm and reached values above 20 °C from Kiel Bight to Bornholm Basin and up to the central Baltic. Compared to the long-term mean, the SST values were 3 K too warm in the western Baltic Sea. The typical summer thermal stratification developed throughout the Baltic Sea. The seasonal thermocline was found at depths of about 15-20 m in the western Baltic Sea. Below the thermocline warm baroclinic summer inflows had replaced the former cool bottom water in the Arkona Basin. Maximum bottom water temperature ranged between 11.48 °C in the Arkona Basin and 7,13 °C in the Bornholm Basin, which was not influenced by inflow at the bottom of 90 m water depth. The baroclinic inflow water was found in the Bornholm Basin below the halocline in 50-70 m water depth, but did not have reached the bottom. The general temperature distribution in November 2024 reflected the autumnal cooling and the erosion of the seasonal thermocline in the surface layer. From the Kiel Bight to Bornholm Basin surface temperatures were about 11-11.5 °C, which was about 2-2.5 K higher than normal. From the Belt Sea to the Bornholm Basin the bottom water was only slightly warmer than the surface water layer, all areas within 1 K difference. The bottom layer of the Arkona Basin showed a maximum deep-water temperature of 12.1 °C and 11.6 °C at the surface.

Inorganic nutrients

The concentration of inorganic nutrients in the water column was measured on the Thalweg transect between the Mecklenburg Bight and the western Gotland Sea during all cruises in 2024.

Considering the winter (February) nutrient data as reservoir for the phytoplankton of the year, neither nitrate nor phosphate data showed a significant trend in surface waters of the western Baltic Sea during the last decade. Inter- and intraannual variations of nutrient concentrations are quite high in the shallow water due to mixing, upwelling and nutrient load from rivers. Nitrate surface concentrations ranged between 2.8 µmol l^-1 (2019) and 5.8 µmol l^-1 (2013, 2022) in February in Mecklenburg Bight. Phosphate surface concentrations ranged between 0.6 µmol l^-1 (2014, 2017) and 0.9 µmol l^-1 (2013) in February in Mecklenburg Bight between 2013 and 2022.

In 2024, the February surface concentrations of nitrate and phosphate were 6.43 µmol l^-1 and 0.63 µmol l^-1 respectively in Mecklenburg Bight and thus within the decadal range. In February 2024, the phosphate concentration ranged in the western Baltic Sea from 0.52 in the Kiel Bight to 0.63 µmol l^-1 in the Mecklenburg Bight in the surface mixed layer. The Arkona Basin and Bornholm Deep showed 0.60 µmol l^-1 and 0.61 µmol l^-1 phosphate concentrations at the surface. A phosphate maximum of 1.68 µmol l^‑1 at bottom near water depths of about 88 m was visible in the Bornholm Sea. The nitrate concentration in surface water decreased from 6.43 µmol l^-1 in the Mecklenburg Bight to 3.05 µmol l^-1 at the Bornholm Basin. In the entire area of the western Baltic Sea nitrate concentrations were determined in bottom near water depths between 6.04-6.93 µmol l^-1, with exception of 4.61 µmol l^-1 in the Kiel Bight. In March, nitrate was considerably consumed in the western Baltic Sea and declined to 0.11 µmol l^-1 in the Kiel Bight to 0.64 µmol l^-1 in Bornholm Sea surface water. The concentration distribution of phosphate reflected as well consumption in the western Baltic Sea surface water. The measured range was from 0.06 µmol l^-1 in Kiel Bight to 0.37-0.41 µmol l^-1 in the Arkona and Bornholm Seas. In May, surface water showed depleted nitrate concentrations in the entire area, in the Arkona Basin even up to the mid layer of 20 m water depth. Phosphate concentrations showed as well low concentrations but only local close to the detection limit (Kiel Bight to Mecklenburg Bight). Arkona Basin and Bornholm Basin showed surface concentrations of 0.13-0.29 µmol l^-1 of phosphate. In August surface water phosphate was depleted in the western Baltic Sea (0.0-0.01 µmol l^-1). The nitrate concentrations stayed on depleted level like in May. In the bottom near water layer phosphate ranged from 0.53 µmol l^-1 (Kiel Bight) to 1.51 µmol l^-1 (Bornholm Deep). Nitrate concentrations showed bottom near values low levels of 0.11 µmol l^-1 (Kiel Bight) to higher levels of 5.63 µmol l^-1 at Mecklenburg Bight to 8.88 µmol l^-1 at the Bornholm Deep. In November, the concentrations of nitrate and phosphate stayed low like in summertime because of more intensive than usual consumption of ongoing primary production.

The surface water DIN/DIP ratio (sum of ammonium, nitrate, and nitrite concentrations versus the phosphate concentration) ranged between about 10 mol mol^-1 in the western part and 6 mol mol^-1 in the Bornholm Sea in February 2024. The N/P ratio showed a decreasing trend from west to east: Kiel Bight /Mecklenburg Bight 9-11 mol mol^-1, Arkona Sea 6 mol mol^-1 and Bornholm Sea 5 mol mol^-1. The distribution pattern is similar to the situation in the previous year and confirmed again that nitrogen was a limiting factor from the western part up to the Baltic Proper, giving diazotrophic cyanobacteria an advantage compared to primary producers that depend on nitrate.