The Turkish Strait System, which is the only connection between the Black Sea and the Mediterranean Sea, is a challenging region for ocean circulation models due to topographic constraints and water mass structure. We present a newly developed high resolution unstructured finite element grid model to simulate the Turkish Strait System using realistic atmospheric forcing and lateral open boundary conditions. We find that the jet flowing from the Bosphorus Strait into the Marmara creates an anticyclonic circulation. The eddy kinetic energy field is high around the jets exiting from the Bosphorus Strait, Dardanelles Strait, and also the leeward side of the islands in the Marmara Sea. The model successfully captures the two-layer structure of the Sea of Marmara. The volume transport at the Bosphorus is around 120 km${}^3$/year which is consistent with the recent observations. The largest bias in the model is at the interface depth due to the shallower mixed layer.

With the repetition of mucilage event, which is triggered by many different anthropogenic, climatic and microbiological factors, in the Marmara Sea in 2021, the importance of water quality in the seas has come to the fore again. To present the spatial distribution of the mucilage, a feasibility study has been carried out with point-based water quality measurements and remote sensing data. In-situ measurements are collected routinely within the scope of the Integrated Marine Pollution Monitoring Program (DEN-IZ) which was conducted in cooperation with the Ministry of Environment, Urbanisation and Climate Change and the Scientific and Technological Research Council of Turkey - Marmara Research Center (TUBITAK-MAM). In this preliminary study, 16 in-situ measurements, 5 of which were taken from water containing mucilage, on 29 April 2021 in the Gulf of Gemlik were used. Then, univariate regression analyzes were performed in two different scenarios (i.e. 5 mucilage points and all in-situ points) with Sentinel-2 satellite imagery and in-situ water quality measurements for 2 different parameters (i.e. chlorophyll-a – Chl-a) and turbidity). According to R² and accuracy assessment measures (f- and t- statistics etc.), the most suitable models were determined for two scenarios and two parameters. Finally, the performances of the selected models were tested with 2 different in-situ measurements and satellite images (dated 22 and 27 April) taken from dates close to the data set used; and it was concluded that the models created with 16 points were successful for both Chl-a and turbidity estimation for this preliminary study.

This study investigated the taxonomic composition, abundance, and biomass values of mesozooplankton in the marine area in front of two important rivers (Sakarya River and Yeşilırmak River) and the largest port located on the Black Sea coast in Türkiye (Samsun Port) in July 2019 and January 2020. The average mesozooplankton abundance and biomass were 4187.3 ind. m-3 and 89.7 mg m-3 in Sakarya River, 3638.5 ind. m-3 and 78.2 mg m-3 in Samsun Port, and 3327.6 ind.m-3 and 77.6 mg m-3 in Yeşilırmak River, respectively. In July 2019, the highest abundance value of mesozooplankton (8581 ind. m-3) was recorded at SAK08 station off Sakarya River due to the copepod $Acartia$ $clausi$ (3279 ind. m-3). In July 2019, the highest biomass value of mesozooplankton (209.34 mg m-3) was found at YSL07 station off Yeşilırmak River with the contribution of the copepod $Centropages$ $ponticus$ (77.90 mg m-3). In January 2020, the highest abundance and biomass values of mesozooplankton (4035 ind. m-3 and 66.45 mg m-3) were detected at SLI05 station off Samsun Port due to copepod $Acartia$ $clausi$. A difference in mesozooplankton species composition between the two sampling periods was identified. While Cladocera species and copepod $Acartia$ $tonsa$ were exclusive at the sampling stations in July 2019, copepod $Calanus$ $euxinus$, $Pseudocalanus$ $elongatus$ and $Oithona$ $similis$ were observed at the sampling stations in January 2020. Also, in January 2020, the presence of freshwater Cladocera species was detected off Sakarya River. The changes in biodiversity were determined to depend on temperature changes and riverine input.

İzmit Bay, a 50 km long inlet at the eastern end of the Marmara Sea, is crucial for the region’s economy, culture, and marine ecosystem. The bay’s water circulation regulates nutrient distribution, stratification, sedimentation, oxygen levels, heat, and pollution levels. It is also influenced by meteorological events, such as short-term moderate to strong wind conditions. This study investigated the sensitivity of İzmit Bay Water Circulation to wind speed, direction, and duration using the MITgcm model with Orlanski boundary conditions and process-oriented modeling. The simulations showed that under weak forcing conditions, seawater temperature, salinity, and stratification do not significantly vary. However, strong forcing and wind speeds (statistically defined by percentiles of observation data) of 4.9 m/s (75%), 6.7 m/s (90%), and 10.1 m/s (99%) generate significant mesoscale and sub-mesoscale processes, depending on the direction. Westerly component winds cause downwelling at the eastern coastline, while easterly component winds bring sub-surface water to the surface. Strong winds from N, NE, and E sectors lead to the rise in lower-layer waters in the western basin, forcing them to overflow through the Hersek Delta sill into the central basin. Overall, severe wind events greater than 4.9 m/s (75%) significantly affect the bay’s hydrography by transforming the upper layer, with a decrease in temperature up to 5 °C and an increase in salinity up to 10 ppt.

Based on in situ measurements between 2017 and 2022 in the Sea of Marmara, three "warm and saline" upwelling events were observed during autumn and winter scientific expeditions. These observations were crucial and worth mentioning since the surface salinity value increased and even reached the lower layer value, which is ~38 psu, during the process. Other characteristics of the lower layer were also detected, either as they are or in between upper- and lower-layer values due to mixing. After analysing the effects of coastal upwellings on the upper layer, it was concluded that coastal upwelling, which has been underestimated for a long time, has to be well studied because it could be another reason or way of nutrient enrichment in the upper layer and salinity increase in winters. This phenomenon also has huge potential to differ the upper layer characteristics with a rise in frequency. Therefore, it is advised that coastal upwelling should always be considered with other well-known features during marine studies and for future engineering solutions in the basin.