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The hazardous substances listed in the EU Water Framework Directive (Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for the Community action in the field of water policy) as priority and priority hazardous substances, HELCOM substances of specific concern to the Baltic Sea and some other pollutants which our team deemed potentially problematic for the Baltic Sea were analyzed in the botom boundary layer (BBL) in some coastal areas of the Gulf of Finland (GOF). All of the substances under observation are persistent, toxic, bioaccumulative and, because of these properties, are hazardous to the water environment. Chemical analyses indicated that the content of the most toxic HELCOM metals -Hg, Cd and Pb was very low at most stations, but considerable amounts of Zn and Cu were found at some stations. The highest concentrations of PCDD/Fs (WHO-TEQ 2005 value up to 6 ng/kg d.m.) and dl-PCB-s were found in some deeper areas of Tallinn Bay and Muuga Bay. Based on the circumstance that the maximum concentrations of all investigated metals and compounds were found in lower layers of sediments (6 -15 cm), it may be concluded that the environmental status of the catchments and also of the sea has improved considerably during the last decades.The results on the content of toxic metals in Estonian sediment samples are in good agreement with those obtained by other investigators of the Baltic Sea and the Gulf of Finland. It seems that the environmental status of the near bottom water and bottom surface sediment is quite good in the North-Estonian coastal sea area.

The Gulf of Riga is a shallow basin in the eastern Baltic Sea connected to the Central Baltic Sea via shallow straits. Seasonal oxygen depletion occurs in the deep layer of the gulf. We conducted hourly measurements of dissolved oxygen, temperature, and salinity in the deep layer (50 m) of the gulf and observed the full cycle of development and relaxation of hypoxia in 2021. Hypoxia (<2.9 mg l-1) first occurred on 27 June and was observed for 71 days until its complete decay on 22 October. Average oxygen decline of 0.10 mg l-1 d-1 from saturation in mid-April until mid-July and 0.04 mg l-1 d-1 onwards until the end of August were observed. This seasonal pattern was superimposed by short-term variability in time scales from hours to days and was probably caused by inertial oscillations, (sub)mesoscale processes, deep layer currents, and pycnocline movements. Ventilation events with a relatively low impact and duration of up to ten days occurred in the deep layer due to the inflows of the saltier water from the Central Baltic. The inflowed water originated from the upper layer in winter and the thermocline in summer and was almost saturated in oxygen. Mostly mixing with existing oxygen-depleted water in the Gulf of Riga, but also local consumption declined the oxygen levels in the inflow water before it arrived at the observing station. Monthly standard deviations in oxygen varied from 0.3 to 2.8 mg l-1 and illustrated the added value of Eulerian measurements to complement the conventional monitoring.