Geochemical assessment of an M-100 fuel oil spill in the Black Sea (December 2024): Elemental signatures in seawater and macroalgae inform a novel bioindicator

This study investigates the elemental composition of seawater and macroalgae (red, green, and brown species) in the Russian Black Sea coastal zone following an M-100 fuel oil spill in December 2024. Our findings indicate a limited transfer of chemical elements from the fuel oil into the dissolved phase of the ambient marine environment. Conversely, the sorption and subsequent bioaccumulation of chemical elements by macrophytes proved significantly more pronounced. The most substantial accumulation was consistently noted in the brown alga Ectocarpus siliculosus. The clear and measurable differentiation between spill-affected and unaffected areas, based on the algal elemental signature, facilitated the development of a novel geochemical indicator of fuel oil pollution (GIP). The GIP is defined as the ratio of the sum of the elements concentration coefficients (Сc – multiplicity of elements' contents excess relative to the background) associated with fuel oil pollution (Mn, Co, and rare earth elements) to the sum for elements characteristic of standard shipping-related pollution (Cd, Zn, V, Ni). A GIP value exceeding one signifies the presence of fuel oil contaminants in the water column and associated biota, enabling a clear distinction from pre-existing background anthropogenic inputs.