From nutrients to nutrition: nitrogen pathways and seafood nutritional quality in the Black Sea - a systematic review

Small pelagic fish and bivalves from the Black Sea represent a primary dietary source of high-quality protein, omega-3 fatty acids (EPA and DHA), and bioavailable micronutrients. Whether this nutritional contribution is changing under ongoing environmental degradation remains an open and consequential question. A key but underexplored mechanism is the nitrogen pathway: how marine nitrogen dynamics — from riverine inputs through trophic transfer to biochemical incorporation — shape the nutritional quality of seafood remains poorly quantified. This systematic review, conducted in accordance with PRISMA 2020 guidelines, synthesises evidence on nitrogen pathways from ecosystem inputs to seafood nutritional composition. A structured literature search across Web of Science yielded 265 records, of which 100 peer-reviewed studies were selected for analysis and classified into core (n = 23), supporting (n = 59), and contextual (n = 18) evidence categories. The results reveal a pronounced imbalance: while nitrogen pathways at the ecosystem level are relatively well documented, direct biochemical measurements in key edible species — including horse mackerel (Trachurus mediterraneus), red mullet (Mullus barbatus), anchovy (Engraulis encrasicolus), and Mediterranean mussel (Mytilus galloprovincialis) remain scarce, and for sprat (Sprattus sprattus) are virtually absent. The available evidence suggests that eutrophication-driven shifts in phytoplankton composition and reduced trophic transfer efficiency may contribute to changes in seafood nutritional quality. These findings highlight a critical gap at the interface of marine biogeochemistry and nutritional science. Integrating direct biochemical assessments with ecological monitoring is essential for evaluating seafood as a source of high-quality nutrients and for supporting sustainable marine food systems.