Dataset contains hydrochemistry data from the Romanian shelf waters (bottom depths between 15 and 224 m), in August 2021. Data were collected onboard R/V Mare Nigrum, during a cruise conducted within the national project "Multidisciplinary research to improve knowledge of the climate change and human pressure interaction and its effects on the Black Sea ecosystem" – PN 19 20 01 02. Thirty stations distributed along 4 main transects (Sf. Gheorghe-SE, Portita-SE, Constanta-SE, and Mangalia-E) were performed for water column investigations by the GeoEcoMar team. In situ measurements of physical-chemical seawater parameters were done at each sampling station using a CTD SBE 25, with SBE 32 Rosette. Seawater samples were collected with 5L Niskin bottles from different depths and further analyzed in the GeoEcoMar lab for nutrients, chlorophylls and phaeophytin, TOC, and total mercury.