Nitrogenous discharges to the eastern Gulf of Finland, the Baltic Sea: Elemental flows, stable isotope signatures, and their estuarine modification

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We studied the nutrient input to the Gulf of Finland via River Neva, the largest river discharging freshwater to the Baltic Sea, and characterised the isotopic signatures ( 15N, 18O, 13C) in dissolved and particulate substances (NO 3 −, PON, POC, DIC) in the River Neva over two seasonal cycles, as well as in samples from St. Petersburg wastewater treatment plants (NO 3 −, NH 4 +, PON, POC). These riverine and municipal discharges account for 40% of terrestrial inorganic N loading to the Gulf of Finland, representing annually 7% of the total nitrogen pool in the water mass of the whole Gulf. To describe and evaluate the modification of these isotopic signals along a Gulf of Finland transect towards the Baltic Proper, two cruises were arranged, one in late spring after the annual maximum in River Neva runoff, and one in autumn, in the late phase of the annual growth season. River Neva nitrate signatures of 15N and 18O indicated major agricultural fertilizer origin of nitrogen, and the isotopic composition was clearly lighter (δ 15N-NO 3 − mean of 2.4‰ air) than previously measured from more southern rivers discharging into the Baltic Sea. Because of the light composition of the River Neva N source, close to the 15N signatures of the open Gulf, as well as of the efficient depletion of the inorganic load already in the innermost estuary, straightforward end-member tracer analysis of the transport of N in the basin is problematic. St. Petersburg wastewater ammonium showed, however, high δ 15N values (ca. 13‰), which gives a first estimate of 5.8‰ for δ 15N of the easternmost estuarine total inorganic N source. The available sediment data from the basin (δ 15N 6 to 8‰) somewhat exceeds the average source signature. This emphasizes the significance of biological transformation processes, most importantly assimilation of inorganic nitrogen, food web interactions and denitrification, which all involve isotopic fractionation, for the mass balance models describing the dynamics of the sources and sinks of the N cycle of the basin.