Long-term dynamics of water-borne nitrogen, phosphorus and suspended solids in the lower Don River basin (Russian Federation)

A long-term study (1986–2002) of water-borne nutrient and suspended solids dynamics was undertaken on the lower Don River, which plays an extremely important role in the water supply of the Black Sea and Azov Sea basin. Suspended solids were greatest in spring and summer and were correlated to river discharge. Mean annual nitrogen concentrations increased from 1986 to 1995 and then decreased from 1996 to 2002. Unlike nitrogen, phosphorus concentrations (both phosphates and total phosphorus) gradually increased throughout the study period changing the river from an oligotrophic to upper mesotrophic status. If this trend continues phytoplankton could become nitrogen-limited leading to the development of nitrogen-fixing cyanobacterial blooms. No obvious relation between fertiliser usage over the Rostovskaya Oblast and nutrient dynamic patterns was identified, probably because only 10% of the water in the river comes from this area. The reason for the unusual and contradictory nitrogen and phosphorus changes remain largely unknown for this regulated river.