In this study, biochemical (nutrients, total phosphorus (TP), biological oxygen demand) parameters were determined seasonally at downstream points of the five regional rivers (major/small ones: Ceyhan, Seyhan, Goksu, Berdan, Lamas) flowing into wide shelf zone of the northeastern (NE) Mediterranean. Long-term chemical data obtained seasonally in the 2008-2015 period were examined to assess seasonal variations in the chemical concentrations and annual mass influxes to the studied coastal sites, leading to better understanding of impacts of riverine nutrient inputs on the development of eutrophication in the Mersin Bay coastal regions. Expectedly, seasonal and annual variations were recorded in both volume fluxes and nutrient concentrations of the major rivers (Seyhan, Ceyhan, Goksu). The higher nutrient concentrations were consistently recorded in late winter-spring periods. Silicate concentrations, ranging between 95-140 µM in 2008-2011 period, decreased by about 20-40% to 90-110 µM levels in 2012-2015 as the NO3 consistently increased by about 20%, leading to apparent decreases in the Si/NO3 ratio due to new dam constructions on the regional rivers and enhanced fertilization and domestic wastewater discharges to the rivers. The total annual nutrient loads of the regional rivers were calculated as TP: 1990 tonnes/yr, PO4: 1024 tonnes/yr, NO3: 19420 tonnes/yr and reactive-Si: 38780 tonnes/yr. This long-term decreasing trends in the reactive silicate loads of the nitrate-laden major rivers are very likely to modify Diatom/Dinoflagellate ratio of algal production and abundance in the phosphorus deficient NE Mediterranean shelf waters
