Field observation of the axial distributions of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) contents, as well as of ammonia, nitrite, nitrate, phosphate, O₂ and d¹⁸O in water, were conducted at different seasons in the highly stratified Riiana estuary (Gulf of Trieste, Northern Adriatic) from the freshwater entrance to its mouth (- 35 salinity). The upper estuary contains high levels of nitrate. Distinctive mixing patterns, as illustrated by chemical property - salinity plots, show non-conservative mixing pattern for all constituents, except for nitrate during high to normal flow of the river and at lower temperatures. In these cases, a nearly-conservative mixing of nitrate in the estuary was observed. High values of DON, DOP, ammonia and phosphate found in the middle of the estuary are the consequence of the primary treated sewage inflow and as such is considered as an estuarine source. The estuarine removal of DOC is controlled by mineralization and chemical precipitation. The sequential appearance of DON, ammonia and nitrite and nitrate, and DOP and phosphate in parallel with increased bacterial density indicates that these chemical constituents in the estuary are bacterially controlled. DON and DOP are mineralized and ammonia is nitrified. The oxygen level is near saturation throughout the estuary, except in the summertime. In this period, characterised by a low flow rate and a high water temperature, oxygen and nitrate depletion prevail, showing that during such stratified periods denitrification proceeds in the upper layer of the Riiana estuary. Using a one-dimensional geochemical modelling approach it was concluded that the Riiana estuary is a moderately efficient recycling system for DOM and phosphate, but rather inefficient for inorganic nitrogen. The behaviour of DOM is bacterially controlled and related to the water temperature and the river discharge.