Organic matter stoichiometry based on oxygen consumption-nutrients regeneration during a stagnation period in Jabuka Pit (middle Adriatic Sea)

The Jabuka Pit, a continental shelf depression in the middle Adriatic Sea, acts as a collection basin for the dense water formed during winter in the northern Adriatic. Its deep waters are usually renewed at least every second year. The deep waters of Jabuka Pit remained practically isolated after the strong flushing occurred in early spring 1993. Our data from May 1993 to November 1994, permitted to follow, step by step, the oxygen consumption and the nutrient and carbon regeneration in the deep layer of Jabuka Pit during this period of isolation. Extremely high oxygen consumption and nutrients and carbon regeneration rates were estimated; the oxygen reduction rate is about 66.5 μmol/kg/yr, while the accompanying increasing rate of nitrate is 2.64 μmol/kg/yr, of silicate 4.2 μmol/kg/yr, of phosphate 0.24 μmol/kg/yr and of ΣCO2 81.0 μmol/kg/yr. The temporal evolution of the regeneration ratios of nitrogen and carbon, as well as of the Ninorg/PO4 ratio, imply that possibly other processes, besides aerobic respiration, like denitrification, take place and modulate the values of the ratios. Furthermore, the changes of the concentrations of oxygen, nitrate, phosphate and ΣCO2 in the deep layers of Jabuka Pit were exploited in order to estimate the elemental composition of the organic matter that is remineralised. Our results suggest that the most probable empirical formula that corresponds to the composition of the organic matter in the study system is (C4H6ON)8; (C6H10O5)3; (CH2)108 being 11.4% carbohydrates, 20.3% proteins and 68.3% lipids. Furthermore, the investigation of our results show that there is evidence that the observed changes of the concentrations of the chemical parameters are better interpreted if additionally to the remineralisation of organic matter, small-scale CaCO3 dissolution with simultaneous denitrification processes are considered.