An analysis of factors affecting oxygen depletion in the northern Adriatic Sea

Abstract

        In the northern Adriatic Sea the occurrence of hypoxic conditions and related benthic mortalities have considerably increased in frequency for the last 20 years. The causal relationship of factors which create conditions near the bottom favourable for the development of hypoxia is discussed.

       The main causative agent of the northern Adriatic hypoxia is sedimentation of organic matter from the pelagic strata and subsequent benthic respiration on the bottom. During winter, the entire water column is well saturated in oxygen. Towards summer, the oxygen demand at the bottom increases with the increasing sedimentation and temperature while the reoxygenation through the pycnocline decreases with the increasing stability of the water column. As a consequence the oxygen content at the bottom decreases. The oxygen content of the bottom "cold pool" decreases also due to the fact that pycnocline gradually progresses towards the deeper parts of the water column. On occasion, it may lie just few metres above the bottom. The phenomenon of near bottom pycnocline is important in the shallower northern and western subareas. Interestingly, it is regularly associated with the occurrence of hypoxia in the Gulf of Trieste. The development of hypoxic conditions at the bottom coincides with the periods of low wind frequency. Apparently, the pronounced hypoxia may develop only if the wind speed at the surface is not higher than 4 m/s. Stronger wind tends to destroy stratification and this process seems to progress linearly with the increasing wind speed.

      Accordingly, the wind faster than 8 m/s is able to mix the water column down to the depth of 30 m, which is an average depth of the northern Adriatic Sea.

      To analyze relative importance between a set of factors which create conditions near the bottom, favourable for the development of hypoxia, a two-box mathematical model has been used. The model describes decay of organic matter as a function of primary production at some earlier time, where the time lag is caused due to the sinking of detritus from the euphotic zone to the bottom. The balance equation for oxygen is given as the difference between the resupply of oxygen across the pycnocline and the oxygen consumption due to benthic and epibenthic respiration. The decay constant depends primarily on ambient water temperature, critical oxygen flux is estimated from Fickian diffusive model where the vertical eddy coefficient has a nonlinear dependence on the stability of the water column. The model has been calibrated on the asis of 10-year data series. Comparing measured oxygen concentrations and the simulation results one can see that the model complies with the data relatively well. Simulations of perturbed dynamics gave an indication of relative importance of a set of ecological factors in formation of hypoxia.