Preliminary chemical and hydrological observations in the Ionian Sea

Abstract

The paper contains the results of the investigation on nutrients and hydrological parameters at 11 stations at standard depths covering all the water column in the Ionian Sea in March 1983. The following characteristics can be observed from the vertical distribution of dissolved oxygen during the Spring: from 0 to 75 m the waters are always saturated; a maximum gradient depletion occurs between 75-150 m, where easily oxidizable material is decomposed and respiration processes prevail; the concentration gradient is very small from 150 m to the bottom. The distrubution of nutrients can be considered as a normal pattern for Mediterranean waters. In the euphotic zone, nutrients are practically depleted by the phytoplankton uptake. The oxidation of organic material induces a progressive enrichment of reactive phosphate, nitrate and silica from 100 m to the bottom. A nitrate maximum is observed at the compensation depth for photosynthesis. The N:P and Si:P atomic ratios are characterized by a wide variability with depth at the euphotic zone. On the contrary, from 75 m to about 80 m, the ratios increase and afterwards they tend to constant value of 21 for N:P and 35 for Si:P. Some conclusions are also drawn about the levantine and deep waters filling the Ionian basin.