Carbon, nitrogen, silicium and phosphorus nutrients in the eastern part of the Gulf of Trieste (northern Adriatic)

Abstract

       The distribution of inorganic carbon, nitrogen, silicom and phosphorus nutrients in the Eastern part of the Gulf of Trieste (Northern Adriatic - Yugoslavia) were investigated in the period from March 1978 - July 1979 monthly using standard methods. From the results it is evident that these nutrients, essential for primary bioproduction, are widely distributed in this area, origi­nating from rivers, rain, run-off, regeneration processes, excretion and local sewage discharges, particularly in the inner part of the Bay of Koper. Some­what higher concentrations were observed in the Bay of Piran (Station MA) and Bay of Koper (Station K-1) and in the bottom layer in the vicinity of the Piran submarine sewage outfall (Station PI) in comparison with the open sea environment (Station 14). The highest concentrations of nitrogen, silicon and phosphorus nutrients were detected in the Rižana and Dragonja estuaries, as well as in the inner part of the Bay of Koper in the civinity of the Koper sewage discharge (Station 8). The mean values of nutrient concentrations in the Gulf of Trieste were up to three times higher than those reported from the Southern and Central Adriatic, and approximately equal to those in the Western part of the Northern Adriatic under the influence of the river Po.

        A comparison was made in the determination of total CO₂ using the direct determination by means of the manometric Van Slyke apparatus, and indi­rectly via alkalinity and pH, when temperature and salinity were known. It is evident from the results that the indirect determination when applied to the waters of the Gulf of Trieste, causes nonnegligible errors due to the noncarbonate contributions to the alkalinity, probably from organic matter.

       In the seasonal dynamics of nitrogen and phosphorus nutrients the highest concentrations were observed in early spring, while those for silicon occured in early autumn. The temporal dynamics of total CO₂ exhibited a maximum in winter due to the enhanced dissolution of atmospheric CO₂ in seawater, and a minimum in early summer due to increased photosynthetic activity. The inverse correlation between ammonia or nitrate and phytoplankton cell density suggested these inorganic nitrogen compounds as the most effective nitrogen nutrients in phytoplankton assimilation, although was not possible to identify the most active inorganic nitrogen nutrient.

       From the high atomic C : N : Si : P ratio in seawater from the Gulf of Trieste it was concluded that phosphorus is the limiting nutrient for primary bioproduction in the Gulf of Trieste although the role of the high concentra­tions of total phosphorus in the upper layers (0 - 2 cm) of sediments remains unexplained. To resolve these questions, kinetic studies using labelled com­pounds are suggested.