Monthly riverine load of inorganic nutrients and carbon into the Gulf of Trieste (north-eastern Mediterranean Sea): insights from flood and drought periods

River discharge of freshwater and nutrients regulates coastal ecosystems, as it exerts a significant impact on their hydrology, biogeochemistry and productivity. Moreover, rivers act as important sources of carbon dioxide (CO₂) to the atmosphere and play an important role in the estuarine carbonate system.

Hydrology, biogeochemistry and productivity of the Gulf of Trieste (GoT), a shallow semi-enclosed basin of the northernmost part of the Mediterranean Sea, are mainly influenced by rivers, which represent the major allochthonous source of freshwater, total alkalinity (TA) and nutrients in the area. The Isonzo River is the main freshwater input into the GoT. It generally shows significant seasonal variations in discharge, with two main flooding periods related to snowmelt and rainfall, developing a turbidity plume strongly influenced by wind. The second freshwater source to the GoT is the Timavo River, which flows underground for about 38 km before re-emerging in proximity of its mouth. The Timavo has particularly complex hydrological characteristics related to its karstic nature, as the flow at the mouth is also influenced by underground circulation within the karst aquifer, moreover several minor springs are scattered along the coastline. Despite the relevance of the input of these rivers to the GoT, data on discharges of TA, dissolved inorganic carbon (DIC) and nutrients are scarce and fragmented. The aim of this study is to fill these knowledge gaps by providing a monthly biogeochemical characterisation of Isonzo and Timavo in terms of nutrients and carbonate system parameters, to shed light on their dynamics at the end of the catchment and estimate the input of nutrients, alkalinity and DIC into the marine system. Here we present the data from monthly sampling carried out during a dry (2022) and a rainy (2023) year at the mouth of Isonzo and Timavo rivers. Considering the karstic nature of Timavo, samples were also collected at one of its underwater springs.

The average annual discharge of AT, DIC and nutrients under different hydrological conditions (2022 vs. 2023) highlighted that riverine nutrients and inorganic carbon load is highly related to the runoff which strongly varies on interannual scale. Moreover, we observed that the Timavo waters generally had lower pH and higher DIC and TA values than Isonzo, likely as a consequence of the different catchment basins and the nature of the rivers’ course (hypogeous vs. surface). The implementation of this study, which is still ongoing, will continue over the next two years. The results will be also useful to assess the real influence of the numerous coastal and underwater springs of the Timavo River, which are often underestimated or neglected, on this coastal ecosystem. As river discharge dynamics are one of the main drivers influencing the biogeochemistry of marine ecosystem, our results will provide a basis for assessing the impact of rivers on the GoT, and support future studies on oligotrophication and acidification.