Long-Term Assessment of Trophic State and Environmental Drivers in the Gulf of Trieste (Northern Adriatic Sea) Within the MSFD (D5) Implementation

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Abstract
The trophic status of coastal environments is largely controlled by nutrient inputs, particularly nitrogen and phosphorus, whose excess may lead to eutrophication. The northern Adriatic Sea has historically been affected by these processes, with notable impacts on water quality. This study analyses a time series (2015–2024) collected at six offshore sites in the Gulf of Trieste within the Marine Strategy Framework Directive (MSFD) Descriptor 5. Dissolved Inorganic Nitrogen (DIN) showed marked spatial variability, with 19.9% of samples exceeding the threshold of 6.85 µM. Phosphate concentrations were low (mean 0.17 µM), indicating strong P-limitation (mean N:P ratio = 277). Chlorophyll a concentrations (mean 0.9 ± 0.1 µg L−1) reflected oligotrophic conditions, although 17% of samples exceeded 1.5 µg L−1. Time-series analyses revealed a significant warming trend (+0.1 °C yr−1; p = 0.022) and a significant decrease in chlorophyll a (p = 0.038), while no significant trends were observed for nutrients, dissolved oxygen or TRIX. TRIX values (0.8–6.9) indicated overall good to high ecological status. A trophic–hydrological gradient highlighted the positive influence of river inputs and precipitation on nutrient availability and trophic conditions. These results provide a solid quantitative baseline for MSFD assessments and underscore the role of hydrological and meteorological forcing in shaping trophic variability in the Gulf of Trieste.
Funding Information
  • Italian Ministry for Environment, Land and Sea Protection in accordance with the Marine Strategy Project (2008/56/EC)

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