Despite the extensive and rapidly growing literature on microplastics in oceans and coastal seas, little information exists on microplastic distribution through the salinity gradient. This study is the first one to evaluate microplastic distribution through the salinity gradient of a highly stratified estuary. A total of 910 microplastic particles were collected from 12 different sampling events in the Krka River estuary, Croatia. The number of detected particles ranged from 389 in the surface layer to 63 in the deepest marine layer. The highest plastic abundance was found in the surface layer (3.68 particles/m³) and the lowest in the marine layer (0.13 particles/m³). The measured values of the cross-sectional area indicated an ellipsoidal cross-sectional shape of the particles. It was also found that the majority of microplastic particles belonged to the small microplastic class (<1 mm). The Nile Red (NR) staining method was used to visualize fluorescent microplastic particles, while quantification was performed using ImageJ/Fiji software. The strong salinity stratification in the studied estuary did not alter the usual distribution of microplastic particles in the water column, and there was no significant accumulation on the halocline.

Microplastics (MPs), commonly defined as particles less than 5 mm, are a persistent ubiquitous anthropogenic contaminant that can be found in every environment, making it a global environmental, health, and socioeconomic problem. Due to their high surface area, MPs adsorb toxic pollutants that become bioavailable to organisms upon ingestion as they are often mistaken for food leading to biomagnification (Bule et al., 2020). The sampling area represents the lower part of the Krka River Estuary and is under direct anthropogenic influence from the city of Šibenik runoff waters, nautical and communal ports, city harbor, tourism, mariculture, and fishing. Estuaries and harbors have been recognized as hotspots and transfer pathways for MPs primarily because of the vicinity of the urban environment that emits contaminants from various sources (Miller et al., 2021). The main focus of this research was to determine MPs size, shape, color, surface area, and abundance in surface water using volume-reduced samples collected by a net. Laboratory protocol included sieving, wet peroxidation (H2O2), density separation (saturated NaCl solution), sonication, and filtration. Filter papers were then visually inspected for MPs. Image processing and measurements were carried out with ImageJ/Fiji open-source software.

This study evaluates the effectiveness of Sentinel-2 MSI imagery, corrected using the Case-2 Regional Coast Colour (C2RCC) algorithm in SNAP software (Sentinel Application Platform), for estimating chlorophyll-a (Chl-a) and total suspended solids (TSS) in Deran Lake, a karstic lake with minimal anthropogenic pressure within Hutovo Blato Nature Park, Bosnia and Herzegovina. In situ measurements were collected in March 2025 using a YSI EXO2s multiparameter probe at ten monitoring stations under the SMART-Water project. TSS was used as a proxy for total suspended matter (TSM). A regression analysis between satellite-derived and measured values showed a strong correlation for TSS/TSM (R² = 0.76) and a moderate correlation for Chl-a (R² = 0.61). Spatial averaging over 5×5-pixel windows improved estimation accuracy, yielding R² values of 0.85 for TSS/TSM and 0.69 for Chl-a. Thematics maps of SNAP-derived Chl-a and TSS/TSM distribution was performed using QGIS 3.40. Chl-a concentrations confirmed oligotrophic conditions, while TSS patterns aligned with known hydrodynamic features such as inflow zones and sediment resuspension areas. These findings align with other regional studies demonstrating Sentinel-2’s potential for monitoring small and optically complex waterbodies when paired with appropriate atmospheric correction, statistical estimators (e.g., mean), and spatial windowing strategies. This work reinforces the value of remote sensing for cost-effective, high-resolution monitoring of inland waters, improving water resource management across diverse ecosystems.