water Article Nitrogen and Phosphorus Loads in Greek Rivers: Implications for Management in Compliance with the Water Framework Directive Konstantinos Stefanidis 1 , Aikaterini Christopoulou 2, Serafeim Poulos 3, Emmanouil Dassenakis 2 and Elias Dimitriou 1,* 1 Hellenic Centre for Marine Research, Institute of Marine Biological Resources and Inland Waters, 46.7 km of Athens—Sounio Ave., 19013 Anavyssos, Attiki, Greece;
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[email protected]; Tel.: +30-229-107-6389 Received: 8 April 2020; Accepted: 25 May 2020; Published: 27 May 2020 Abstract: Reduction of nutrient loadings is often prioritized among other management measures for improving the water quality of freshwaters within the catchment. However, urban point sources and agriculture still thrive as the main drivers of nitrogen and phosphorus pollution in European rivers. With this article we present a nationwide assessment of nitrogen and phosphorus loads that 18 large rivers in Greece receive with the purpose to assess variability among seasons, catchments, and river types and distinguish relationships between loads and land uses of the catchment. We employed an extensive dataset of 636 field measurements of nutrient concentrations and river discharges to calculate nitrogen and phosphorus loads. Descriptive statistics and a cluster analysis were conducted to identify commonalties and differences among catchments and seasons. In addition a network analysis was conducted and its modularity feature was used to detect commonalities among rivers and sampling sites with regard to their nutrient loads.