When Clear Water Lies: Filamentous Algae overgrow Macrophytes

As primary producers, submerged macrophytes depend on the availability of ambient light within the water column. This study evaluates underwater vertical PAR profiles measured in May 2025 at seven floodplain lakes and channels in the Danube Delta Biosphere Reserve, Romania, focusing on the depth of optimum light exposure rather than solely on minimal light requirements (euphotic depth). Alongside submerged macrophytes, dense accumulations of filamentous algae frequently develop in the water column, creating a ‘false transparency’ effect in which visually clear water overestimates the light available at the macrophyte canopy. Effective management and restoration of the Danube Delta therefore require supporting macrophyte communitiescapable of establishing and persisting under low-nutrient conditions. Only such resilient stands – free from filamentous algal overgrowth – can ensure that open-water transparency once again reflects true ecological quality.