La nature en ville : l’improbable biodiversité Nature in the city: the unlikely biodiversity

Human perception of riverscapes with and without large wood (LW) has been investigated with a photo‐questionnaire submitted to 2250 students in ten countries, capturing reactions to 20 pictures in terms of naturalness, danger, aesthetics and need for improvement. Principal component analysis performed on average scores per country per item showed that the primary discriminating factors are human modification, turbulence and extent of water shown in the scenes. Wood discriminated the perception of student groups, but other factors are also critical. Features associated with human activity (channel deepening and straightening, rip‐rapping) were perceived to be less aesthetically pleasing than wood. Perception of the most dangerous riverscapes is not specifically affected by wood, but by turbulent flows and wide rivers. The presence of wood clearly discriminated the need for human intervention. Perceptions differed among countries, reflecting different cultural contexts. Students from Germany, Sweden and Oregon perceived LW more positively as a natural and wild component of watercourses. These perceptions are linked to positive attitudes towards natural riverscapes. Other students, such as those from China, Russia or India, perceived rivers with wood as needing regulation and maintenance. Whatever the socio‐cultural context, large wood elicits an emotional influence on how one perceives riverscapes.

Green tides of macroalgae have been negatively affecting the coasts of Brittany, France, for at least five decades, caused by excessive nitrogen inputs from the farming sector. Regular areal estimates of green tide surfaces are publicly available but only from 2002 onwards. Using free and openly accessible Landsat satellite imagery archives over 35 years (1984–2019), this study explores the potential of remote sensing for detection and long-term monitoring of green macroalgae blooms. By using a Google Earth Engine (GEE) script, we were able to detect and quantify green tide surfaces using the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) at four highly affected beaches in Northern Brittany. Mean green tide coverage was derived and analyzed from 1984 to 2019, at both monthly and annual scales. Our results show important interannual and seasonal fluctuations in estimated macroalgae cover. In terms of trends over time, green tide events did not show a decrease in extent at three out of four studied sites. The observed decrease in nitrogen concentrations for the rivers draining the study sites has not resulted in a reduction of green tide extents.