Application of aerial digital photography for macrophyte cover and composition survey in small rural streams

Macrophytes are a key biological component in many fluvial ecosystems. In altered streams, they can become highly dominant features, covering extensive parts of the channel with cascading effects on stream conditions and biological composition. The aim of this study is to examine the possibility of using information collected by aerial digital photography-spectral analysis (ADP-SA) as an alternative, cost-effective methodology to the commonly used spectral procedure at a large (section) scale, and to the ground level visual survey (gridded frame) at a smaller (organism) scale. Cladophora glomerata and Nasturtium officinale, were selected as the targeted macrophyte species and classified at the same time (species relative cover) by using the three methodologies. Our findings show that the ADP-SA methodology was able to detect species and relative cover in similar accuracy (≤10% differences) at the two spatial scales. ADP-SA had a better spatial resolution than both the hyperspectral and the visual methodology (4cm vs. 1m and 10cm, respectively) and was capable to differentiate submerged from emergent plants. However, on a smaller scale, ground level work is suitable and essential for detecting rarer species and is not hindered by weather, canopy cover or multi-layered plant composition. ADP-SA can therefore add a cost-effective and nonsubjective practice to the existing tool kit of macrophyte surveys, particularly in small streams, which require high spatial resolution.