Concave islands: Habitat heterogeneity of parafluvial ponds in a gravel-bed river

Floodplain pond distribution, density, and diversity were investigated along the active river corridor of the Tagliamento River in northeastern Italy, the last major semi-natural river in Central Europe. Along the corridor, ponds peaked in bar- and island-braided floodplains but were absent in constrained sections. Within the main study area, a 1.0-km(2) large braided floodplain, the number of ponds containing water ranged between 18 and 39 depending on the water level in the main channel. Thermal properties and water-level fluctuations were the most important variables determining pond characteristics and heterogeneity. Results from a PCA on environmental variables distinguished four groups of ponds distributed along a hydrologic and thermal gradient. Parafluvial ponds are short-lived, discrete, aquatic "islands" within the floodplain matrix. They are expected to contribute disproportionately to aquatic biodiversity; however, they are very sensitive landscape elements that disappear as a consequence of river regulation, wood removal, and flow control.