Functional classification of artificial alluvial ponds driven by connectivity with the river: Consequences for restoration

Abstract

Chemical and biological functioning of artificial wetlands and lakes in alluvial plains is investigated based on the example of small artificial ponds located within dike fields along the Rhône River (France). The objective is to identify the diversity of metabolism in these artificial aquatic ecosystems and to propose a functional classification linked to the connectivity with the river. A comprehensive characterization of chemical and biological parameters in six shallow ponds was conducted, along with 4-month continuous monitoring of temperature and dissolved oxygen concentration, both in the sub-surface and at depth. Metabolism intensity is quantified using the diel oxygen method. Multivariate analysis of chemical and biological characteristics is performed to identify the functional heterogeneity between the ponds, which is shown to be consistent with the intensity of primary production (GPP) and ecosystem respiration (ER). Some ponds are permanently connected to the river, like backwater, and are very turbid, because of suspended matter and the presence of phytoplankton. The oxygenation is limited to the surface layer with anoxia rapidly building up at the bottom due to an intense ER. Ponds that are rarely connected become eutrophic with large concentration of phytoplankton and large GPP and ER values. Anoxia and even denitrification occur close to the bottom sediment. Finally, in the ponds with intermediate degree of connection with the river, macrophytes grow in clear water with limited amount of phytoplankton, resulting in a gradient of GPP and ER values, controlled by nutrient availability. Rehabilitation of various connection patterns with the river is thus crucial to maintain a functional diversity in large river alluvial plains.