River Continuity Restoration and Diadromous Fishes: Much More than an Ecological Issue.

Ecosystem fragmentation is a serious threat to biodiversity and one of the main challenges in ecosystem restoration. River continuity restoration (RCR) has often targeted diadromous fishes, a group of species supporting strong cultural and economic values and especially sensitive to river fragmentation. Yet it has frequently produced mixed results and diadromous fishes remain at very low levels of abundance. Against this background, this paper presents the main challenges for defining, evaluating and achieving effective RCR. We first identify challenges specific to disciplines. In ecology, there is a need to develop quantitative and mechanistic models to support decision making, accounting for both direct and indirect impacts of river obstacles and working at the river catchment scale. In a context of dwindling abundances and reduced market value, cultural services provided by diadromous fishes are becoming increasingly prominent. Methods for carrying out economic quantification of non-market values of diadromous fishes become ever more urgent. Given current challenges for rivers to meet all needs sustainably, conflicts arise over the legitimate use of water resources for human purposes. Concepts and methods from political science and geography are needed to develop understandings on how the political work of public authorities and stakeholders can influence the legitimacy of restoration projects. Finally, the most exciting challenge is to combine disciplinary outcomes to achieve a multidisciplinary approach to RCR. Accordingly, the co-construction of intermediary objects and diagrams of flows of knowledge among disciplines can be first steps towards new frameworks supporting restoration design and planning.

Trajectories of river chemical quality issues over the Longue Durée: the Seine River (1900S-2010).

River quality trajectories are presented for (i) organic pollution, (ii) eutrophication, (iii) nitrate pollution, and (iv) metal contamination over the Longue Durée (130 to 70 years). They are defined by a quantified state indicator (S) specific to each issue, compared to drivers (D) or pressures (P) and to social responses (R) that reflect the complex interactions between society and river quality. The Lower Seine River, naturally sensitive to anthropogenic pressures, greatly impacted by Paris urban growth, industrialization, and intensive agriculture, and well documented by the PIREN-Seine 25-year research program, was chosen to illustrate these trajectories. State indicators, dissolved oxygen, algal pigments, nitrate, and heavy metals (Cd, Cr, Hg, Pb, Zn) in sediments have only been monitored by river basin authorities since 1971. Therefore, their past changes have been reconstructed using three approaches: (i) reassessment of historical sources, (ii) pressure-state models that reconstruct past water quality, and (iii) sedimentary archives of past persistent contamination from dated floodplain cores. The indicators were then transformed into river quality status using contemporary water quality criteria throughout these records. Each environmental issue shows specific trajectories because each has its own relationship between the issue evidence and the social response, but all are characterized by very poor quality in the past, largely ignored: the long-term summer hypoxia (<1880-1995), the summer eutrophication peak (1965-2005), the growing nitrate level since the 1950s, recently stabilized but still high, and the extreme metal contamination (>1935-2000) that peaked in the 1960s. The efficiency of social responses has been highly variable but more efficient in the last 15-25 years.