Ecological Engineering 101 (2017) 612–624 Contents lists available at ScienceDirect Ecological Engineering jo ur nal home p ag e: www.elsevier.com/locate/ecoleng Spatially diverse recovery (1986–2012) of fish fauna in the Warta River, Poland: The role of recolonizers’ availability after large-area degradation Andrzej Kruk ∗ , Michał Ciepłucha, Grzegorz Zi˛ eba, Dagmara Bło ´ nska, Szymon Tybulczuk, Mariusz Tszydel, Lidia Marszał, Bartosz Janic, Dariusz Pietraszewski, Mirosław Przybylski, Tadeusz Penczak Department of Ecology and Vertebrate Zoology, Faculty of Biology and Environmental Protection, University of Lód´ z, 12/16 Banacha Str., Lód´ z 90-237, Poland a r t i c l e i n f o Article history: Received 26 November 2016 Received in revised form 16 January 2017 Accepted 18 January 2017 Keywords: Point-source pollution Impoundment Rheophilic fish Kohonen artificial neural network Self-organizing map (SOM) IndVal a b s t r a c t The Warta River (795 km long) is the largest tributary of the Odra (Oder) River. This study presents results from one of the best-documented long-term monitoring projects in Poland, based on three periods of electrofishing: 1986–88 (T1), 1996–98 (T2) and 2011–12 (T3). After decades of severe point-source pollu- tion, water quality has been improving since the early 1990s. However, the recovery of fish assemblages was recorded with a considerable delay, i.e. not in T2 but in T3. Species richness in T3 increased by ≥50% in relation to T2. The recovery process proceeded at a different pace in the upper (section X), middle (sec- tion Y) and lower (section Z) river courses. The good status recorded in X in T3 was qualitatively different from the good status observed in T1 as migratory and/or lithophilic species were less common. Section X, with relatively clean tributaries, was isolated from the rest of the Warta system by the Jeziorsko Reservoir constructed between X and Y in 1986 (without fish passage). Ichthyofauna in Y was not only in the poor- est condition but was also recovering very slowly because of migration barriers and polluted tributaries, resulting in a severe shortage of sources of recolonizers. The quickest positive changes were recorded in Z because of the beneficial role played by certain less degraded tributaries and the direct connection to the recipient Odra River. Significant increases in biomass were recorded in Z for both limnophilic and rheophilic species; the former may be linked to zones of stagnant water existing between groynes and the latter to fast-flowing water near groyne heads. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Historically, the distribution of organisms in undisturbed rivers followed natural gradients, of which the most distinct was the downstream gradient. It manifested itself in many mutually related variables, including (1) decreasing channel slope, water velocity and bed substratum coarseness, and (2) increasing channel size, discharge, summer water temperature, biological productivity and variability of dissolved oxygen concentration. As a result of the responses of fish assemblages to this gradient – namely a gradual increase in the abundance of some species and a decrease in others along the longitudinal profile of natural rivers – the concept of fish ∗ Corresponding author. E-mail address: a.kruk@biol.uni.lodz.pl (A. Kruk). fauna zonation of rivers was formulated (Friˇ c, 1872; Borne, 1877; Nowicki, 1883). However, soon after the popularization of this concept by Huet (1954, 1962), it was treated as historic for several decades. Its potential for explaining fish distribution decreased as the natu- ral gradients of physical and chemical parameters along rivers were seriously disturbed and fish assemblages started to follow human pressure gradients. The chemical variability of streams was impacted by toxic sewage, often discharged to surface waters in its untreated or pre-treated form (Kostrzewa and Penczak, 2002; Mourato et al., 2005; Jurajda et al., 2010), and by detergent release and run-off of natural and artificial fertilizers that were inten- sively used in agriculture (Zalewski et al., 2001; Décamps et al., 2004). The structure of river channels was also severely modified, which usually resulted in reduced diversity of habitats available for aquatic organisms. Rivers were deepened and straightened in order to make them navigable (Backiel et al., 2000; Hajós, 2001). For flood http://dx.doi.org/10.1016/j.ecoleng.2017.01.019 0925-8574/© 2017 Elsevier B.V. All rights reserved.