Temporal changes in fish diversity in lotic and lentic environments along a reservoir cascade

Abstract

Long‐term modifications in hydrological conditions due to river damming result in varied effects on fish diversity, which can be stronger in a reservoir cascade. Therefore, we assessed changes in fish diversity over a 28‐year period, in a series of reservoirs along the Araguari River Basin, a Neotropical drainage in Brazil, in both lentic environments (reservoirs) and the lotic stretches between them. We used linear mixed models to assess long‐term changes in fish diversity. Diversity was measured as α diversity (fish species richness) and β diversity (Jaccard's and Raup–Crick's dissimilarity index). Beta diversity was then partitioned into turnover (species replacement) and nestedness (species loss). To understand fish species replacement along the cascade, we modelled turnover with four covariates: age, richness of native and non‐native fish species, and longitudinal position of sampling site along the cascade. The reduction of total richness and the increase in non‐native richness were related to reservoir age. These effects were similar in both lotic and lentic environments, with no significant difference between them. We also observed a decrease in native fish richness over time, with a slight tendency for lentic environments to lose more species than lotic environments. Dissimilarity in fish assemblage increased over time and β Raup–Crick results corroborated those derived from β Jaccard, yielding similar significant temporal trends. Changes in fish species composition occurred predominantly due to turnover, compared to nestedness. We observed a greater species loss in reservoirs, suggesting that the transformation from a lotic environment into a lentic had greater impact on fish assemblages than the flow regulation of the lotic stretches downstream of the dams. Our results suggest that species replacement can be explained mainly by the increase of non‐native fish richness, followed by the longitudinal position along the cascade in an upstream–downstream direction, and reservoir age. This study reveals that, even after 2 decades of monitoring, we still observe important changes in fish assemblages, demonstrating the importance of long‐term monitoring to assess biodiversity. Our findings reinforce the importance of long‐term fish monitoring schemes for the investigation of the effects of non‐native fish species on native fish assemblages.

The species–area relationship for a highly fragmented temperate river system

“…If this is indeed one of the first studies to document a riverine fragment SAR, more case studies are needed to establish whether the Trent River is a unique case, or whether steeper‐than‐average slopes are typical of dammed river fragments. While we acknowledge that dam density on many river networks may be lower than observed in the Trent River, settings with dense networks of dams operating together (i.e., cascade dams) are present in many places (Loures and Pompeu 2019, Zhang et al 2019). In addition to verifying the existence of additional river fragment SARs, explicit tests are warranted of causal drivers of SARs in fragmented rivers, and the role of tributaries for mitigating negative demographic effects on small population sizes within fragments.…”

“…Total species richness and richness of functional guilds are frequently used metrics of community change in studies of river fragmentation (Loures and Pompeu 2019), yet very little work has connected species richness within river fragments to the extensive literature on species–area relationships (SARs). The observation of increasing species richness with increasing area is a foundational pattern in ecology that has been a central theme in biogeography theory (MacArthur and Wilson 1967, Rosenzweig 1995, Hubbell 2001, McGill et al 2006, Whittaker and Fernández‐Palacios 2007).…”

Effects of upstream cascade dams and longitudinal environmental gradients on variations in fish assemblages of the Three Gorges Reservoir

“…This not only transforms many river reaches into lentic environmental areas with unnoticeable current velocity, but also produces multiple “barrier effects” resulting from the dam blocking and ecological filtering effect of large reservoirs (Nobile et al., 2019; Pelicice et al., 2015). Considering the remarkably altered habitats, the reservoir area has always been one of the important research areas to determine the short‐ and long‐term changes in fish assemblage structure (Franssen & Tobler, 2013; Gao et al., 2010; Loures & Pompeu, 2019; Sá‐Oliveira et al., 2015).…”

“…Moreover, after impoundment, numerous exotic fish species emerge in the reservoirs and become the dominant taxa (Lin et al., 2019; Maceda‐Veiga et al., 2010; Santos et al., 2017). In addition, the long‐term effects of non‐native fish species on native fish assemblages in reservoirs have been investigated (Loures & Pompeu, 2019).…”

“…Few studies have determined the actual effects of upstream cascade reservoirs on the variations in the composition and abundance of fish assemblages in three distinct zones of a downstream reservoir, and most of these studies have focused on the effects resulting from the impoundment of a reservoir itself (Gao et al, 2010; Sá‐Oliveira et al, 2015; dos Santos et al, 2018; Lin et al, 2019; Loures & Pompeu, 2019). Whether upstream dams and reservoirs can significantly affect the variations in fish assemblages of a downstream reservoir, as well as whether they have different degrees of impact on fish assemblages of the three distinct zones (riverine, transitional, and lacustrine zones; Thornton et al, 1990) of a downstream reservoir, is not yet known.…”