Fragmentation of lateral connectivity and fish population dynamics in large rivers

Abstract

Lateral Hydrological Connectivity (LHC) has been extensively fragmented in river–floodplain ecosystems of the world. Uncertainties about how LHC affects fishes are great, impeding the design of effective rehabilitation strategies. Existing conceptual frameworks do not provide sufficient mechanistic detail to support the novel decision problems river managers face. We offer a framework of how LHC affects fishes in river–floodplain ecosystems that is, process‐based, integrates all life‐stages and is spatial; these features, we argue, are required to assess risks and opportunities associated with different LHC rehabilitation strategies. Within river–floodplain segments, LHC affects population processes through five ‘effect‐classes’: effects of floodplain habitat (1) and channel habitat (2); effects of material subsidies from the floodplain to the channel (3) and vice versa (4); and effects of connectivity on lateral dispersal (5). The relative influence of these effect‐classes on processes varies among species and life‐stages. At the scale of the drainage basin, inter‐segment variation in geomorphology generates a longitudinal source‐sink structure to habitat quality and quantity, pointing to a need to better understand fish metapopulation dynamics in river–floodplain ecosystems. Given the significant investment in trying to restore river–floodplain ecosystems, we highlight potentially costly and ineffective LHC management decisions. These include certain heavily engineered LHC rehabilitation strategies that do not promote critical population processes at a local scale, and at basin scales implementing strategies that do not facilitate the metapopulation processes that promote species’ persistence.

River Damming Impacts on Fish Habitat and Associated Conservation Measures

“…Dam construction also reduces lateral connectivity of rivers, which decreases the interactions between rivers and floodplains/wetlands in river basins (Latrubesse et al., 2017; Stoffels et al., 2022), and thereby affects the productivity of floodplain and wetland ecosystems (Palmer & Ruhi, 2019). In the Atreyee River basin in India, the active floodplain area was reduced by 66.2%, and 48.9% of total pre‐dam wetland was completely obliterated, due to the reduction in lateral connectivity caused by the Mohanpur Dam (Saha et al., 2022).…”

Time for decisive actions to protect freshwater ecosystems from global changes

“…Finally, the loss of lateral hydrological connectivity between rivers and their floodplains through flood-control infrastructures (e.g. levees) to satisfy human demand for freshwater and infrastructure development, not only decreases habitat availability but also affects food webs and threatens species that benefit from facultative ontogenetic niche shifts between floodplain and main channel, jeopardizing at the end the entire ecosystem productivity (Opperman et al, 2009;Stoffels et al, 2022).…”

River connectivity increases the diversity of fish communities in gravel pit lakes

“…This landscape can be considered in four dimensions: longitudinal (upstream–downstream), lateral (river channel and floodplain), vertical (river channel and contiguous groundwater), and temporal (Ward 1989). Connectivity among the various elements of a river system, especially between a river and its floodplain, can influence aquatic community assemblages via immigration and emigration (Stoffels et al 2022). Periodic disturbances and connection events like floods facilitate the dispersal of individuals, facilitate linkages among fish communities, and modify community assemblages, resulting in increased species richness and diversity in some systems (Dembkowski and Miranda 2011; Fernandes et al 2014; Zeigler and Fagan 2014).…”

“…In general, modification and regulation of river channels have reduced river–floodplain connectivity (Edwards et al 2016). Disruptions to connectivity regimes can particularly impact the natural processes of colonization that structure fish assemblages and maintain biodiversity in floodplain lakes (Bayley 1995; Stoffels et al 2022).…”