Do novel ecosystems follow predictable trajectories? Testing the trophic surge hypothesis in reservoirs using fish

Abstract

Hydroelectric reservoirs are novel ecosystems that provide a variety of important services. To manage these ecosystems and their fish populations effectively, we need to develop conceptual frameworks for predicting their short‐ and long‐term responses. To advance this goal, we revisited and tested the “trophic surge hypothesis, TSH.” The TSH has been widely cited in the literature, but has not been empirically tested across numerous reservoirs. The TSH suggests that fish populations should show a hump‐shaped pattern (i.e., the non‐equilibrium phase) after river impoundment. As such, we assembled 40 recruitment and 109 adult fish abundance time series from 19 species distributed across seven reservoirs from temperate and boreal regions, and applied curve fitting analyses and model selection. We found that the hump‐shaped pattern was the predominant pattern across individual time series, providing moderate support for the TSH. Fish recruitment increased substantially during reservoir filling and was followed by an increase in adult fish lagging 3–4 years behind. The non‐equilibrium phase was transient and lasted roughly eight years for recruits, whereas it could be much longer for adults. When time series were combined across regions and sites, the support for the TSH was weaker. However, we observed significant variability in the duration, timing, and magnitude of the surge across individual time series and found that the total flooded area was the most influential predictor to explain this variability. In conclusion, the TSH and related metrics can be a useful and general predictive framework to understand how fish populations may respond to impoundment. In particular, long‐term management recommendations could be short‐sighted if formulated before convincing evidence has emerged to show that the reservoir reached its new trophic equilibrium.

The impact of a hydroelectric dam on Neotropical fish communities: A spatio‐temporal analysis of the Trophic Upsurge Hypothesis

“…The rapid response of these riverine fishes corresponds with previous evidence from tropical systems (Agostinho et al, 2016, ; Cottet et al, ; Henderson, Ryder, & Kudhongania, 1; Williams, Winemiller, Taphorn, & Balbas, ) and fits the general expectation of Turgeon et al () with fish populations in this tropical reservoir peaking considerably faster (0.9–1.4 years) than temperate (4.1 years) and boreal (4.5 years) systems. Although the short fill time (4 months) was clearly an important factor, the quick ecological response was primarily a consequence of the higher productivity of warm‐water ecosystems (Brown, Gillooly, Allen, Savage, & West, ; Lowe‐McConnell ).…”

“…Overall, the ecological process is characterised by three trophic phases: (a) equilibrium—in accordance with the natural river before closure; (b) nonequilibrium—an upsurge, peak and decline in productivity; (c) equilibrium—in accordance with the environmental conditions of the new reservoir environment (Baranov, ). While the rise and fall of fish production following dam installation has been widely documented (Agostinho, Gomes, Santos, Ortega, & Pelicice, ; Balon & Coche, ; Benson, ; Cottet, Descloux, Guédant, Cerdant, & Vigouroux, ; Lima et al, ), its explicit time‐series character has, until recently, escaped quantitative description (Turgeon, Solomon, Nozais, & Gregory‐Eaves, ).…”

“…The progressive transformation of environmental characteristics following dam closure presents a complex spatio‐temporal template for the ecological response (Agostinho et al, ; Kimmel & Groeger, ; Turgeon et al, ). The upstream reservoir takes time to fill.…”

“…In consequence, the response patterns of fishes vary on a local and regional basis (Agostinho et al, ; Poff & Zimmerman, ). Turgeon et al () provided evidence that the upsurge of fish populations was faster in temperate systems compared to boreal systems, speculating that the response would be even faster in warm‐water, tropical systems.…”

Turbine entrainment and passage of potadromous fish through hydropower dams: Developing conceptual frameworks and metrics for moving beyond turbine passage mortality

“…The proportional nature of the metric lends itself to tagging methods (see Table and case‐study 1), where assuming that the entrainment event detection efficiency is known, and vulnerability to entrainment is equal among tagged sample and equivalent populations, then the proportion of tagged fish entrained can be considered equal the proportion of population entrained. Furthermore, the proportional nature of the metric means that it is resilient to changes in upstream abundance, such as those that occur as a consequence of trophic upsurge which is common in newly constructed temperate reservoirs (Turgeon, Solomon, Nozais, & Gregory‐Eaves, ). Alternatively, Entrain v can also be calculated by comparison of Turb Entry count , with upstream abundance estimates (see case‐study 2).…”