Ecological thresholds and regime shifts within hydrologically modified rivers: A 75+ year retrospective analysis. (10th December 2020)

• Record Type: Journal Article
• Authors: Delong, Michael D.; Thorp, James H.; Anderson, Jeffrey R.
• Is Part Of: River research and applications. Volume 37:Number 1(2021)
• Page Start: 54
• Page End: 63
• Publication Date: 2020-12-10
• Subjects: heterogeneity -- metastability -- regime shift -- resilience -- thresholds -- variability; Rivers -- Regulation -- Periodicals; Rivers -- Periodicals; 551.483
• DOI: 10.1002/rra.3749
• Languages: English
• ISSNs: 1535-1459

Abstract

Abstract: Theoretical models have spurred empirical testing to understand how anthropogenic disturbances cause sudden shifts between alternative functional states. Most studies are done over short periods of time, making it difficult to determine the occurrence of ecological thresholds or regime shifts. This study used carbon stable isotope ratios (δ 13 C) from museum specimens of fish, mussels, and snails over a timeline of 75+ years to ascertain if trophic structure of two hydrologically modified rivers changed between premodification and postmodification periods. An alternative state model following Scheffer's shallow lake model was developed. As long‐lived species, mussels and snails are time integrators with long tissue turnover time and serve as a surrogate for the balance of benthic and pelagic basal food resource contributions to fish biomass. Hydrological datasets over the period of record for the Upper Mississippi (UMR) and lower Ohio (LOR) rivers were placed in 5‐year intervals, as were δ 13 C data for invertivorous and piscivorous fish. The model indicated changes in δ 13 C of fish relative to changes in mean stage height (UMR) and mean minimum stage height (LOR) over time. Evidence based on the model and empirical data of δ 13 C‐stage height relationship suggests a regime shift occurred during the postdam period in the LOR. Postdam, fish from the UMR revealed little change in δ 13 C of invertivores, whereas postdam decreases in variability of δ 13 C for piscivores shifted from a threshold to an unstable equilibrium. Decreases in hydrological variability led to a loss of resilience, with both rivers becoming metastable due to hydrological modification.