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Regime shifts - persistent changes in the structure and function of an ecosystem - are well-documented for some ecosystems and have informed research and management of these ecosystems. In floodplain-river ecosystems, there is growing interest from restoration practitioners in ecological resilience, yet regime shifts remain poorly understood in these ecosystems. To understand how regime shifts may apply to floodplain-river ecosystems, we synthesize our understanding of ecosystem dynamics using an alternate regimes conceptual framework. We present three plausible sets of alternate regimes relevant to natural resource management interests within the Upper Mississippi River and Illinois River. These alternate regimes include: 1) a clear water and abundant vegetation regime vs. a turbid water and sparse vegetation regime in lentic, off-channel areas, 2) a diverse native fish community regime vs. an invasive-dominated fish community regime, and 3) a regime characterized by a diverse and dynamic mosaic of floodplain vegetation types vs. one characterized as a persistent invasive wet meadow monoculture. For each set of potential alternate regimes, we review available literature to synthesize known or hypothesized feedback mechanisms that reinforce regimes, controlling variables that drive regime transitions, and current restoration pathways. Our conceptual models provide preliminary support for the existence of alternate regimes in floodplain-river ecosystems. Quantitatively testing hypotheses contained within the conceptual model are important next steps in evaluating the model. Ultimately, the synthesis and evaluation of alternate regimes can inform the utility of resilience concepts in restoration and management on the Upper Mississippi River and Illinois River and improve our understanding of ecosystem dynamics in other large, heavily managed floodplain-river ecosystems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier Ltd.)

We compared processing and fate of dissolved NO3- in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of approximately 1-4 micromol NO3-/ L and the other receiving experimentally fertilized flood tides containing approximately 70-100 micromol NO3-/ L. We conducted simultaneous 15NO3- (isotope) tracer additions from 23 to 28 July 2005 in the reference (8.4 ha) and fertilized (12.4 ha) systems to compare N dynamics and fate. Two full tidal cycles were intensively studied during the paired tracer additions. Resulting mass balances showed that essentially 100% (0.48-0.61 mol NO3-N.ha(-1).h(-1)) of incoming NO3- was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO3- concentrations were 1.3-1.8 micromol/L. In contrast, only 50-60% of incoming NO3- was processed in the fertilized system when NO3- concentrations were 84-96 micromol/L; the remainder was exported in ebb tidewater. Gross NO3- processing was approximately 40 times higher in the fertilized system at 19.34-24.67 mol NO3-N.ha(-1).h(-1). Dissimilatory nitrate reduction to ammonium was evident in both systems during the first 48 h of the tracer additions but <1% of incoming 15NO3- was exported as 15NH4+. Nitrification rates calculated by 15NO3- dilution were 6.05 and 4.46 mol.ha(-1).h(-1) in the fertilized system but could not be accurately calculated in the reference system due to rapid (<4 h) NO3- turnover. Over the five-day paired tracer addition, sediments sequestered a small fraction of incoming NO3-, although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 mol.ha(-1).d(-1), respectively. Macrophyte NO3- uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO3- by plants was lower in the reference system (1.75 mol NO3-.ha(-1).d(-1)) than the fertilized system (approximately 10 mol NO3-.ha(-1).d(-1)). Nitrogen processing efficiency decreased with NO3- load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

We use relationships between modern Pacific salmon (Oncorhynchus spp.) escapement (migrating adults counted at weirs or dams) and riparian tree-ring growth to reconstruct the abundance of stream-spawning salmon over 150-350 years. After examining nine sites, we produced reconstructions for five mid-order rivers and four salmon species over a large geographic range in the Pacific Northwest: chinook (O. tschwatcha) in the Umpqua River, Oregon, USA; sockeye (O. nerka) in Drinkwater Creek, British Columbia, Canada; pink (O. gorbuscha) in Sashin Creek, southeastern Alaska, USA; chum (O. keta) in Disappearance Creek, southeastern Alaska, USA; and pink and chum in the Kadashan River, southeastern Alaska, USA. We first derived stand-level, non-climatic growth chronologies from riparian trees using standard dendroecology methods and differencing. When the chronologies were compared to 18-55 years of adult salmon escapement we detected positive, significant correlations at five of the nine sites. Regression models relating escapement to tree-ring growth at the five sites were applied to the differenced chronologies to reconstruct salmon abundance. Each reconstruction contains unique patterns characteristic of the site and salmon species. Reconstructions were validated by comparison to local histories (e.g., construction of dams and salmon canneries) and regional fisheries data such as salmon landings and aerial surveys and the Pacific Decadal Oscillation climate index. The reconstructions capture lower-frequency cycles better than extremes and are most useful for determination and comparison of relative abundance, cycles, and the effects of interventions. Reconstructions show lower population cycle maxima in both Umpqua River chinook and Sashin Creek pink salmon in recent decades. The Drinkwater Creek reconstruction suggests that sockeye abundance since the mid-1990s has been 15-25% higher than at any time since 1850, while no long-term deviations from natural cycles are detected for salmon in the Kadashan River or in Disappearance Creek. Decadal-scale cycles in salmon abundance with periods of 25-68 years were detected in all of the reconstructions. This novel approach provides river-specific, long-term perspectives on salmon abundance and cycles. Additionally, it provides a new frame of reference for maintaining and rebuilding individual stocks and for striking a balance between societal demands and the limited, always-changing salmon resource.

We introduced an 15N-NH4+ tracer to the riparian forest of a salmon-bearing stream (Kennedy Creek, Washington, USA) to quantify the cycling and fate of a late-season pulse of salmon N and, ultimately, mechanisms regulating potential links between salmon abundance and tree growth. The 15N tracer simulated deposition of 7.25 kg of salmon (fresh) to four 50-m2 plots. We added NH4+ (the initial product of salmon carcass decay) and other important nutrients provided by carcasses (P, S, K, Mg, Ca) to soils in late October 2003, coincident with local salmon spawning. We followed the 15N tracer through soil and tree pools for one year. Biological uptake of the 15N tracer occurred quickly: 64% of the 15N tracer was bound in soil microbiota within 14 days, and roots of the dominant riparian tree, western red cedar (Thuja plicata), began to take up 15N tracer within seven days. Root uptake continued through the winter. The 15N tracer content of soil organic matter reached a maximum of approximately 52%, five weeks after the application, and a relative equilibrium of approximately 40% within five months. Six months after the addition, in spring 2004, at least 37% of the 15N tracer was found in tree tissues: approximately 23% in foliage, approximately 11% in roots, and approximately 3% in stems. Within the stems, xylem and phloem sap contained approximately 96% of the tracer N, and approximately 4% was in structural xylem N. After one year, at least 28% of the 15N tracer was still found in trees, and loss from the plots was only approximately 20%. The large portion of tracer N taken up in the fall and reallocated to leaves and stems the following spring provides mechanistic evidence for a one-year-lagged tree-growth response to salmon nutrients. Salmon nutrients have been deposited in the Kennedy Creek system each fall for centuries, but the system shows no evidence of nutrient saturation. Rates of N uptake and retention are a function of site history and disturbance and also may be the result of a legacy effect, in which annual salmon nutrient addition may lead to increased efficiency of nutrient uptake and use.

Detrimental effects of introduced fishes on native amphibian populations have prompted removal of introduced cutthroat (Oncorhynchus clarki), rainbow (Oncorhynchus mykiss), and brook trout ( Salvelinus fontinalis) from naturally fishless lakes at Mt. Rainier National Park, Washington ( U.S.A.). Using paleolimnological indicators (diatoms, invertebrates, and sediment characteristics) in eight 480-year-old sediment cores from eight lakes, we (1) derived estimates of baseline environmental conditions and natural variation, (2) assessed the effects of stocking naturally fishless lakes, and (3) determined whether lakes returned to predisturbance conditions after fish removal (restoration). Diatom floras were relatively stable between 315 and 90 years before present in all lakes; we used this time period to define lake-specific "baseline" conditions. Dissimilarity analyses of diatoms revealed sustained, dramatic changes in diatom floras that occurred approximately 80 years ago (when fish were introduced) in four of five stocked lakes, whereas the diatom floras in two unstocked lakes had not changed significantly in the last 315 years. Diatoms were not preserved in an eighth lake. State changes also occurred in two lakes over 200 years before European settlement of the Pacific Northwest. Preserved invertebrate densities fluctuated dramatically over time in all cores, providing a poor reference for assessing the effects of fishes. Nevertheless, fish-invertebrate interactions have been demonstrated in other paleolimnological studies and may be useful for lower-elevation or more productive lakes. Because diatom communities have not returned to predisturbance assemblages in restored lakes, even 20-30 years after fish removal, we conclude that Mt. Rainier lakes were not successfully restored by the removal of fishes.