Flood Disturbance Research Articles

Convergence in floodplain pond communities indicates different pathways to community assembly

Disturbance can strongly influence ecosystems, yet much remains unknown about the relative importance of key processes (selection, drift, and dispersal) in the recovery of ecological communities following disturbance. We combined field surveys with a field experiment to elucidate mechanisms governing the recovery of aquatic macroinvertebrates in habitats of an alluvial floodplain following flood disturbance. We monitored macroinvertebrates in 24 natural parafluvial habitats over 60 days after a major flood, as well as the colonization of 24 newly-built ponds by macroinvertebrates over 45 days in the same floodplain. We examined the sources of environmental variation and their relative effects on aquatic assemblages using a combination of null models and Mantel tests. We also used a joint species distribution model to investigate the importance of primary metacommunity structuring processes during recovery: selection, dispersal, and drift. Contrary to expectations, we found that beta diversity actually decreased among natural habitats over time after the flood or the creation of the ponds, instead of increasing. This result was despite environmental predictors showing contrasting patterns for explaining community variation over time in the natural habitats compared with the experimental ponds. Flood heterogeneity across the floodplain and spatial scale differences between the experimental ponds and the natural habitats seemingly constrained the balance between deterministic and stochastic processes driving the ecological convergence of assemblages over time. While environmental selection was the dominant structuring process in both groups, biotic interactions also had a prominent influence on community assembly. These findings have profound implications towards understanding metacommunity structuring in riverscapes that includes common linkages between disturbance heterogeneity, spatial scale properties, and community composition.

Drivers of periphyton biomass in south-western Cape rivers, South Africa, and the implications for management

In open-canopied temperate rivers, periphyton plays a pivotal role by converting dissolved nutrients into food for biota. International research suggests that periphyton is particularly responsive to flow and nutrient alterations and therefore ideally suited to aiding the prediction and monitoring of the effects of altered flow regimes. Given that one of South Africa’s key focus areas for river management has been the provision of flows to sustain aquatic ecosystems, this study aimed to quantify the relative importance of potential environmental drivers of periphyton, particularly flow, under different nutrient conditions in rivers of the south-western Cape. The study focused on two perennial rivers, the Berg and Molenaars Rivers. Quantitative monthly samples of periphyton biomass were collected from four sites, representative of different flow and nutrient regimes, over a 21-month sampling period between September 2007 and May 2009. The frequency of flood disturbance over the wet season was found to be the single most important driver of periphyton communities. It accounted for almost 75% of the total variation in periphyton biomass under natural flow and nutrient-poor conditions and it was therefore identified as the primary driver of periphyton communities. Its role was smaller under moderately enriched conditions and accounted for 45% of the total variation. These findings suggest that periphyton in south-western Cape rivers has great potential as a tool for predicting and monitoring the effects of altered flow regimes under different nutrient enrichment regimes. Together with some insight into community structure, the next step is to develop periphyton indicators that reflect different aspects of ecosystem integrity, for use in scenario modelling and other management aspects.

Flood disturbance mediates the strength of stream trophic cascades caused by trout

AbstractTrophic cascades are important features of aquatic ecosystems, but much variation in their strength remains unexplained. Disturbance could restrict cascade occurrence by limiting predator distribution, and increase cascade strength by lowering defended herbivore abundance. These possibilities were investigated in 20 New Zealand rivers varying both in flooding disturbance (measured by bed movement) and presence of non‐native predatory trout. Trout were often, but not always, associated with indirectly increasing algal abundance compared to similar rivers with native galaxiid fishes. Floods explained variation in cascades by (1) resetting algal abundance such that algal minima were related to bed movement; (2) preventing occurrence by restricting trout abundance in the most disturbed rivers; and (3) increasing potential cascade strength by removing defended herbivores. These observations indicate strong nonlinear influences of disturbance on both trophic cascade occurrence and maximum strength which will be important to consider as climate change alters disturbance regimes.

Investigating Flood Impact on Crop Production under a Comprehensive and Spatially Explicit Risk Evaluation Framework

Due to the projected increased frequency of occurrence of extreme flood events, it is becoming increasingly important to pay attention to agricultural flood management. The middle and lower reaches of the Yangtze River Basin (MLYRB), as one of the most important agricultural areas in the world, frequently suffer from the ravages of long-duration extreme flood events. Comprehensive flood risk evaluation can provide important support for effective management strategies by focusing on the combination of flood hazard and the consequences of flooding in areas exposed to the inundation. Previous satellite-based flood disturbance detection methods intended for use in single-cropping agricultural systems cannot be applied to the MLYRB with multi-cropping practices and long-duration flood events. Additionally, comprehensive agricultural flood risk evaluations traditionally neglect the characteristics of the impact of flooding with strong spatial and temporal variability. Thus, in this research, an integrated disturbance index (IDI) was developed to detect the impact of flood disturbance on crop growth, aiming to acquire a map of crop damage condition for a multi-cropping agricultural system with long-duration flood events that is spatially explicit and has a sufficiently high spatial resolution. A coupled hydrological and 2D hydraulic model parallelized using the GPU approach was employed to simulate flood flows, aiming at deriving sufficient meaningful detail at the local scale in terms of flood inundation patterns and processes over the whole natural watershed. Additionally, a spatial map of the combined effects of flood hazard and the consequences of flooding was used to investigate the relationship between flood characteristics and associated loss extent with the random forest model. The comprehensive evaluation framework was applied for the 2010 flood event in the MLYRB. The evaluation results indicate that the detection results based on IDI are consistent with the governmental statistics, the most hard-hit areas in related reports, and the spatial characteristics of river floods. The coupled hydrological–hydraulic model offers a clear picture of the flood characteristics over the whole basin, while simultaneously ensuring a sufficiently high spatial resolution. Our findings show that flood duration is the most important predictor in predicting crop damage extent.

Impacts of Flood Disturbance on the Dynamics of Basin-Scale Swimming Fish Migration in Mountainous Streams

This study investigated the response of sweetfish (Plecoglossus altivelis), a species that migrates dynamically throughout a river basin, to two flood events during the summer of 2020 in the Nagara River, located in the central region of Japan. By combining multiple environmental DNA (eDNA) surveys and hydrological modeling, the spatiotemporal distribution of P. altivelis throughout a mountainous river basin was captured and analyzed. The eDNA concentrations at 42 sites in the Nagara River Basin were analyzed five times from August to early October 2020. In addition, Rainfall–Runoff–Inundation model calculations were performed using 1-km resolution precipitation data as input values to analyze the magnitude of the flood disturbance at the eDNA sites. The daily specific discharge Qs (m3/s/km2) was employed as an index of the flood magnitude. The calculation period included Flood Events 1 and 2 with 52 and 38 days of precipitation reaching 1923 and 528 mm, respectively. The results of the eDNA analysis showed that, immediately after Flood Event 1, the spatial distribution of P. altivelis was unevenly distributed in the upper reaches of the Nagara River and some of its tributaries. Subsequently, the distribution expanded to the entire mainstem. The distribution of the maximum daily specific discharge suggested that the river segments with high eDNA concentrations of P. altivelis immediately after the high-magnitude flood event were those with a relatively low intensity of flood disturbance compared with those in the other connected river segments and tributaries. The results of this study indicate that the resilience of riverine communities to extreme floods is supported by the continuity and connectivity between the mainstem and its tributaries in mountainous river basins.

Abiotic drivers shape seed inputs and outputs in a tropical wetland on Croton trinitatis population

BackgroundPredicting how natural and anthropogenic drivers shape different ecological indicators, such as plant populations along environmental gradients, can be a relevant tool for establishing management and conservation criteria of tropical wetlands. We aimed to assess the effects of seasonal flood disturbance, type of grasslands and topographical conditions on Croton trinitatis population distribution in a tropical wetland.MethodsThe study was carried out in a seasonally flooded grassland (Central-West Brazil). We conducted samplings of soil on the dry and flood hydrophases of the Pantanal. We took the samples in eight seasonal ponds, with 1 km interval between them. Transects were marked during the flood period, observing the water level, one in the lowest zone, in the middle of the pond = low (ca. 60 cm deep), one at the pond edge = mid (ca. 30 cm deep) and one in the higher zone, on the external part = high (ca. 1 cm deep).ResultsThe results showed that the topography, seasonality, and types of grassland determine differences in the abundance patterns of adult plants and seedlings, and seed bank and seed predation. The abiotic factors can shape plant population-related ecological processes and patterns, with outputs (germination and predation) and inputs (local dispersion and from neighbouring areas) of proportional seeds for the population maintenance in this environment.ConclusionWe emphasize the importance of these findings, to show that abiotic factors are not the only ones to be considered in ecological studies of distribution and structuring of populations in habitats with extreme seasonal events.

Patchiness in flow refugia use by macroinvertebrates following an artificial flood pulse

AbstractFlow refugia, locations that maintain substrate stability and low hydraulic stress during periods of high flow, can ensure riverine resilience in the face of increasing hydrological unpredictability. Despite their known importance, they have been overlooked in recent years with work on drought refugia currently seeing greater attention. Moreover, research on the role of flow refugia during artificial flood pulses in regulated rivers, where flood disturbances are no longer part of the hydrograph, is essentially absent. Here, we compared flow refugia for benthic macroinvertebrates among six habitats (main channel, side channel, riffle, margin, lentic including a floodplain pond, and inundated floodplain) within four different sites in response to an artificial flood pulse. We found that the grain‐size distribution and macroinvertebrate community composition changed at each site following the flood. Macroinvertebrate assemblages became longitudinally homogeneous, but within‐site beta diversity and taxa richness remained temporally stable following the flood pulse, suggesting the presence of flow refugia. In this respect, margin, inundated floodplain and lentic (a floodplain pond) habitats provided important flow refugia locations, particularly for the mobile mayfly Rhithrogena sp. In contrast, low substrate stability in riffle and side channels resulted in limited refugia potential for most taxa. Refuge use was however patchy with high levels of intra‐habitat variability being evident for Rhithrogena sp. and the amphipod Gammarus fossarum in margin and side channel habitats. Further work is required to advance our knowledge of flow refugia in rivers with differing flow regimes to enable their integration into management and restoration schemes.

Seasonal drought and its effects on frog population dynamics and amphibian disease in intermittent streams

AbstractChytridiomycosis, caused by the pathogenic fungusBatrachochytrium dendrobatidis(Bd), has contributed to amphibian declines globally, but drivers of outbreaks vary locally. Here, we explore the role of drought in population and host‐disease dynamics of the endangered stream‐breeding foothill yellow‐legged frog (Rana boylii). In central California (USA) where severity of seasonal drought is increasing, we observed the non‐native, Bd‐tolerant and lentic‐adapted North American bullfrog (Lithobates catesbeianus) extend into streams when flood disturbance was minimal. Analysis of skin swabs revealed that prevalence and load of Bd infection among bullfrogs was low. Yet, among the native frogs, prevalence and load intensified as the seasonal drought progressed and surface flow became intermittent. When temperatures decreased in autumn and frogs concentrated at a reduced number of water points, we found dozens of dead foothill yellow‐legged frogs (2018–2019). Necropsies suggested chytridiomycosis as the likely cause of death. Despite recent lethal outbreaks, foothill yellow‐legged frog population abundance appeared resilient based on comparison to prior decades when no die‐offs were observed. Wet–dry mapping of the stream channel and retrospective analysis of hydrologic records revealed that the native frogs spawn away from perennial pools, a behaviour that may allow them to avoid bullfrogs and predatory fish. In an ecological trade‐off, tadpoles face the risk of the stream drying before metamorphosis. Fluctuations in population size thus corresponded to extremes of inter‐annual variation in streamflow that limit recruitment rather than disease outbreaks. We conclude that hydrologic constraints, which climate change may exacerbate, appear to override the stressors of non‐indigenous species and chytridiomycosis.

Habitat size influences community stability

Mechanisms linked to demographic, biogeographic, and food-web processes thought to underpin community stability could be affected by habitat size, but the effects of habitat size on community stability remain relatively unknown. We investigated whether those habitat-size-dependent properties influenced community instability and vulnerability to perturbations caused by disturbance. This is particularly important given that human exploitation is contracting ecosystems, and abiotic perturbations are becoming more severe and frequent. We used a perturbation experiment in which 10 streams, spanning three orders of magnitude in habitat size, were subjected to simulated bed movement akin to a major flood disturbance event. We measured the resistance, resilience, and variability of basal resources, and population and community-level responses across the stream habitat-size gradient immediately before, and at 0.5, 5, 10, 20, and 40 d post-disturbance. Resistance to disturbance consistently increased with stream size in all response variables. In contrast, resilience was significantly higher in smaller streams for some response variables. However, this higher resilience of small ecosystems was insufficient to compensate for their lower resistance, and communities of smaller streams were thus more variable over time than those of larger streams. Compensatory dynamics of populations, especially for predators, stabilized some aspects of communities, but these mechanisms were unrelated to habitat size. Together, our results provide compelling evidence for the links between habitat size and community stability, and should motivate ecologists and managers to consider how changes in the size of habitats will alter the vulnerability of ecosystems to perturbations caused by environmental disturbance.

Impacts of Dam Operation on Vegetation Dynamics of Mid-Channel Bars in the Mid-Lower Yangtze River, China

Vegetation dynamics on mid-channel bars (MCBs) is essential for supporting ecosystem functions and associated services in river systems, especially in dammed large rivers. Generally, there are two possible changing patterns that vegetation of MCBs downstream a dam would experience. On one hand, the vegetation area may shrink because of a decrease in the MCB area in the post-dam period, which has been observed in many rivers around the world. On the other hand, the MCB vegetation area may expand because flood disturbances would be weakened by dam operation. However, little evidence has been reported to clarify such confusion. Therefore, vegetation dynamics of MCBs in the mid-lower Yangtze River downstream the Three Gorges Dam (TGD; the world’s largest dam) is selected as a case study to address the issue. Using long-term (1987–2017) Landsat archive images, this study reveals the spatiotemporal variations of vegetation area change intensities (VACIs; indicated by dynamic trends) on MCBs in the mid-lower Yangtze River. Results show that an overall VACI in the post-dam period (2003–2017) is about three times faster than that in the pre-dam period (1987–2002). In other words, the rate of vegetation colonization accelerated after the TGD operation began in 2003. Moreover, the VACIs in the post-dam period are size dependent, where large size MCBs are likely to gain higher VACIs: Small-sized MCBs (0.33 km2/yr), medium-sized MCBs (1.23 km2/yr), large-sized MCBs (1.49 km2/yr). In addition, VACIs of individual MCBs in the post-dam period are distance dependent, where the further a MCB was from the TGD, the higher the VACI. It is also suggested that the weakened flood disturbances in the post-dam could explain the rapid vegetation growth and colonization. This work is not only beneficial for managing and protecting MCBs downstream the TGD after its operation, but is also helpful in understanding vegetation dynamics of MCBs in other dammed river systems around the world.

Floodplain forest tree seedling response to variation in flood timing and duration

The regeneration process is a sensitive period within life cycles of floodplain tree species and can strongly influence forest community composition. Yet, fundamental information remains limited on the relationship between regeneration processes and the flood disturbances that, together, construct floodplain forest landscapes. In a controlled greenhouse experiment we tested the effects of complete submergence on six temperate floodplain forest species to understand how flood timing and duration influence seedling survival. Groups of overcup oak (Quercus lyrata), Nuttall oak (Quercus texana), willow oak (Quercus phellos), sugarberry (Celtis laevigata), green ash (Fraxinus pennsylvanica), and American elm (Ulmus americana) seedlings were submerged for either 5, 15, 25, or 0 days (control) at the ages of 3-weeks, 6-weeks, and 9-weeks post-emergence. All species demonstrated a higher sensitivity to flooding at age 3-weeks compared to 6- and 9- weeks, indicating substantial changes in seedling resilience within the first months following emergence. Additionally, the heavier-seeded Q. lyrata, Q. texana, and Q. phellos were less or equally vulnerable to flooding compared to the lighter-seeded C. laevigata, F. pennsylvanica, and U. americana across all age groups, especially at 3-weeks post-emergence. The results of this study have implications for understanding woody species regeneration ecology and changes in floodplain forest composition, particularly in the context of hydrologic modifications.

Extreme flood disturbance effects on multiple dimensions of river invertebrate community stability.

Multidimensional analysis of community stability has recently emerged as an overarching approach to evaluating ecosystem response to disturbance. However, the approach has previously been applied only in experimental and modelling studies. We applied this concept to an 18-year time series (2000-2017) of macroinvertebrate community dynamics from a southeast Alaskan river to further develop and test the approach in relation to the effects of two extreme flood events occurring in 2005 (event 1) and 2014 (event 2). Five components of stability were calculated for pairs of pre- or post-event years. Individual components were tested for differences between pre- and post-event time periods. Stability components' pairwise correlations were assessed and ellipsoids of stability were developed for each time period and compared to a null model derived from the permuted dataset. Only one stability component demonstrated a significant difference between time periods. In contrast, 80% of moderate and significant correlations between stability components were degraded post-disturbance and significant changes to the form of stability ellipsoids were observed. Ellipsoids of stability for all periods after the initial disturbance (2005) were not different to the null model. Our results illustrate that the dimensionality of stability approach can be applied to natural ecosystem time-series data. The major increase in dimensionality of stability observed following disturbance potentially indicates significant shifts in the processes which drive stability following disturbance. This evidence improves our understanding of community response beyond what is possible through analysis of individual stability components.

Flood disturbance affects morphology and reproduction of woody riparian plants

Riparian forests are structured and maintained by their hydrology. Woody riparian plants typically adapt to the local flood regime to maximise their likelihood of survival and reproductive success. Understanding how extant trees form and reproduce in response to flood disturbance is crucial for predicting vegetation changes and informing restoration. Working in a temperate evergreen riparian forest, we aimed to determine whether disturbance-based responses of plants found in other ecosystems also typify woody plants in riparian forests where disturbances are often mild or chronic, non-lethal, annual events. Using plant surveys and 20-year modelled hydrological data, we examined whether (1) the morphology (main stem diameter, height, crown width, crown extent, stem leaning) and (2) reproduction type (sexual and asexual reproduction) and extent of three dominant woody species (Eucalyptus camphora, Leptospermum lanigerum and Melaleuca squarrosa) vary with flood regime (flood frequency and flood duration); and (3) whether different morphology is associated with different reproductive strategies. Increased flooding generally resulted in increased stem numbers and greater stem leaning—morphologies associated with asexual reproduction—of our study species. More frequent flooding also reduced plant size and sexual reproduction in E. camphora. Sexual reproduction in the studied species was more common in taller plants with single, more upright stems in good condition. Flexible morphology and plastic reproductive strategy may constitute an adaptation of trees to mild or chronic disturbance in floodplains. Our findings suggest that flood regime (i.e. variable frequency and duration of flooding events) is critical to the structural integrity and self-maintenance of species-diverse riparian forests.

Do neighbours matter? The effect of single‐ and mixed‐species sowing density on seed germination of annual wetland plants

AbstractQuestionGermination is one of the decisive processes influencing species presence and plant community structure. Here, we investigated the influence of seed density and single and mixed species on germination percentage and timing using annual wetland plants of the Pantanal in Brazil.LocationSeeds were collected from annual species occupying a seasonally flooded grassland, at Fazenda São Bento (19°29′27,3″ S; 57°01′55,9″ W), in the Abobral sub‐region, Pantanal, Mato Grosso do Sul (Central‐West Brazil).MethodsExperiments on the effects of seed density dependence on the germination time in intraspecific (single‐species) and interspecific (mixed‐species) treatments were conducted with ten annual species at three levels: for intraspecific — scarce (one seed), moderate (10 seeds), and high (20 seeds); and for interspecific — low (one each), medium (10) and abundant (20).ResultsThe germination percentage was independent of density and species mixtures. Germination time was density‐dependent for the mixed‐species treatment, but not the single‐species treatments. Six species had a shorter germination time, suggesting a group priority effect. Other species delayed or did not alter their germination time, indicating that the response to intraspecific seed densities is species‐specific.ConclusionThese findings illustrate that even though species have a history of coexistence and characteristics of rapid germination, seeds can alter germination time if they perceive neighbouring seeds of other species. Species that respond with a faster germination time may have a competitive advantage for recruitment and growth. These terrestrial species show high initial colonization power, which is essential in the regeneration of wetlands after flood disturbance.

Community Trait Responses of Three Dominant Macrophytes to Variations in Flooding During 2011-2019 in a Yangtze River-Connected Floodplain Wetland (Dongting Lake, China).

In lacustrine wetlands connected to rivers, the changes in flood regimes caused by hydrological projects lead to changes in the community traits of dominant macrophytes and, consequently, influence the structure and function of wetland vegetation. However, community trait responses of macrophytes to the timing and duration of flood disturbance have been rarely quantified. In 2011–2019, we investigated plant species diversity, density, and biomass in three dominant macrophyte communities (Carex brevicuspis C.B. Clarke, Miscanthus sacchariflorus (Maxim.) Hackel, and Polygonum hydropiper L.) through monthly field surveys in Dongting Lake wetlands. Partial least squares regressions were used to analyze how the variations in hydrological regimes affected plant community traits. Apparent inter-annual fluctuations in plant community traits were detected during 2011–2019. The species richness and Shannon index of diversity of Miscanthus and Polygonum communities increased, whereas the Shannon index of diversity of Carex community decreased. Variation in flooding had a greater effect on Polygonum and Carex community traits than on Miscanthus community traits. Flooding disturbed all plant communities, especially when the duration and timing varied. Shorter inundation periods caused the biomass of Miscanthus community to decline, and that of Carex and Polygonum communities to increase. Earlier flood recession caused the species richness and Shannon index of diversity of Polygonum and Miscanthus community to increase, and those of Carex community to decrease. These findings imply that shorter inundation durations and earlier flood recession generated by the operation of the Three Gorges Dam have changed the macrophyte growth pattern.

The influence of pastoral and native forest land cover, flooding disturbance, and stream size on the trophic ecology of New Zealand streams

AbstractDescribing trophic structure within freshwater food webs can be a useful tool for understanding relationships to make ecological inferences and to inform management action. A complementary analysis examining both stable isotope (SI) and biomass community components may be useful, because these two responses may be influenced differently by habitat factors and perturbations (e.g. flooding disturbance). To test this, stable isotope‐derived trophic height and biomass were characterised, as was coarse allochthonous, periphyton, invertebrate and fish components, for 27 stream communities in Canterbury, New Zealand. Using piecewise structural equation modelling to test relationships between components, it was found that increased catchment agricultural land cover was associated with increased periphyton biomass and δ15N (trophic height) in stream invertebrates and invertivorous fishes, likely due to nitrate runoff, but did not affect piscivorous fishes. Additionally, native forest land cover was associated with increased instream allochthonous biomass. Increased discharge (i.e. larger habitat size) did not affect the trophic height or biomass per‐unit‐area of large‐bodied piscivorous fishes (non‐native trout and native eels), although it did result in decreased biomass of small‐bodied invertivorous fishes (primarily native benthic taxa), likely due to high water velocities in larger habitats rendering habitat less suitable for small‐bodied fishes. Finally, flooding disturbance negatively affected both trophic height and biomass of large‐bodied fishes, but did not affect small‐bodied invertivorous fishes. Overall, describing stream trophic structure with complementary SI and biomass structural equation models appears to be a useful approach for constructing an integrative picture of how abiotic and biotic habitat factors affect freshwater communities. Our findings indicate that land cover, stream size and flooding disturbance should be taken into consideration by stream managers when conducting habitat restoration efforts or setting fish harvest regulations.

Assessing invertebrate response to an extreme flood event at a regional scale utilizing past survey data

Climate change is causing frequent and large floods, creating a need for information regarding the impact of severe flood disturbances on stream invertebrates. We briefly describe the response of invertebrate assemblages to an extreme flood event in 2018 in the western Ehime Prefecture of Japan. We compared invertebrate data collected in past regional-scale surveys with those gathered by revisiting 27 study sites after the flood and examined the relationship between catchment characteristics and the invertebrate response across the study area. Heavy rainfall was recorded in the catchment of each study site during the flood (range 565–1081 mm). Stream invertebrates may be eliminated by the historical flood, although the degree was small compared with that in previous reports on the effects of severe flood disturbances (abundance − 19.1%, taxon richness − 9.0%), likely owing to the absence of severe bed disturbance. Our results imply that high primary productivity, possibly caused by low elevations and the development of farms, accelerated the recovery of invertebrates after the disturbance. They also suggest that assembling pre-disturbance data is the key to understanding the impact of extreme floods on stream invertebrates and managing stream ecosystems during climate change.

Differential effects of a catastrophic wildfire on downstream fish assemblages in an aridland river

Few studies have evaluated the effects of large wildfires on downstream non-salmonid fish assemblages. Using multi-year (2009–2015) data from fish assemblage surveys and high-frequency water quality monitoring, we analyzed within-site responses of a cypriniform-dominated fish assemblage at two sites located > 20 km downstream of a 633 km2 wildfire in 2011 in the Rio Grande watershed in New Mexico, USA. Season had a weak influence on fish assemblage (fish) catch rate, richness, and evenness at the upstream site, but it had a strong negative influence (during spring/winter) on fish catch rate and richness at the downstream site. Fish species richness and evenness at both sites were not strongly affected by the wildfire, despite numerous post-fire sags in dissolved oxygen (including short-lived hypoxia < 2 mg L−1and anoxia—0 mg L−1) during subsequent monsoon seasons. Although fish catch rate was negatively impacted by the wildfire at the upstream site, it was unaffected at the downstream site. Fish catch rate, richness, and evenness at the upstream site were suppressed following a major flood event that occurred 26 months post-fire, but the downstream fish assemblage was resistant to the flood disturbance. Our study provided a rare opportunity to evaluate how aridland riverine fishes respond to disturbance from wildfire and subsequent flooding. Larger and more severe wildfires are occurring due to climate warming, and responses of water quality and fish community assemblages require study and assessment.

Could spatial heterogeneity in flow disturbance drive temporal stability of native–invasive species co‐occurrence in riverscapes?

Abstract Spatial heterogeneity of abiotic influences like disturbance in riverscapes could play an important role in dispersal‐aided community stability. We tested if higher spatial variability in conditions around river confluences caused by different flood disturbance regimes in branches could possibly influence the stability of a fish assemblage dominated by strong negative interactions between non‐native trout and native galaxiid fishes. We used field surveys to evaluate the effects of disturbance‐driven differences between river branches on fish densities and temporal variation in the relative abundance of galaxiids. Three repeated electrofishing surveys were conducted in two reaches of all branches of eight confluences in Canterbury, New Zealand. We found the differences in flood disturbance regimes between the two stream branches joining at a confluence were associated with higher mean fish abundance and more temporal stability in the relative abundance of native fish. Conversely, when flood disturbance conditions in the two streams joining were similar, resulting in a homogeneous confluence, fish abundance was lower and temporal variability was higher. For homogeneous confluences, strong interactions between native galaxiids and invasive trout probably produced lower fish densities where rivers with low flow disturbance met, whereas when two highly flow‐disturbed rivers met, environmental harshness probably lowered fish densities. By comparison, enhanced galaxiid temporal stability at confluences containing both disturbed and stable river branches probably reflected the combined effects of refugia from trout predation in disturbed branches and refugia from flooding in stable branches. Overall, these results are likely to reflect the combined effects of population‐specific responses to physical heterogeneity and spatially variable interspecific interactions. They may mean that river network configuration, and confluences especially, could influence community stability through a habitat portfolio effect. Furthermore, it will be worth investigating further whether preservation or restoration of differences in flow disturbance conditions in riverscapes encourages higher ecosystem resilience in the face of global change.

Endemic macrophyte is more plastic than two cosmopolitan species in fluctuating water levels and nutrient-enriched conditions

ABSTRACT Plant communities within many of the world’s waterways are losing diversity where flows are stabilised for security and enriched from land-use intensification. Understanding the phenotypic plasticity of plant species that protect and/or store their below-ground biomass during floods may help to promote them if problematic, overabundant species – like Phragmites australis and Typha domingensis – are less plastic. To investigate mechanisms underlying the plasticity of two cosmopolitan (Phragmites, Typha) and two endemic (Cycnogeton procerum, Cyperus gymnocaulos) macrophytes in response to disturbance (fluctuating hydrological regimes) across a nutrient gradient, we analysed historical data from pond experiments with single-species pots. Our analyses showed fluctuating water levels in nutrient-enriched conditions reduced total biomass in all species, with 65% and 46% reduction in Phragmites and Typha respectively. Contrary to expectations, only Cycnogeton allocated a higher proportion of biomass to protect (root) and store (rhizome) resources in fluctuating water levels and nutrient-enriched conditions (45% versus 23% in stable regime), and was the most plastic by allocating 75% more biomass to belowground in the least favourable conditions. Our results indicate that removing impediments to stochastic flooding disturbance could benefit some endemic species while reducing the productivity of overabundant Phragmites and Typha in environments enriched by human activity.