1. Drought disturbances can have strong but variable effects on aquatic communities and little is understood about the impacts of drought, fragmentation and habitat reconnectivity on the dynamics of intermittent stream fish metacommunities. 2. We performed two experiments using outdoor stream mesocosms to test the effects of drought-mediated connectivity and habitat heterogeneity on realistic stream fish assemblages at local (pool/patch) and regional (stream unit) scales under non-drought versus drought conditions (Exp1) and under drought conditions with 'pulse' flow connectivity versus 'non-pulse' (Exp2). 3. Survivorship, a-diversity and c-diversity differed little at the unit level between treatments and experiments, but significant interactions between treatments and pool position were observed for species richness and abundances at the pool-level. Specifically, drought (Exp1) and non-pulse (Exp2) treatments had consistently higher species richness and abundances in deeper downstream pools due to downstream-biased immigration during the onset of drought and higher residency among fishes in downstream pools. Species-specific responses in these treatments resulted in downstream pools that were characterised by species pre-adapted to lentic conditions (e.g. sunfishes), whereas upstream pools were characterised by smaller-bodied top-water and pelagic species. Non-drought (Exp1) and pulse (Exp2) treatments showed no difference in richness or abundances among pool positions and assemblages were generally well-mixed, indicating that connectivity (even a brief pulse) of pool refugia was important for determining local and regional assemblage structure and mediating the impacts of drought. 4. These experiments demonstrate that stream fish assemblages responded strongly to reduced flows and fragmentation, and that metacommunity dynamics were structured by differential emigration and immigration rates and directionality among heterogeneous and variably connected pool habitats.
Hydrological regimes are primary drivers of community structure and dynamics in streams with strong seasonal or annual flood and drought cycles. In the current study, we investigated the dynamics of fish metacommunities in two intermittent streams (Hickory Creek and Clear Creek) in north Texas, USA, by examining changes in diversity, abundance, assemblage structure and temporal stability associated with prolonged seasonal drought and reconnectivity. Diversity (α and γ), abundance and stability increased with initial isolation during summer drought but dramatically declined as drought or drying persisted through the winter (November–December). During post-drought reconnectivity in Hickory Creek, diversity and abundance increased and approached pre-drought levels. Abundance and body size varied greatly among species and indicated species-specific responses (i.e. mortality, recruitment, dispersal) to hydrologic fragmentation and connectivity. Ultimately, assemblage structures were significantly altered by drought in Hickory and Clear creeks, and despite a trend towards recovery in Hickory Creek, assemblages did not fully recover during the present study. Intermittent-stream fishes may be generally adapted to natural drought dynamics; however, climate change and human-mediated habitat alterations may result in prolonged and intensified drought conditions that exceed many species mechanisms of resistance or resilience having potentially large impacts on biodiversity across spatial and temporal scales.
Water-resource managers are challenged to balance growing water demand with protecting aquatic ecosystems and biodiversity. Management decisions can benefit from improved understanding of water-withdrawal impacts on hydrologic regimes and ecological assemblages. This study used ecological limit functions for fish groups within the Tennessee and Cumberland River basins to predict species richness responses under simulated constant-rate (CR) and percent-of-flow (POF) withdrawals and for different minimum flow level protections. Streamflow characteristics (SFC) and richness were generally less sensitive to POF withdrawals than CR withdrawals among sites, fish groups, and ecoregions. Species richness generally declined with increasing withdrawals, but responses were variable depending on site-specific departures of SFCs from reference conditions, drainage area, fish group, ecoregion, and minimum flow level. Under POF withdrawals, 10% and 20% daily flow reductions often resulted in loss of <1 species and/or ≤5% richness among fish groups. Median ecological withdrawal thresholds ranged from 3.5–31% for POF withdrawals and from 0.01–0.92 m3/s for CR withdrawals across fish groups and ecoregions. Application of minimum flow level cutoffs often resulted in damping effects on SFC and richness responses, indicating that protection of low streamflows may mitigate hydrologic alteration and fish species richness loss related to water withdrawals. Site-specific and regionally summarized responses of flow regimes and fish assemblages under alternative withdrawal strategies in this study may be useful in informing water-management decisions regarding streamflow allocation and maintaining ecological flows.
Background A freshly deceased mud crab (Scylla serrata) exhibiting multiple white spots under the carapace was found in Pumicestone Passage, northern Moreton Bay in May 2018. This crab was taken from within a biosecurity zone established due to a recent incursion of White Spot Syndrome Virus (WSSV) into populations of wild penaeids (Penaeus spp., Metapenaeus spp.) and crabs (Thalamita crenata) in the area. Because grossly visible white spots have been previously observed under the carapace of moribund S. serrata with white spot disease (WSD) in India, an investigation into the cause of death was undertaken. Case report The affected S. serrata was negative for WSSV DNA when gill samples were tested by real-time PCR. Histopathology found no evidence of WSD lesions in the form of basophilic hypertrophied intranuclear inclusions in any tissues of ectodermal or mesodermal origin. Histopathology of the affected carapace showed that the white spots consisted of multiple lighter coloured foci in the exocuticle formed from concentric crystalline-like rings, which extended into the endocuticle. These were interpreted as evidence of mineral mobilisation within the carapace during the pre-moult (D1 or D2) stage of the moult cycle. The cause of death in this case therefore may have been due to moult-related complications. Conclusion These observations confirm that formation of grossly visible white spots under the carapace of S. serrata are not pathognomonic for infection with WSSV. Similar observations in previous studies where WSSV was detected by PCR in this same host may have been incidental findings.
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