Towards a mechanistic understanding of fish species niche divergence along a river continuum

Citation: Troia, M. J., & Gido, K. B. (2014). Towards a mechanistic understanding of fish species niche divergence along a river continuum. Ecosphere, 5(4), art41. https://doi.org/10.1890/ES13-00399.1Environmental niche modeling is a valuable tool but it often fails to identify causal links between environmental gradients and individual- or population-level performance that drive species' distributions. Correlation between the abundances of stream fish species and longitudinal position in stream networks is well documented and is hypothesized to occur through differential environmental filtering of trophic traits. Still, trophically similar congeners often exhibit complementary distributions along stream size gradients, suggesting that other mechanisms are important. We present niche models to test the hypothesis that four congeneric pairs (Teleostei: Cyprinidae) exhibit complementary distributions along a gradient of stream size in the central Great Plains of Kansas, USA. Stream size was the strongest predictor of abundance compared to five other environmental variables tested and three of the four species pairs exhibited complementary distributions along a stream size gradient. We carried out field experiments to quantify potentially causal environmental gradients (food resources, temperature, and turbidity) and four measures of individual performance (adult spawning success and juvenile survival, condition, and growth) along a stream size gradient for one congeneric pair: Pimephales notatus, a tributary species and P. vigilax, a river mainstem species. These experiments revealed an increase in temperature and food resources with stream size, along with a corresponding increase in adult spawning success, juvenile condition, and juvenile growth for both species. We conclude that these congeners respond similarly to abiotic gradients associated with the river continuum and that complementary distributions are a consequence of biotic interactions, differential environmental filtering evident in an unmeasured performance metric, or differential environmental filtering by a direct environmental gradient operating at longer timescales

Data from: Extreme heat events and the vulnerability of endemic montane fishes to climate change

Identifying how close species live to their physiological thermal maxima is essential to understand historical warm-edge elevational limits of montane faunas and forecast upslope shifts caused by future climate change. We used laboratory experiments to quantify the thermal tolerance and acclimation potential of four fishes (Notropis leuciodus, N. rubricroceus, Etheostoma rufilineatum, E. chlorobranchium) that are endemic to the southern Appalachian Mountains (USA), exhibit different historical elevational limits, and represent the two most species-rich families in the region. All-subsets selection of linear regression models using AICc indicated that species, acclimation temperature, collection location and month, and the interaction between species and acclimation temperature were important predictors of thermal maxima (Tmax), which ranged from 28.5 to 37.2°C. Next, we implemented water temperature models and stochastic weather generation to characterize the magnitude and frequency of extreme heat events (Textreme) under historical and future climate scenarios across 25,379 stream reaches in the upper Tennessee River system. Lastly, we used environmental niche models to compare warming tolerances (acclimation-corrected Tmax minus Textreme) between historically occupied versus unoccupied reaches. Historical warming tolerances, ranging from +2.2 to +10.9°C, increased from low to high elevation and were positive for all species, suggesting that Tmax does not drive warm-edge (low elevation) range limits. Future warming tolerances were lower (-1.2 to +9.3°C) but remained positive for all species under the direst warming scenario except for a small proportion of reaches historically occupied by E. rufilineatum, indicating that Tmax and acclimation potentials of southern Appalachian minnows and darters are adequate to survive future heat waves. We caution concluding that these species are invulnerable to 21st century warming because sublethal thermal physiology remains poorly understood. Integrating physiological sensitivity and warming exposure demonstrates a general and fine-grained approach to assess climate change vulnerability for freshwater organisms across physiographically diverse riverscapes

Spatial and Temporal Correlates of Greenhouse Gas Diffusion from a Hydropower Reservoir in the Southern United States

Emissions of CO2 and CH4 from freshwater reservoirs constitute a globally significant source of atmospheric greenhouse gases (GHGs), but knowledge gaps remain with regard to spatiotemporal drivers of emissions. We document the spatial and seasonal variation in surface diffusion of CO2 and CH4 from Douglas Lake, a hydropower reservoir in Tennessee, USA. Monthly estimates across 13 reservoir sites from January to November 2010 indicated that surface diffusions ranged from 236 to 18,806 mg·m−2·day−1 for CO2 and 0 to 0.95 mg·m−2·day−1 for CH4. Next, we developed statistical models using spatial and physicochemical variables to predict surface diffusions of CO2 and CH4. Models explained 22.7% and 20.9% of the variation in CO2 and CH4 diffusions respectively, and identified pH, temperature, dissolved oxygen, and Julian day as the most informative predictors. These findings provide baseline estimates of GHG emissions from a reservoir in eastern temperate North America, a region for which estimates of reservoir GHGs emissions are limited. Our statistical models effectively characterized non-linear and threshold relationships between physicochemical predictors and GHG emissions. Further refinement of such modeling approaches will aid in predicting current GHG emissions from unsampled reservoirs and forecasting future GHG emissions

A stream classification system to explore the physical habitat diversity and anthropogenic impacts in riverscapes of the eastern United States

<div><p>Describing the physical habitat diversity of stream types is important for understanding stream ecosystem complexity, but also prioritizing management of stream ecosystems, especially those that are rare. We developed a stream classification system of six physical habitat layers (size, gradient, hydrology, temperature, valley confinement, and substrate) for approximately 1 million stream reaches within the Eastern United States in order to conduct an inventory of different types of streams and examine stream diversity. Additionally, we compare stream diversity to patterns of anthropogenic disturbances to evaluate associations between stream types and human disturbances, but also to prioritize rare stream types that may lack natural representation in the landscape. Based on combinations of different layers, we estimate there are anywhere from 1,521 to 5,577 different physical types of stream reaches within the Eastern US. By accounting for uncertainty in class membership, these estimates could range from 1,434 to 6,856 stream types. However, 95% of total stream distance is represented by only 30% of the total stream habitat types, which suggests that most stream types are rare. Unfortunately, as much as one third of stream physical diversity within the region has been compromised by anthropogenic disturbances. To provide an example of the stream classification’s utility in management of these ecosystems, we isolated 5% of stream length in the entire region that represented 87% of the total physical diversity of streams to prioritize streams for conservation protection, restoration, and biological monitoring. We suggest that our stream classification framework could be important for exploring the diversity of stream ecosystems and is flexible in that it can be combined with other stream classification frameworks developed at higher resolutions (meso- and micro-habitat scales). Additionally, the exploration of physical diversity helps to estimate the rarity and patchiness of riverscapes over large region and assist in conservation and management.</p></div

Disturbance regimes mapped to stream reaches.

<p>Disturbance regimes mapped to stream reaches.</p

Case study examining streams similar to Walker Branch, Tennessee, and the Lower Roanoke River, North Carolina.

<p>(A) Locations of Walker Branch and the Lower Roanoke River and associated Ecoregions. Study side maps of (B) Walker Branch watershed and (C) the Lower Roanoke River. Streams in the (D) Ridge and Valley and (E) Mid-Atlantic and Southeastern Plains were sequentially filtered based on whether they matched the same typology as Walker Branch and the Lower Roanoke River, respectively. Sequential filtering started with size classes, and then was followed by gradient, hydrology, temperature, confinement, and substrate (i.e., in order of hypothesized importance in structuring stream ecosystems). Total length of streams matching filters is provided.</p

Proportion of stream length having various levels of % agriculture or % urban land cover in their upstream watersheds according to stream classes.

<p>Proportion of stream length having various levels of % agriculture or % urban land cover in their upstream watersheds according to stream classes.</p

Proportion of stream length either impounded or regulated by various degrees by impoundments according to stream classes.

<p>Proportion of stream length either impounded or regulated by various degrees by impoundments according to stream classes.</p

Substrate classes developed from weighted mean particle sizes at field locations and extrapolated to stream reaches.

<p>(median particle size not available). Total length of streams per class reported as 10³ km.</p

Measures of association between stream classes and disturbance classes.

<p>Disturbance classes are a categorical indication of the anthropogenic stressor, if present, inducing the largest influence on a stream and includes (in order of greatest to least influence): Impoundment, dam regulation, dam fragmentation, landscape alteration, and no disturbance.</p