Mark Pyron

Large-scale Flow Experiments for Managing River Systems

Experimental manipulations of streamflow have been used globally in recent decades to mitigate the impacts of dam operations on river systems. Rivers are challenging subjects for experimentation, because they are open systems that cannot be isolated from their social context. We identify principles to address the challenges of conducting effective large-scale flow experiments. Flow experiments have both scientific and social value when they help to resolve specific questions about the ecological action of flow with a clear nexus to water policies and decisions. Water managers must integrate new information into operating policies for large-scale experiments to be effective. Modeling and monitoring can be integrated with experiments to analyze long-term ecological responses. Experimental design should include spatially extensive observations and well-defined, repeated treatments. Large-scale flow manipulations are only a part of dam operations that affect river systems. Scientists can ensure that experimental manipulations continue to be a valuable approach for the scientifically based management of river systems.

Are large-scale flow experiments informing the science and management of freshwater ecosystems?

Greater scientific knowledge, changing societal values, and legislative mandates have emphasized the importance of implementing large-scale flow experiments (FEs) downstream of dams. We provide the first global assessment of FEs to evaluate their success in advancing science and informing management decisions. Systematic review of 113 FEs across 20 countries revealed that clear articulation of experimental objectives, while not universally practiced, was crucial for achieving management outcomes and changing dam-operating policies. Furthermore, changes to dam operations were three times less likely when FEs were conducted primarily for scientific purposes. Despite the recognized importance of riverine flow regimes, four-fifths of FEs involved only discrete flow events. Over three-quarters of FEs documented both abiotic and biotic outcomes, but only one-third examined multiple taxonomic responses, thus limiting how FE results can inform holistic dam management. Future FEs will present new opportunities to advance scientifically credible water policies.

DISTRIBUTION AND ABUNDANCE OF TROPICAL FRESHWATER SHRIMP ALONG A STREAM CORRIDOR : RESPONSE TO DISTURBANCE

Different intensities and frequencies of disturbances can alter spatial and temporal patterns of abundances for coexisting species. We observed the effects of a high-flow event generated by Hurricane Hugo (19 September 1989) and belownormal stream flow during 1994 on freshwater shrimp populations in a tropical, montane stream. To determine if these different hydrologic regimes altered distributions of populations, we compared shrimp densities during three periods: pre-Hurricane Hugo (20 mo), post-Hurricane Hugo (50 mo), and low-flow (12 mo). There were significant differences in the relationships between locations of stream pools along an elevational gradient (300 to 470 m) and the abundances of two species of shrimp (Atya lanipes and Xiphocaris elongata) during these three periods. Atya increased in density with increasing elevation in a consistent fashion during all three periods. Densities also increased during the post-Hugo and low-flow periods relative to the pre-Hugo baseline. Xiphocaris increased in density with increasing elevation during the pre- and post-Hugo periods, but density decreased with elevation during the low-flow period. Palaemonid species of predatory shrimp, Macrobrachium carcinus and Macrobrachium crenulatum, consistently decreased in density with elevation during all three periods of observation. Multiple regression analyses demonstrated high predictability of Atya density distributions based on volume, pool depth, and coefficient of variation of pool depth during the pre-Hugo baseline observations. Xiphocaris densities were positively associated with pool width and negatively associated with the coefficient of variation of pool width in the pre-Hugo period. However, physical variables did not predict Atya or Xiphocaris densities during the post-Hugo period. During the dry period, there was a negative association between Atya densities and pool depth and width, and a positive association with the coefficient of variation of pool depth. During the dry period, Xiphocaris densities were best predicted as a function of maximum depth; there was a negative association with the coefficient of variation of pool width. Pre-Hugo Macrobrachium densities were negatively associated with the coefficient of variation of pool width, pool width-to-depth ratio, and elevation; in the post-Hugo and dry periods, Macrobrachium densities were best predicted by elevation. Mechanisms that likely cause these patterns of distribution include avoidance of predators coupled with active preference by prey species for pool habitats with low frequency of washout by storm flows, and with sufficient storage of food resources (microbially conditioned leaf detritus).

Role of the fish Astyanax aeneus (Characidae) as a keystone nutrient recycler in low-nutrient Neotropical streams

Nutrient recycling by animals is a potentially important biogeochemical process in both terrestrial and aquatic ecosystems. Stoichiometric traits of individual species may result in some taxa playing disproportionately important roles in the recycling of nutrients relative to their biomass, acting as keystone nutrient recyclers. We examined factors controlling the relative contribution of 12 Neotropical fish species to nutrient recycling in four streams spanning a range of phosphorus (P) levels. In high-P conditions (135 microg/L soluble reactive phosphorus, SRP), most species fed on P-enriched diets and P excretion rates were high across species. In low-P conditions (3 microg/L SRP), aquatic food resources were depleted in P, and species with higher body P content showed low rates of P recycling. However, fishes that were subsidized by terrestrial inputs were decoupled from aquatic P availability and therefore excreted P at disproportionately high rates. One of these species, Astyanax aeneus (Characidae), represented 12% of the total population and 18% of the total biomass of the fish assemblage in our focal low-P study stream but had P excretion rates > 10-fold higher than other abundant fishes. As a result, we estimated that P excretion by A. aeneus accounted for 90% of the P recycled by this fish assemblage and also supplied approximately 90% of the stream P demand in this P-limited ecosystem. Nitrogen excretion rates showed little variation among species, and the contribution of a given species to ecosystem N recycling was largely dependent upon the total biomass of that species. Because of the high variability in P excretion rates among fish species, ecosystem-level P recycling could be particularly sensitive to changes in fish community structure in P-limited systems.

Hydrological variation and fish assemblage structure in the middle Wabash River

Two years of fish assemblage data from 28 sites in the Wabash River Indiana, were examined for relationships with environmental variables using a multivariate approach, correspondence analysis. Upstream sites had lower mean daily discharge and lower coefficient of variation of daily discharge when compared to downstream sites. Although the fish assemblage changed along this 230-km river distance gradient, patterns were in contrast to the patterns in streams with unaltered flow regimes. We compared functional variables of fish species (species traits that describe habitat, trophic, morphological, and tolerance characteristics) by examining the proportion of their occurrences along the hydrological variability gradient (upstream–downstream). The general pattern showed assemblages from hydrologically stable (upstream) sites had higher proportions of generalist species that tend to occur in small to medium streams, prefer fast current velocities, generally occur over rocky, gravel, and sand substrates, and have low silt tolerance. In addition, there was a pattern relating the hydrological/longitudinal regime and the overall morphology of species: species with higher caudal peduncle/caudal fin ratios and more fusiform body shapes occurred in higher proportion in upstream sites.

Habitat Influence on Fish Community Assemblage in an Agricultural Landscape in Four East Central Indiana Streams

ABSTRACT The objective of this study was to relate the quality of the fish community with habitat using the Index of Biotic Integrity (IBI) and Qualitative Habitat Evaluation Index (QHEI) in four agriculturally influenced streams in east central, Indiana. A total of 48 species was collected from 42 sites. IBI scores ranged from 14 to 48, and QHEI scores ranged from 29 to 83. There was a significant positive correlation between IBI and QHEI scores. Furthermore, we found significant positive correlations between IBI scores and four individual QHEI metrics (channel morphology, substrate, poollglide and riffletrun quality, and in-stream cover). Habitat influenced the fish assemblages with channelization and substrate being the primary structuring factors. The land use in this area is 70% agriculture, which has heavily influenced lotic character through anthropogenic practices.

Cryptic phenotypic plasticity in populations of the freshwater prosobranch snail, Pleurocera canaliculata

We sampled four populations of the robustly shelled Pleurocera canaliculata from large rivers and five pleurocerid populations bearing more fusiform shells (nominally P. acuta and P. pyrenellum) from smaller streams in a study area extending from upstate New York to northern Alabama, USA. Gene frequencies at 9 allozyme-encoding loci revealed that each population of P. acuta or P. pyrenellum was more genetically similar to the P. canaliculata population inhabiting the larger river immediately downstream than to any nominal conspecific. Thus, the extensive intraspecific variation in shell robustness displayed by these nine populations has apparently been rendered cryptic by taxonomic confusion. We then employed geometric morphometrics to explore a gradient in shell morphology from the acuta form to the typical canaliculata form in 18 historic samples collected down the length of Indiana’s Wabash River. The shell forms appeared generally distinctive on the major axes yielded by relative warp analysis (increasing robustness and decreasing spire elongation), although some overlap was apparent. MANCOVA returned a significant relationship between multivariate shape variation and stream size, as measured by drainage area. Possible drivers for this phenomenon include an environmental cline in the risk of dislodgement due to hydrodynamic drag and shifts in the community of predators.

Temporal and spatial variation in an index of biological integrity for the middle Wabash River, Indiana

Few studies have addressed within-year temporal variation of IBI scores. We compared index of biotic integrity (IBI) scores for two summer sampling events from a large river during 25 annual periods. The results indicated that IBI scores calculated from June samples were not significantly different from July samples. Spatial autocorrelation was present, such that sites that were closer together produced similar IBI scores and could not be considered independent. Temporal autocorrelation was present, but was not strong. Lower quality sites (low IBI score) did not have higher variation than higher quality sites. Our results show that a single sample of fishes by boat electrofisher during the summer in a large river such as the Wabash River can produce a repeatable estimate of IBI score. Thus, repeated or additional sampling within the summer season to improve the quality of the evaluation is not warranted.

Evolution of Form and Function: Morphology and Swimming Performance in North American Minnows

ABSTRACT We tested for the ability to predict burst swimming performance in 15 species of North American minnows from body morphology. We used principal components analysis (PCA) to reduce 14 morphological variables into five axes that were used as predictors of swimming performance among species. Comparisons of swimming performance were among fish species with and without phylogenetic control. Four of our PCA axes were significant predictors of swimming performance with no phylogenetic control. However, comparisons with independent contrasts did not result in any significant predictors of performance from our PCA axes of body morphology. Thus, analyses with phylogenetic control can provide results very different from analyses without such control.

Female Zebrafish (Danio rerio) Do Not Prefer Mutant Longfin Males

ABSTRACT When we presented longfin and normal male zebrafish to females in an aquarium experiment, females did not spend more time adjacent to longfin than normal males and did not interact more with longfin than normal males. When we tested for effects of longfin mutants on male-male interactions by comparing aggressive interactions between two normal males to interactions between a normal male and a longfin mutant male, there was no difference in male-male aggression. These results suggest that neither male nor female zebrafish use fin size in evaluating male status.