Mary C. Freeman

Regional Effects of Hydrologic Alterations on Riverine Macrobiota in the New World: Tropical–Temperate Comparisons

temperate zones, paradigms and theories of how hydrologic modifications caused by dams alter the ecological dynamics of rivers are based largely on studies of temperate basins (e.g., Poff et al. 1997). Little is known about biotic responses to hydrologic modifications in tropical streams; generalizations about the effects of dams in the tropics are constrained by limited data on recently constructed, and relatively few, dams. Moreover, general ecological understanding of the effects of dams in both tropical and temperate zones is constrained by a lack of baseline information on the distribution and ecology of aquatic biota before dam construction, as well as by an overemphasis on economically important species. This article has two main objectives: to examine what is known about regional effects of hydrologic modifications in temperate and tropical areas of the New World (i.e., North and South America and the Caribbean), with an emphasis on fishes and molluscs; and to discuss research needs regarding regional effects of hydrologic alterations in temperate and tropical regions. A better understanding of regional effects of cumulative hydrologic alterations could help inform decisions on the nature and location of future hydrologic modifications. We begin with a brief description of the scope of hydrologic alterations in the New World, emphasizing dams. This is followed by a summary of biotic patterns that have emerged in hydrologically altered rivers draining temperate regions. We use the highly regulated Mobile River basin in southeastern North America as a temperate-zone case study to discuss specific biological effects. We then focus

FLOW AND HABITAT EFFECTS ON JUVENILE FISH ABUNDANCE IN NATURAL AND ALTERED FLOW REGIMES

Conserving biological resources native to large river systems increasingly depends on how flow-regulated segments of these rivers are managed. Improving management will require a better understanding of linkages between river biota and temporal variability of flow and instream habitat. However, few studies have quantified responses of native fish populations to multiyear (>2 yr) patterns of hydrologic or habitat variability in flow-regulated systems. To provide these data, we quantified young-of-year (YOY) fish abundance during four years in relation to hydrologic and habitat variability in two segments of the Tallapoosa River in the southeastern United States. One segment had an unregulated flow regime, whereas the other was flow-regulated by a peak-load generating hydropower dam. We sampled fishes annually and explored how continuously recorded flow data and physical habitat simulation models (PHABSIM) for spring (April–June) and summer (July–August) preceding each sample explained fish abundances. Patt...

ASSEMBLAGE ORGANIZATION IN STREAM FISHES: EFFECTS OF ENVIRONMENTAL VARIATION AND INTERSPECIFIC INTERACTIONS

We assessed the relative importance of environmental variation, interspecific competition for space, and predator abundance on assemblage structure and microhabitat use in a stream fish assemblage inhabiting Coweeta Creek, North Carolina, USA. Our study encompassed a 10–yr time span (1983–1992) and included some of the highest and lowest flows in the last 58 years. We collected 16 seasonal samples which included data on: (1) habitat availability (total and microhabitat) and microhabitat diversity, (2) assemblage structure (i.e., the number and abundances of species comprising a subset of the community), and (3) microhabitat use and overlap. We classified habitat availability data on the basis of year, season, and hydrologic period. Hydrologic period (i.e., pre–drought [PR], drought [D], and post–drought [PO]) represented the temporal location of a sample with respect to a four–year drought that occurred during the study. Hydrologic period explained a greater amount of variance in habitat availability data...

ESTIMATING SPECIES OCCURRENCE, ABUNDANCE, AND DETECTION PROBABILITY USING ZERO-INFLATED DISTRIBUTIONS

Researchers have developed methods to account for imperfect detection of species with either occupancy (presence absence) or count data using replicated sampling. We show how these approaches can be combined to simultaneously estimate occurrence, abundance, and detection probability by specifying a zero-inflated distribution for abundance. This approach may be particularly appropriate when patterns of occurrence and abundance arise from distinct processes operating at differing spatial or temporal scales. We apply the model to two data sets: (1) previously published data for a species of duck, Anas platyrhynchos, and (2) data for a stream fish species, Etheostoma scotti. We show that in these cases, an incomplete-detection zero-inflated modeling approach yields a superior fit to the data than other models. We propose that zero-inflated abundance models accounting for incomplete detection be considered when replicate count data are available.

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.

Stream fish occurrence in response to impervious cover, historic land use, and hydrogeomorphic factors

We evaluated competing models explaining the occurrence of five stream fishes in an urbanizing watershed to determine the relative importance of (a) impervious surface and other indicators of current land use, (b) historic land use (e.g., agriculture, impoundments), and (c) hydrogeomorphic characteristics (e.g., stream size, elevation, geology). For four of five species, the best-supported models were those that included both current effective impervious cover and historic land use predictor variables, although models with only effective impervious cover were equally well supported for two of those species. For the best-supported models for three species, occurrence probability was predicted to approach zero at levels of development equivalent to about 2%-4% effective impervious cover in the surrounding region. Data were drawn from 357 fish collections made in the Etowah River basin, Georgia, USA, between 1998 and 2003 and analyzed using hi- erarchical logistic regression accounting for imperfect species detection. This is the first study we know of to examine the response of individual fish species to both increasing impervious cover and historic land use. Such individual species as- sessments will be increasingly necessary to guide policies for managing urban effects and preventing extirpations of sensi- tive species.

Movements by two small fishes in a large stream

Movements by adult Percina nigrofasciata and juvenile Lepomis auritus were examined in a large Coastal Plain stream in the southeastern United States. I marked fishes with subcutaneous injections of acrylic paints to indicate capture location within a 550-m long study site. Recaptures over an 18-month period primarily occurred within 33 m of the original capture location, suggesting longterm residence in relatively small areas. However, 11 Percina and three Lepomis moved at least 100 m (maximum distance moved = 200 m for Lepomis, 420 m for Percina), and individuals of both species shifted between distinctly different mesohabitats (boulder riffle, sand pool, and gravel riffle). Distance moved did not strongly relate to time between captures. Long-distance movements (> 33 m) by Percina mostly occurred from Nov. through June. The winter and springtime movements by darters occurred in upstream and downstream directions and into all three mesohabitats, and at least 40% of these movements were unassociated with periods of extreme high flow. Periodic long-range movements may allow small fishes to respond to variation in resources over a large area and across a variety of stream habitats.

Principal Component Analyses of Assemblage Structure Data: Utility of Tests Based on Eigenvalues

We examined the ability of eigenvalue tests to distinguish field-collected from random, assemblage structure data sets. Eight published time series of species abun- dances were used in the analysis, including data sets for: fishes, birds, mammals, stream benthos, and crabs. To test the efficacy of eigenvalue tests, we constructed 1000 randomly generated data sets for each real data set, whose means and variances were identical to the means and variances of the original data matrices. The data sets were then subjected to a principal components analysis (PCA) and eigenvalue tests used to identify significant ei- genvalues for both correlation and covariance matrix solutions. We also examined the effects of: (1) number of species (= number of variables), (2) number of samples (= rep- lication), and (3) variance structure, on the performance of the test. Using PCAs based on the correlation matrix and with sample sizes typically encountered in the field, the eigenvalue tests generally performed at the .05 level when a = .01. Slightly poorer results were obtained with the covariance matrix. Increasing the number of samples to at least three times the number of species generally gave a level coverage for an a level test (i.e., a = .05, .01). Increasing variance in the data set only affected test outcomes at levels of replication less than twice the number of species. We conclude that the eigenvalue tests can be used to detect patterns in PCAs of assemblage structure data, if the number of samples is at least three times the number of species and either a covariance or correlation matrix solution is used. It is assumed that these patterns represent ecologically meaningful patterns of variation.

Evaluation of Generalized Habitat Criteria for Assessing Impacts of Altered Flow Regimes on Warmwater Fishes

Abstract Assessing potential effects of flow regulation on southeastern warmwater fish assemblages is problematic because of high species richness and poor knowledge of habitat requirements for most species. Our study investigated relationships between availability and temporal persistence of key habitats and fish assemblage structure at regulated and unregulated sites in the Tallapoosa River system. Fish assemblage characteristics at seven sites were quantified based on 1,400 electrofishing samples collected during 1994 and 1995. Physical Habitat Simulation (PHABSIM) programs were used to model availability and persistence of key habitats at regulated and unregulated sites. Associations between fish assemblages and availability or persistence of key habitats were identified via regression and analysis of variance. We found that hydropeaking dam operation reduced the average length of time that shallow-water habitats persisted and also reduced year-to-year variation in the persistence of shallow-water hab...