Judith L. Meyer

Investigating hydrologic alteration as a mechanism of fish assemblage shifts in urbanizing streams

Abstract Stream biota in urban and suburban settings are thought to be impaired by altered hydrology; however, it is unknown what aspects of the hydrograph alter fish assemblage structure and which fishes are most vulnerable to hydrologic alterations in small streams. We quantified hydrologic variables and fish assemblages in 30 small streams and their subcatchments (area 8–20 km2) in the Etowah River Catchment (Georgia, USA). We stratified streams and their subcatchments into 3 landcover categories based on imperviousness (<10%, 10–20%, >20% of subcatchment), and then estimated the degree of hydrologic alteration based on synoptic measurements of baseflow yield. We derived hydrologic variables from stage gauges at each study site for 1 y (January 2003–2004). Increased imperviousness was positively correlated with the frequency of storm events and rates of the rising and falling limb of the hydrograph (i.e., storm “flashiness”) during most seasons. Increased duration of low flows associated with imperviousness only occurred during the autumn low-flow period, and this measure corresponded with increased richness of lentic tolerant species. Altered storm flows in summer and autumn were related to decreased richness of endemic, cosmopolitan, and sensitive fish species, and decreased abundance of lentic tolerant species. Species predicted to be sensitive to urbanization, based on specific life-history or habitat requirements, also were related to stormflow variables and % fine bed sediment in riffles. Overall, hydrologic variables explained 22 to 66% of the variation in fish assemblage richness and abundance. Linkages between hydrologic alteration and fish assemblages were potentially complicated by contrasting effects of elevated flows on sediment delivery and scour, and mediating effects of high stream gradient on sediment delivery from elevated flows. However, stormwater management practices promoting natural hydrologic regimes are likely to reduce the impacts of catchment imperviousness on stream fish assemblages.

Summary of Findings and Recommendations

This book responds to questions in three general areas: characterization of hypoxia; characterization of nutrient fate, transport and sources; and the scientific basis for goals and management options. In the sections below, these questions (shown in italics below) are addressed very briefly with references to those sections of this book where more detailed science on that particular question may be found.

Urban Food Webs: Predators, Prey, and the People Who Feed Them

A prevailing image of the city is of the steel and concrete downtown skyline. The more common ex‐ perience of urban residents, however, is a place of irrigated and fertilized green spaces, such as yards, gardens, and parks, surrounding homes and business‐ es where people commonly feed birds, squirrels, and other wildlife. Within these highly human-modified environments, researchers are becoming increasingly curious about how fundamental ecological phenom‐ ena play out, such as the feeding relationships among species. While food webs have long provided a tool for organizing information about feeding relation‐ ships and energy flows through natural habitats, they have not been applied to urban ecosystems until re‐ cently (Faeth et al. 2005).

Characterization of Hypoxia

The hypoxic region along the northern Gulf of Mexico (NGOM) extends up to 125 km offshore and to 60 m water depth, has substantial variability with an average midsummer areal extent of 16,500 km2 (2001–2007), and extends in some years from the Mississippi River mouth westward to Texas coastal waters (Rabalais et al., 2007). This hypoxic region (Fig. 1.1) occurs along a relatively shallow, open coastline with complex circulation and water column structure typical of many coastal regions and includes massive inputs of freshwater, weak tidal energies, seasonally varying stratification strength, generally high water temperature, wind effects from both frontal weather systems and hurricanes, and mixing of river plumes from the Atchafalaya and Mississippi Rivers and other smaller sources (DiMarco et al., 2006; Hetland and DiMarco, 2007).

Nutrient Fate, Transport, and Sources

The Study Group was asked to review the available literature and information, especially that developed since 2000, that would allow them to assess any changes and improvements in the understanding of nutrient sources and flux estimates within the Mississippi and Atchafalaya River basins (MARB) (see Fig. Fig 1.2) and the current ability to use watershed models to route and predict nutrient delivery to the Gulf of Mexico. The following sections discuss the current levels of understanding and provide brief summaries of the Study Group’s key findings and recommendations.

Scientific Basis for Goals and Management Options

Adaptive management offers a way to address the pressing need to take steps to manage for factors affecting hypoxia in the NGOM in the face of uncertainties. The authors of a recent study undertaken by the National Research Council of the National Academy of Sciences identified six elements of adaptive management that are directly relevant to goal setting and research needs (National Research Council, 2004): (1) resources of concern are clearly defined; (2) conceptual models are developed during planning and assessment; (3) management questions are formulated as testable hypotheses to guide inquiry; (4) management actions are treated like experiments that test hypotheses to answer questions and provide future management guidance; (5) ongoing monitoring and evaluation is necessary to improve accuracy and completeness of knowledge; and (6) management actions are revised with new cycles of learning.