Proposal for Research Funds Administered by the Office of Academic Programs

Human use of freshwater resources has altered the dynamics of hydrologic regimes in rivers around the world. Frequency, timing, and periodicity of extreme fluctuations in water flow have been modified, disrupting the flow regimes to which lotic biota have adapted over evolutionary time. The goal of the proposed research is to increase our understanding of how the hydrologic regime modulates links between biological communities and cycling of chemical elements, and by extension, to improve our predictions of ecosystem-scale consequences of alterations to flow regimes. I will use the theoretical framework of ecological stoichiometry to measure the relative incongruence of elemental ratios of biological supply and demand in the benthos of streams, and assess how these patterns are related to water flow characteristics. I will generate a comprehensive measurement of hydrologic disturbance that includes actual and potential substrate movement, and determine the stoichimetric relationships between benthic consumers and their resources in streams with distinct hydrologic regimes. The proposed research will integrate the traditionally diSt>arate field§ of or~anismal biology. community ecology, and ecosystem ecology to increase mechanistic understanding of dynamics operating at the levels of communities and ecosystems. INTRODUCnON Human impacts on water flow patterns of rivers are global in extent, and human-mediated change in water retention, land uSe, and climate can have often unanticipated consequences for lotic communities and ecosystem processes (Poff et al. 1997). Extreme fluctuations in flow and the accompanying movements of substrate are one of the primary sources of hydrologic disturbance in rivers and streams (Poff and Ward 1989) and are widely regarded as an important driver of community structure (Resh et al. 1988, Lake 2000). Given the knowledge that streams are hydrologically dynamic, studies of interactions between lotic communities and ecosystem processes hould incorporate the importance of extreme flow variation. The overarching goal of my research is to increase our understanding of how hydrologic regime modulates links between aquatic communities and cycles of biologically important chemical elements. The proposed research will use the the~s;,~ framework of ecological stoichiometry to integrate community and ecosystem processes in streams. BACKGROUND AND LITERATURE REVIEW Ecological stoichiometry and the balance of resource supply and demand Stoichiometry is the measurement of quantitative relationships between chemical constituents that are involved in, and produced by, chemical reactions. In ecological stoichiometry it is the balance between biologically important chemical elements in ecological interactions that is of interest. Ecological stoichiometry has been put forth as a conceptual framework that increases mechanistic understanding of complex ecological dynamics by distilling them into mass balance relationships (Elser et al. 1996, Sterner and Elser 2002). Using ecological stoichiometry, for example, food web interactions can be examined by congruence of elemental ratios of biological supply and demand, and causes and consequences of a lack thereof (Sterner et al. 1996, Schade et al. 2005). Under stable conditions a species in relative stoichiometric balance with a given resource supply will have fewer food quality constraints on growth and reproduction, and thus gain a competitive advantage over other species within a functional group (Sterner and Elser 2002). Yet conditions are not often stable in natural systems, and temporal and spatial heterogeneity can alter supply ratios at different scales. Factors 1 J. Moslemi; Andrew W. Mellon Research Grant external to communities-such as temperature, disturbance regime, and solar radiation-provide a "stoichiometric template" (Schade et al. 2005), defining the range of stoichiometric responses of communities to supply ratios (Elser et al. 2000, Woods et al. 2003). Such factors may drive and/or maintain stoichiometric imbalances, acting against forces such as natural selection that over evolutionary time should bring organisms towards stoichiometric balance with their food (Redfield 1958, Sterner et al. 2004). Despite the influence of these external factors on elemental cycles in ecosystems, their identities and importance are often unclear. Hydrologic disturbance and stochiometric balance in benthic systems Ecologists are just beginning to recognize external factors that form the stoichiometric template at broad scales in streams (e.g anthropogenic nutrient enrichment; Bowman et al. 2005). Hydrologic regime is likely a significant determinant of the stoichiometric template because of its influence on community structure and dissolved nutrient availability on broad scales. During floods, high discharge can suspend sediments, reduce nutrient availablity, move and redistribute benthic material, remove algae by scouring the streambed, and kill or displace biota (Lake 2000, Holmes et al. 1998). By subjecting organisms to a harsh environment characterized by scouring, extreme flow events can alter relative densities of consumers and their prey (Peckarsky 1983). Since benthic species often differ in elemental composition (Cross et al. 2003, Evans-White et al. 2005), hydrologic events that drive the identity and relative densities of biota can change systemwide stoichiometric patterns of resource supply and demand (Schade et al. 2005). Hydrologic disturbance can also reduce dissolved nutrient availability, which can change the stoichiometry of benthic producers that serve as food resources to higher trophic levels (Holmes et al. 1998). The hydrologic regime has the potential to alter elemental patterns in benthic systems by mediating community structure and dissolved nutrient availability, but links between stoichiometric imbalance and hydrologic disturbance have not been examined. The proposed research would provide information to improve our ability to predict consequences-ranging from impacts on biogeochemical cycles to food quality constraints on individual organisIn£-Of human modification of hydrologic regimes. RESEARCH OBJECTIVE AND HYPOTHESIS ~bjective: Identify watershed-scale relationships of hydrologic disturbance regime and stoichiometric imbalance between resource supply and demand in benthic communities. Hypothesis: Relatively high levels of hydrologic disturbance will be associated with increased stoichiometric imbalance between consumers and resources in benthic systems. Rationale: Communities associated with relatively harsh hydrologic regimes experience a reduction in resource availability due to the scouring caused by flo~ events. These organisms may be forced to draw from lower quality food patches, likely increasing stoichiometric imbalance between resource supply and demand relative to systems associated with more physically benign conditions.