Effects of Flow Regime Shifts Anticendent Hydrology Nitrogen Pulses and Resource Quantity and Quality on Food Chain Length in Rivers

OverviewFood chain length (FCL) is a key measure of the vertical structure of food webs that determines energy flow through ecosystems, carbon exchange between freshwater ecosystems and the atmosphere, and rates of nutrient cycling. FCL also has a strong bearing on the biomass of green plants in ecosystems and hence on water quality in aquatic ecosystems. Broad-scale syntheses of controls on FCL in stream ecosystems indicate that FCL declines with discharge variation but, counter to theory, does not vary significantly with energy supply. The mechanisms linking discharge and energy to FCL are largely unresolved in streams. We propose that lack of a relationship between energy supply and FCL may be due to variation in efficiency of energy transfer caused by constraints of food quality, or to a temporal mismatch between measures of energy inputs and FCL. Alternatively, the effects of flow variation on FCL may simply be paramount to energy supply, but potential mechanisms linking flow to FCL remain untested. Regime shiftspunctuated change between strings of high- and low-flow eventsmay cause comprehensive faunal replacement across trophic levels and collapse of the vertical structure of food webs. FCL may change as a result of loss (or gain) of an apex predator, or as a result of changes in feeding relationships leading to apex predators that eat higher on the food chain. Finally, flow variation may indirectly influence FCL through inputs of limiting nutrients during floods. In desert streams, algae typically provide the primary source of energy, and algal production is limited by nitrogen (N). N loading from terrestrial ecosystems is strongly related to flow variation, particularly to the inter-flood interval (IFI) or duration of baseflow between floods. Long IFI leads to larger N pulses and potentially greater net ecosystem production (NEP), thereby providing an indirect effect of flow variation on FCL. Specific aims of the research include: 1) Quantify the effect of food quality, energy supply and energetic efficiencies on FCL and trophic structure, 2) Quantify the effects of IFI on FCL and trophic structure caused by episodic stimulation of NEP by N inputs, and 3) Quantify the effect of flow regime shifts on FCL and trophic structure via direct mortality, shifts in the trophic base of production, and reassembly of the vertical structure of the food web. Proposed research activities include characterization of the hydrologic regime, analysis of food webs across a hydroclimate gradient, and manipulation of N supply. Extreme event statistics and spectral analyses will characterize properties of flood intervals and flow regime shifts across 12 study sites spanning a gradient in timing of rainfall and hydrologic variation (monsoon vs. winter precipitation dominance) in Arizona. Derived hydrologic metrics will be used in combination with measures of ecosystem metabolism, N supply, secondary consumer energetic efficiencies, resource stoichiometry, and the proportion of autochthonous energy sources as predictor variables of FCL and trophic structure to understand the mechanisms linking energetics and hydrology to FCL. This comparative study includes 12 streams within the same biogeographic province that feature an algal-dominated food source and similar ecosystem size (1st-3rd order streams). Finally, a novel N fertilization experiment in 6 of the same 12 streams will eliminate the indirect effects of hydrology on N loading by simulating pulsed input of N to isolate the direct effects of hydrology from a potential indirect fertilization effect on river food webs. Intellectual Merit This work will transform our understanding of food webs by directly testing the interaction between two putative controls (disturbance and energy supply), thereby broadening empirical support for mechanisms determining the number of trophic levels in ecological systems. The work will also unify related research in community ecology, ecosystem science and hydrology to provide a new causal, ecohydrologic framework for understanding riverine food webs. Broader Impacts The proposed project includes training of several undergraduates, graduate students, and a postdoc. Work with a non-profit group will integrate project findings with an existing citizen science program on river drying sponsored by an NGO and a K-5 environmental outreach program. Finally, the PIs will establish an innovative open source graduate distributed seminar on application of spectral methods in ecology.

We propose a Research Experience for Undergraduates supplement to support the inclusion of an undergraduate researcher on the Food Chain Length (FCL) in Rivers project currently supported by the Division of Environmental Biology (#1457689). The FCL project has been designed to understand how variation of streamflow affects energy transfer through food webs. Research focuses specifically on floods, droughts, and nitrogen pulses during floods as drivers of variation in stream food chain length. Ongoing research conducted in streams across a gradient of seasonal precipitation in Arizona includes repeated measures of variation in food chain length, ecosystem metabolism, and nitrate concentrations, as well as experimental nitrogen pulse manipulations. A REU student will contribute both to the field campaign and laboratory sample processing while working daily with experienced project personnel to learn techniques for quantifying food chain length, and deploying and maintaining instream sensors. The student will collaborate closely with a principal investigator, graduate students, and technicians to develop an independent research project that will build on the planned research. Furthermore, the student will also have the opportunity to collaborate with other researchers studying different aspects of desert streams, such as biogeochemistry and plant community ecology.