DOI: 10.6073/pasta/eb28ac2a44ea8c17084e229d431d17bd
title: Food-chain length in desert streams of central and southern Arizona, USA

description: ## overview Food 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 shifts—punctuated change between strings of high- and low-flow events—may 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. ## streamwater chemistry Water chemistry was monitored in quarterly in 11 desert streams of Arizona for two to three years. Sample collection occurred at an approximately monthly resolution in two of the streams, with additional sample collection following storms. Six streams were also sampled during fertilization experiments during which nitrate was added to a target concentration of 0.3 mg N/L. ## whole-stream metabolism Whole-stream metabolism was modeled from dissolved oxygen, light, and water temperature in 11 desert streams at a quarterly frequency for 1-3 years. In addition, metabolism of six streams was monitored during 12-14 d fertilization experiments, conducted once each in spring and autumn. During experiments, a control reach was paired with a reach fertilized with nitrate at a target concentration of 0.3 mg N/L. ## benthic organic matter Coarse and fine particulate benthic organic matter were collected and analyzed from 10 desert streams during baseflow conditions. All samples were analyzed for organic matter content, and fine particulate organic matter samples were analyzed for carbon:nitrogen ratio and delta-C-13. ## nutrient-diffusing substrata Nutrient-diffusing substrata (NDS) were incubated in nine streams in Arizona. Replicates (4) of control (no nutrient addition), nitrogen, phosphorus, and nitrogen + phosphorus additions were incubated for three weeks in each stream. NDS were incubated during summer baseflow (June 2017) and in autumn (October 2018). Samples were frozen until extraction and measurement of chlorophyll a concentration.