Print PageLong-term hydrologic and disturbance regimes determine biogeomorphic structure by influencing the prevalence and persistence of wetland plant-dominated and gravel-dominated stream reaches, which represent alternative ecosystem states in desert streams. Disturbance regimes also affect nutrient status through variations in the degree of terrestrial-aquatic connection. We expect both nutrient status and ecosystem state to influence ecosystem functioning (nutrient retention and ecosystem metabolism), and have established a long-term research program in Sycamore Creek, Arizona to test these hypotheses. Two integrative objectives of this research are 1) to determine how climate variability and change (over several years to decades) influence the factors controlling vegetation biomass over multiple years, and hence, the prevalence of the wetland state, and 2) to identify the nature of flow variation (floods) associated with interannual differences in ecosystem metabolism, nutrient retention, algal biomass in the two different reach types. We measured wetland patches over a 12-km reach in multiple years to correlate the hydrologic regime with changes in vegetation structure over time. Nitrate and phosphate uptake (using short-term nutrient release experiments), ecosystem metabolism (diel oxygen), and algal biomass were measured in 3 wetland and 3 gravel reaches in 2010 and 2011, two years with contrasting patterns of floods and drying.
Results/Conclusions
A large flood in 2010 appeared to wipe out most of the wetland vegetation that was present in 2009. However, as discharge declined and summer progressed, wetland patches recovered rapidly. During summer 2010, nitrate concentration in gravel reaches was 5 times lower (0.01 mg/l) compared to the wetland reaches (0.06 mg/l), while there was no difference in phosphate concentration between reaches (0.06 mg/l for both). Uptake velocity of nitrate was higher in two wetland reaches compared to gravel reaches in May, but the opposite was true in June. There were no differences in phosphate uptake, either between reaches or months. Nutrient-enrichment experiments (adding nitrogen and phosphorus, alone and in combination) demonstrated that nitrogen was limiting in both gravel-bed and wetland reaches; however, there was greater uptake of nitrogen in the wetland reaches (P = 0.03). Phosphorus was not secondarily limiting, as response to combined nitrogen and phosphorus enrichment was the same as for nitrogen alone. We expect these patterns to vary among years, given the importance of a highly variable disturbance regime in delivering nutrients and controlling vegetation biomass in this arid-land stream ecosystem.
