Both ecosystem metabolism and nutrient concentrations are important indicators of aquatic ecosystem functioning and health, and they are often linked to each other. Ecosystem metabolism encompasses the transfer of energy into and out of the system via gross primary production (GPP) and ecosystem respiration (ER). Nutrient and dissolved organic carbon (DOC) concentrations indicate the system’s capacity to support that production through biotic assimilation, but also reflect levels of external inputs that may exceed assimilative capacity. Urbanization and the engineering of stream channels can have a significant impact on nutrient and DOC input to ecological systems and thus metabolism. Using long-term data provided by the Salt River Project and the Central Arizona-Phoenix Long Term Ecological Research program, we examined spatial and temporal variability in rates of GPP and ER and dissolved nutrient and DOC concentrations in the Phoenix metropolitan area canal system that brings water for home use into the city from the Salt, Verde, and Colorado River basins. We predicted that rates of metabolism and nutrient and DOC concentrations would differ among sites within the canal system. Specifically, we expected the influence of urbanization to increase as canal channels progressed further into the city, with higher rates of GPP and ER supported by higher nutrient and DOC concentrations, respectively.
We found significant differences in total nitrogen, dissolved inorganic nitrogen, dissolved organic nitrogen, DOC, and total phosphorus across canal sites. We observed spatial and temporal differences in dissolved oxygen concentration and metabolic rates within and across sites. Rates of GPP and ER were generally lower at canal sites (mean GPP 11.1 g O2 m-2 day-1, mean ER -5.0 g O2 m-2 day-1) than in comparable natural streams (mean GPP 14.4-28.8 g O2 m-2 day-1, mean ER -10.56 g O2 m-2 day-1), but indicate that the canal system is autotrophic and has a high productive capacity. Concentration of nitrate in particular varied among sites, with higher concentrations being found in canals where groundwater pumping is prevalent. These variable nitrate inputs may account for spatial variation in GPP and ER. This study has implications for drinking water treatment and quality in the Phoenix metropolitan area, as elevated levels of nitrate and primary productivity can contribute negatively to human health and wellbeing.