Collection and treatment of sewage in wastewater treatment plants (WWTPs) reduces the most deleterious effects of sewage on streams, but the treated effluent still has potentially important effects on stream ecosystems. Very little is known about the fate of these treated effluents in tropical streams. Short-term nutrient additions often are used to assess the fate of nutrients in streams, but long-term additions, such as those from sewage treatment plants, are likely to follow different uptake kinetics. Here we examined the longitudinal changes in Cl−, NO3−-N, NH4 +-N, dissolved organic N (DON), total dissolved N (TDN), soluble reactive P (SRP), and dissolved organic C (DOC) concentrations downstream from chronic WWTP inputs in 6 streams across the island of Puerto Rico. We applied the nutrient-uptake framework to these long-duration and high-magnitude nutrient additions to examine net nutrient uptake in these streams. On average, nutrient concentrations were increased more than 2× by sewage effluent at the study streams while DOC concentrations increased 30%. Net nutrient uptake was sporadic, and on many dates, no evidence of measurable uptake was found. Biogeochemical processes, such as nitrification and remineralization, appeared to be responsible for some of the longitudinal increases in nutrient concentrations. Limited nutrient uptake and increasing nutrient concentrations along the reaches suggested that most of the time, nutrients were being transported downstream with no significant net retention. This result suggests that downstream transport of nutrients is large with the potential to create water-quality problems in downstream ecosystems. Our study highlighted the profound effects that inputs from point sources may have on stream chemistry and nutrient cycling in tropical receiving streams.
Figueroa-Nieves D., McDowell, W.H., Potter, J.D., Martínez, G. (2016): Limited uptake of nutrient input from sewage effluent in a tropical landscape. Freshwater Science. DOI: 10.1086/684992