Nitrogen stimulation of periphyton biomass in rivers: Differential effects of ammonium-N and nitrate-N

Abstract

The ecological effects of wastewater treatment plant (WWTP) discharges into rivers typically include elevated periphyton biomass resulting from enrichment by dissolved reactive phosphorus (DRP), dissolved inorganic nitrogen (DIN), or both. WWTP discharges may transfer DIN to receiving water as ammonium-N (NH4-N) and, following nitrification steps, nitrate-N (NO3-N). The potential for NH4-N to exacerbate nuisance periphyton biomass compared to NO3-N is not well understood. Theoretically, NH4-N is the more energy-efficient source of N for algae, but its effect on algal growth rates depends on concentration, interaction with NO3-N, and environmental conditions, such as temperature. We investigated the relative importance of NH4-N and NO3-N in WWTP discharges by tracking periphyton biomass and algal assemblage composition in streamside channels for 27 d under 500 µg DIN/L comprising different proportions of NH4-N: 0.5% (low-NH4, ambient levels of NH4-N, 2.2 µg/L), 30% (medium-NH4, 160 µg/L), and 77% NH4-N (high-NH4, 389 µg/L). The DIN treatments had replete DRP (25 µg/L) and were compared to controls with no enrichment (control) and DRP enrichment only (P-control). Biomass was measured as chlorophyll a (Chl a) and ash-free dry mass (AFDM), which showed contrasting responses to different proportions of NH4-N enrichment. Chl a was higher under high-NH4 than under low-NH4 (p < 0.01, 50% higher at maximum Chl a on d 20). Under medium-NH4 and low-NH4, Chl a was not different from the controls and P-controls (p > 0.15). AFDM did not differ among the 3 DIN treatments (p > 0.11) but was higher in all DIN treatments than in the controls (p < 0.05). Algal assemblage composition discriminated the controls from all other treatments and the low-NH4 from high-NH4 treatments. The assemblage analyses indicated that high-NH4 favored enhanced cell division rates in small diatoms, especially Encyonema minutum. Higher Chl a/unit volume of small taxa than larger taxa likely explained elevated Chl a under high-NH4. The results illustrated that, with replete DRP, elevating DIN through enrichment with NH4-N may cause additional increases in periphyton biomass as Chl a compared to elevating DIN through NO3-N enrichment. Consequently, enhanced management to specifically reduce NH4-N outputs in WWTP discharges may assist in the control of nuisance periphyton downstream.