Evaluating biological nutrient removal for dissolved organic nitrogen quality and its impact on phytoplankton bloom dynamics in a eutrophic, freshwater reservoir
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Date
2019-07-31
Advisors
Journal Title
Series/Report No.
UNC-WRRI;474
WRRI Project;18-10-U
WRRI Project;18-10-U
Journal ISSN
Volume Title
Publisher
NC Water Resources Research Institute
Abstract
There is increasing evidence that dissolved organic nitrogen (DON) can stimulate phytoplankton growth and play an important role in structuring phytoplankton community composition. Specifically, high ratios of DON to dissolved inorganic nitrogen (DIN) may select for potentially toxic harmful algal blooms (HABs) which continue to plague lakes, rivers and freshwater reservoirs. Recent attention has been focused on the composition and quality of DON discharged in waste water treatment plant (WWTP) effluent, particularly as WWTPs switch from conventional activated sludge (CAS) to biological nutrient removal (BNR) processes in response to increasing pressure to reduce total nitrogen (TN) loading in WWTP effluent. BNR systems significantly reduce TN discharged in effluent, but increases the concentration of DON that may be bio-reactive and capable of stimulating phytoplankton growth. The goals of this project were to determine whether BNR processes produce bio-reactive DON and determine whether these compounds stimulate phytoplankton growth, particularly HAB species. A combination of bulk DON analyses, fluorescent spectroscopy, humic, and protein DON analyses were used to characterize the DON in the influent and effluent waters of a representative BNR WWTP to assess whether DON is produced during BNR treatment or is inherent to the influent water. Two experimental nutrient additions of influent and effluent waters were made during summer and fall 2018 to natural phytoplankton and microbial assemblages from a eutrophic reservoir (Jordan Lake, North Carolina) to determine the potential stimulatory role of effluent DON and to assess whether bio-stimulatory DON is produced by the BNR process or is inherent to the received influent water. Influent and effluent characterization revealed that influent waters contain high concentrations of potentially bioreactive protein. However, about 80% of the protein is removed within the WWTP process, and protein contained in the effluent appeared largely refractory. During the summer experiment, effluent additions stimulated total phytoplankton biomass beyond the stimulation that could be attributed to the effluent inorganic nutrient content. Accessory pigment analysis, indicated that diatoms/chrysophytes, chlorophytes, and dinoflagellates were the primary taxa that were stimulated. During the fall experiment, only the diatom/chrysophyte group was stimulated by effluent additions. Potentially harmful cyanobacteria taxa were either slightly inhibited (summer) or showed no response to effluent organic matter additions (fall). Based on canonical phytoplankton stoichiometry and lack of decline in the DON pool, phytoplankton growth during the experiments could be best explained solely by observed uptake of DIN. Therefore, it seems possible that the observed stimulation of phytoplankton growth is due to some other substance, e.g. a vitamin, other co-factor, or micronutrient, rather than direct incorporation of effluent DON. Results from these experiments indicate that although BNR effluent may contain a growth stimulatory substance, it does not contain significant quantities of highly labile DON that could fuel high biomass attainment beyond the carrying capacity set by its DIN concentration. Thus, in nature, BNR effluent would not be expected to strongly stimulate phytoplankton biomass increases in highly N limited reservoirs like Jordan Lake.
