Novel Technique for Assessing Ammonium Utilization by Phytoplankton in the San Francisco Bay-Delta Estuary

Abstract:

High concentrations of NH₄⁺ in the San Francisco Bay-Delta Estuary (SFE) have been shown to inhibit the growth of phytoplankton, which are an important food source to zooplankton at the base of the pelagic food web. Here we present results from a study which used a stable isotope mixing model to quantify the proportion of nitrogen assimilated as NH₄⁺ by phytoplankton in situ in a portion of the Sacramento River where NH₄⁺ concentration is elevated downstream of the Sacramento Regional Wastewater Treatment Plant (SRWTP). To determine the δ¹⁵N value of phytoplankton, a novel method was developed to isolate phytoplankton from bulk particulate organic matter using flow cytometry prior to isotopic analysis. Modifications were made to an elemental analyzer to allow measurement of the δ¹⁵N values of samples containing as little as 0.5 µg N with an analytical precision of 0.2‰ (determined from replicate analysis of standards). During fall and spring field campaigns, two parcels of Sacramento River water (one with wastewater effluent and one without) were tracked and sampled in a Lagrangian sampling scheme over ~80 hours of travel downstream of the SRWTP. Water samples were analyzed for nutrient and chlorophyll concentrations as well δ¹⁵N-NO₃ and δ¹⁵N-NH₄⁺. In addition, approximately ten million phytoplankton cells were sorted from each sample for analysis of δ¹⁵N-phytoplankton. In parcels of Sacramento River water without wastewater effluent, NH₄⁺ concentrations remained low and trends in δ¹⁵N-phytoplankton followed trends in δ¹⁵N-NO₃^-. In contrast, in the parcels containing SRWTP effluent phytoplankton uptake of N as NH₄⁺ gradually increased from 15% immediately downstream of the SRWTP to as high as 90% after 80 hours of downstream transit. Previous mesocosm incubation experiments have demonstrated depressed growth rates and a rapid switch from NO₃^- to NH₄⁺ uptake downstream of the SRWTP, suggesting that the apparent gradual increase in the proportion of N uptake as NH₄⁺ observed in this study may reflect the time required to overprint phytoplankton δ¹⁵N values at relatively low growth rates.