Urea analysis in coastal waters: comparison of enzymatic and direct methods

Abstract

This study presents a comparison of two existing methods for the determination of urea concentration in seawater. These methods are referred to here as the enzymatic method, which is based on the use of the enzyme urease, and the direct method, which is based on the reaction of urea with diacetylmonoxime. A room temperature modification of the direct method was adapted for a single reagent and both the enzymatic and the direct method were tested in artificially prepared solutions and in natural samples from estuaries and shelf waters. We were particularly interested in the effects of salinity and humic acids on the accuracy of both methods. The effect of humic acids was negligible. In contrast, salinity ~34 caused a 15% to 40% underestimation in the urea concentrations measured by the enzymatic method and the degree of underestimation varied among enzyme batches. Urea concentrations corrected for the salt effect should, however, be considered estimations, as other factors also interfered with the enzymatic method in natural samples. The direct method as modified in this study presented a low detection limit (0.04 µM urea‐N) and high precision (standard deviation: 0.02 µM urea‐N; coefficient of variation: 1.1%) comparable to those of the enzymatic method. The direct method was more accurate and less salinity dependent than the enzymatic method. As urea levels could have been underestimated by the enzymatic method, our findings support previous conclusions regarding the important role of urea in the nitrogen cycle and its link with some harmful algal bloom phenomena.

Comparative effects of urea, ammonium, and nitrate on phytoplankton abundance, community composition, and toxicity in hypereutrophic freshwaters

“…Urea concentrations were measured using the diacetylmonoxime method of Revilla et al (2005), and dissolved inorganic carbon and DOC analyses followed standard Environment Canada procedures using a Shimadzu model 5000A total carbon analyzer (Finlay et al 2009). …”

Elevated ammonium concentrations from wastewater discharge depress primary productivity in the Sacramento River and the Northern San Francisco Estuary

“…These samples were also immediately frozen for later analysis by the colorimetric method of Solorzano (1969) using a Hewlett Packard diode array spectrophotometer and 10-cm path length cell. Samples for urea were prepared in the same manner as NH 4 samples with analysis performed according to Revilla et al (2005). Two size fractions were collected for analysis of extracted chlorophyll-a concentration using 25-mm Whatman GF/F filters (nominally cells >0.7-lm, referred to here as the ''whole community'' fraction) and 25-mm diameter 5.0-lm Nuclepore pore-sized polycarbonate filters.…”

Grazing by Karenia brevis on Synechococcus enhances its growth rate and may help to sustain blooms

“…Particulate N in filtered samples (GF/D or GF/F) was determined using a Control Equipment CHN analyzer and dissolved nutrients were analyzed using autoanalysis (Lane et al 2000), except for urea, which was measured using the protocol of Revilla et al (2005).…”