The Rate of Utilization of Urea, Ammonium, and Nitrate by Natural Populations of Marine Phytoplankton in a Eutrophic Environment

Abstract

The utilization rates of ammonium, nitrate ion, and urea were determined for 18 samples of water from the southern sector of Kaneohe Bay, Oahu, Hawaiian Islands. The samples were collected from 14 May through 23 August 1974. The mean daytime uptake rates for this period were 0.040,0.033, and 0.013 hr-1 for ammonium, urea, and nitrate, respectively. Dark uptake rates for ammonium, urea, and nitrate from two samples were approximately 50, 30, and 0 percent of the daytime uptake rates. The uptake data indicate that the phytoplankton growth rate is not limited by the availability of fixed nitrogen. This conclusion is supported by the data on the carbon: nitrogen ratio of the phytoplankton, which show that the plants were more heavily enriched in nitrogen than they had been during previous studies of this part of the bay. Mass balance calculations show that the supply of fixed nitrogen to the nutrient pool from stream runoff and municipal waste discharge was only 3.5 percent of the total uptake rate by phytoplankton, and, therefore, suggest that the in situ regeneration of nutrients is far larger than the new nutrients added to the bay from these sources.