“…Incubations with trace amounts of 15 N isotopes added to water samples have been used for evaluating nitrate, ammonium, urea and mixed amino acid uptake by phytoplankton in environmental since the 1960s (Dugdale & Goering, 1967;Neess, Dugdale, Dugdale, & Goering, 1962). Given that historically N has been thought to limit phytoplankton growth in marine systems and phosphorus more so in freshwaters, most direct uptake studies on natural communities have focused on marine samples, with fewer examples carried out in lakes and reservoirs (Berman, Sherr, Sherr, Wynne, & McCarthy, 1984;du Plooy, Perissinotto, Smit, & Muir, 2015;Takamura, Iwakuma, & Yasuno, 1987;Toetz, Varga, & Loughran, 1973).…”