Nitrogenous nutrition of the plankton in the Chesapeake Bay. 1. Nutrient availability and phytoplankton preferences

Eight stations in the main body of Chesapeake Bay and one on the continental shelf were sampled seven times over a period of 13 months to investigate the nitrogenous nutrition of the phytoplankton. The rates at which the phytoplankton were utilizing NO 3 − , NO 2 − , NH 4 + , and urea N were determined. The data demonstrate that for a large portion of the year there is inadequate N nutrient available to permit a single doubling of the particulate N. Over temperatures from 4°–28°C and salinities from 2–32‰, there was a universally high phytoplankton preference for NH 4 + and urea N over NO 3 − and NO 2 − . A relative preference index indicated that NH 4 + concentrations in excess of 0.5–1.0 µ g‐atom N liter −1 almost totally suppressed NO 3 − utilization. Urea N was used after NH 4 + in order of preference, and when the sum of available NH 4 + and urea N was insufficient to meet the phytoplankton N nutrient demand, NO 3 − was used. When the sum of all available N nutrients was less than that required to satiate the phytoplankton demand, NH 4 + , urea N, NO 3 − , and NO 2 − were all utilized at rates proportional to their availability. For the midbay region in October 1973, NO 2 − was the dominant N nutrient present both in the water and in the diet of the phytoplankton.