Intracellular ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ pools associated with deposited phytoplankton in a marine sediment (Aarhus Bight, Denmark)

Concentration profiles of ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ in pore water and particulate matter were determined at high spatial resolution (mm scale) in surface sediment from a coastal bay area (Aarhus Bight, Denmark) at 15 m depth during an annual cycle. Pore water pools of ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ were always considerably lower than particulate pools in the surface sediment. Particulate ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ were apparently intracellular pools in deposited microalgae and were extracted after freezing sediment samples in liquid N₂ (–196 °C). Pore water ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and most of the adsorbed (KCl-extractable) ${\mathrm{N}\mathrm{H}}_{4}^{+}$ were also extracted by the freezing technique, and an estimate of the intracellular ${\mathrm{N}\mathrm{H}}_{4}^{+}$ pool was obtained by difference. In the absence of an adsorbed ${\mathrm{N}\mathrm{O}}_{3}^{-}$ pool, intracellular ${\mathrm{N}\mathrm{O}}_{3}^{-}$ was determined by subtraction of the pore water pool from the liquid N₂-extractable pool. Highest concentrations of intracellular ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ were always observed in the upper 2 mm of sediment, declining sharply with depth. A distinct seasonal maximum for both pools, ca 200 nmol cm⁻³ at 0 to 2 mm depth, appeared after sedimentation of a phytoplankton bloom in early spring, and should be compared to a minimum of only 25 nmol cm⁻³ or less in fall and winter. The freeze-extraction technique is proposed for a reliable estimate of intracellular ${\mathrm{N}\mathrm{H}}_{4}^{+}$ and ${\mathrm{N}\mathrm{O}}_{3}^{-}$ pools in surface sediments rich in microalgae, and may thus be used as an indicator of sedimentation of phytoplankton blooms. The significance of intracellular pools for sediment nitrogen cycling is discussed.