Intracellular NH4 and NO5 pools associated with deposited phytoplankton in a marine sediment (Aarhus Bight, Denmark)

Abstract

Concentration profiles of NH4+ and NO3- 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 NH4+ and NO3- were always considerably lower than particulate pools in the surface sediment Particulate NH4+ and NO3- were apparently intracellular pools in deposited microalgae and were extracted after freezing sediment samples in liquid N2 (-196.degree. C). Pore water NH4+ and most of the adsorbed (KCl-extractable) NH4+ were also extracted by the freezing technique, and an estimate of the intracellular NH4+ pool was obtained by difference. In the absence of an adsorbed NO3-pool, intracellular NO3- was determined by subtraction of the pore water pool from the liquid N2-extractable pool. Highest concentrations of intracellular NH4+ and NO3- 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 NH4+ and NO3- 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.