DISTRIBUTION AND FORMS OF NITROGEN IN A LAKE ONTARIO SEDIMENT CORE

Fixed ammonium nitrogen is the dominant inorganic form of nitrogen in the scdimcnts of Lake Ontario. The fixed ammonium nitrogen concentration is around 300 ,ug/g of sediment at the top of a 10-m core and increases gradually to 525 pg at 150 cm, below which it remains constant. As nitrification is precluded in the rcduccd sediments below 3 cm, the ammonium ion is tither fixed within the scdimcnt clay lattices or migrates upward in the sediment interstitial waters. The uniform fixed and exchangcablc ammonium concentrations below 150 cm in the core indicate that the sediment is saturated with rcspcct to ammonium fixation, and the dccrcasc in thcsc concentrations above 150 cm in the core suggest that equilibrium is not attained with the ammonium ion. The dccpcr scdimcnts probably do not regenerate nitrogen to Lake Ontario, but most of the nitrogen relcascd to the hypolimnion by the sediments is from nitrification, denitrification, and ammonification reactions at the sediment-water intcrfacc. A minimum of 20% of the organic nitrogen input to the scdimcnts is rcgcneratcd to the lake from the top G cm of sediments. About 90% of the nitrogen in the surface muds is organic: 28-46% as amino acid-N, 4-7s as hcxosamine-N, and 21-3170 as hydrolyzable unidentified-N, From 29-57s of the total nitrogen could not be accounted for as amino acids, hcxosamincs, fixed and cxchangeablc amnonia, nitrate, and nitrite in the surface sediments.