Nitrogen uptake by heterotrophic bacteria and phytoplankton in the nitrate-rich Thames estuary

The uptake of ammonium, nitrate, amino acids and urea was examined in the n itrate-rich Thames estuary and adjacent area in the North Sea during Februa ry 1999. The majority of uptake was by heterotrophic bacteria, as demonstra ted by addition of a prokaryotic inhibitor that lowered uptake rates by 82, 66, 49 and 86 % for ammonium, nitrate, amino acids and urea, respectively. Amino acids were preferred over ammonium and urea, which in turn were pref erred over nitrate. Urea was not important as nitrogen substrate. Amino aci ds were the main nitrogen substrate offshore and at the mouth of the estuar y, but in the inner estuary nitrogen was mainly taken up as ammonium and, e ven more, as nitrate. Nitrate appeared to be the main substrate in the inne r estuary (60 to 90 %) despite its low affinity for nitrate. The high nitra te uptake by heterotrophic bacteria is probably due to high ambient concent rations of nitrate (up to 650 mu M).