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 nitrate-rich Thames estuary and adjacent area in the North Sea during February 1999. The majority of uptake was by heterotrophic bacteria, as demonstrated 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 preferred over nitrate. Urea was not important as nitrogen substrate. Amino acids were the main nitrogen substrate offshore and at the mouth of the estuary, but in the inner estuary nitrogen was mainly taken up as ammonium and, even more, as nitrate. Nitrate appeared to be the main substrate in the inner estuary (60 to 90 %) despite its low affinity for nitrate. The high nitrate uptake by heterotrophic bacteria is probably due to high ambient concentrations of nitrate (up to 650 mu M).