THE IMPACT OF NITROGEN AND PHOSPHORUS RELEASE FROM A SILICEOUS SEDIMENT ON THE OVERLYING WATER

Abstract

The release (+) or uptake (−) of ammonia (NH4+), nitrite (NO2−), nitrate (NO3+), phosphate (PO4−3) and dissolved organic phosphorus, nitrogen and carbon (DOP, DON, DOC) from a subtidal (18.3 m depth) siliceous sediment off La Jolla, California was studied over two years. The mean net exchange values (and the total range of rates obtained) were: NH4+ +872μ mol m−2d−1 (−47 to +3290); NO2− +34 μ mol m−2 d−1 (−5 to +97); NO3− −77 μ mol m−2d−1 (−720 to +647); PO4−3 +77 μ mol m−2 d−1 (−438 to +502); DOP +12 μ mol m−2 d−1 (−28 to +59); DON −75 μ mol m−2 d−1 (−1326 to +1280); and DOC −583 μ mol m−2 d−1 (−30,800 to +23,800). Using published primary production rates and C:N:P ratios it was calculated that 15,000 μ mol N m−2 d−1 and 935 μ mol P m−2 d−1 were taken up by the phytoplankton. The benthos released 786 μ mol N m−2 d−1 and 90 μ mol P m−2 d−1 or 5% and 10% of the required N and P. It was shown, however, that sediment exchange released only an insignificant fraction of both N (0.4%) and P (0.5%) already contained in the water. By using a conservative mean net advection rate of 1.5 km d−1 it must be presumed that the N and P released into the water at this site was superfluous to the needs of the phytoplankton and was, therefore, exported from the area.