Dissolved Inorganic Nutrients on GLOBEC Cruises NBP01-03 and NBP01-04

K. Fanning, Y. Serebrennikova, and R. Masserini

University of South Florida

Concentrations of most of the dissolved nutrients measured along the transects of the 2001 GLOBEC cruises changed very little between the cruises (NBP01-03 and NBP01-04):
Change (µM)
Upper Phosphate ~0.4 (or ~25% increase)
Deep Phosphate ~0.0
Upper Nitrate ~6.0 (or ~30% increase)
Deep Nitrate ~0.0
Upper Silica ~10 (or ~20% increase)
Deep Silica ~0

The 20-30% increases in upper concentrations of phosphate, nitrate, and silica between the two cruises suggest the occurrence of remineralization or regeneration of nutrients between the cruises. Deep values showed little change, as did the silica-to-nitrate ratio.

The nutrient that showed the greatest amount of relative change in concentration was ammonia, with values declining by 50-60% between the two cruises. All of the ammonia was in the upper water column. Integration to estimate ammonia stocks revealed that the reduction of available ammonia in the water column was even more drastic, with standing stocks declining from values of >400 µmol/sq. meter to <130 µmol/sq. meter. Marguerite Bay was the site of the highest standing stocks of ammonia on both cruises.

Two types of processes that could have affected ammonia are (1) mixing with surrounding waters containing lower ammonia concentrations and (2) biochemical reactions. The latter include production by ammonification and consumption because of further remineralization to nitrate (i.e., nitrification) and/or uptake by phytoplankton. Phytoplankton uptake was probably minimal because of the winter season and the fact that other nutrient concentrations increased between the two cruises. A composite regression of nitrate concentration versus ammonia concentration for both cruises suggested that the overall ratio of the changes to nitrate and ammonia was approximately -3 moles of nitrate to 1 mole of ammonia. This ratio is higher than the -1:1 ratio that would be expected if nitrification were the principal cause of ammonia reduction. Therefore some other processes were at work and might have included a more intense and varied mixing regime due to the greater scatter of points from the first cruise around the overall trend. The nitrate-ammonia regression for the second cruise is much more orderly and is consistent with two distinguishable mixing regimes - the first in the upper water column and the second between waters at the bottom of the upper water column and the deep water column.