Diel variability in nitrogenous nutrient uptake at photic and sub-photic depths

Diel patterns in nitrogen uptake were investigated at three positions along an offshore orientated transect in the Southern Benguela Upwelling System. Both photic and sub-photic depths were targeted in the ¹⁵N tracer experiment. An appreciable proportion of nitrogen uptake was found to occur at sub-photic depths during both the day and the night. The importance of sub-photic uptake decreased offshore but remained non-trivial at all locations. Although NH₄ dominated this uptake. NO₃ removal at depth did account for an important fraction of the total water column NO₃ uptake, particularly over the shelf at night. For example, sub-photic NO₃ uptake at night for the inshore stations averaged 68% of the total for the water column declining to 3% offshore. The impact of sub-photic NO₃ uptake on new production will depend on whether the NO₃ is incorporated into primary assimilation pathways or is transported to the sediments in inorganic form. Whereas sub-photic/photic uptake decreased offshore, night/day uptake ratios increased with distance from the coast, approaching unity at the offshore location. This effect was most obvious for the photic communities. This finding supports the belief that dark nitrogen uptake assumes increased importance as nutrients become less available. Photic zone f-ratios were smaller at night (mean = 0.14) than during the day (mean = 0.28) indicating a greater importance of reduced nitrogen uptake during the dark hours. This emphasis on NH₄ utilization at night can be explained purely in terms of energy efficiency. Such diel phasing in f-ratios needs to be accounted for in scaling total production to new production.