Phytoplankton drives nitrite dynamics in the Gulf of Aqaba, Red Sea

Kurzfassung

This study focuses on the seasonal patterns of nitrite concentration changes and its relationship with phytoplankton activity that is mainly controlled by the alternation of the water column stratification and mixing. Within the euphotic zone during thermal summer stratification, nutrient depletion was severe, and no nitrite could be detected. However, during stratification nitrite was always associated with the nutriclines and formed a deep maximum at the bottom of the euphotic zone. By contrast, within the mixed water column during winter nitrite accumulated, closely following the development of phytoplankton biomass. In the Gulf of Aqaba, the maximum of nitrite accumulation occurred when winter mixing reached its greatest depth, which in turn was coincident with the height of the phytoplankton spring bloom. Thus, our field data suggest that the accumulation of nitrite is associated with nutrient-replete phytoplankton growth. This hypothesis was supported by nutrient enrichment bioassays performed concomitantly: Only when phytoplankton growth was stimulated by nutrient additions, nitrite accumulated in the water. In the bioassays, the time-course of nitrite accumulation closely followed the development of phytoplankton biomass during the incubation period. We therefore suggest that the accumulation of nitrite in the mixed water column during winter was due to the excretion by algal cells. Our field and experimental data show that between 10 to 15 % of the total amount of nitrogen introduced to the mixing water column is released as nitrite by phytoplankton. Further, our field and experimental data support the hypothesis that nitrite excretion by phytoplankton has a significant role in the formation of the DNM during stratification in summer. In the bioassays, phytoplankton cells did excrete nitrite even when ammonia was the nitrogen source. This gives evidence, that there is a so far unrecognised physiological pathway involved in nitrite excretion by phytoplankton cells.