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

Abstract

This study focuses on the seasonal changes in the Gulf of Aquaba, Red Sea, in nitrite concentration and their relationship with phytoplankton activity, which is mainly controlled by an alternation of water-column stratification with vertical mixing. Within the euphotic zone, during thermal summer stratification, nutrient depletion was severe, and no nitrite could be detected in the upper 70 m. However, during stratification, nitrite was always associated with the nutriclines and formed a deep maximum at the bottom of the euphotic zone. In contrast, nitrite accumulated in the mixed water column during winter, closely paralleling the development of phytoplankton biomass. In the Gulf of Aqaba, maximum 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 accumulation of nitrite is associated with nutrient-stimulated phytoplankton growth. This hypothesis was supported by nutrient-enrichment bioassays performed concomitantly: only when phytoplankton growth was stimulated by nutrient additions, did nitrite accumulate in the water. In the bioassays, the time-course of nitrite accumulation closely paralleled the development of phytoplankton biomass during the incubation period. We therefore suggest that the accumulation of nitrite in the mixed water column during winter is due to excretion by algal cells. Our field and experimental data show that between 10 and 15% of the total amount of nitrogen entering the mixed-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 deep nitrite maximum (DNM) during stratification in summer. In the bioassays, phytoplankton cells excreted nitrite even when ammonia was the nitrogen source. This indicates a so far unrecognised physiological pathway involved in nitrite excretion by phytoplankton cells. KEY WORDS: Nutrients · Nitrogen species · Nitrite · PhytoplanktonResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 239: 233-239, 2002 In the well-oxygenated water column of the Gulf of Aqaba, the dissimilatory reduction of nitrate to nitrite by denitrifying bacteria is likely to be negligible. Thus, we consider the following processes as potentially responsible for the generation and consumption of ambient nitrite in the Gulf of Aqaba: (1) Excretion of nitrite during nitrate reduction, i.e. incomplete assimilatory reduction of nitrate by phytoplankton and bacteria (Vaccaro & Ruyther 1960, Wada & Hattori, 1971, Miyazzaki et al. 1973, Miyazzaki et al. 1975, Kiefer et al. 1976, Herbland & Voituriez 1979, Olson 1981a, Dore & Karl, 1996b, Collos 1998. (2) Ammonium oxidation to nitrite by autotrophic nitrifying bacteria (Brandhorst 1959, Miyazzaki et al. 1975, Olson 1981a,b, Ward 1986, Dore & Karl 1996b, Enoksson et al. 1996. (3) Nitrite assimilation by p...

Hypoxia in deep waters of moderately eutrophic marine lakes, Island of Mljet, eastern Adriatic Sea

“…Higher t and Chl a are positively related to phytoplankton growth (Agawin et al 2000). Increased NO 2 in the photic zone might be notably related to excretion during phytoplankton growth (Al-Qutob et al 2002). This fits with the detection of maximum NO 2 concentrations in the photic zone of BL and G in March, but is less likely to be relevant for the maximum value of NO 2 in deep water of SL in October.…”

Forming the primary nitrite maximum: Nitrifiers or phytoplankton?

“…Under very similar physical conditions in the Red Sea, Al-Qutob et al (2002) also observed elevated NO 2 2 concentrations through the entire euphotic zone. In fact, NO 2 2 constituted up to 80% of the total oxidized nitrogen present in the Red Sea.…”

“…During the highly stratified summer months, the northwestern Sargasso Sea (Zafiriou et al 1992;Lipschultz et al 1996), just like many other ocean regions (Kiefer et al 1976;Dore and Karl 1996a;Al-Qutob et al 2002), shows a distinct PNM near the base of the euphotic zone that is surprisingly high (mean 58 nmol L 21 ; range 0-200 nmol L 21 ) in concentration, given the very oligotrophic nature of the system (Fig. 1).…”

“…A critical question arising from the Sargasso Sea data set, as well as that of Al-Qutob et al (2002) for the Red Sea, is the interaction between physical forcing and NO 2 2 cycling when the water column is not stratified and convective mixing exceeds the depth of the euphotic zone. Can active convective mixing convert phytoplankton from a net sink for NO 2 2 in the upper euphotic zone to a net source?…”

Quantum Yields in Aquatic Photosynthesis

“…David Iluz 1,2 and Zvy Dubinsky 1 1 The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel…”