The influence of light on nitrogen cycling and the primary nitrite maximum in a seasonally stratified sea

In the seasonally stratified Gulf of Aqaba Red Sea, both NO 2 - release by phytoplankton and NH 4 + oxidation by nitrifying microbes contributed to the formation of a primary nitrite maximum (PNM) over different seasons and depths in the water column. In the winter and during the days immediately following spring stratification, NO 2 - formation was strongly correlated (R2 = 0.99) with decreasing irradiance and chlorophyll, suggesting that incomplete NO 3 - reduction by light limited phytoplankton was a major source of NO 2 - . However, as stratification progressed, NO 2 - continued to be generated below the euphotic depth by microbial NH 4 + oxidation, likely due to differential photoinhibition of NH 4 + and NO 2 - oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of the δ15N and δ18O in the combined NO 3 - and NO 2 - pool, suggesting that assimilation and nitrification were co-occurring in surface waters. As stratification progressed, the δ15N of particulate N below the euphotic depth increased from −5‰ to up to +20‰. ke rates were also influenced by light; based on 15N tracer experiments, assimilation of NO 3 - , NO 2 - , and urea was more rapid in the light (434 ± 24, 94 ± 17, and 1194 ± 48 nmol N L−1 day−1 respectively) than in the dark (58 ± 14, 29 ± 14, and 476 ± 31 nmol N L−1 day−1 respectively). Dark NH 4 + assimilation was 314 ± 31 nmol N L−1 day−1, while light NH 4 + assimilation was much faster, resulting in complete consumption of the 15N spike in less than 7 h from spike addition. The overall rate of coupled urea mineralization and NH 4 + oxidation (14.1 ± 7.6 nmol N L−1 day−1) was similar to that of NH 4 + oxidation alone (16.4 ± 8.1 nmol N L−1 day−1), suggesting that mineralization of labile dissolved organic N compounds like urea was not a rate limiting step for nitrification. Our results suggest that assimilation and nitrification compete for NH 4 + and that N transformation rates throughout the water column are influenced by light over diel and seasonal cycles, allowing phytoplankton and nitrifying microbes to contribute jointly to PNM formation. We identify important factors that influence the N cycle throughout the year, including light intensity, substrate availability, and microbial community structure. These processes could be relevant to other regions worldwide where seasonal variability in mixing depth and stratification influence the contributions of phytoplankton and non-photosynthetic microbes to the N cycle.