Mechanisms of P* Reduction in the Eastern Tropical South Pacific

Abstract

Water masses influenced by oxygen minimum zones (OMZ) feature low inorganic nitrogen (N) to phosphorus (P) ratios. The surplus of P over N is thought to favor non-Redfield primary production by bloom-forming phytoplankton species. Additionally, excess phosphate (P * ) is thought to provide a niche for nitrogen fixing organisms. In order to assess the effect of low inorganic nutrient ratios on the stoichiometry and composition of primary producers, biogeochemical measurements were carried out in 2012 during a research cruise in the eastern tropical South Pacific (ETSP). Based on pigment analyses, a succession of different phytoplankton functional groups was observed along onshore-offshore transects with diatoms dominating the productive upwelling region, and prymnesiophytes, cryptophytes, and Synechococcus prevailing in the oligotrophic open ocean. Although inorganic nutrient supply ratios were below Redfield proportions throughout the sampling area, the stoichiometry of particulate organic nitrogen to phosphorus (PON:POP) generally exceeded ratios of 16:1. Despite PON:POP ≥ 16, high P * -values in the surface layer (0-50 m) above the shelf rapidly decreased as water masses were advected offshore. There are three mechanisms which can explain these observations: (1) non-Redfield primary production, where the excess phosphorus in the biomass is directly released as dissolved organic phosphorus (DOP), (2) non-Redfield primary production, which is masked by a particulate organic matter pool mainly consisting of P-depleted detrital biomass, and/or (3) Redfield primary production combined with dinitrogen (N 2 ) fixation. Our observations suggest that the three processes occur simultaneously in the study area; quantifying the relative importance of each of these mechanisms needs further investigation. Therefore, it remains uncertain whether the ETSP is a net sink for bioavailable N or whether the N-deficit in this area is replenished locally.

Metabolic versatility of a novel N2‐fixing Alphaproteobacterium isolated from a marine oxygen minimum zone

“…4), indicating that they were not actively fixing N 2 at the time our study was performed. These results seem paradoxical, provided the significant N 2 fixation rates measured at these study sites (Meyer et al, 2017) and the confirmed presence of S. castanea at this site. But even if diazotrophy is a facultative trait of the presented isolate, we cannot define the extent to which the behaviour of an organism in the laboratory predicts activities in natural habitats.…”

“…Subsamples for nanoSIMS (50 ml) had been previously incubated in non‐sulfidic conditions with 15 N 2 and 13 C‐glucose for 24 hr (following Mohr et al . method and 15 N 2 supplied by Cambridge Isotopes; detailed description in Meyer et al ., ). Water samples were incubated in on‐deck incubators, which were connected to a flow‐through seawater system and were shaded with blue lagoon (Lee filters) foil to simulate light levels at the corresponding water depth.…”

“…The Peruvian OMZ has been subject to several N 2 fixation studies due to the possibility of diazotrophy being coupled to, and greatest, in areas of denitrification (P* hypothesis, Deutsch et al, 2007). At the time of isolation, N 2 fixation rates were measurable throughout the water column of the Peruvian shelf and were highest in the chemocline (1-5 nmol L 21 d 21 ; Meyer et al, 2017). In order to determine whether S. castanea was contributing to the measured rates, we analysed the isotopic composition of microscopically identified S. castanea cells from subsamples of the stable isotope incubations ( 15 N 2 ) performed by Meyer and colleagues (Meyer et al, 2017).…”

“…At the time of isolation, N 2 fixation rates were measurable throughout the water column of the Peruvian shelf and were highest in the chemocline (1-5 nmol L 21 d 21 ; Meyer et al, 2017). In order to determine whether S. castanea was contributing to the measured rates, we analysed the isotopic composition of microscopically identified S. castanea cells from subsamples of the stable isotope incubations ( 15 N 2 ) performed by Meyer and colleagues (Meyer et al, 2017). A total of 60 positively-hybridized S. castanea cells were analysed from different depths: surface oxic waters (5 m), surface anoxic waters (10 m), the chemocline (30 m) and deep sulfidic waters (60 m) ( Fig.…”

Submesoscale CO<sub>2</sub> variability across an upwelling front off Peru

“…A detailed description of the data processing, including the calibration of oxygen can be found in Thomsen et al (2016b). As the Chl a concentrations measured by the WET Labs (USA) fluorometer did not differ significantly from Chl a concentrations determined from water samples (Meyer et al, 2017), no further calibration than the factory calibration was applied to the fluorometer data (see Loginova et al, 2016, for details). In order to obtain further subsurface hydrographic information, 53 temperature profiles organized in the two cross-frontal transects B and C with a horizontal resolution of 0.3-0.5 km were measured using the CTD sensors mounted on a microstructure profiler.…”