Distribution of inorganic and organic nutrients in the South Pacific Ocean − evidence for long-term accumulation of organic matter in nitrogen-depleted waters

Abstract

Abstract. During the BIOSOPE cruise the RV Atalante was dedicated to study the biogeochemical properties in the South Pacific between the Marquesas Islands (141 • W-8 • S) and the Chilean upwelling (73 • W-34 • S). Over the 8000 km covered by the cruise, several different trophic situations were encountered, in particular strong oligotrophic conditions in the South Pacific Gyre (SPG, between 123 • W and 101 • W). In this isolated region, nitrate was undetectable between the surface and 160-180 m and only trace quantities (<20 nmoles l −1 ) of regenerated nitrogen (nitrite and ammonium) were detected, even in the subsurface maximum. Integrated nitrate over the photic layer, which reached 165 m, was close to zero. Despite this severe nitrogendepletion, phosphate was always present in significant concentrations (≈0.1 µmoles l −1 ), while silicic acid was maintained at low but classical oceanic levels (≈1 µmoles l −1 ). In contrast, the Marquesas region (MAR) to the west and Chilean upwelling (UPW) to the east were characterized by high nutrient concentrations, one hundred to one thousand fold higher than in the SPG. The distribution of surface chlorophyll reflected the nitrate gradient, the lowest concentrations (0.023 nmoles l −1 ) being measured at the centre of the SPG, where integrated value throughout the photic layer was very low (≈ 10 mg m −2 ). However, due to the relatively high concentrations of chlorophyll-a encountered in the DCM (0.2 µg l −1 ), chlorophyll-a concentrations throughout the photic layer were less variable than nitrate concentrations (by a factor 2 to 5). In contrast to chlorophyll-a, integrated particulate organic matter (POM) remained more or less constant along the study area (500 mmoles m −2 , 60 mmoles m −2 and 3.5 mmoles m −2 for particulate organic carbon, particulate organic nitrogen and particulate organic phosphorus, Correspondence to: P. Raimbault (patrick.raimbault@univmed.fr) respectively), with the exception of the upwelling, where values were two fold higher. The residence time of particulate carbon in the surface water was only 4-5 days in the upwelling, but up to 30 days in the SPG, where light isotopic δ 15 N signal noted in the suspended POM suggests that N 2 -fixation provides a dominant supply of nitrogen to phytoplankton. The most striking feature was the large accumulation of dissolved organic matter (DOM) in the SPG compared to the surrounding waters, in particular dissolved organic carbon (DOC) where concentrations were at levels rarely measured in oceanic waters (>100 µmoles l −1 ). Due to this large pool of DOM in the SPG photic layer, integrated values followed a converse geographical pattern to that of inorganic nutrients with a large accumulation in the centre of the SPG. Whereas suspended particulate matter in the mixed layer had a C/N ratio largely conforming to the Redfield stochiometry (C/N≈6.6), marked deviations were observed in this excess DOM (C/N≈16 to 23). The marked geographical trend suggests that a net in situ source exists, mainly due to biologic...

Measurements of nitrite production in and around the primary nitrite maximum in the central California Current

“…1e, f). Co-localization of these features has been observed in isolated profiles (Olson, 1981a;Lipschultz et al, 1996;Beman et al, 2012), and across the South Pacific Gyre (Raimbault et al, 2008). The localization of sequential maxima in Chl a, NH , however, were measured below and not within the peak of the PNM, consistent with previous reports from the North Pacific (Dore and Karl, 1996a).…”

“…It fuels microbial metabolism as an intermediate in both nitrification and denitrification, and is a substrate for anaerobic ammonium oxidation (anammox). NO Lipschultz, 2006;Karl et al, 2008) ranging in magnitude from tens of nmol L −1 in the oligotrophic gyres (Wada and Hattori, 1971;Dore and Karl, 1996b;Raimbault et al, 2008) to several µmol L −1 in productive coastal upwelling areas (Codispoti et al, 1986;Naqvi et al, 1998;Newell et al, 2011).…”

The composition and flux of particulate and dissolved carbohydrates from the Rhone River into the Mediterranean Sea

“…To remove inorganic carbon, the GF/F filters with collected particles were covered with 100 µl of 0.5 N H 2 SO 4 and then dried at 60 • C. The POC analyses on the filters were carried out using high temperature combustion (900 • C) performed on a CN Integra mass spectrometer (Raimbault et al, 2008). Analytical errors based on duplicate analysis were approximately 5 %.…”

Carbon sources in suspended particles and surface sediments from the Beaufort Sea revealed by molecular lipid biomarkers and compound-specific isotope analysis

“…Total Chl a was obtained by using the method described in Ras et al (2008). Nutrient concentrations (nitrate, phosphate and silicate) were determined onboard using the methods described in Raimbault et al (2008). The dissolved CO 2 concentration was derived from alkalinity, pH, temperature, salinity and the concentrations of silicate and phosphate using the CO2SYS program developed for CO 2 system (Lewis and Wallace, 1998).…”