Abstract
Abstract. N2 fixation rates were measured daily in large (∼ 50 m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L−1 d−1 over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L−1 d−1. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10 % of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 µm) UCYN-C cells into large (100–500 µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labeling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.
Highlights
To light, nitrogen (N) is the major limiting factor for primary productivity in much of the low-latitude surface ocean (Falkowski, 1997; Moore et al, 2013)
Maximum diatom abundances were reached on day 15–16 at the very beginning of P2, and declined by day 18 to reach abundances similar to those observed during P1. These results suggest that diatoms were the primary beneficiaries of diazotrophderived nitrogen (DDN) in the mesocosms at the start of P2, when N2 fixation rates and unicellular N2-fixing cyanobacteria from group C (UCYN-C) abundances increased dramatically
While studies on the fate of DDN in the ocean are rare, the contribution of DDN to particle export based on the δ15N signatures of exported material indicates that N2 fixation can efficiently contribute to export production in the oligotrophic ocean (Dore et al, 2008)
Summary
Nitrogen (N) is the major limiting factor for primary productivity in much of the low-latitude surface ocean (Falkowski, 1997; Moore et al, 2013). Nitrate (NO−3 ) is the dominant form of fixed nitrogen (N) in seawater and derives from the remineralization of sinking organic N in the dark ocean. NO−3 is supplied to photic waters by upward mixing and transport, and constitutes the main source of fixed N for photosynthetic organisms in the temperate and high-latitude ocean. In the oligotrophic tropical and subtropical oceans, vertical mixing and transport of NO−3 is generally low and surface waters are often depleted in NO−3. In these ocean deserts, specialized organisms termed dinitrogen (N2) fixers (or diazotrophs) are able to use N in its simplest and most abundant form on Earth and in seawater, namely dinitrogen (N2). N2 fixation introduces a source of new bioavailable N to surface waters, and is considered to be the most important external source of N to the ocean, more significant than atmospheric and riverine inputs (Gruber, 2004)
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