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Realistic model representation of ocean phytoplankton is important for simulating nutrient cycles and the biological carbon pump, which affects atmospheric carbon dioxide (pCO2) concentrations and, thus, climate. Until recently, most models assumed constant ratios (or stoichiometry) of phosphorous (P), nitrogen (N), silicon (Si), and carbon (C) in phytoplankton, despite observations indicating systematic variations. Here, we investigate the effects of variable stoichiometry on simulated nutrient distributions, plankton community compositions, and the C cycle in the preindustrial (PI) and glacial oceans. Using a biogeochemical model, a linearly increasing P:N relation to increasing PO4 is implemented for ordinary phytoplankton (PO), and a nonlinearly decreasing Si:N relation to increasing Fe is applied to diatoms (PDiat). C:N remains fixed. Variable P:N affects modeled community composition through enhanced PO4 availability, which increases N-fixers in the oligotrophic ocean, consistent with previous research. This increases the NO3 fertilization of PO, the NO3 inventory, and the total plankton biomass. The accuracy of modeled surface nutrients is relatively unchanged. Conversely, variable Si:N shifts south the Southern Ocean’s meridional surface silicate gradient, which aligns better with observations, but depresses PDiat growth globally. In Last Glacial Maximum simulations, PO respond to more oligotrophic conditions by increasing their C:P. This strengthens the biologically mediated C storage such that dissolved organic (inorganic) C inventories increase by 34-40 (38-50) Pg C and 0.7-1.2 Pg yr-1 more particulate C is exported into the interior ocean. Thus, an additional 13-14 ppm of pCO2 difference from PI levels results, improving model agreement with glacial observations.
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The Diel Cycle of Surface Ocean Elemental Stoichiometry has Implications for Ocean Productivity
Abstract Environmentally driven variability in the elemental stoichiometry of ocean plankton plays a key role in ocean biogeochemical processes. Recent studies have identified clear regional variability in C:N:P, but less is known about the environmental regulation of diel variability in plankton elemental stoichiometry. Here, we quantified the amplitude of the diel variability in C:N of surface ocean particles (<30 μm,C:Namp) across large latitudinal gradients in the Indian and Atlantic Oceans. We commonly observed diel oscillations in C:N and biome‐specific variability inC:Namp. Temperature emerged as the strongest predictor ofC:Namp, relative to the supply of nitrate. We propose thatC:Nampis positively related to photosynthesis and respiration and thus phytoplankton growth rates. We find that independent growth rate proxies and an ecosystem model support this hypothesis. In addition, the temperature sensitivity ofC:Namphas aQ10of 1.78 corroborating studies of phytoplankton growth rates. Surface communities across the Indian Ocean transect had a very small dependency on nitrate, whereas recycled nitrogen sources were by far the most preferred and the ratio of recycled‐N:nitrate utilization increased with increasingC:Namp. To predict future changes inC:Namp, we combined our statistical model with data from the fifth Coupled Model Intercomparison Project for the years 1990 and 2090. The results suggest that future rising temperatures will yield increasedC:Namp. Collectively, our results imply that rising surface ocean temperatures lead to elevated phytoplankton growth rates supported by recycled nutrients.
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- PAR ID:
- 10442936
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 36
- Issue:
- 3
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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