ALBERT

Description: Dinitrogen fixation is now recognized as one of the major sources of bio-available nitrogen in the ocean. Thus, N2 fixation sustains a significant part of the global primary production by supplying the most common limiting nutrient for phytoplankton growth. The “Oligotrophy to UlTra-oligotrophy PACific Experiment” (OUTPACE) improved the data coverage of the western tropical South Pacific, an area recently recognized as a hotspot of N2 fixation. This new development leads us to develop and test an explicit N2 fixation formulation based on the Trichodesmium physiology (the most studied nitrogen fixer) within a 3-D coupled dynamical–biogeochemical model (ROMS-PISCES). We performed a climatological numerical simulation that is able to reproduce the main physical (e.g. sea surface temperature) and biogeochemical patterns (nutrient and chlorophyll concentrations, as well as N2 fixation) in the tropical Pacific. This simulation displayed a Trichodesmium regional distribution that extends from 150∘ E to 120∘ W in the south tropical Pacific, and from 120∘ E to 140∘ W in the north tropical Pacific. The local simulated maximuma were found around islands (Hawaii, Fiji, Samoa, New Caledonia, Vanuatu). We assessed that 15 % of the total primary production may be due to Trichodesmium in the low-nutrient low-chlorophyll regions (LNLC) of the tropical Pacific. Comparison between our explicit and the often used (in biogeochemical models) implicit parameterization of N2 fixation showed that the latter leads to an underestimation of N2 fixation rates by about 25 % in LNLC regions. Finally, we established that iron fluxes from island sediments control the spatial distribution of Trichodesmium biomasses in the western tropical South Pacific. Note, this last result does not take into account the iron supply from rivers and hydrothermal sources, which may well be of importance in a region known for its strong precipitation rates and volcanic activity.

Description: Dinitrogen fixation is now recognized as one of the major sources of bio-available nitrogen in the ocean. Thus, nitrogen fixation sustains a significant part of the global primary production by providing an input of the most common limiting nutrient for phytoplankton growth. Evidences of the Western Tropical South Pacific being a hotspot of nitrogen fixation, and a data coverage complemented by OUTPACE, lead us to develop an explicit nitrogen fixation compartment based on the Trichodesmium physiology (the most studied nitrogen fixer) within a 3D coupled dynamical-biogeochemical model (ROMS-PISCES). We performed a first 20-year tropical Pacific simulation that is able to reproduce the main physical (e.g. Sea Surface Temperature) and biogeochemical conditions (nutrients, and chlorophyll concentrations as well as dinitrogen fixation). This simulation showed a possible Trichodesmium regional distribution that extends from 150° E to 120° W in the south tropical Pacific, and from 120° E to 140° W in the north tropical Pacific. The local simulated maximums were around islands (Hawaii, Fiji, Samoa, New Caledonia, Vanuatu). We assessed that 15 % of the total primary production may be due to Trichodesmium in the Low Nutrient, Low Chlorophyll regions (LNLC). We also argue that implicit parameterization of N2 fixation (often used in biogeochemical models) leads to underestimate nitrogen fixation rates by about 25% in LNLC regions compared to our explicit formulation. Finally, we showed that iron fluxes from island sediments control the spatial distribution and the abundance of Trichodesmium in the western tropical south Pacific. Noteworthy, this last result does not take into account the iron supply from rivers and hydrothermal sources, which may well be of importance in a region known for its strong precipitation rates and volcanic activity.

Description: Marine particulate organic carbon stable isotope ratios (δ13CPOC) provide insights into understanding carbon cycling through the atmosphere, ocean and biosphere. They have for example been used to trace the input of anthropogenic carbon in the marine ecosystem due to the distinct isotopically light signature of anthropogenic emissions. However, δ13CPOC is also significantly altered during photosynthesis by phytoplankton, which complicates its interpretation. For such purposes, robust spatio-temporal coverage of δ13CPOC observations is essential. We collected all such available data sets and merged and homogenized them to provide the largest available marine δ13CPOC data set (https://doi.org/10.1594/PANGAEA.929931; Verwega et al., 2021). The data set consists of 4732 data points covering all major ocean basins beginning in the 1960s. We describe the compiled raw data, compare different observational methods, and provide key insights in the temporal and spatial distribution that is consistent with previously observed large-scale patterns. The main different sample collection methods (bottle, intake, net, trap) are generally consistent with each other when comparing within regions. An analysis of 1990s median δ13CPOC values in a meridional section across the best-covered Atlantic Ocean shows relatively high values (≥-22 ‰) in the low latitudes (