scholarly journals Modelling N<sub>2</sub> fixation related to <i>Trichodesmium</i> sp.: driving processes and impacts on primary production in the tropical Pacific Ocean

2018 ◽  
Vol 15 (14) ◽  
pp. 4333-4352 ◽  
Author(s):  
Cyril Dutheil ◽  
Olivier Aumont ◽  
Thomas Gorguès ◽  
Anne Lorrain ◽  
Sophie Bonnet ◽  
...  
Keyword(s):  
Primary Production ◽  
N2 Fixation ◽  
New Caledonia ◽  
Regional Distribution ◽  
South Pacific ◽  
Tropical Pacific ◽  
Biogeochemical Model ◽  
Available Nitrogen ◽  
Biogeochemical Models ◽  
The Tropical Pacific

Abstract. 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.

scholarly journals Modelling the processes driving <i>Trichodesmium</i> sp. spatial distribution and biogeochemical impact in the tropical Pacific Ocean

2018 ◽  
Author(s):  
Cyril Dutheil ◽  
Olivier Aumont ◽  
Thomas Gorguès ◽  
Anne Lorrain ◽  
Sophie Bonnet ◽  
...  
Keyword(s):  
Nitrogen Fixation ◽  
Spatial Distribution ◽  
Primary Production ◽  
New Caledonia ◽  
Regional Distribution ◽  
South Pacific ◽  
Tropical Pacific ◽  
Dinitrogen Fixation ◽  
Biogeochemical Model ◽  
Biogeochemical Models

Abstract. 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.

scholarly journals Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)

2018 ◽  
Vol 15 (23) ◽  
pp. 7273-7297 ◽  
Author(s):  
François Carlotti ◽  
Marc Pagano ◽  
Loïc Guilloux ◽  
Katty Donoso ◽  
Valentina Valdés ◽  
...  
Keyword(s):  
Primary Production ◽  
Food Web ◽  
N2 Fixation ◽  
New Caledonia ◽  
Zooplankton Community ◽  
South Pacific ◽  
Zooplankton Biomass ◽  
Top Down ◽  
Coral Sea ◽  
High Contribution

Abstract. The western tropical South Pacific (WTSP) is one of the most understudied oceanic regions in terms of the planktonic food web, despite supporting some of the largest tuna fisheries in the world. In this stratified oligotrophic ocean, nitrogen fixation may play an important role in supporting the plankton food web and higher trophic level production. In the austral summer (February–April) of 2015, the OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project conducted a comprehensive survey of 4000 km along 20∘ S, from New Caledonia to Tahiti, to determine the role of N2 fixation on biogeochemical cycles and food web structure in this region. Here, we characterize the zooplankton community and plankton food web processes at 15 short-duration stations (8 h each) to describe the large-scale variability across trophic gradients from oligotrophic waters around Melanesian archipelagoes (MAs) to ultra-oligotrophic waters of the South Pacific gyre (GY). Three long-duration stations (5 days each) enabled a more detailed analysis of processes and were positioned (1) in offshore northern waters of New Caledonia (MA), (2) near Niue Island (MA), and (3) in the subtropical Pacific gyre (GY) near the Cook Islands. At all stations, meso-zooplankton was sampled with a bongo net with 120 µm mesh size to estimate abundance, biomass, community taxonomy and size structure, and size fractionated δ15N. Subsequently, we estimated zooplankton carbon demand, grazing impact, excretion rates, and the contribution of diazotroph-derived nitrogen (DDN) to zooplankton biomass. The meso-zooplankton community showed a general decreasing trend in abundance and biomass from west to east, with a clear drop in the GY waters. Higher abundance and biomass corresponded to higher primary production associated with complex mesoscale circulation in the Coral Sea and between 170–180∘ W. The taxonomic structure showed a high degree of similarity in terms of species richness and abundance distribution across the whole region, with, however, a moderate difference in the GY region, where the copepod contribution to meso-zooplankton increased. The calculated ingestion and metabolic rates allowed us to estimate that the top–down (grazing) and bottom–up (excretion of nitrogen and phosphorous) impact of zooplankton on phytoplankton was potentially high. Daily grazing pressure on phytoplankton stocks was estimated to remove 19 % to 184 % of the total daily primary production and 1.5 % to 22 % of fixed N2. The top–down impact of meso-zooplankton was higher in the eastern part of the transect, including GY, than in the Coral Sea region and was mainly exerted on nano- and micro-phytoplankton. The regeneration of nutrients by zooplankton excretion was high, suggesting a strong contribution to regenerated production, particularly in terms of N. Daily NH4+ excretion accounted for 14.5 % to 165 % of phytoplankton needs for N, whereas PO43- excretion accounted for only 2.8 % to 34 % of P needs. From zooplankton δ15N values, we estimated that the DDN contributed to up to 67 % and 75 % to the zooplankton biomass in the western and central parts of the MA regions, respectively, but strongly decreased to an average of 22 % in the GY region and down to 7 % in the easternmost station. Thus, the highest contribution of diazotrophic microorganisms to zooplankton biomass occurred in the region of highest N2 fixation rates and when Trichodesmium dominated the diazotrophs community (MA waters). Our estimations of the fluxes associated with zooplankton were highly variable between stations and zones but very high in most cases compared to literature data, partially due to the high contribution of small forms. The highest values encountered were found at the boundary between the oligotrophic (MA) and ultra-oligotrophic regions (GY). Within the MA zone, the high variability of the top–down and bottom–up impact was related to the high mesoscale activity in the physical environment. Estimated zooplankton respiration rates relative to primary production were among the highest cited values at similar latitudes, inducing a high contribution of migrant zooplankton respiration to carbon flux. Despite the relatively low biomass values of planktonic components in quasi-steady state, the availability of micro- and macronutrients related to physical mesoscale patterns in the waters surrounding the MA, the fueling by DDN, and the relatively high rates of plankton production and metabolism estimated during OUTPACE may explain the productive food chain ending with valuable fisheries in this region.

scholarly journals Mesozooplankton structure and functioning in the western tropical South Pacific along the 20° parallel south during the OUTPACE survey (February–April 2015)

2018 ◽  
Author(s):  
François Carlotti ◽  
Marc Pagano ◽  
Loïc Guilloux ◽  
Katty Donoso ◽  
Valentina Valdés ◽  
...  
Keyword(s):  
Primary Production ◽  
New Caledonia ◽  
Size Structure ◽  
South Pacific ◽  
French Polynesia ◽  
Grazing Impact ◽  
Coral Sea ◽  
Carbon Demand ◽  
Excretion Rates ◽  
Density And Biomass

Abstract. This paper presents results on the spatial and temporal distribution patterns of mesozooplankton in the western tropical South Pacific along the 20 °S south visited during austral summer (February–April 2015). By contributing to the interdisciplinary OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project (Moutin et al., 2017), the specific aims of this study dedicated to mesozooplankton observations were (1) to document the responses of zooplankton in terms of species diversity, density and biomass along the transect, and (2) to characterize the trophic pathways from primary production to large mesozooplanktonic organisms. Along a West-East transect of 4000 km from New Caledonia to the French Polynesia, 15 short-duration stations (SD-1 to SD-15, 8 hours each) dedicated to a large-scale description, and three long-duration stations (LD-A to LD-C, 5days each), respectively positioned (1) in offshore northern waters of New Caledonia, (2) near Niue Island, and (3) in the subtropical Pacific gyre near the Cook Islands, were sampled with a Bongo Net with 120 μm mesh size net for quantifying mesozooplankton abundance, biomass, community taxonomy and size structure, and size fractionated content of δ15N. Subsequently, the contribution of Diazotroph Derived Nitrogen (DDN (%) to zooplankton δ15N (ZDDN) values at each station was calculated, as well as an estimation of zooplankton carbon demand and grazing impact and of zooplankton excretion rates. The mesozooplankton community showed a general decreasing trend in abundance and biomass from West to East, with a clear drop in the ultra-oligotrophic waters of the subtropical Pacific gyre (LD-C, SD-14 and SD-15). Higher abundance and biomass corresponded to higher primary production of more or less ephemeral blooms linked to complex mesoscale circulation in the Coral Sea and between the longitudes 170–180 °W. Copepods were the most abundant group (68 to 86 % of total abundance), slightly increasing in contribution from west to east while, in parallel, gelatinous plankton decreased (dominated by appendicularians) and other holoplankton. Detritus in the net tow samples represented 20–50 % of the biomass, the lowest and the highest values being obtained in the subtropical Pacific gyre and in the Coral Sea, respectively, linked to the local primary production and the biomass and growth rates of zooplanktonic populations. Taxonomic compositions showed a high degree of similarity across the whole region, however, with a moderate difference in subtropical Pacific gyre. Several copepod taxa, known to have trophic links with Trichodesmium, presented positive relationships with Trichodesmium abundance, such as the Harpacticoids Macrosetella, Microsetella and Miracia, and the Poecilostomatoids Corycaeus and Oncaea. At the LD stations, the populations initially responded to local spring blooms with a large production of larval forms, reflected in increasing abundances but with limited (station LD-A) or no (station LD-A) biomass changes. Diazotrophs contributed up to 67 and 75 % to zooplankton biomass in the western and central Melanesian Archipelago regions respectively, but strongly decreased to an average of 22 % in the subtropical Pacific gyre (GY) and down to 7 % occurring in the most eastern station (SD-15). Using allometric relationships, specific zooplankton ingestion rates were estimated between 0.55 and 0.64 d−1 with the highest mean value at the bloom station (LD-B) and the lowest in GY, whereas estimated weight specific excretion rates ranged between 0.1 and 0.15 d−1 for NH4 and between 0.09 and 9.12 d−1 for PO4. Daily grazing pressure on phytoplankton stocks and daily regeneration by zooplankton were as well estimated for the different regions showing contrasted impacts between MA and GY regions. For the 3 LD stations, it was not possible to find any relationship between the abundance and biomass in the water column and swimmers found in sediment traps. Diel vertical migration of zooplankton, which obviously occurs from observed differences in day and night samples, might strongly influence the community of swimmers in traps.

scholarly journals Seasonal patterns in phytoplankton biomass across the northern and deep Gulf of Mexico: a numerical model study

2018 ◽  
Vol 15 (11) ◽  
pp. 3561-3576 ◽  
Author(s):  
Fabian A. Gomez ◽  
Sang-Ki Lee ◽  
Yanyun Liu ◽  
Frank J. Hernandez Jr. ◽  
Frank E. Muller-Karger ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Primary Production ◽  
Phytoplankton Biomass ◽  
Mississippi Delta ◽  
Three Dimensional ◽  
Rate Of Change ◽  
Integrated Analysis ◽  
Biogeochemical Model ◽  
Seasonal Patterns ◽  
Biogeochemical Models

Abstract. Biogeochemical models that simulate realistic lower-trophic-level dynamics, including the representation of main phytoplankton and zooplankton functional groups, are valuable tools for improving our understanding of natural and anthropogenic disturbances in marine ecosystems. Previous three-dimensional biogeochemical modeling studies in the northern and deep Gulf of Mexico (GoM) have used only one phytoplankton and one zooplankton type. To advance our modeling capability of the GoM ecosystem and to investigate the dominant spatial and seasonal patterns of phytoplankton biomass, we configured a 13-component biogeochemical model that explicitly represents nanophytoplankton, diatoms, micro-, and mesozooplankton. Our model outputs compare reasonably well with observed patterns in chlorophyll, primary production, and nutrients over the Louisiana–Texas shelf and deep GoM region. Our model suggests silica limitation of diatom growth in the deep GoM during winter and near the Mississippi delta during spring. Model nanophytoplankton growth is weakly nutrient limited in the Mississippi delta year-round and strongly nutrient limited in the deep GoM during summer. Our examination of primary production and net phytoplankton growth from the model indicates that the biomass losses, mainly due to zooplankton grazing, play an important role in modulating the simulated seasonal biomass patterns of nanophytoplankton and diatoms. Our analysis further shows that the dominant physical process influencing the local rate of change of model phytoplankton is horizontal advection in the northern shelf and vertical mixing in the deep GoM. This study highlights the need for an integrated analysis of biologically and physically driven biomass fluxes to better understand phytoplankton biomass phenologies in the GoM.

The Contribution of Boreal Spring South Pacific Atmospheric Variability to El Niño Occurrence

2020 ◽  
Vol 33 (19) ◽  
pp. 8301-8313
Author(s):  
Qingye Min ◽  
Renhe Zhang
Keyword(s):  
El Niño ◽  
El Nino ◽  
South Pacific ◽  
Tropical Pacific ◽  
The South ◽  
Atmospheric Variability ◽  
Boreal Spring ◽  
El Niño Events ◽  
El Nino Events ◽  
The Tropical Pacific

AbstractDespite the fact that great efforts have been made to improve the prediction of El Niño events, it remains challenging because of limited understanding of El Niño and its precursors. This research focuses on the influence of South Pacific atmospheric variability on the development of the sea surface temperature anomaly (SSTA) in the tropical Pacific. It is found that as early as in the boreal spring of El Niño years, the sea level pressure anomaly (SLPA) shows a configuration characterized by two significant negative anomaly centers in the north and a positive anomaly center in the south between the subtropics and high latitudes in South Pacific. Such an anomalous SLPA pattern becomes stronger in the following late boreal spring and summer associated with the strengthening of westerly anomalies in the tropical Pacific, weakening the southeasterly trade winds and promoting the warming of tropical eastern Pacific, which is conducive to the development of El Niño events. It is demonstrated that the SLPA pattern in boreal spring revealed in this study is closely associated with boreal summer South Pacific Oscillation (SPO) and South Pacific meridional mode (SPMM). As a precursor in boreal spring, the prediction skill of the South Pacific SLPA in boreal spring for the SSTA in the eastern equatorial Pacific is better than that of the SPMM. This study is helpful to deepen our understanding of the contribution of South Pacific extratropical atmospheric variability to El Niño occurrence.

scholarly journals Evolution of primary production and its drivers on the Lebanese coast between 1986 and 2013

2020 ◽  
Vol 20 (4) ◽  
pp. 207-217
Author(s):  
Ali Fadel ◽  
Lama Salameh ◽  
Malak Kanj ◽  
Ahmad Kobaissi
Keyword(s):  
Primary Production ◽  
Phytoplankton Community ◽  
Controlling Factors ◽  
Sea Surface ◽  
Biogeochemical Model ◽  
Average Temperature ◽  
Biogeochemical Models ◽  
Increasing Trend ◽  
Production Dynamics ◽  
Lebanese Coast

AbstractPhysical-biogeochemical models help us to understand the dynamics and the controlling factors of primary production. In this study, the outputs of a validated hydrodynamic and biogeochemical model were used to elucidate the primary production dynamics between 1992 and 2012 for three studied sites on the Lebanese coast: Naqoura, Beirut, and Tripoli. The results showed that primary production presents a homogeneous spatial distribution along the Lebanese coastline. The phytoplankton community has a low optimal temperature. The thermocline develops in March, with maximum stratification in August and fades in October. Chlorophyll, dissolved oxygen and salinity were positively correlated throughout the water column. A significant increasing trend of sea surface temperature was found on the Lebanese coast over 27 years, between 1986 and 2013. Annual averages increased from 22°C in 1986 to 23.1°C in 2013 with the highest recorded average temperature of 23.7 °C in 2010.

scholarly journals Impacts of Interhemispheric Asymmetric Thermal Forcing on Tropical Pacific Climate: Surface Air–Sea Coupling and Subduction

2013 ◽  
Vol 26 (2) ◽  
pp. 575-582 ◽  
Author(s):  
Chun Li ◽  
Lixin Wu ◽  
Shang-Ping Xie
Keyword(s):  
General Circulation ◽  
Cold Water ◽  
Circulation Model ◽  
South Pacific ◽  
Feedback Mechanism ◽  
Tropical Pacific ◽  
Second Stage ◽  
The North ◽  
Mean Circulation ◽  
The Tropical Pacific

Abstract Paleoclimate observations and modeling studies suggest that extratropical climate change affects the tropical Pacific. A global coupled general circulation model is used to investigate the equatorial Pacific response to extratropical surface heat flux forcing that is downward (upward) poleward of 40°N (S). The equatorial response consists of two distinct stages: the zonal sea surface temperature (SST) gradient strengthens for the first two to three decades and then weakens afterward. In the first stage, fast surface air–sea coupling feedback mechanism communicates the extratropical warming (cooling) from the North (South) Pacific toward the equator. The second stage is characterized by a basinwide shoaling of the tropical Pacific thermocline as the subtropical cell (STC) advects cold water from the South Pacific along the thermocline. This preference of Southern Hemisphere anomalies is due to the meridional asymmetry in the mean circulation: the interior pathway for STC is open south but partially blocked north of the equator. Paleoclimate implications are discussed.

scholarly journals Oceanic Response to Idealized Net Atmospheric Freshwater in the Pacific at the Decadal Time Scale*

2005 ◽  
Vol 35 (12) ◽  
pp. 2467-2486 ◽  
Author(s):  
Boyin Huang ◽  
Vikram M. Mehta ◽  
Niklas Schneider
Keyword(s):  
Pacific Ocean ◽  
South Pacific ◽  
Tropical Pacific ◽  
Temperature Anomalies ◽  
The Pacific ◽  
Basin Scale ◽  
The Pacific Ocean ◽  
The Tropics ◽  
North And South ◽  
The Tropical Pacific

Abstract In the study of decadal variations of the Pacific Ocean circulations and temperature, the role of anomalous net atmospheric freshwater [evaporation minus precipitation minus river runoff (EmP)] has received scant attention even though ocean salinity anomalies are long lived and can be expected to have more variance at low frequencies than at high frequencies. To explore the magnitude of salinity and temperature anomalies and their generation processes, the authors studied the response of the Pacific Ocean to idealized EmP anomalies in the Tropics and subtropics using an ocean general circulation model developed at the Massachusetts Institute of Technology. Simulations showed that salinity anomalies generated by the anomalous EmP were spread throughout the Pacific basin by mean flow advection. This redistribution of salinity anomalies caused adjustments of basin-scale ocean currents, which further resulted in basin-scale temperature anomalies due to changes in heat advection caused by anomalous currents. In this study, the response of the Pacific Ocean to magnitudes and locations of anomalous EmP was linear. When forced with a positive EmP anomaly in the subtropical North (South) Pacific, a cooling occurred in the western North (South) Pacific, which extended to the tropical and South (North) Pacific, and a warming occurred in the eastern North (South) Pacific. When forced with a negative EmP anomaly in the tropical Pacific, a warming occurred in the tropical Pacific and western North and South Pacific and a cooling occurred in the eastern North Pacific near 30°N and the South Pacific near 30°S. The temperature changes (0.2°C) in the tropical Pacific were associated with changes in the South Equatorial Current. The temperature changes (0.8°C) in the subtropical North and South Pacific were associated with changes in the subtropical gyres. The temperature anomalies propagated from the tropical Pacific to the subtropical North and South Pacific via equatorial divergent Ekman flows and poleward western boundary currents, and they propagated from the subtropical North and South Pacific to the western tropical Pacific via equatorward-propagating coastal Kelvin waves and to the eastern tropical Pacific via eastward-propagating equatorial Kelvin waves. The time scale of temperature response was typically much longer than that of salinity response because of slow adjustment times of ocean circulations. These results imply that the slow response of ocean temperature due to anomalous EmP in the Tropics and subtropics may play an important role in the Pacific decadal variability.

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