scholarly journals Contribution and pathways of diazotroph derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

2016 ◽  
Author(s):  
B. P. V. Hunt ◽  
S. Bonnet ◽  
H. Berthelot ◽  
B. J. Conroy ◽  
R. A. Foster ◽  
...  
Keyword(s):  
N2 Fixation ◽  
New Caledonia ◽  
Austral Summer ◽  
Zooplankton Community ◽  
Zooplankton Biomass ◽  
Diazotrophic Cyanobacteria ◽  
Subtropical Oceans ◽  
Isotope Mixing Models

Abstract. In oligotrophic tropical and subtropical oceans, where strong stratification can limit the replenishment of surface nitrate, dinitrogen (N2) fixation by diazotrophs can represent a significant source of nitrogen (N) for primary production. The VAHINE experiment was designed to examine the fate of diazotroph derived nitrogen (DDN) in such ecosystems. In austral summer 2013 three large (~ 50 m3) in situ mesocosms were deployed for 23 days in the New Caledonia lagoon, an ecosystem that typifies the low-nutrient, low-chlorophyll environment, to stimulate diazotroph production. The zooplankton component of the study aimed to measure the incorporation of DDN into zooplankton biomass, and assess the role of direct diazotroph grazing by zooplankton as a DDN uptake pathway. Inside the mesocosms the diatom-diazotroph association (DDA) het-1 predominated during day 5-15 while the unicellular diazotrophic cyanobacteria UCYN-C predominated during day 15-23. A Trichodesmium bloom was observed in the lagoon (outside the mesocosms) towards the end of the experiment. The zooplankton community was dominated by copepods (63 % of total abundance) for the duration of the experiment. Using two source N isotope mixing models we estimated a mean ~ 30 % contribution of DDN to zooplankton biomass at the start of the experiment, indicating that the natural summer peak of N2 fixation in the lagoon was already contributing significantly to the zooplankton. Stimulation of N2 fixation BNF in the mesocosms corresponded with a generally low level enhancement of DDN contribution to zooplankton biomass, but with a peak of ~ 70 % in Mesocosm 1 following the UCYN-C bloom. qPCR analysis targeting four of the common diazotroph groups present in the mesocosms (Trichodesmium, het-1, het-2, UCYN-C) demonstrated that all were ingested by copepod grazers and that target abundance generally corresponded with their in situ abundance. 15N2 labeled grazing experiments provided evidence for direct ingestion and assimilation of UCYN-C-derived N by the zooplankton, but not for het-1 and Trichodesmium, supporting an important role of secondary pathways of DDN to the zooplankton for the latter groups, i.e., DDN contributions to the dissolved N pool and uptake by non-diazotrophs. This study appears to provide the first evidence of direct UCYN-C grazing by zooplankton, and indicates that UCYN-C-derived N contributes significantly to the zooplankton food web in the New Caledonia lagoon though a combination of direct grazing and secondary pathways.

scholarly journals Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

2016 ◽  
Vol 13 (10) ◽  
pp. 3131-3145 ◽  
Author(s):  
Brian P. V. Hunt ◽  
Sophie Bonnet ◽  
Hugo Berthelot ◽  
Brandon J. Conroy ◽  
Rachel A. Foster ◽  
...  
Keyword(s):  
N2 Fixation ◽  
New Caledonia ◽  
Austral Summer ◽  
Zooplankton Community ◽  
Zooplankton Biomass ◽  
Diazotrophic Cyanobacteria ◽  
Subtropical Oceans ◽  
Isotope Mixing Models

Abstract. In oligotrophic tropical and subtropical oceans, where strong stratification can limit the replenishment of surface nitrate, dinitrogen (N2) fixation by diazotrophs can represent a significant source of nitrogen (N) for primary production. The VAHINE (VAriability of vertical and tropHIc transfer of fixed N2 in the south-wEst Pacific) experiment was designed to examine the fate of diazotroph-derived nitrogen (DDN) in such ecosystems. In austral summer 2013, three large ( ∼  50 m3) in situ mesocosms were deployed for 23 days in the New Caledonia lagoon, an ecosystem that typifies the low-nutrient, low-chlorophyll environment, to stimulate diazotroph production. The zooplankton component of the study aimed to measure the incorporation of DDN into zooplankton biomass, and assess the role of direct diazotroph grazing by zooplankton as a DDN uptake pathway. Inside the mesocosms, the diatom–diazotroph association (DDA) het-1 predominated during days 5–15 while the unicellular diazotrophic cyanobacteria UCYN-C predominated during days 15–23. A Trichodesmium bloom was observed in the lagoon (outside the mesocosms) towards the end of the experiment. The zooplankton community was dominated by copepods (63 % of total abundance) for the duration of the experiment. Using two-source N isotope mixing models we estimated a mean  ∼  28 % contribution of DDN to zooplankton nitrogen biomass at the start of the experiment, indicating that the natural summer peak of N2 fixation in the lagoon was already contributing significantly to the zooplankton. Stimulation of N2 fixation in the mesocosms corresponded with a generally low-level enhancement of DDN contribution to zooplankton nitrogen biomass, but with a peak of  ∼  73 % in mesocosm 1 following the UCYN-C bloom. qPCR analysis targeting four of the common diazotroph groups present in the mesocosms (Trichodesmium, het-1, het-2, UCYN-C) demonstrated that all four were ingested by copepod grazers, and that their abundance in copepod stomachs generally corresponded with their in situ abundance. 15N2 labelled grazing experiments therefore provided evidence for direct ingestion and assimilation of UCYN-C-derived N by the zooplankton, but not for het-1 and Trichodesmium, supporting an important role of secondary pathways of DDN to the zooplankton for the latter groups, i.e. DDN contributions to the dissolved N pool and uptake by nondiazotrophs. This study appears to provide the first evidence of direct UCYN-C grazing by zooplankton, and indicates that UCYN-C-derived N contributes significantly to the zooplankton food web in the New Caledonia lagoon through a combination of direct grazing and secondary pathways.

scholarly journals Biogeochemical fluxes and fate of diazotroph derived nitrogen in the food web after a phosphate enrichment: Modeling of the VAHINE mesocosms experiment

2016 ◽  
Author(s):  
A. Gimenez ◽  
M. Baklouti ◽  
S. Bonnet ◽  
T. Moutin
Keyword(s):  
Primary Production ◽  
Food Web ◽  
N2 Fixation ◽  
New Caledonia ◽  
Mechanistic Model ◽  
In Situ Experiment ◽  
Planktonic Food ◽  
Biogeochemical Fluxes

Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Noumea lagoon (New Caledonia) aimed to assess the role of the nitrogen input through N2 fixation on carbon production and export, and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1D-vertical biogeochemical mechanistic model was used in addition to the in situ experiment to complement our comprehension of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N), phosphate (P) fluxes. The mesocosms were intentionally enriched with 0.8 μmol.L−1 of P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N2 fixation, primary production and C export increased by 201, 208 and 87 %, respectively, consistent with the observed trends observed in the mesocosms (+ 124 %, + 141 %, + 261 % for N2 fixation, PP and C export, respectively). The increase in primary and export productions became significant 10 days after the DIP enrichment, indicating that i) several days were necessary to obtain a significant response at the population scale, and ii) classical methods (short-term microscosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model allowed to follow the fate of fixed N2 by providing over time the proportion of DDN in each compartment (mineral and organic) of the model. At the end of the simulation (25 days), 43 % of the DDN was found in the non-diazotroph organisms, 33 % in diazotrophs, 16 % in the dissolved organic nitrogen pool, 3 % in the particulate detrital organic pool and 5 % in traps, indicating that N2 fixation efficiently benefitted to non-diazotrophic organisms and contributed to C export.

scholarly journals Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment

2016 ◽  
Vol 13 (17) ◽  
pp. 5103-5120 ◽  
Author(s):  
Audrey Gimenez ◽  
Melika Baklouti ◽  
Sophie Bonnet ◽  
Thierry Moutin
Keyword(s):  
Primary Production ◽  
Food Web ◽  
N2 Fixation ◽  
New Caledonia ◽  
Mechanistic Model ◽  
In Situ Experiment ◽  
Planktonic Food ◽  
Biogeochemical Fluxes

Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Nouméa lagoon (New Caledonia), where high N2 fixation rates and abundant diazotroph organisms were observed, aimed to assess the role of the nitrogen input through N2 fixation in carbon production and export and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1-D vertical biogeochemical mechanistic model was used in addition to the in situ experiment to enrich our understanding of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N) and phosphate (P) fluxes. The mesocosms were intentionally enriched with  ∼  0.8 µmol L−1 of inorganic P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N2 fixation, primary production (PP) and C export increased by 201, 208 and 87 %, respectively, consistent with the trends observed in the mesocosms (+124, +141 and +261 % for N2 fixation, PP and C export, respectively). In total, 5–10 days were necessary to obtain an increase in primary and export productions after the dissolved inorganic phosphate (DIP) enrichment, thereby suggesting that classical methods (short-term microcosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model enabled us to monitor the fate of fixed N2 by providing the proportion of DDN in each compartment (inorganic and organic) of the model over time. At the end of the simulation (25 days), 43 % of the DDN was found in the non-diazotroph organisms, 33 % in diazotrophs, 16 % in the dissolved organic nitrogen pool, 3 % in the particulate detrital organic pool and 5 % in traps, indicating that N2 fixation was of benefit to non-diazotrophic organisms and contributed to C export.

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 Zooplankton Community Responses to Oxygen Stress

Water ◽  
2020 ◽  
Vol 12 (3) ◽  
pp. 706 ◽  
Author(s):  
Maciej Karpowicz ◽  
Jolanta Ejsmont-Karabin ◽  
Joanna Kozłowska ◽  
Irina Feniova ◽  
Andrew R. Dzialowski
Keyword(s):  
Zooplankton Community ◽  
Oxygen Depletion ◽  
Zooplankton Biomass ◽  
Oxygen Stress ◽  
Carbon Transfer ◽  
Oxygen Conditions ◽  
Anoxic Layer ◽  
Zooplankton Communities ◽  
The Impact

Recent changes in climate and eutrophication have caused increases in oxygen depletion in both freshwater and marine ecosystems. However, the impact of oxygen stress on zooplankton, which is the major trophic link between primary producers and fish, remains largely unknown in lakes. Therefore, we studied 41 lakes with different trophic and oxygen conditions to assess the role of oxygen stress on zooplankton communities and carbon transfer between phytoplankton and zooplankton. Samples were collected from each lake at the peak of summer stratification from three depth layers (the epilimnion, metalimnion, and hypolimnion). Our results revealed that freshwater zooplankton were relatively tolerant to anoxic conditions and the greatest changes in community structure were found in lakes with the highest oxygen deficits. This caused a switch in dominance from large to small species and reduced the zooplankton biomass in lower, anoxic layers of water, but not in the upper layers of water where the oxygen deficits began. This upper anoxic layer could thus be a very important refuge for zooplankton to avoid predation during the day. However, the reduction of zooplankton in the lower water layers was the main factor that reduced the effectiveness of carbon transfer between the phytoplankton and zooplankton.

Cladoceran- and copepod-dominated zooplankton communities graze at similar rates in low-productivity lakes

1998 ◽  
Vol 55 (2) ◽  
pp. 414-422 ◽  
Author(s):  
Hélène Cyr
Keyword(s):  
Zooplankton Community ◽  
Grazing Rate ◽  
Taxonomic Composition ◽  
Zooplankton Biomass ◽  
Phytoplankton Assemblage ◽  
Different Types ◽  
The Difference ◽  
Zooplankton Communities ◽  
Copepod Biomass

Many studies suggest that the taxonomic composition of a zooplankton community should determine its grazing rate and selectivity for different types of particles. It is generally believed that copepod-dominated communities should (i) have lower grazing rates and (ii) consume larger particles than communities dominated by large cladocerans. I tested these hypotheses in situ by comparing zooplankton grazing in 19 communities from low-productivity lakes where the zooplankton ranged from >99% copepod biomass to >90% large cladoceran biomass (Holopedium gibberum, Daphnia spp.). The zooplankton grazed 1-14% of total chlorophyll per day and 0-17% of the chlorophyll in algae <35 µm per day. Grazing rates increased with increasing zooplankton biomass (r2 = 0.34, P < 0.01), but once the effect of zooplankton biomass was accounted for, similar grazing rates were found in copepod- and in cladoceran-dominated communities. The difference in grazing rates on small algae and on the whole phytoplankton assemblage, on the other hand, varied systematically with zooplankton taxonomic composition. Holopedium-dominated communities were most efficient at grazing algae <35 µm, Bosmina-dominated communities had similar grazing rates on algae <35 µm and on the whole phytoplankton assemblage, and copepod-dominated communities had similar or slightly higher grazing rates on the whole phytoplankton assemblage. Qualitative differences in grazing selectivity of different zooplankton taxa are observed in complex natural communities.

scholarly journals Diazotrophy as the main driver of planktonic production and biogeochemical C, N, P cycles in theWestern Tropical South Pacific Ocean: results from a 1DV biogeochemical-physical coupled model

2018 ◽  
Author(s):  
Audrey Gimenez ◽  
Melika Baklouti ◽  
Thierry Moutin
Keyword(s):  
Seasonal Variations ◽  
N2 Fixation ◽  
Mixed Layer Depth ◽  
Coupled Model ◽  
Austral Summer ◽  
South Pacific ◽  
Inorganic Phosphorus ◽  
South Pacific Ocean ◽  
Surface Production

Abstract. The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the Western Tropical South Pacific (WTSP) during the austral summer (March–April 2015). The aim of the OUTPACE project is to investigate a longitudinal gradient of biological and biogeochemical features in the WTSP, and especially the role of N2 fixation on the C, N, P cycles. Two contrasted regions were considered: the Western Melanesian Archipelago (WMA), characterized by high N2 fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the Western south Pacific GYre (WGY), characterized by very low N2 fixation rates, low surface production rates and high DIP concentrations. A one-dimensional biogeochemical – physical coupled model was used to investigate the role of N2 fixation in the WTSP by running two identical simulations, only differing by the presence or not of diazotrophs. We evidenced that the nitracline and the phosphacline had to be respectively deeper and shallower than the Mixed-Layer Depth (MLD) to bring N-depleted and P-repleted waters to the surface during winter mixing, thereby creating favorable conditions for the development of diazotrophs. We also concluded that a preferential regeneration of the detrital phosphorus (P) matter was necessary to obtain this gap between the nitracline and the phosphacline depths, as the nutricline depths significantly depend on the regeneration of organic matter in the water column. Moreover, the model enabled us to highlight the presence of seasonal variations in upper surface waters in the simulation standing for WMA, where diazotrophs provided a new source of nitrogen (N) to their ecosystem, whereas no seasonal variations were obtained in the simulation standing for WGY, in absence of diazotrophs. These main results emphasized the fact that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on N2 fixation in a crucial way.

{10-12} Twinning Mechanism During In Situ Micro-Tensile Loading of Pure Mg: Role of Basal Slip and Twin-Twin Boundary Interaction

2020 ◽  
Author(s):  
Nicolò Maria della Ventura ◽  
Szilvia Kalácska ◽  
Daniele Casari ◽  
Thomas Edward James Edwards ◽  
Johann Michler ◽  
...  
Keyword(s):  
Twin Boundary ◽  
Basal Slip ◽  
Tensile Loading ◽  
Boundary Interaction

The role of (bio)surfactant sorption in promoting the bioavailability of nutrients localized at the solid-water interface

1999 ◽  
Vol 39 (7) ◽  
pp. 91-98 ◽  
Author(s):  
Ryan N. Jordan ◽  
Eric P. Nichols ◽  
Alfred B. Cunningham
Keyword(s):  
Mass Transfer ◽  
Cell Membrane ◽  
Substrate Concentration ◽  
Water Interface ◽  
Industrial Systems ◽  
Solid Liquid Interface ◽  
Solid Liquid ◽  
Surfactant Sorption

Bioavailability is herein defined as the accessibility of a substrate by a microorganism. Further, bioavailability is governed by (1) the substrate concentration that the cell membrane “sees,” (i.e., the “directly bioavailable” pool) as well as (2) the rate of mass transfer from potentially bioavailable (e.g., nonaqueous) phases to the directly bioavailable (e.g., aqueous) phase. Mechanisms by which sorbed (bio)surfactants influence these two processes are discussed. We propose the hypothesis that the sorption of (bio)surfactants at the solid-liquid interface is partially responsible for the increased bioavailability of surface-bound nutrients, and offer this as a basis for suggesting the development of engineered in-situ bioremediation technologies that take advantage of low (bio)surfactant concentrations. In addition, other industrial systems where bioavailability phenomena should be considered are addressed.

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