Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA

Abstract Mesozooplankton biomass in the North Pacific Subtropical Gyre (NPSG), as measured by the Hawaii Ocean Time-series program at Station ALOHA (22.45°N 158°W), increased significantly from 1994 to 2002. The changes occurred at a rate of 60 mg DW m−2 yr−1 for night-time collections and 45 mg DW m−2 yr−1 for daytime collections. Principal components analysis indicates that the 9-year trend was driven by an increase in small (0.5–2.0 mm) zooplankton that do not migrate on a diel cycle. This plankton class is known to increase during the summer at Station ALOHA when the water column is most stratified, and a strong summertime response is also apparent within the long-term trend from 1998 through 2002. Both long-term and seasonal changes in zooplankton biomass at Station ALOHA can be linked to an enhanced role of nitrogen fixation in ecosystem productivity. Climate forcing from El Niño–Southern Oscillation (ENSO) events may have influenced nitrogen fixation, general ecosystem productivity, and thus zooplankton biomass in the NPSG. However, it is difficult to evaluate the effect of climate cycles in this region without the benefit of a longer time-series at Station ALOHA. Because biomass trends in higher-level consumers like mesozooplankton can have cascading influences on lower levels, understanding the relative roles of bottom-up climate influences and top-down trophic processes will be important in resolving long-term trends in community composition and structure in the subtropical North Pacific Ocean.

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Massive export of diazotrophs across the tropical south Pacific Ocean

10.1101/2021.05.07.442706 ◽

2021 ◽

Author(s):

Sophie Bonnet ◽

Mar Benavides ◽

Mercedes Camps ◽

Antoine Torremocha ◽

Olivier Grosso ◽

...

Keyword(s):

Pacific Ocean ◽

South Pacific ◽

Sediment Traps ◽

Marine Snow ◽

Turnover Rates ◽

Sequencing Data ◽

Particulate Nitrogen ◽

Nitrogen Export ◽

South Pacific Ocean ◽

Species Specific

Diazotrophs are widespread microorganisms that regulate marine productivity in 60% of our oceans by alleviating nitrogen limitation. Yet, their contribution to organic carbon and nitrogen export fluxes has never been quantified, making an assessment of their impact on the biological carbon pump impossible. Here, we examine species-specific fates of several groups of globally-distributed unicellular (UCYN) and filamentous diazotrophs in the mesopelagic ocean. We used an innovative approach consisting of the combined deployment of surface-tethered drifting sediment traps, Marine Snow Catcher, and Bottle-net, in which we performed nifH sequencing and quantitative PCR on major diazotroph groups across the subtropical South Pacific Ocean. nifH sequencing data from sediment traps deployed at 170 m, 270 m and 1000 m provide clear evidence that cyanobacterial and non-cyanobacterial diazotrophs are systematically present in sinking particles down to 1000 m, with export fluxes being the highest for the UCYN-A1 symbiosis, followed by UCYN-B or Trichodesmium (depending on station and depth), Gamma A and UCYN-C. Specific export turnover rates (a metric similar to the export efficiency adapted to organisms) point to a more efficient export of UCYN groups relative to the filamentous Trichodesmium. This is further confirmed by Marine Snow catcher data showing that the proportion of sinking cells was significantly higher for UCYN compared to Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were indeed recurrently found embedded in large (> 50 micrometers) seemingly organic aggregates, or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, facilitating the export. Overall, diazotrophs accounted for 6-13% (170 m) to 45-100% (1000 m) of the total particulate nitrogen export fluxes in our study. We thus conclude that diazotrophs are important contributors to carbon sequestration in the subtropical South Pacific Ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.

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Seasonal Changes in Nitrogen Compartments of Lakes Under Different Loading Conditions

Journal of the Fisheries Research Board of Canada ◽

10.1139/f78-173 ◽

1978 ◽

Vol 35 (8) ◽

pp. 1095-1101 ◽

Cited By ~ 13

Author(s):

C. F.-H. Liao ◽

D. R. S. Lean

Keyword(s):

Nitrogen Fixation ◽

Chlorophyll A ◽

Seasonal Changes ◽

Weight Ratio ◽

Organic N ◽

Lake Ecosystem ◽

Potential Contribution ◽

Pore Waters ◽

Particulate Nitrogen ◽

Total N

To develop a better conceptual picture for nitrogen dynamics in lake ecosystems, the seasonal changes in the concentrations of nitrate, ammonia, dissolved organic and particulate nitrogen were monitored in the water contained in three limnocorrals located in the Bay of Quinte, Ontario. Following the decline in summer particulate N, there was an increase in ammonia. This was followed by an increase in nitrate throughout the winter suggesting that nitrification was a significant process. To assess the potential contribution of the sediments, ammonia, nitrate, and dissolved organic N were measured for pore waters and total N, exchangeable ammonia, and nitrate in the solid fraction. The influence of nutrient addition was evaluated by comparing corral I, receiving no nutrient additions, with corral II to which phosphate was added (0.92 g P/m2 per yr) and corral III which received the same amount of phosphate plus nitrate at an N:P weight ratio of 13. Increases and decreases in phosphorus, nitrogen, and chlorophyll a seemed to take place in all three corrals at the same time. There was a considerable delay in the response to nutrient enrichment, but the addition of nitrate and phosphate eventually resulted in a chlorophyll a level greater than that for phosphate enrichment alone. The importance of sediments as a net source of N for the planktonic community was insignificant, but the flux rate for nitrogen to and from the sediment was important. During the summer an input–output budget showed that nitrate loading plus N2 fixation was approximately equal to the net change in the total combined N in the water. This implied that denitrification was not important, but over 12 mo most of the nitrogen that entered the system was lost; hence denitrification must be important at certain times. A lake ecosystem tends to compensate for low or high levels of NO3 loading. Without the addition of nitrogen the requirement was partly met through nitrogen fixation and this can be enhanced by P enrichment. Key words: ammonia, nitrate, nutrient kinetics, nitrogen fixation, denitrification, sediments

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Supplemental Material for Distractor Intrusions Are the Result of Delayed Attentional Engagement: A New Temporal Variability Account of Attentional Selectivity in Dynamic Visual Tasks

Journal of Experimental Psychology General ◽

10.1037/xge0000789.supp ◽

2020 ◽

Keyword(s):

Temporal Variability ◽

Visual Tasks ◽

Attentional Engagement ◽

Attentional Selectivity

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Factors controlling nitrogen fixation in temperate seagrass beds

Marine Ecology Progress Series ◽

10.3354/meps11247 ◽

2015 ◽

Vol 525 ◽

pp. 41-51 ◽

Cited By ~ 15

Author(s):

PLM Cook ◽

V Evrard ◽

RJ Woodland

Keyword(s):

Nitrogen Fixation ◽

Seagrass Beds

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Fine-scale taxonomic and temporal variability in the energy density of invertebrate prey of juvenile Chinook salmon Oncorhynchus tshawytscha

Marine Ecology Progress Series ◽

10.3354/meps13513 ◽

2020 ◽

Vol 655 ◽

pp. 185-198

Author(s):

J Weil ◽

WDP Duguid ◽

F Juanes

Keyword(s):

Energy Density ◽

Chinook Salmon ◽

Temporal Variability ◽

Energy Content ◽

Single Point ◽

Single Species ◽

Fine Scale ◽

Temporal Differences ◽

Juvenile Chinook Salmon ◽

Juvenile Chinook

Variation in the energy content of prey can drive the diet choice, growth and ultimate survival of consumers. In Pacific salmon species, obtaining sufficient energy for rapid growth during early marine residence is hypothesized to reduce the risk of size-selective mortality. In order to determine the energetic benefit of feeding choices for individuals, accurate estimates of energy density (ED) across prey groups are required. Frequently, a single species is assumed to be representative of a larger taxonomic group or related species. Further, single-point estimates are often assumed to be representative of a group across seasons, despite temporal variability. To test the validity of these practices, we sampled zooplankton prey of juvenile Chinook salmon to investigate fine-scale taxonomic and temporal differences in ED. Using a recently developed model to estimate the ED of organisms using percent ash-free dry weight, we compared energy content of several groups that are typically grouped together in growth studies. Decapod megalopae were more energy rich than zoeae and showed family-level variability in ED. Amphipods showed significant species-level variability in ED. Temporal differences were observed, but patterns were not consistent among groups. Bioenergetic model simulations showed that growth rate of juvenile Chinook salmon was almost identical when prey ED values were calculated on a fine scale or on a taxon-averaged coarse scale. However, single-species representative calculations of prey ED yielded highly variable output in growth depending on the representative species used. These results suggest that the latter approach may yield significantly biased results.

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